Liquid crystal diffraction element and optical device

Abstract

The present invention addresses the problem of providing: a liquid crystal diffraction element that, in a liquid crystal alignment pattern included in a reflection type liquid crystal diffraction element, has excellent diffraction efficiency even when one cycle, in which the direction of an optical axis derived from a liquid crystal compound changes while continuously rotating along one direction in a plane, is short; and an optical device using the liquid crystal diffraction element.　A liquid crystal diffraction element according to the present invention is provided with a cholesteric liquid crystal layer. The cholesteric liquid crystal layer has a liquid crystal alignment pattern in which the direction of an optical axis derived from a liquid crystal compound changes while continuously rotating along at least one direction in a plane. The liquid crystal compound has, on at least one surface of the cholesteric liquid crystal layer, a region having a tilt angle with respect to the surface of the cholesteric liquid crystal layer. Further, the cholesteric liquid crystal layer has a region in which the tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer varies in a plane.

Description

Liquid crystal diffraction element and optical device

[0001] The present invention relates to a liquid crystal diffraction element used in a head-mounted display or the like, and to an optical device having this liquid crystal diffraction element.

[0002] Display devices such as AR (Augmented Reality) glasses and head-mounted displays (HMDs) have been proposed as means for providing a virtual reality (VR) experience to an observer. These display devices are relatively small and easy to carry and wear, and are expected to serve as multifunctional devices that can replace smartphones, tablets, and the like.

[0003] An example of a diffraction element used in the display device is a reflective structure using a cholesteric liquid crystal layer formed by fixing a cholesteric liquid crystal phase, as described in Patent Document 1. Patent Document 1 describes a reflective structure that includes a plurality of spiral structures each extending along a predetermined direction, and has a first incident surface that intersects with the predetermined direction and on which light is incident, and a reflective surface that intersects with the predetermined direction and reflects the light incident from the first incident surface, the reflective surface being non-parallel to the first incident surface.

[0004] International Publication No. 2016 / 194961

[0005] The reflective structure (cholesteric liquid crystal layer) described in Patent Document 1, in short, has a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane, and is therefore able to diffract incident circularly polarized light and reflect circularly polarized light that is more tilted relative to the incident direction.

[0006] Here, further thinning and a wider viewing angle are required for the optical systems of display devices such as head-mounted displays. To achieve this, it is necessary to increase the diffraction angle of light at the end of a reflective diffraction element. In a reflective liquid crystal diffraction element having a cholesteric liquid crystal layer, such as the reflective structure described in Patent Document 1, where one period is the length of a 180° rotation of the optical axis direction derived from the liquid crystal compound along one in-plane direction, the diffraction angle of light can be increased by shortening this period. However, shortening the length of this period results in a problem of reduced diffraction efficiency. In particular, in the high diffraction angle region where the length of one period is as short as 1 μm, the diffraction efficiency deteriorates and sufficient diffraction efficiency cannot be obtained.

[0007] The present invention aims to solve the above-mentioned conventional problems and to provide a reflective liquid crystal diffraction element having excellent diffraction efficiency, even when the length of one period in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along one direction in the plane is short in the liquid crystal orientation pattern of the reflective liquid crystal diffraction element. Another object of the present invention is to provide an optical device using the above liquid crystal diffraction element.

[0008] To solve this problem, the present invention has the following configurations. [1] A liquid crystal diffraction element including a cholesteric liquid crystal layer, the cholesteric liquid crystal layer having a liquid crystal orientation pattern in which the orientation of an optical axis derived from a liquid crystal compound changes while continuously rotating along at least one direction in a plane, and at least one surface of the cholesteric liquid crystal layer has a region in which the liquid crystal compound has a tilt angle with respect to the surface of the cholesteric liquid crystal layer, and the cholesteric liquid crystal layer further has regions in which the tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer varies within the plane. [2] The liquid crystal diffraction element according to [1], wherein, when the length of a 180° rotation of the orientation of the optical axis derived from the liquid crystal compound in a plane is defined as one period, the length of one period in the liquid crystal orientation pattern has regions in which the length of the one period varies within the plane. [3] The liquid crystal diffraction element according to [2], wherein the length of one period in the liquid crystal orientation pattern gradually changes along the one direction, and the tilt angle of the liquid crystal compound gradually changes along the one direction. [4] The liquid crystal diffraction element according to [3], wherein the tilt angle of the liquid crystal compound increases along the one direction as the length of one period in the liquid crystal orientation pattern decreases along the one direction. [5] The liquid crystal diffraction element according to any one of [1] to [4], wherein the cholesteric liquid crystal layer has light and dark portions extending from one surface to the other surface in a cross-sectional image obtained by observing a cross section cut along the one direction and a thickness direction of the cholesteric liquid crystal layer with a scanning electron microscope, and there is a region in the thickness direction of the cholesteric liquid crystal layer where the tilt angle of the dark portion with respect to the one surface differs from the tilt angle of the liquid crystal compound. [6] The liquid crystal diffraction element according to any one of [1] to [5], comprising at least two of the cholesteric liquid crystal layers, each of the at least two cholesteric liquid crystal layers having a light portion and a dark portion extending from one surface to the other surface in a cross-sectional image obtained by observing a cross section cut along the one direction and a thickness direction of the cholesteric liquid crystal layer with a scanning electron microscope, and the inclination angles of the dark portions of the at least two cholesteric liquid crystal layers are different from each other.[7] A liquid crystal diffraction element comprising a cholesteric liquid crystal layer, wherein the cholesteric liquid crystal layer has a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in a plane, the cholesteric liquid crystal layer has a region in which the direction of minimum retardation is tilted with respect to the normal direction when retardation is measured from a normal direction to a main surface of the cholesteric liquid crystal layer and a direction tilted with respect to the normal direction, and the cholesteric liquid crystal layer has regions in which the direction of minimum retardation varies within the plane. [8] The liquid crystal diffraction element according to [7], wherein, when the length of a 180° rotation of the direction of the optical axis derived from the liquid crystal compound in a plane is defined as one period, the length of one period in the liquid crystal orientation pattern varies within the plane. [9] The liquid crystal diffraction element according to [8], wherein the length of one period in the liquid crystal orientation pattern gradually changes along the one direction, and the angle from the normal to the principal surface of the cholesteric liquid crystal layer in the direction in which the retardation of the cholesteric liquid crystal layer is minimized gradually changes along the one direction.

[10] The liquid crystal diffraction element according to [8] or [9], wherein, in the liquid crystal orientation pattern, as the length of one period becomes shorter, the angle from the normal to the principal surface of the cholesteric liquid crystal layer in the direction in which the retardation of the cholesteric liquid crystal layer is minimized increases.

[11] The liquid crystal diffraction element according to any one of [8] to

[10] , wherein in a cross-sectional image obtained by observing a cross section cut along the one direction and the thickness direction with a scanning electron microscope, the cholesteric liquid crystal layer has light and dark areas extending from one surface to the other surface, and there is a region in the thickness direction of the cholesteric liquid crystal layer where the inclination angle of the dark area with respect to the one surface differs from the angle formed by the direction in which the retardation of the cholesteric liquid crystal layer is minimized and the normal direction to the main surface of the cholesteric liquid crystal layer.

[12] The liquid crystal diffraction element according to

[11] , comprising at least two of the cholesteric liquid crystal layers, each of which has a light portion and a dark portion extending from one surface to the other surface in a cross-sectional image obtained by observing a cross section cut in a thickness direction along the one direction with a scanning electron microscope, and the dark portion of each of the at least two cholesteric liquid crystal layers has a different inclination angle with respect to the one surface.

[13] An optical device comprising the liquid crystal diffraction element according to any of [1] to

[12] , and a light source that irradiates light to the liquid crystal diffraction element.

[14] An optical device comprising the liquid crystal diffraction element according to any one of [1] to [6], and a light source that incidents light onto the liquid crystal diffraction element, wherein the angle of incident light from the light source onto the liquid crystal diffraction element is θin, the angle of emergence of first-order light reflected by the liquid crystal diffraction element is θm, and the refractive index of the cholesteric liquid crystal layer is nG, the tilt angle θP [°] of the liquid crystal compound satisfies the following formulas (A1) to (A3).

[15] An optical device comprising the liquid crystal diffraction element according to any one of [7] to

[12] , and a light source that incidents light onto the liquid crystal diffraction element, wherein the angle of incident light from the light source onto the liquid crystal diffraction element is θin, the angle of emergence of first-order light emerging from the liquid crystal diffraction element is θm, and the refractive index of the cholesteric liquid crystal layer is nG, the retardation of the cholesteric liquid crystal layer is minimized in the direction D. Ｒ and the normal direction of the principal surface of the cholesteric liquid crystal layer, the angle θ2 [°] between the

[0009] According to the present invention, a reflective liquid crystal diffraction element having excellent diffraction efficiency can be provided, even when the length of one period in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along one in-plane direction in the liquid crystal orientation pattern of the reflective liquid crystal diffraction element is short.Furthermore, according to the present invention, an optical device using the above liquid crystal diffraction element can be provided.

[0010] FIG. 1 is a plan view conceptually showing a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 2 is a view conceptually showing a cross section of the liquid crystal diffraction element shown in FIG. 1. FIG. 3 is a conceptual view for explaining a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 4 is a conceptual view for explaining an example of a cholesteric liquid crystal layer. FIG. 5 is a conceptual view for explaining a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 6 is a conceptual view for explaining the function of a cholesteric liquid crystal layer. FIG. 7 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 8 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 9 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 10 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 11 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 12 is a view conceptually showing another example of a liquid crystal diffraction element according to one embodiment of the present invention. FIG. 1 is a diagram conceptually showing another example of an image display device. FIG. 2 is a diagram conceptually showing another example of an image display device. FIG. 3 is a diagram conceptually showing another example of an image display device. FIG. 4 is a diagram conceptually showing an exposure device for producing a liquid crystal diffraction element. FIG. 5 is a diagram conceptually showing another example of an image display device.

[0011] The liquid crystal diffraction element and optical device of the present invention will be described in detail below based on preferred embodiments shown in the accompanying drawings. The following description of the components may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment. Furthermore, all of the figures shown below are conceptual diagrams for explaining the present invention, and the shape, size, thickness, and positional relationship of each component do not necessarily correspond to actual objects. In this specification, a numerical range expressed using "to" refers to a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In this specification, "same" is intended to include a generally acceptable error range in the technical field. Furthermore, in this specification, terms such as "all," "all," and "entire surface" are intended to include not only 100% but also generally acceptable error ranges in the technical field, such as 99% or more, 95% or more, or 90% or more. Regarding angles, "orthogonal" and "perpendicular" refer to a range of 90°±5°, and "parallel" refers to a range of 0°±5°. Similarly, unless otherwise specified, the angle means that the difference from the exact angle is within 5°. The difference in the angle is preferably within 4°, and more preferably within 3°.

[0012] In this specification, in-plane retardation Re(λ) (also referred to as "Re") represents the in-plane retardation at a wavelength λ. Unless otherwise specified, the wavelength λ is 550 nm. In this specification, Re(λ) is a value measured at a wavelength λ using a polarization phase difference analyzer AxoScan manufactured by Axometrics. By inputting the average refractive index ((nx + ny + nz) / 3) and film thickness (d (μm)) into AxoScan, the slow axis direction (°) Re(λ) = R0(λ) is calculated. Note that R0(λ) is displayed as a numerical value calculated by AxoScan, but it means Re(λ). In the present invention, the refractive indices nx, ny, and nz are measured using an Abbe refractive index (NAR-4T, manufactured by Atago Co., Ltd.) and a sodium lamp (λ = 589 nm) as a light source. Furthermore, when measuring wavelength dependency, measurements can be made using a multi-wavelength Abbe refractometer (DR-M2, manufactured by Atago Co., Ltd.) in combination with an interference filter. Values ​​from the Polymer Handbook (John Wiley & Sons, Inc.) and catalogs for various optical films can also be used. Examples of average refractive index values ​​for major optical films are listed below: cellulose acylate (1.48), cycloolefin polymer (1.52), polycarbonate (1.59), polymethyl methacrylate (1.49), and polystyrene (1.59).

[0013] [First embodiment] A liquid crystal diffraction element according to a first embodiment of the present invention is a liquid crystal diffraction element including a cholesteric liquid crystal layer, in which the cholesteric liquid crystal layer has a liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in a plane, and at least one surface of the cholesteric liquid crystal layer has a region in which the liquid crystal compound has a tilt angle with respect to the surface of the cholesteric liquid crystal layer, and further the cholesteric liquid crystal layer has regions in which the tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer varies in the plane.

[0014] 1 and 2 conceptually illustrate an example of a liquid crystal diffraction element according to this embodiment. FIG. 1 is a plan view, and FIG. 2 is a cross-sectional view in the thickness direction. This liquid crystal diffraction element is used as a reflective liquid crystal diffraction element (concave mirror). As shown in FIGS. 1 and 2, the liquid crystal diffraction element 18 includes a substrate 32, an alignment film 34, and a cholesteric liquid crystal layer 36 formed by fixing a cholesteric liquid crystal phase. In the liquid crystal diffraction element 18, the cholesteric liquid crystal layer 36 functions as a reflective liquid crystal diffraction element (concave mirror). Therefore, the liquid crystal diffraction element 18 may be constructed with only the cholesteric liquid crystal layer 36, with the substrate 32 and alignment film 34 removed; alternatively, the liquid crystal diffraction element 18 may be constructed with only the alignment film 34 and the cholesteric liquid crystal layer 36, with the substrate 32 removed; or alternatively, the substrate 32 and alignment film 34 may be removed from the cholesteric liquid crystal layer 36, and the cholesteric liquid crystal layer 36 may be laminated on another substrate.

[0015] In the liquid crystal diffraction element 18 shown in FIGS. 1 and 2 , the cholesteric liquid crystal layer 36 is a liquid crystal layer formed on an alignment film 34 using a composition containing a liquid crystal compound 38, and the liquid crystal compound 38 is aligned and fixed in the following liquid crystal alignment pattern. Specifically, the cholesteric liquid crystal layer 36 has a liquid crystal alignment pattern in which the orientation of the optical axis derived from the liquid crystal compound 38 changes while continuously rotating in one direction, radially from the inside to the outside. That is, the liquid crystal alignment pattern of the cholesteric liquid crystal layer 36 shown in FIGS. 1 and 2 is a concentric pattern in which the orientation of the optical axis derived from the liquid crystal compound 38 changes while continuously rotating in one direction, concentrically from the inside to the outside. Note that in FIGS. 1 and 2 , a rod-shaped liquid crystal compound is exemplified as the liquid crystal compound 38, and therefore the direction of the optical axis coincides with the longitudinal direction of the liquid crystal compound 38.

[0016] More specifically, in the cholesteric liquid crystal layer 36, the direction of the optical axis of the liquid crystal compound 38 changes while continuously rotating in multiple directions, such as the direction indicated by arrow A1, the direction indicated by arrow A2, the direction indicated by arrow A3, the direction indicated by arrow A4, and so on, radially outward from the center of the cholesteric liquid crystal layer 36 (i.e., the optical axis when functioning as a concave mirror). In the cholesteric liquid crystal layer 36, the rotation direction of the optical axis of the liquid crystal compound 38 is the same in all directions (one direction). In the illustrated example, the rotation direction of the optical axis of the liquid crystal compound 38 is counterclockwise in all directions indicated by arrow A1, the direction indicated by arrow A2, the direction indicated by arrow A3, and the direction indicated by arrow A4. In other words, if arrow A1 and arrow A4 are considered to be a single line, the rotation direction of the optical axis of the liquid crystal compound 38 is reversed at the center of the cholesteric liquid crystal layer 36 on this line. As an example, let us assume that the line formed by arrows A1 and A4 points to the right in the drawing (the direction of arrow A1). In this case, the optical axis of liquid crystal compound 38 initially rotates clockwise from the outside toward the center of cholesteric liquid crystal layer 36, reverses its direction of rotation at the center of cholesteric liquid crystal layer 36, and then rotates counterclockwise from the center toward the outside of cholesteric liquid crystal layer 36. When cholesteric liquid crystal layer 36 functions as a concave mirror, the center of cholesteric liquid crystal layer 36 corresponds to the optical axis of the concave mirror.

[0017] Also, in FIG. 2 , to clarify the configuration of the cholesteric liquid crystal layer 36, the liquid crystal compound 38 is shown parallel to the surface of the cholesteric liquid crystal layer 36. However, in the liquid crystal diffraction element 18, which is the liquid crystal diffraction element of this embodiment, the liquid crystal compound 38 has a region on at least one surface of the cholesteric liquid crystal layer 36 that has a tilt angle with respect to the surface, i.e., the main surface, of the cholesteric liquid crystal layer 36. Note that the main surface is the largest surface of the layer (sheet, membrane, film), and usually refers to both surfaces in the thickness direction. In the liquid crystal diffraction element 18 of the illustrated example, as conceptually shown in FIG. 3 , in the central region of the concentric circles, the liquid crystal compound 38 is oriented parallel to both surfaces of the cholesteric liquid crystal layer 36. In contrast, in regions away from the center of the concentric circles, the liquid crystal compound 38 is in a state of having a tilt angle in which it is oriented at an angle with respect to both surfaces of the cholesteric liquid crystal layer 36, i.e., a tilt-oriented state. In the illustrated example, the liquid crystal compound 38 has a tilt angle so that it rises from the outside to the inside toward the center of the concentric circles. The liquid crystal compound "having a tilt angle" with respect to the surface of the cholesteric liquid crystal layer means that the angle of the optical axis of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer is greater than 0°.

[0018] Furthermore, as conceptually shown in FIG. 3 , in the illustrated liquid crystal diffraction element 18, as a preferred example, the tilt angle of the liquid crystal compound 38 gradually increases from the inside to the outside of the concentric circles. That is, in the liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 gradually increases from the center of the concentric circles to the outside. As will be described later, the cholesteric liquid crystal layer 36 has a liquid crystal orientation pattern in which one period is the length of a 180° rotation of the optical axis direction derived from the liquid crystal compound in the liquid crystal orientation pattern, and one period gradually shortens from the inside to the outside of the concentric circles. As described above, the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 gradually increases from the inside to the outside of the concentric circles. That is, in the cholesteric liquid crystal layer 36 of the liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 increases as one period of the liquid crystal orientation pattern shortens.

[0019] 3, in order to clearly show the tilt alignment state of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36, the liquid crystal compound is shown in a state in which it does not have a liquid crystal alignment pattern. This also applies to FIGS. 7 to 10 described later.

[0020] As is well known, a cholesteric liquid crystal layer (liquid crystal layer) having a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound 38 changes while continuously rotating in one direction selectively reflects light of a specific wavelength, and acts as a reflective liquid crystal diffraction element that reflects light in a direction different from regular reflection (specular reflection). Hereinafter, reflecting light in a direction different from regular reflection will also be referred to as diffracting (bending) the reflected light.

[0021] Specifically, in a cholesteric liquid crystal layer 36 having a liquid crystal orientation pattern in which the orientation of the optical axis of the liquid crystal compound 38 changes while continuously rotating in one direction, the diffraction direction (refraction direction) of reflected light relative to the direction of specular reflection depends on the rotation direction of the optical axis of the liquid crystal compound 38. That is, in this liquid crystal orientation pattern, if the rotation direction of the optical axis of the liquid crystal compound 38, which is oriented in one direction, is reversed, the diffraction direction of reflected light relative to the direction of specular reflection will be opposite to the direction of rotation of the optical axis. As is well known, cholesteric liquid crystal phases exhibit selective reflectivity (wavelength-selective reflectivity) for either left- or right-handed circularly polarized light at a specific wavelength. Whether the reflected light is right- or left-handed circularly polarized light depends on the helical twist direction (sense) of the cholesteric liquid crystal phase. The selective reflection of circularly polarized light by the cholesteric liquid crystal phase reflects right-handed circularly polarized light when the helical twist direction of the cholesteric liquid crystal phase is right-handed, and left-handed circularly polarized light when the helical twist direction is left-handed. For example, in the liquid crystal diffraction element 18 shown in FIG. 2, when the cholesteric liquid crystal layer 36 has a selective reflection center wavelength in the green wavelength region and selectively reflects right-handed circularly polarized green light, the cholesteric liquid crystal layer 36 has a right-handed helical twist direction of the cholesteric liquid crystal phase, and selectively reflects right-handed circularly polarized green light G. Ｒ The direction of rotation of the cholesteric liquid crystal phase can be adjusted by the type of liquid crystal compound forming the cholesteric liquid crystal layer and / or the type of chiral agent added.

[0022] In the cholesteric liquid crystal layer 36 of the liquid crystal diffraction element 18, one period (one period Λ) is defined as the length over which the optical axis direction rotates 180° in one direction, in which the orientation of the optical axis originating from the liquid crystal compound 38 in the liquid crystal orientation pattern changes while continuously rotating. That is, in the cholesteric liquid crystal layer 36, which is a liquid crystal diffraction element, this one period corresponds to one period of the diffraction structure. In the illustrated liquid crystal diffraction element 18, the length of this one period of the cholesteric liquid crystal layer 36 gradually shortens from the inside to the outside. Here, in a liquid crystal layer having a liquid crystal orientation pattern in which the orientation of the optical axis of the liquid crystal compound 38 changes while continuously rotating in one direction, the shorter the length of one period, the larger the diffraction angle. Therefore, in the cholesteric liquid crystal layer 36 having a concentric liquid crystal orientation pattern, the diffraction angle gradually increases from the center of the concentric circle toward the outside. As described above, in the illustrated cholesteric liquid crystal layer 36, the tilt angle of the liquid crystal compound 38 increases as the period of the liquid crystal orientation pattern shortens.

[0023] Therefore, the cholesteric liquid crystal layer 36 having a concentric liquid crystal orientation pattern in which the optical axis derived from the liquid crystal compound has a radially changing liquid crystal orientation pattern that continuously rotates can reflect incident light (light beam) in a divergent or convergent manner depending on the rotation direction of the optical axis of the liquid crystal compound 38 and the twist direction of the helix of the cholesteric liquid crystal phase. In other words, the liquid crystal diffraction element 18 having such a cholesteric liquid crystal layer 36 acts as a concave or convex mirror that selectively reflects right-handed circularly polarized light when the twist direction of the helix of the cholesteric liquid crystal phase is right-handed, and acts as a concave or convex mirror that selectively reflects left-handed circularly polarized light when the twist direction of the helix of the cholesteric liquid crystal phase is left-handed.

[0024] 1 and 3, in order to simplify the drawings and clearly show the configuration of the liquid crystal diffraction element 18, the cholesteric liquid crystal layer 36 only shows the liquid crystal compound 38 (liquid crystal compound molecules) on the surface of the alignment film 34. However, as conceptually shown in FIG. 2, the cholesteric liquid crystal layer 36 has a helical structure in which the liquid crystal compound 38 is spirally wound and stacked, and the helical structure has a structure in which the helical wound liquid crystal compound 38 is stacked at one pitch (helical pitch), where one pitch (helical pitch) of the helical structure is defined as the configuration in which the liquid crystal compound 38 is spirally wound and stacked. The cholesteric liquid crystal layer 36 preferably has a structure in which the helical wound liquid crystal compound 38 is stacked at multiple pitches. Furthermore, in the illustrated liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 may be the same throughout the thickness direction at the same in-plane position, as shown in the upper part of FIG. 4, or may vary in the thickness direction, as shown in the lower part of FIG. 4.

[0025] The tilt angle (inclination angle) of the liquid crystal compound relative to the surface of the cholesteric liquid crystal layer can be measured by observing the direction of the optical axis of the liquid crystal compound at a cross section obtained by cutting the cholesteric liquid crystal layer along the thickness direction of the cholesteric liquid crystal layer using a polarizing microscope. Specifically, the tilt angle of the liquid crystal compound within the thickness direction of the cholesteric liquid crystal layer described below can be measured by cutting the cholesteric liquid crystal layer along the direction in which the optical axis continuously rotates and the thickness direction to prepare a sample, and then rotating the cut surface of the obtained sample while observing the direction of the optical axis of the liquid crystal compound using a polarizing microscope. In addition, the tilt angle (inclination angle) of the liquid crystal compound relative to the surface of the cholesteric liquid crystal layer can be measured by analyzing the surface of the cholesteric liquid crystal layer using SHG (Second Harmonic Generation).

[0026] The function of this cholesteric liquid crystal layer 36 will be described in detail below with reference to FIG. 5. FIG. 5 is a plan view conceptually illustrating the configuration of a cholesteric liquid crystal layer. The cholesteric liquid crystal layer 36A shown in FIG. 5 has a liquid crystal orientation pattern in which an optical axis 38A derived from the liquid crystal compound 38 changes while continuously rotating in one direction indicated by arrow A. Note that with regard to the one direction in which the optical axis changes while continuously rotating, the concentric liquid crystal orientation pattern shown in FIG. 1 also exhibits the same optical effect as the liquid crystal orientation pattern shown in FIG. 5. In the following description, the optical axis 38A derived from the liquid crystal compound 38 will also be referred to as the "optical axis 38A of the liquid crystal compound 38" or "optical axis 38A."

[0027] In the cholesteric liquid crystal layer 36A, the liquid crystal compound 38 is two-dimensionally oriented in a plane parallel to one direction indicated by arrow A and the Y direction perpendicular to the direction of arrow A. In the cholesteric liquid crystal layer 36 shown in FIG. 1, the circumferential direction of the concentric circles in the concentric liquid crystal orientation pattern corresponds to the Y direction in FIG. 5. In the cholesteric liquid crystal layer 36 shown in FIG. 2 and FIG. 6 described later, the direction perpendicular to the paper surface corresponds to the Y direction in FIG. 5. In the following description, the "one direction indicated by arrow A" may also be simply referred to as the "direction of arrow A."

[0028] Cholesteric liquid crystal layer 36A has a liquid crystal orientation pattern in which, within the plane of cholesteric liquid crystal layer 36A, the orientation of optical axis 38A derived from liquid crystal compound 38 changes while continuously rotating along the direction of arrow A. The orientation of optical axis 38A of liquid crystal compound 38 changes while continuously rotating in the direction of arrow A (a predetermined direction), specifically, means that the angle formed between optical axis 38A of liquid crystal compound 38 aligned along the direction of arrow A and the direction of arrow A varies depending on the position in the direction of arrow A, and the angle formed between optical axis 38A and the direction of arrow A changes sequentially from θ to θ+180° or θ−180° along the direction of arrow A.

[0029] On the other hand, the liquid crystal compounds 38 forming the cholesteric liquid crystal layer 36A are arranged at equal intervals in the Y direction perpendicular to the direction of arrow A, i.e., the Y direction perpendicular to the direction in which the optical axes 38A continuously rotate. In other words, the liquid crystal compounds 38 forming the cholesteric liquid crystal layer 36 arranged in the Y direction have the same angle between the direction of the optical axes 38A and the direction of arrow A. In the cholesteric liquid crystal layer 36 shown in FIG. 1 , regions in which the optical axes 38A are oriented in the same direction are formed in the shape of rings whose centers coincide, forming a concentric liquid crystal orientation pattern.

[0030] As described above, in a liquid crystal orientation pattern in which the optical axis 38A continuously rotates in one direction, the length (distance) over which the optical axis 38A of the liquid crystal compound 38 rotates 180° is the length Λ of one period in the liquid crystal orientation pattern. That is, in the case of the cholesteric liquid crystal layer 36A shown in FIG. 5 , the length (distance) over which the optical axis 38A of the liquid crystal compound 38 rotates 180° in the direction of arrow A, in which the orientation of the optical axis 38A continuously rotates and changes in the plane, is defined as one period Λ of the liquid crystal orientation pattern. In other words, one period Λ of the liquid crystal orientation pattern is defined as the distance from the angle θ between the optical axis 38A of the liquid crystal compound 38 and the direction of arrow A to the angle θ + 180°. That is, one period Λ is the distance between the centers of two liquid crystal compounds 38 in the direction of arrow A that have the same angle with respect to the direction of arrow A. Specifically, as shown in FIG. 5 , one period Λ is the distance between the centers of two liquid crystal compounds 38 in the direction of arrow A whose optical axes 38A coincide with the direction of arrow A. In the cholesteric liquid crystal layer 36A (cholesteric liquid crystal layer 36), the liquid crystal orientation pattern repeats this one period Λ in the direction of arrow A, i.e., in one direction in which the direction of the optical axis 38A continuously rotates and changes. As described above, the cholesteric liquid crystal layer 36A having such a liquid crystal orientation pattern is also a reflective liquid crystal diffraction element, and this one period Λ is the period (one period) of the diffraction structure.

[0031] In the cholesteric liquid crystal layer 36A, the liquid crystal compounds aligned in the Y direction have the same angle between their optical axes 38A and the direction of arrow A. The region in which the liquid crystal compounds 38 aligned in the Y direction and having the same angle between their optical axes 38A and the direction of arrow A are arranged is referred to as region R. In the liquid crystal diffraction element 18 having a concentric liquid crystal orientation pattern in which the optical axes 38A are continuously rotated radially in one direction, the regions formed in annular shapes with the same center and in which the optical axes 38A are oriented in the same direction correspond to region R in FIG.

[0032] When light is incident on such a cholesteric liquid crystal layer 36A, light of a specific wavelength is selectively converted (diffracted) in a direction different from regular reflection (specular reflection) and reflected. The function of the cholesteric liquid crystal layer will be explained in more detail below with reference to FIG. 6. Note that in FIG. 6, only the cholesteric liquid crystal layer 36 is shown to clearly show the function of the liquid crystal diffraction element 18A. For the same reason, it is assumed that light is incident on the liquid crystal diffraction element 18A from the normal direction (front). For the sake of explanation, the cholesteric liquid crystal layer 36 is configured to diffract right-handed circularly polarized green light G R The liquid crystal diffraction element 18 selectively reflects light having a concentric circular liquid crystal orientation pattern, in which the optical axis 38A of the liquid crystal compound 38 continuously rotates radially in one direction, and transmits other light. As described above, this effect is exactly the same in the liquid crystal diffraction element 18 having a concentric circular liquid crystal orientation pattern. Also, in FIG. 6, in order to clearly show the orientation state of the liquid crystal compound 38, the liquid crystal compound 38 is shown in a state without a tilt angle. However, in the liquid crystal diffraction element of this embodiment, the cholesteric liquid crystal layer has a region in which the liquid crystal compound has a tilt angle relative to the surface of the cholesteric liquid crystal layer, and further, as described above, there are regions in the plane of the cholesteric liquid crystal layer in which the liquid crystal compound has a different tilt angle.

[0033] In the portion shown in Fig. 6, the cholesteric liquid crystal layer 36 has three regions A0, A1, and A2 from the left in Fig. 6, and the helical pitch length and the length of one period Λ are different in each region. Specifically, the helical pitch increases in the order of regions A0, A1, and A2, and the length of one period Λ decreases in the order of regions A0, A1, and A2. However, Fig. 6 illustrates only one example of the configuration of the cholesteric liquid crystal layer 36, and the cholesteric liquid crystal layer 36 may have two, four, or more regions with different helical pitch lengths and different length of one period Λ.

[0034] In the liquid crystal diffraction element 18A, right-handed circularly polarized green light G R1 When right-handed circularly polarized green light G enters the region A1 in the plane of the cholesteric liquid crystal layer 36, it is reflected in the direction of arrow A with respect to the direction of incidence, that is, in a direction tilted by a predetermined angle in one direction in which the orientation of the optical axis of the liquid crystal compound changes while continuously rotating, as described above. R2 When the right-handed circularly polarized light G enters the area A2 in the plane of the cholesteric liquid crystal layer 36, it is reflected in a direction tilted at a predetermined angle in the direction of arrow A with respect to the incident direction. R2 When light enters an area A0 in the plane of the cholesteric liquid crystal layer 36, it is reflected in a direction tilted at a predetermined angle in the direction of arrow A with respect to the incident direction. A1 than one period Λ of the liquid crystal alignment pattern in the region A2 A2 6, the angle of reflection of the incident light by the cholesteric liquid crystal layer 36 is the angle θ of the reflected light in the region A2. A2 The angle θ of the reflected light from the area A1 is A1 In addition, one period Λ of the liquid crystal alignment pattern in the region A1 becomes larger than A1 than one period Λ of the liquid crystal alignment pattern in the region A0 A0 Since the angle of reflection of the incident light is long, as shown in FIG. 6, the angle of the reflected light in the area A0 is θ A0 The angle θ of the reflected light from the area A1 is A1 will be smaller than

[0035] As described above, the cholesteric liquid crystal layer 36A changes one period Λ of the liquid crystal orientation pattern formed, thereby converting the right-handed circularly polarized light G R Specifically, the shorter the period Λ of the liquid crystal orientation pattern, the stronger the interference between the lights that have passed through the adjacent liquid crystal compounds 38, and the greater the diffraction angle of the reflected light.

[0036] In the liquid crystal diffraction element of this embodiment, there is no limitation on the period Λ of the liquid crystal orientation pattern in the cholesteric liquid crystal layer. That is, the period of the liquid crystal orientation pattern can be appropriately set to a period that achieves the desired optical characteristics depending on the application of the liquid crystal diffraction element, the optical characteristics required for the liquid crystal diffraction element, such as focal length, and the size of the liquid crystal diffraction element. Furthermore, as in the illustrated example, when the cholesteric liquid crystal layer has a liquid crystal orientation pattern in which the period Λ changes in-plane, the degree of change can also be set similarly. Here, as the period of the liquid crystal orientation pattern becomes shorter, the diffraction efficiency, described later, decreases accordingly. That is, the shorter the period of the liquid crystal orientation pattern, the greater the effect of the present invention in tilting the liquid crystal compound. Considering this point, the liquid crystal orientation pattern in the cholesteric liquid crystal layer preferably includes a region where the length of the period Λ is 100 μm or less, more preferably 10 μm or less, even more preferably 2 μm or less, and particularly preferably 1 μm or less. Although there is no lower limit for the period Λ of the liquid crystal orientation pattern in the cholesteric liquid crystal layer, taking into consideration the accuracy of the liquid crystal orientation pattern, diffraction efficiency, etc., it is preferable that the period Λ be 0.1 μm or more. The preferred period of the liquid crystal orientation pattern varies depending on the application of the liquid crystal diffraction element, etc. For example, in the case of a liquid crystal diffraction element used to diverge reflected light over a wide angle, as shown in Figures 7 and 9, the effect of tilting the liquid crystal compound can be suitably obtained even if the period Λ is several tens of μm or more.

[0037] Furthermore, the cholesteric liquid crystal layer 36A can reverse the direction in which reflected light is diffracted relative to regular reflection (specular reflection) by reversing the rotation direction of the optical axis 38A of the liquid crystal compound 38, which rotates along the direction of arrow A. Furthermore, the cholesteric liquid crystal layer 36A can select circularly polarized light to be reflected depending on the twist direction of the helix of the cholesteric liquid crystal phase.

[0038] As described above, the same applies to the cholesteric liquid crystal layer 36 (liquid crystal diffraction element 18) having a concentric liquid crystal orientation pattern. Therefore, the liquid crystal diffraction element 18 including the cholesteric liquid crystal layer 36 having such a concentric liquid crystal orientation pattern acts as a concave mirror that focuses reflected light or a convex mirror that diverges reflected light, depending on the rotation direction of the optical axis 38A and the twist direction of the helical spiral of the cholesteric liquid crystal phase.

[0039] As described above, when a liquid crystal diffraction element 18 (liquid crystal lens) is used in a head-mounted display, the focal length of the liquid crystal diffraction element 18 must be shortened in order to make the optical system thinner and more compact. Furthermore, as described above, in a liquid crystal diffraction element having a liquid crystal orientation pattern in which the optical axis 38A of the liquid crystal compound 38 continuously rotates in one direction, the shorter the period Λ in which the optical axis 38A rotates 180°, the larger the diffraction angle of light. However, in a liquid crystal diffraction element having this liquid crystal orientation pattern, as the period Λ becomes shorter, the diffraction efficiency decreases, as the amount of zero-order light that is not diffracted by the liquid crystal diffraction element increases. In particular, as the period Λ becomes shorter, down to the 1 μm level, the diffraction efficiency decreases significantly.

[0040] The present inventors conducted extensive research to solve this problem. As a result, they discovered that by making the liquid crystal compound constituting the cholesteric liquid crystal layer of the liquid crystal diffraction element have an angle with respect to the surface of the cholesteric liquid crystal layer, i.e., by making the liquid crystal compound 38 have a tilt angle with respect to the surface of the cholesteric liquid crystal layer 36, it is possible to suppress a decrease in diffraction efficiency even when one period of the liquid crystal diffraction element is shortened. The first embodiment of the present invention was made based on this finding. The cholesteric liquid crystal layer that mainly functions as a diffraction element in the liquid crystal diffraction element has a region on at least one surface of the cholesteric liquid crystal layer where the liquid crystal compound has a tilt angle with respect to the surface of the cholesteric liquid crystal layer. Furthermore, the cholesteric liquid crystal layer has regions in its plane where the tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal is different.

[0041] The liquid crystal diffraction element of this embodiment has such a configuration, and can achieve excellent diffraction efficiency even when one period Λ of the liquid crystal orientation pattern in the cholesteric liquid crystal layer 36 is short, such as 1 μm or less. Therefore, the liquid crystal diffraction element of this embodiment can focus light with high focusing efficiency when used as, for example, a short-focus concave mirror.

[0042] Because the liquid crystal diffraction element 18 in the illustrated example acts as a concave mirror, one period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer 36 gradually shortens from the inside to the outside of the concentric circles. Accordingly, in the cholesteric liquid crystal layer 36 of the liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 gradually increases from the inside to the outside of the concentric circles. Specifically, as conceptually shown in FIG. 3 , in the central region of the concentric circles, the cholesteric liquid crystal layer 36 aligns the liquid crystal compound 38 parallel to both surfaces of the cholesteric liquid crystal layer 36 (0°). From regions slightly away from the center of the concentric circles, the liquid crystal compound 38 has a tilt angle, and the tilt angle of the liquid crystal compound 38 gradually increases from the inside to the outside of the concentric circles. As mentioned above, the tilt angle of the liquid crystal compound 38 may be uniform throughout the thickness direction at the same position in the plane of the cholesteric liquid crystal layer 36, or may vary in the thickness direction.

[0043] That is, in the liquid crystal diffraction element of this embodiment, the cholesteric liquid crystal layer may have a region in at least a part of its plane where the liquid crystal compound 38 does not have a tilt angle, i.e., a region in at least a part of its plane where the liquid crystal compound is not tilt-oriented. Also, the liquid crystal diffraction element of this embodiment may have a configuration in which the liquid crystal compound 38 has a tilt angle throughout the entire plane of the cholesteric liquid crystal layer, i.e., a configuration in which the liquid crystal compound 38 is tilt-oriented throughout the entire plane of the cholesteric liquid crystal layer.

[0044] In this gradual change in the tilt angle of the liquid crystal compound 38, the change in the tilt angle may be continuous or stepwise with a region where the tilt angle is the same, or may be a mixture of a region where the tilt angle changes continuously and a region where the tilt angle changes stepwise. This also applies to other configurations in which the tilt angle of the liquid crystal compound 38 changes gradually.

[0045] Furthermore, the tilt (inclination) direction of the liquid crystal compound in the cholesteric liquid crystal layer is preferably a direction in which the liquid crystal compound rises toward the diffraction direction of reflected light relative to the specular reflection by the liquid crystal diffraction element. That is, when the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36) acts as a concave mirror that focuses light, as shown in Figure 3, the liquid crystal compound 38 tilts so as to rise toward the center, i.e., the focusing direction. Conversely, when the liquid crystal diffraction element acts as a convex mirror that diverges reflected light, as conceptually shown in Figure 7, the liquid crystal compound 38 tilts so as to rise from the inside toward the outside, i.e., the diverging direction, in contrast to Figure 3.

[0046] The liquid crystal diffraction element of this embodiment can also be used as a liquid crystal diffraction element that reflects and focuses light incident at wide angles, as conceptually illustrated by the cholesteric liquid crystal layer 36C in FIG. 8 . In such applications, the diffraction efficiency decreases when light is incident obliquely onto the liquid crystal diffraction element. However, this decrease in diffraction efficiency can be suppressed by having the liquid crystal compound have a tilt angle relative to the surface of the cholesteric liquid crystal layer. Here, the larger the angle of incidence of light onto the liquid crystal diffraction element (cholesteric liquid crystal layer), the lower the diffraction efficiency of the liquid crystal diffraction element. Therefore, in a liquid crystal diffraction element that reflects and focuses light incident at wide angles, it is preferable to increase the tilt angle of the liquid crystal compound in the region where the angle of incidence of light onto the liquid crystal diffraction element is larger. This also applies to the following embodiment in which divergent light is incident. The incident angle is the angle relative to the normal to the liquid crystal diffraction element, i.e., the polar angle. The normal is a line perpendicular to the surface of the sheet-like material. The liquid crystal diffraction element of this embodiment can also be used for divergent light. For example, as shown in FIG. 9 , divergent light may be incident on a liquid crystal diffraction element (only the cholesteric liquid crystal layer is shown in FIG. 9 ), and the reflected light may be further diverged by the liquid crystal diffraction element. When the liquid crystal diffraction element is used as a convex mirror, the liquid crystal compound tilts so that it rises from the inside to the outside, i.e., in the direction of light divergence, as described above. Alternatively, as conceptually shown in FIG. 10 , divergent light may be incident on a liquid crystal diffraction element (same as above) acting as a concave mirror, and the reflected light may be focused by the liquid crystal diffraction element, thereby weakening the divergence. Furthermore, the light to be focused may be incident on a liquid crystal diffraction element acting as a concave mirror, thereby strengthening the focusing, as shown in FIG. 8 . Furthermore, although not shown, the light to be focused may be incident on a liquid crystal diffraction element acting as a convex mirror, thereby weakening the focusing.

[0047] As described above, in the illustrated liquid crystal diffraction element 18, the cholesteric liquid crystal layer 36 has a liquid crystal orientation pattern in which one period Λ gradually shortens from the inside to the outside of the concentric circles. Accordingly, in a preferred embodiment of the liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 gradually increases from the inside to the outside. That is, in the cholesteric liquid crystal layer of the illustrated liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 increases as one period Λ of the liquid crystal orientation pattern shortens. In other words, in the cholesteric liquid crystal layer of the illustrated liquid crystal diffraction element 18, the tilt angle of the liquid crystal compound 38 increases in conjunction with the shortening of one period Λ of the liquid crystal orientation pattern.

[0048] However, the liquid crystal diffraction element of this embodiment is not limited to the above-mentioned aspect. That is, in the liquid crystal diffraction element of this embodiment, the tilt angle of the liquid crystal compound in the plane of the cholesteric liquid crystal layer may be constant, or the tilt angle may decrease as the period Λ of the liquid crystal orientation pattern becomes shorter, or the tilt angle may not be linked to the change in the period Λ of the liquid crystal orientation pattern. Furthermore, in at least a part or all of the liquid crystal diffraction element, the tilt angle of the liquid crystal compound in the cholesteric liquid crystal layer may gradually change as the period Λ of the liquid crystal orientation pattern gradually changes along one direction in which the optical axis changes while continuously rotating. Here, in the liquid crystal diffraction element (cholesteric liquid crystal layer), the diffraction efficiency decreases as the period Λ of the liquid crystal orientation pattern becomes shorter. Considering this point, it is preferable that, in at least a part (more preferably all) of the cholesteric liquid crystal layer, the tilt angle of the liquid crystal compound increases as the period Λ of the liquid crystal orientation pattern becomes shorter.

[0049] In the illustrated liquid crystal diffraction element 18, the cholesteric liquid crystal layer 36 preferably functions as a concave mirror, and the period Λ of the liquid crystal orientation pattern gradually shortens from the inside to the outside. That is, the illustrated cholesteric liquid crystal layer 36 has regions in which the length of the period Λ varies within the plane. However, the liquid crystal diffraction element of this embodiment is not limited to this, and may have a region in which the period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer is uniform throughout and the tilt angle of the liquid crystal compound relative to the surface of the cholesteric liquid crystal layer varies within the plane.

[0050] In the liquid crystal diffraction element 18 of this embodiment, there is no limitation on the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36, as long as there is a region on the surface of the cholesteric liquid crystal layer 36 where the liquid crystal compound 38 has a tilt angle with respect to the surface of the cholesteric liquid crystal layer 36. That is, the tilt angle of the liquid crystal compound 38 may be set appropriately depending on the optical characteristics required of the liquid crystal diffraction element 18, the size of the liquid crystal diffraction element 18, the liquid crystal orientation pattern of the cholesteric liquid crystal layer, the angle of incidence of light onto the liquid crystal diffraction element, etc. In the region where the liquid crystal compound 38 has a tilt angle, i.e., the region where the angle formed between the liquid crystal compound 38 and the surface of the cholesteric liquid crystal layer 36 is greater than 0°, the tilt angle of the liquid crystal compound 38 is preferably 5 to 85°, more preferably 10 to 80°, and even more preferably 15 to 70°. By setting the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 to 5° or more, it is possible to obtain excellent diffraction efficiency even when one period Λ of the liquid crystal orientation pattern is short, and it is preferable to obtain excellent diffraction efficiency even when the angle of incidence of light onto the liquid crystal diffraction element is large, etc. Furthermore, it is preferable to set the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 to 85° or less in terms of alignment stability, etc.

[0051] Furthermore, in the liquid crystal diffraction element 18 of this embodiment, the cholesteric liquid crystal layer 36 has regions in its plane where the tilt angles of the liquid crystal compounds 38 are different. There are no limitations on the difference in tilt angles of the liquid crystal compounds 38 in the cholesteric liquid crystal layer 36. That is, the difference in tilt angles of the liquid crystal compounds 38 in the cholesteric liquid crystal layer 36 may be appropriately set depending on the optical characteristics required of the liquid crystal diffraction element 18, the size of the liquid crystal diffraction element 18, the liquid crystal orientation pattern of the cholesteric liquid crystal layer, the angle of incidence of light on the liquid crystal diffraction element, and the like. The difference in tilt angles of the liquid crystal compounds 38 in the cholesteric liquid crystal layer 36 is preferably 5 to 85°, more preferably 10 to 80°, and even more preferably 15 to 70°. By setting the difference in tilt angles of the liquid crystal compounds 38 in the cholesteric liquid crystal layer 36 to 5° or more, excellent diffraction efficiency can be obtained even when the period Λ of the liquid crystal orientation pattern is short, and excellent diffraction efficiency can be obtained even when the angle of incidence of light on the liquid crystal diffraction element is large, which is preferable. Furthermore, it is preferable to set the difference in tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 to 85° or less in terms of in-plane alignment stability, etc. In this case, the difference in tilt angle of the liquid crystal compound 38 also includes the liquid crystal compound 38 having a minimum tilt angle of 0°, i.e., no tilt angle.

[0052] In this embodiment, the tilt angle of the liquid crystal compound is specifically the acute angle formed between one surface of the cholesteric liquid crystal layer and the longitudinal direction (optical axis (slow axis)) of the rod-shaped liquid crystal compound when the liquid crystal compound is a rod-shaped liquid crystal compound. Furthermore, when the liquid crystal compound is a discotic liquid crystal compound, it is the acute angle formed between one surface of the cholesteric liquid crystal layer and the disc surface of the discotic liquid crystal compound. In this definition, the acute angle also includes a right angle.

[0053] In the liquid crystal diffraction element of this embodiment, it is preferable that the tilt angle of the liquid crystal compound 38 is close to the traveling direction of light inside the cholesteric liquid crystal layer 36. That is, in the liquid crystal diffraction element of this embodiment, it is preferable that the tilt angle of the liquid crystal compound 38 is close to the angle formed by the traveling direction of light inside the cholesteric liquid crystal layer 36 and the normal direction to the principal surface of the cholesteric liquid crystal layer 36.

[0054] The relationship between the tilt angle of the liquid crystal compound 38 and the propagation direction of light inside the cholesteric liquid crystal layer 36 will be described with reference to the drawings. FIG. 11 is a conceptual diagram illustrating an example of an optical device of this embodiment having the liquid crystal diffraction element and a light source of this embodiment. The optical device 50 shown in FIG. 11 includes a cholesteric liquid crystal layer 36 constituting the liquid crystal diffraction element of this embodiment and a light source 40. The dashed-dotted line in FIG. 11 represents the normal to the cholesteric liquid crystal layer 36. In such an optical device 50, as conceptually illustrated in FIG. 11 , when the angle of incident light from the light source 40 to the cholesteric liquid crystal layer 36 (liquid crystal diffraction element) is θin, the refractive index of the cholesteric liquid crystal layer 36 is nG, and the exit angle of the primary light reflected by the cholesteric liquid crystal layer 36 (liquid crystal diffraction element) is θm, it is preferable that the tilt angle θP [°] of the liquid crystal compound satisfies the following formulas (A1) to (A3): (A1) θP [°] = (θG [°] - θr [°]) / 2 ± 15 [°] (A2) sin θG = sin θm / nG (A3) sin θr = sin θin / nG The above formula (A1) can be rewritten as the following formula (A4): (A4) (θG [°] - θr [°]) / 2 - 15° ≦ θP [°] ≦ (θG [°] - θr [°]) / 2 + 15° By having such a configuration, a liquid crystal diffraction element having excellent diffraction efficiency can be obtained even if one period Λ of the liquid crystal orientation pattern in the cholesteric liquid crystal layer 36 is short. This is particularly preferable when the diffraction angle is large, from the viewpoint of obtaining a liquid crystal diffraction element having excellent diffraction efficiency and excellent polarization (high degree of circular polarization).

[0055] As will be described later, the liquid crystal diffraction element of this embodiment may have multiple cholesteric liquid crystal layers. In this case, the refractive index nG of the cholesteric liquid crystal layer is the average refractive index of the multiple cholesteric liquid crystal layers. The tilt angle θP [°] is the average tilt angle of the multiple cholesteric liquid crystal layers, taking into account the thickness of each layer, at the position where light is emitted from the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36) into air.

[0056] For example, as conceptually shown in Figure 12, if the liquid crystal diffraction element has cholesteric liquid crystal layers 36a, 36b, and 36c, the refractive index nG of the cholesteric liquid crystal layers is the average refractive index of the refractive indexes of the cholesteric liquid crystal layers 36a, 36b, and 36c. Using this refractive index nG, θr [°] is calculated from the above formula (A2), and θG [°] is calculated from the above formula (A3). Furthermore, the tilt angle θP is determined by taking into account the thickness of each cholesteric liquid crystal layer at the position where light is emitted into air from the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36), i.e., on the normal line (dotted line) to the light emission position shown in Figure 12. The average tilt angle θP [°] of the tilt angles of each cholesteric liquid crystal layer, taking into account the thickness of each cholesteric liquid crystal layer, satisfies the above formula (A1). Specifically, θP is calculated from the following formula (A5): (A5) θP [°] = (θA × dA + θB × dB + θC × dC) / (dA + dB + dC) [°] It is sufficient that θP [°] calculated from formula (A5) satisfies formula (A1) above.

[0057] The function of the liquid crystal diffraction element 18A shown in Figure 6 will be described in more detail. Typically, when light is reflected by a cholesteric liquid crystal layer, the wavelength of the selectively reflected light shifts toward shorter wavelengths depending on the angle of the incident light, a phenomenon known as a blue shift (shortwave shift). Therefore, in a cholesteric liquid crystal layer having a liquid crystal orientation pattern in which the orientation of the optical axis of the liquid crystal compound changes while continuously rotating along at least one direction in the plane, the effect of the blue shift (shortwave shift) causes a problem of a decrease in the amount of reflected light as the reflection angle increases. Therefore, if the configuration has regions where the length of one period in which the orientation of the optical axis of the liquid crystal compound rotates 180° in the plane varies, the reflection angle varies depending on the position of incidence of the light, resulting in differences in the amount of reflected light depending on the position of incidence in the plane. In other words, regions where the reflected light becomes darker occur depending on the position of incidence in the plane.

[0058] In contrast, the liquid crystal diffraction element 18A shown in the figure has regions where the helical pitch of the cholesteric liquid crystal layer varies within the plane. In the example shown in FIG. 6, the pitch length PL of the helical structure of the region A2 of the cholesteric liquid crystal layer 36 is A2 is the pitch length PL of the helical structure of region A1 A1 and the pitch length PL of the spiral structure in the region A0 A0 is the pitch length PL of the helical structure of region A1 A1 This reduces the effect of blue shift, in which the wavelength of selectively reflected light shifts to the shorter wavelength side, and suppresses a decrease in the amount of reflected light in areas where the reflection angle of reflected light increases. Specifically, by lengthening the pitch length of the helical structure so that the selectively reflected wavelength after blue shift matches the wavelength of the incident light, the reflection efficiency at the wavelength of the incident light can be increased. Therefore, it is possible to suppress the occurrence of areas where the reflected light becomes dark depending on the incident position in the plane.

[0059] In the example shown in FIG. 6, the reflection angle θ of the reflected light in the area A1 A1 is the reflection angle θ of the reflected light in the area A0 A0 That is, the length of one period in the region A1 is larger than A1 is the length of one period in the region A0 A0Therefore, the helical pitch PL in the region A1 is A1 The area A0 is the spiral pitch PL A0 The reflection angle θ A2 is the largest, that is, the length of one period Λ A2 The helical pitch PL in the region A2 is the shortest A2 is set to be longer than the helical pitch of the region A0 and the region A1. This makes it possible to suppress a decrease in the amount of reflected light reflected by the regions A1 and A2, and to make the amount of reflected light uniform regardless of the incident position within the surface.

[0060] Thus, in the liquid crystal diffraction element 18A, in areas in the plane where the reflection angle of the cholesteric liquid crystal layer is large, the incident light is reflected by areas with a long pitch of the helical structure. In contrast, in areas in the plane where the reflection angle of the cholesteric liquid crystal layer is small, the incident light is reflected by areas with a short pitch of the helical structure. In other words, in the liquid crystal diffraction element 18A, the reflected light relative to the incident light can be brightened by setting the pitch length of the helical structure in the plane according to the reflection angle of the cholesteric liquid crystal layer. Therefore, the liquid crystal diffraction element 18A can reduce the reflection angle dependence of the amount of reflected light in the plane.

[0061] In the reflective liquid crystal diffraction element, as described above, the shorter the period Λ of the liquid crystal orientation pattern, the larger the angle of reflection, so it is possible to make the reflected light brighter by making the pitch length PL of the helical structure longer in areas where the period Λ of the liquid crystal orientation pattern is shorter. Therefore, in the reflective liquid crystal diffraction element, in areas where the length of one period of the liquid crystal orientation pattern is different, it is preferable that the pitch length of the helical structure is shorter as the length of one period is longer.

[0062] The liquid crystal diffraction element according to this embodiment is not limited to the embodiment shown in Fig. 6, and in the liquid crystal diffraction element, in regions where the length of one period of the liquid crystal orientation pattern is different, the length of one period and the pitch length of the helical structure may be proportional to each other. In the reflective liquid crystal diffraction element, the pitch length of the helical structure has a preferred range depending on one period Λ of the in-plane liquid crystal orientation pattern, and may be set appropriately.

[0063] 13 conceptually shows an image obtained by observing a cross section of the cholesteric liquid crystal layer 36 shown in FIG. 2 cut along the direction of continuous rotation of the optical axis and the thickness direction with a scanning electron microscope (SEM). The cholesteric liquid crystal layer having the above-described liquid crystal orientation pattern has bright areas 42 and dark areas 44 extending from one surface to the other in an image obtained by observing a cross section cut along the direction of continuous rotation of the optical axis and the thickness direction of the cholesteric liquid crystal layer with an SEM (hereinafter, for convenience, also referred to as a "cross-sectional SEM image"). The bright areas 42 and dark areas 44 observed in the cross-sectional SEM image are derived from the liquid crystal phase having the liquid crystal orientation pattern.

[0064] As shown in the cross-sectional SEM image of the cholesteric liquid crystal layer 36, a striped pattern is observed in which light and dark regions 42 and 44 are alternately arranged, with the orientation inclined at a predetermined angle relative to the principal surface. The spacing between the light and dark regions 42 and 44, i.e., the surface pitch P, basically depends on the helical pitch of the cholesteric liquid crystal phase, i.e., the length of one pitch of the helical structure formed by stacking liquid crystal compounds in a helical shape through one full rotation (360° rotation). The wavelength band of light selectively reflected by the cholesteric liquid crystal layer correlates with the surface pitch P, i.e., the spacing between the light and dark regions 42 and 44. That is, if the surface pitch P is long, the helical pitch is long, and therefore the wavelength band of light selectively reflected by the cholesteric liquid crystal layer is long-wavelength. Conversely, if the surface pitch P is short, the helical pitch is short, and therefore the wavelength band of light selectively reflected by the cholesteric liquid crystal layer is short-wavelength. Here, in the cholesteric liquid crystal layer, the helical pitch basically corresponds to two repetitions of the light and dark regions 42 and 44. Therefore, in the cross-sectional SEM image, the spacing between adjacent bright portions 42 or dark portions 44 in the normal direction (orthogonal direction) to the line formed by the bright portions 42 or dark portions 44 corresponds to 1 / 2 pitch of the surface pitch P. In other words, the surface pitch P can be measured by defining the spacing between the bright portions 42 or dark portions 44 in the normal direction to the line as 1 / 2 pitch.

[0065] In a cholesteric liquid crystal layer having dark portions (light portions) tilted relative to the surface of the cholesteric liquid crystal layer, such as the cholesteric liquid crystal layer 36 shown in Figure 13, the tilt angle of the dark portions relative to the surface and the tilt angle of the liquid crystal compound do not necessarily have to be the same. The tilt angle of the light portions is the same as the tilt angle of the dark portions, so a description thereof will be omitted. That is, in the liquid crystal diffraction element of this embodiment, in a cholesteric liquid crystal layer having dark portions tilted relative to the surface of the cholesteric liquid crystal layer, the tilt angle of the dark portions relative to the surface and the tilt angle of the liquid crystal compound may be the same across the entire surface, or may be different across the entire surface, or regions that match and regions that differ may coexist in the surface.

[0066] Furthermore, in the liquid crystal diffraction element according to this embodiment, the tilt angle of the liquid crystal compound in the cholesteric liquid crystal layer 36 is not limited to being uniform in the thickness direction. The cholesteric liquid crystal layer 36 of the liquid crystal diffraction element according to this embodiment may have two or more thickness-wise regions in which the tilt angles of the liquid crystal compound differ from one another. FIG. 14 is a cross-sectional view in the thickness direction conceptually illustrating another example of a liquid crystal diffraction element according to this embodiment. The liquid crystal diffraction element 18B shown in FIG. 14 includes a substrate 32, an alignment film 34, and a cholesteric liquid crystal layer 36B. As shown in the figure, at the interface of the cholesteric liquid crystal layer 36B on the alignment film 34 side, the optical axis 38A of the liquid crystal compound 38 is parallel to the surface (has no tilt angle). In the cholesteric liquid crystal layer 36B, the tilt angle of the liquid crystal compound 38 increases as the distance from the interface on the alignment film 34 side increases in the thickness direction, and then the liquid crystal compound 38 is aligned at a constant tilt angle up to the surface (air interface) side of the liquid crystal diffraction element 18B. As described above, the cholesteric liquid crystal layer of the liquid crystal diffraction element of this embodiment may be configured such that the liquid crystal compound on one surface has a tilt angle relative to that surface and the liquid crystal compound on the other surface does not have a tilt angle relative to that other surface, or may be configured such that the liquid crystal compound has a tilt angle relative to both surfaces. Furthermore, when the liquid crystal compound has a tilt angle relative to both surfaces of the cholesteric liquid crystal layer, the tilt angles of the liquid crystal compound relative to each surface may be the same or different.

[0067] In a cross-sectional SEM image of the cholesteric liquid crystal layer of the liquid crystal diffraction element of this embodiment, when there are bright and dark areas extending from one surface to the other, the inclination angles of the dark areas relative to the surface of the cholesteric liquid crystal layer may be uniform or may vary in the thickness direction. That is, there may be at least two regions in the thickness direction of the cholesteric liquid crystal layer where the inclination angles of the dark areas are different from each other.

[0068] In the cholesteric liquid crystal layer of the liquid crystal diffraction element of this embodiment, the tilt angle of the dark portions relative to the surface of the cholesteric liquid crystal layer obtained from a cross-sectional SEM image and the tilt angle of the liquid crystal compound may be the same or different in the thickness direction of the cholesteric liquid crystal layer. That is, the tilt angle of the dark portions relative to the surface of the cholesteric liquid crystal layer and the tilt angle of the liquid crystal compound may be the same throughout the thickness direction of the cholesteric liquid crystal layer. Furthermore, in the thickness direction of the cholesteric liquid crystal layer, there may be regions where the tilt angle of the dark portions relative to the surface and the tilt angle of the liquid crystal compound are different, or the tilt angle of the dark portions relative to the surface and the tilt angle of the liquid crystal compound may be different throughout the thickness direction.

[0069] In a cholesteric liquid crystal layer, the angle of the dark (bright) area relative to the surface in a cross-sectional SEM image can be adjusted by the length of one period in the above-mentioned liquid crystal alignment pattern and the magnitude of the twist of the liquid crystal compound that is twisted in the thickness direction. The helical twist alignment of the liquid crystal compound in the cholesteric liquid crystal layer can be achieved by adding a chiral agent to the liquid crystal composition for forming the cholesteric liquid crystal layer, as described below. The twist direction and degree of the liquid crystal compound can be adjusted by selecting and adjusting the type and amount of chiral agent added.

[0070] When the retardation of the cholesteric liquid crystal layer is measured in the normal direction of the main surface of the cholesteric liquid crystal layer and in the direction inclined relative to the normal direction, the cholesteric liquid crystal layer preferably has a region in which the direction in which the retardation Re is minimum is inclined relative to the normal direction. Ｒ"The retardation Re of the cholesteric liquid crystal layer is inclined with respect to the normal direction" means that the minimum value of the retardation Re of the cholesteric liquid crystal layer is a measured value when the measuring light is incident from a direction inclined with respect to the normal line of the main surface of the cholesteric liquid crystal layer. Note that the retardation refers to the retardation in a plane perpendicular to the direction in which the measuring light is incident.

[0071] The cholesteric liquid crystal layer has a direction D in which the retardation Re is minimum. Ｒ In at least one region inclined with respect to the normal direction, the direction D Ｒ The absolute value of the optical axis tilt angle φ calculated from the measurement angle θ2, which is the angle between the normal direction and the incident direction, and the average refractive index n of the cholesteric liquid crystal layer using the following formula (1), is preferably 5 to 85°, more preferably 10 to 80°, and even more preferably 15 to 70°: sin|θ2|=n·sinφ (1) Note that the measurement angle θ2 is the angle between the normal direction and the incident direction of the measurement light at which the retardation Re is minimum before the measurement light passes through the air-side interface of the cholesteric liquid crystal layer.

[0072] Furthermore, the cholesteric liquid crystal layer has a direction D in which the retardation Re is minimum. Ｒ has a region inclined with respect to the normal direction, and the retardation Re is the smallest direction D Ｒ When the cholesteric liquid crystal layer has the above-described regions in its plane, the decrease in diffraction efficiency can be further suppressed even in a region where one period of the liquid crystal diffraction element is short, and the amount of reflected light relative to incident light can be further improved.

[0073] In the plane of the cholesteric liquid crystal layer, the direction D ＲIt is preferable that the angle θ2 formed between the normal to the principal surface of the cholesteric liquid crystal layer and the axial direction of the cholesteric liquid crystal layer gradually changes. In particular, it is more preferable that the angle θ2 gradually changes as the period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer gradually changes in at least a portion (more preferably the entirety) of the plane of the cholesteric liquid crystal layer, and it is even more preferable that the angle θ2 increases as the period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer shortens. By having the cholesteric liquid crystal layer have the above configuration, the diffraction efficiency can be improved even in regions where the period of the liquid crystal diffraction element is short, and the amount of reflected light relative to the incident light can be further improved. The gradual change in the angle θ2 may be a continuous change or a stepwise change having regions where the angle θ2 is the same, or a mixture of regions where the angle θ2 changes continuously and regions where it changes stepwise.

[0074] Also, direction D Ｒ is preferably inclined in a direction toward the diffraction direction of reflected light for regular reflection by the liquid crystal diffraction element with respect to the normal direction of the main surface of the cholesteric liquid crystal layer. That is, when the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36) shown in FIG. 1 acts as a concave mirror that collects light, the direction D Ｒ is preferably inclined in a direction toward the center, i.e., the light-collecting direction, with respect to the normal direction (the direction perpendicular to the paper surface in FIG. 1). In addition, when the liquid crystal diffraction element acts as a convex mirror that diverges reflected light, the direction D Ｒ is preferably inclined from the inside to the outside, that is, in the direction in which light diverges, with respect to the normal direction of the main surface.

[0075] The cholesteric liquid crystal layer has a polarization direction D Ｒ In addition, the cholesteric liquid crystal layer may have a region in which the direction D Ｒ may be configured to be inclined.

[0076] The direction D in which the retardation Re in the liquid crystal diffraction element (cholesteric liquid crystal layer) is minimum Ｒ and the direction D in which the retardation Re is minimum.Ｒ A method for producing a cholesteric liquid crystal layer having a region inclined with respect to the normal direction of the principal surface will be described in the second embodiment.

[0077] 2, the liquid crystal diffraction element 18 has only one cholesteric liquid crystal layer 36, but the liquid crystal diffraction element of this embodiment may have multiple cholesteric liquid crystal layers. When the liquid crystal diffraction element has multiple cholesteric liquid crystal layers, it is preferable that the multiple cholesteric liquid crystal layers have different lengths of one period and helical pitches at any one point in the plane.

[0078] For example, when a liquid crystal diffraction element reflects light of multiple wavelengths irradiated by an image display element, the liquid crystal diffraction element preferably has a cholesteric liquid crystal layer that reflects light of each wavelength. The selective reflection wavelength in a cholesteric liquid crystal layer depends on the helical pitch. Therefore, multiple cholesteric liquid crystal layers can reflect light of each wavelength by setting different helical pitches according to the wavelength. In this case, it is necessary to align the diffraction directions (diffraction angles) of light of each wavelength at a certain point (region) within the plane of the liquid crystal diffraction element 18A. Here, the reflection angle of light by a cholesteric liquid crystal layer having a liquid crystal orientation pattern also depends on the wavelength of the light. Therefore, by setting an appropriate helical pitch for each cholesteric liquid crystal layer at any point within the plane, light of different wavelengths can be reflected at the same diffraction angle. This enables the liquid crystal diffraction element to broaden the wavelength band of light reflected and diffracted and to expand the range of incident angles of light reflected and diffracted, thereby improving the dependence of reflection diffraction efficiency on the incident angle of incident light. For example, when the image display element emits light of three colors, red light, green light, and blue light, the liquid crystal diffraction element preferably has three cholesteric liquid crystal layers corresponding to the three colors.

[0079] When the first cholesteric liquid crystal layer reflects blue light, the second cholesteric liquid crystal layer reflects green light, and the third cholesteric liquid crystal layer reflects red light, the first to third cholesteric liquid crystal layers have different periods Λ and helical pitches at any one point in their planes. When the lengths of the periods Λ at any one point in their planes are Λ1, Λ2, and Λ3, the first to third cholesteric liquid crystal layers preferably have a region where Λ1 < Λ2 < Λ3. In other words, the longer the helical pitch and the longer the wavelength of light reflected by the cholesteric liquid crystal layer, the longer the period Λ should be.

[0080] Furthermore, when the liquid crystal diffraction element has multiple cholesteric liquid crystal layers, it may have only one or at least two cholesteric liquid crystal layers having light and dark regions extending from one surface to the other in the cross-sectional SEM image. When the liquid crystal diffraction element has at least two cholesteric liquid crystal layers having light and dark regions extending from one surface to the other in the cross-sectional SEM image of the cholesteric liquid crystal layer, the inclination angles of the dark regions with respect to the surface in the at least two cholesteric liquid crystal layers may be the same or different from each other. When the liquid crystal diffraction element has at least two cholesteric liquid crystal layers, it is preferable that the inclination angles of the dark regions with respect to the surface in the at least two cholesteric liquid crystal layers are different from each other. This configuration can improve the light diffraction efficiency of the liquid crystal diffraction element (optically anisotropic layer).

[0081] In the liquid crystal diffraction element 18 of this embodiment, the cholesteric liquid crystal layer 36 is formed using a liquid crystal composition containing a rod-shaped or discotic liquid crystal compound, and the liquid crystal compound 38 has a liquid crystal orientation pattern oriented as described above. Furthermore, the liquid crystal compound 38 has regions with tilt angles and regions with different tilt angles within the plane. Such a liquid crystal diffraction element can be fabricated by forming an alignment film 34 having an orientation pattern corresponding to the above-described liquid crystal orientation pattern on a substrate 32, and then applying and curing a liquid crystal composition on the alignment film 34 to form a cholesteric liquid crystal layer 36 consisting of a cured layer of the liquid crystal composition. The liquid crystal composition for forming the cholesteric liquid crystal layer 36 contains a rod-shaped or discotic liquid crystal compound, and may further contain other components such as a leveling agent, an alignment control agent, a polymerization initiator, and an alignment aid. Furthermore, the cholesteric liquid crystal layer 36 only needs to have a structure that maintains the orientation state of the liquid crystal compound that forms the cholesteric liquid crystal phase. Typically, a preferred structure is one in which the polymerizable liquid crystal compound is oriented in a predetermined liquid crystal phase, and then polymerized and cured by ultraviolet irradiation, heating, or the like to form a layer with no fluidity, and at the same time, the structure is changed to a state in which the orientation does not change due to an external field or external force. Note that in a structure in which the cholesteric liquid crystal phase is fixed, it is sufficient that the optical properties of the cholesteric liquid crystal phase are maintained, and the liquid crystal compound in the liquid crystal layer does not need to exhibit liquid crystallinity. For example, the polymerizable liquid crystal compound may be polymerized by a curing reaction and lose its liquid crystallinity.

[0082] Furthermore, the cholesteric liquid crystal layer 36 preferably has a broad bandwidth relative to the wavelength of the incident light and is preferably constructed using a liquid crystal material with reverse dispersion birefringence. Examples of materials used to form the cholesteric liquid crystal layer include liquid crystal compositions containing a liquid crystal compound. The liquid crystal compound is preferably a polymerizable liquid crystal compound. The liquid crystal composition used to form the cholesteric liquid crystal layer may also contain a chiral agent, and may further contain, as necessary, surfactants, polymerization initiators, crosslinking agents, polymerization inhibitors, antioxidants, UV absorbers, light stabilizers, colorants, metal oxide particles, and the like, within limits that do not impair optical performance, etc.

[0083] The cholesteric liquid crystal layer may also have a pitch gradient structure. As is well known, the selective reflection center wavelength of a cholesteric liquid crystal layer is determined according to the pitch of the helical structure in the cholesteric liquid crystal phase. A pitch gradient structure is a structure in which the helical pitch varies in the film thickness direction. For example, Figure 13 is a schematic diagram showing a striped pattern in which light areas 42 and dark areas 44 are alternately arranged, as seen when a cross section of a cholesteric liquid crystal layer is observed with a scanning electron microscope (SEM). The spacing between the light areas 42 and the spacing P between the dark areas 44 correspond to the helical pitch.

[0084] When the cholesteric liquid crystal layer has a pitch gradient structure, the helical pitch gradually increases (or decreases) from one surface side to the other surface side of the cholesteric liquid crystal layer. The cholesteric liquid crystal layer can broaden the selective reflection wavelength band by changing the helical pitch P in the film thickness direction.

[0085] Polymerizable Liquid Crystal Compounds Polymerizable liquid crystal compounds may be rod-shaped or discotic. Examples of rod-shaped polymerizable liquid crystal compounds include rod-shaped nematic liquid crystal compounds. Preferred rod-shaped nematic liquid crystal compounds include azomethines, azoxys, cyanobiphenyls, cyanophenyl esters, benzoates, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, alkoxy-substituted phenylpyrimidines, phenyldioxanes, tolanes, and alkenylcyclohexylbenzonitriles. In addition to the low-molecular-weight liquid crystal molecules described above, polymeric liquid crystal molecules can also be used. Polymerizable liquid crystal compounds can be obtained by introducing a polymerizable group into a liquid crystal compound. Examples of polymerizable groups include unsaturated polymerizable groups, epoxy groups, and aziridinyl groups. Unsaturated polymerizable groups are preferred, and ethylenically unsaturated polymerizable groups are more preferred. Polymerizable groups can be introduced into liquid crystal compound molecules using various methods. The number of polymerizable groups that the polymerizable liquid crystal compound has is preferably 1 to 6, and more preferably 1 to 3.

[0086] As the polymerizable rod-like liquid crystal compound, there are mentioned Makromol. Chem. , Vol. 190, p. 2255 (1989), Advanced Materials Vol. 5, p. 107 (1993), U.S. Pat. No. 4,683,327, U.S. Pat. No. 5,622,648, U.S. Pat. No. 5,770,107, WO 95 / 022586, WO 95 / 024455, WO 97 / 000600, WO 98 / 023580, WO 98 / 052905, JP-A-1-272551, JP-A-6-016616, JP-A-7-110469, JP-A-11-080081, and compounds described in JP-A-2001-328973 can be used. Furthermore, as the rod-shaped liquid crystal compound, for example, those described in JP-A-11-513019 and JP-A-2007-279688 can also be preferably used.

[0087] Other examples of polymerizable liquid crystal compounds that can be used include cyclic organopolysiloxane compounds having a cholesteric phase, such as those disclosed in JP-A-57-165480. Examples of the polymeric liquid crystal compounds that can be used include polymers having mesogen groups exhibiting liquid crystallinity introduced into the main chain, side chain, or both the main chain and side chain, polymeric cholesteric liquid crystals having cholesteryl groups introduced into the side chain, liquid crystalline polymers such as those disclosed in JP-A-9-133810, and liquid crystalline polymers such as those disclosed in JP-A-11-293252.

[0088] As the discotic liquid crystal compound, for example, those described in JP-A Nos. 2007-108732 and 2010-244038 can be preferably used. When a discotic liquid crystal compound is used in the liquid crystal layer, liquid crystal compound 38 stands up in the thickness direction in the liquid crystal layer, and optical axis 38A derived from the liquid crystal compound is defined as an axis perpendicular to the disc surface, i.e., a so-called fast axis.

[0089] The amount of the polymerizable liquid crystal compound added to the liquid crystal composition is preferably 75 to 99.9% by mass, more preferably 80 to 99% by mass, and even more preferably 85 to 98% by mass, based on the solid content mass (mass excluding the solvent) of the liquid crystal composition. Two or more types of polymerizable liquid crystal compounds may be used in combination. Using two or more types of polymerizable liquid crystal compounds in combination can lower the alignment temperature.

[0090] --Chiral Agent (Optically Active Compound)-- A chiral agent functions to induce a helical structure in a cholesteric liquid crystal phase. The chiral agent can be selected according to the purpose, since the twist direction or helical pitch (i.e., tilted plane pitch) induced by the chiral agent varies depending on the compound. The chiral agent is not particularly limited, and known compounds (e.g., those described in "Liquid Crystal Device Handbook," Chapter 3, Section 4-3, "Chiral Agents for TN (Twisted Nematic) and STN (Super Twisted Nematic)," p. 199, edited by the 142nd Committee of the Japan Society for the Promotion of Science, 1989), isosorbide, and isomannide derivatives can be used. Chiral agents generally contain an asymmetric carbon atom, but axially asymmetric or planar asymmetric compounds without an asymmetric carbon atom can also be used as chiral agents. Examples of axially asymmetric or planar asymmetric compounds include binaphthyl, helicene, paracyclophane, and their derivatives. The chiral agent may have a polymerizable group. When both the chiral agent and the liquid crystal compound have a polymerizable group, a polymer having a repeating unit derived from the polymerizable liquid crystal compound and a repeating unit derived from the chiral agent can be formed by a polymerization reaction between the polymerizable chiral agent and the polymerizable liquid crystal compound.In this embodiment, the polymerizable group of the polymerizable chiral agent is preferably the same type of group as the polymerizable group of the polymerizable liquid crystal compound.Therefore, the polymerizable group of the chiral agent is also preferably an unsaturated polymerizable group, an epoxy group, or an aziridinyl group, more preferably an unsaturated polymerizable group, and even more preferably an ethylenically unsaturated polymerizable group.In addition, the chiral agent may be a liquid crystal compound.

[0091] When the chiral agent has a photoisomerizable group, it is possible to form a pattern of a desired reflection wavelength corresponding to the emission wavelength by irradiating the chiral agent with actinic rays or the like through a photomask after coating and alignment. The photoisomerizable group is preferably an isomerization site of a compound exhibiting photochromic properties, an azo group, an azoxy group, or a cinnamoyl group. Specific compounds that can be used include those described in JP-A-2002-080478, JP-A-2002-080851, JP-A-2002-179668, JP-A-2002-179669, JP-A-2002-179670, JP-A-2002-179681, JP-A-2002-179682, JP-A-2002-338575, JP-A-2002-338668, JP-A-2003-313189, and JP-A-2003-313292.

[0092] The content of the chiral agent in the liquid crystal composition is preferably 0.01 to 200 mol %, more preferably 1 to 30 mol %, based on the molar amount of the liquid crystal compound contained.

[0093] A cholesteric liquid crystal layer having regions with different helical pitches in the plane can be formed, for example, by using a chiral agent that undergoes back-isomerization, dimerization, isomerization and dimerization, etc., upon irradiation with light, thereby changing the helical twisting power (HTP), and by irradiating different regions in the plane with light of a wavelength that changes the HTP of the chiral agent before or during curing of the liquid crystal composition, with different irradiation doses. For example, by using a chiral agent whose HTP decreases upon irradiation with light, the HTP of the chiral agent decreases upon irradiation with light. Here, when the irradiation dose is varied for each region, for example, in regions with a high irradiation dose, the HTP decreases significantly, resulting in less helical induction and therefore a longer helical pitch PT. On the other hand, in regions with a low irradiation dose, the decrease in HTP is small, and helical induction is induced by the HTP inherent to the chiral agent, resulting in a shorter helical pitch PT.

[0094] There are no particular limitations on the method for changing the amount of light irradiation for each region within a surface, and methods that can be used include irradiating light through a gradation mask, changing the irradiation time for each region, and changing the irradiation intensity for each region. Note that a gradation mask is a mask whose transmittance for the irradiated light varies within its surface.

[0095] Materials for forming the cholesteric liquid crystal layer, a manufacturing method, and a method for exposing an alignment film to align the cholesteric liquid crystal layer are described in WO 2019 / 189852, etc.

[0096] There is no restriction on the thickness of the cholesteric liquid crystal layer, and the thickness that provides the required light reflectance can be set appropriately depending on the application of the liquid crystal diffraction element 18, the light reflectance required for the cholesteric liquid crystal layer, and the material from which the cholesteric liquid crystal layer is formed, etc.

[0097] Below, using FIG. 2 as an example, we will explain other components besides the cholesteric liquid crystal layer that the liquid crystal diffraction element may have, and a manufacturing method for the liquid crystal diffraction element. As described above, the liquid crystal diffraction element 18 shown in FIG. 2 has a substrate 32, an alignment film 34, and the above-mentioned cholesteric liquid crystal layer 36. The substrate 32 constituting such a liquid crystal diffraction element 18 can be any sheet-like material as long as it can support the alignment film 34 and the cholesteric liquid crystal layer 36. A transparent support is preferred for the substrate 32, and examples include polyacrylic resin films such as polymethyl methacrylate, cellulose resin films such as cellulose triacetate, cycloolefin polymer films (e.g., "Arton" manufactured by JSR Corporation, "Zeonor" manufactured by Nippon Zeon Co., Ltd.), polyethylene terephthalate (PET), polycarbonate, and polyvinyl chloride. The support is not limited to a flexible film, and may also be a non-flexible substrate such as a glass substrate.

[0098] An alignment film 34 is formed on the surface of such a substrate 32. The liquid crystal alignment pattern in the cholesteric liquid crystal layer 36 follows the alignment pattern formed on the alignment film 34. Therefore, the alignment film 34 for forming a liquid crystal layer having such a liquid crystal alignment pattern has the same alignment pattern as the liquid crystal alignment pattern in the cholesteric liquid crystal layer 36. The alignment film 34 having such an alignment pattern can be formed, for example, by forming a coating film containing a compound having a photoalignable group, drying the coating film, and then exposing it with an exposure device described below.

[0099] Compounds having a photo-alignment group, that is, photo-alignment materials used in photo-alignment films, include those described in, for example, JP-A Nos. 2006-285197, 2007-076839, 2007-138138, 2007-094071, 2007-121721, 2007-140465, and 2007-156439. azo compounds described in JP-A-2007-133184, JP-A-2009-109831, Japanese Patent Nos. 3,883,848 and 4,151,746; aromatic ester compounds described in JP-A-2002-229039; polymers having photo-alignable units described in JP-A-2002-265541 and JP-A-2002-317013; Preferred examples include methylimide and / or alkenyl-substituted nadimide compounds, photocrosslinkable silane derivatives described in Japanese Patent Nos. 4205195 and 4205198, photocrosslinkable polyimides, photocrosslinkable polyamides and photocrosslinkable esters described in JP-T-2003-520878, JP-T-2004-529220 and JP-T-4162850, and photodimerizable compounds described in JP-A-9-118717, JP-A-10-506420, JP-A-2003-505561, WO 2010 / 150748, JP-A-2013-177561 and JP-A-2014-12823, particularly cinnamate compounds, chalcone compounds and coumarin compounds. Among these, azo compounds, photocrosslinkable polyimides, photocrosslinkable polyamides, photocrosslinkable esters, cinnamate compounds, and chalcone compounds are preferably used.

[0100] In this way, the coating film that will become the alignment film 34 (photo-alignment film) for forming the cholesteric liquid crystal layer 36 is exposed to light to form an alignment pattern corresponding to a concentric liquid crystal alignment pattern in which the optical axis changes by continuously rotating radially. Before exposing this concentric liquid crystal alignment pattern, the alignment film 34 is irradiated with unpolarized light while changing the irradiation amount and irradiation angle, thereby forming an alignment film 34 that tilt-aligns the liquid crystal compound in the cholesteric liquid crystal layer 36. Specifically, the tilt angle of the liquid crystal compound 38 in the cholesteric liquid crystal layer 36 can be increased by increasing the irradiation amount of unpolarized light and the irradiation angle with respect to the surface (i.e., by decreasing the polar angle).

[0101] As an example, the normal direction of the alignment film 34 is set to 0° (polar angle 0°), and the plane direction of the alignment film 34 is set to 90° (polar angle 90°). Unpolarized light is incident on the alignment film 34 so that the incident angle gradually decreases and the irradiation amount gradually increases from the center of the alignment film 34 outward. That is, in a reflective liquid crystal diffraction element as shown in FIG. 1 , unpolarized light is incident on the alignment film 34 in a concentric pattern so that the incident angle gradually decreases and the irradiation amount gradually increases from the central optical axis toward the radially outward direction (concentric circles). This allows for the formation of an alignment film 34 in which the liquid crystal compound 38 has no tilt angle in the center and tilts the liquid crystal compound 38 so that the tilt angle of the liquid crystal compound 38 gradually increases from the inside to the outside, as shown in FIG. 3 .

[0102] In this way, after exposing the alignment film 34 to light in order to tilt align the liquid crystal compound 38, the alignment film 34 is exposed to light to form an alignment pattern corresponding to a concentric liquid crystal alignment pattern in which the optical axis continuously rotates and changes radially.

[0103] 15 conceptually shows an example of an exposure device that exposes a coating film that will become the alignment film 34 (photo-alignment film) for forming the cholesteric liquid crystal layer 36 to light to form an alignment pattern that corresponds to a concentric liquid crystal alignment pattern in which the optical axis changes by continuously rotating radially. The exposure device 80 shown in Fig. 15 includes a light source 84 equipped with a laser 82, a polarizing beam splitter 86 that splits laser light M from the laser 82 into S-polarized light MS and P-polarized light MP, a mirror 90A arranged in the optical path of the P-polarized light MP and a mirror 90B arranged in the optical path of the S-polarized light MS, a lens 92 arranged in the optical path of the S-polarized light MS, a polarizing beam splitter 94, and a λ / 4 plate 96.

[0104] The P-polarized light MP split by the polarizing beam splitter 86 is reflected by a mirror 90A and enters a polarizing beam splitter 94. On the other hand, the S-polarized light MS split by the polarizing beam splitter 86 is reflected by a mirror 90B, collected by a lens 92, and enters the polarizing beam splitter 94. The P-polarized light MP and the S-polarized light MS are combined by the polarizing beam splitter 94 and converted into right- and left-circularly polarized light according to the polarization direction by a λ / 4 plate 96. The combined light then enters the alignment film 34 on the substrate 32. Here, due to interference between the right- and left-circularly polarized light, the polarization state of the light irradiating the alignment film 34 changes periodically in the form of interference fringes. The crossing angle between the left- and right-circularly polarized light changes from the inside to the outside of the concentric circles, resulting in an exposure pattern whose pitch changes from the inside to the outside. This results in a radial (concentric) alignment pattern in the alignment film 34, in which the alignment state changes periodically.

[0105] In this exposure device 80, the period Λ of the liquid crystal orientation pattern in which the optical axis of the liquid crystal compound 38 continuously rotates 180° along one direction can be controlled by changing the refractive power of the lens 92, the focal length of the lens 92, and the distance between the lens 92 and the orientation film 34. Furthermore, by adjusting the refractive power of the lens 92 (the F-number of the lens 92), the length of one period of the liquid crystal orientation pattern in one direction in which the optical axis continuously rotates can be changed. Specifically, the length of one period of the liquid crystal orientation pattern in one direction in which the optical axis continuously rotates can be changed by adjusting the spread angle of the light expanded by the lens 92, which interferes with the parallel light. More specifically, when the refractive power of the lens 92 is weakened, the light becomes closer to parallel light, and the length Λ of one period of the liquid crystal orientation pattern gradually shortens from the inside to the outside. Conversely, when the refractive power of the lens 92 is strengthened, the length Λ of one period of the liquid crystal orientation pattern suddenly shortens from the inside to the outside. That is, by adjusting the refractive index of the lens 92, it is possible to adjust the refractive index of the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36), which acts as a concave or convex lens depending on the rotation direction of the incident circularly polarized light.

[0106] A liquid crystal composition for forming the above-mentioned cholesteric liquid crystal layer 36 is applied to the exposed alignment film 34 thus formed, dried, and cured by ultraviolet irradiation or the like as necessary, thereby forming a cholesteric liquid crystal layer 36 having the above-mentioned concentric liquid crystal alignment pattern, a region in which the liquid crystal compound 38 has a tilt angle, and a region in the plane in which the tilt angle of the liquid crystal compound 38 is different, thereby making it possible to produce the liquid crystal diffraction element 18 as shown in Figures 1 and 2.

[0107] In the liquid crystal diffraction element of the present embodiment described above, the cholesteric liquid crystal layer has a concentric liquid crystal orientation pattern as shown in FIG. 1 , but this embodiment is not limited thereto. For example, in the liquid crystal diffraction element of the present embodiment, the cholesteric liquid crystal layer may have a linear liquid crystal orientation pattern directed in one direction (the direction of arrow A) as shown in FIG. 5 . Such a linear liquid crystal orientation pattern can be formed by exposing the alignment film using a known method, such as a method using an exposure apparatus described in FIG. 8 of Japanese Patent No. 7200383. Furthermore, when one period of the linear liquid crystal orientation pattern directed in one direction (the direction of arrow A) changes within the plane, the liquid crystal diffraction element functions, for example, as a reflective liquid crystal diffraction element that focuses light linearly, or as a reflective liquid crystal diffraction element that diverges light in two opposite directions.

[0108] [Second embodiment] A liquid crystal diffraction element according to a second embodiment of the present invention is a liquid crystal diffraction element including a cholesteric liquid crystal layer, in which the cholesteric liquid crystal layer has a liquid crystal orientation pattern in which the direction of an optical axis derived from a liquid crystal compound changes while continuously rotating along at least one direction in a plane, and the cholesteric liquid crystal layer has a region in which the direction in which the retardation is minimum is tilted with respect to the normal direction when the retardation is measured from the normal direction of a main surface of the cholesteric liquid crystal layer and from a direction tilted with respect to the normal direction, and the cholesteric liquid crystal layer has a region in which the direction in which the retardation Re is minimum is tilted with respect to the normal direction Ｒ has different regions in the plane.

[0109] For the configuration of the liquid crystal diffraction element of this embodiment, the drawings used to explain the liquid crystal diffraction element of the first embodiment can be referred to, and this embodiment will be explained with reference to the drawings as necessary.

[0110] The present inventors have discovered that the liquid crystal diffraction element is a cholesteric liquid crystal layer having a liquid crystal orientation pattern in which the direction of the optical axis changes while continuously rotating along at least one direction in the plane, and that the direction of the optical axis changes along the direction D Ｒ has a region that is inclined with respect to the normal direction, and the direction D ＲThe present inventors have found that, when a cholesteric liquid crystal layer having regions with different diffraction efficiency is provided, the decrease in diffraction efficiency can be further suppressed even in regions where the period Λ of the liquid crystal orientation pattern is short, particularly in regions where the period Λ of the liquid crystal orientation pattern is 1 μm or less, and the amount of reflected light relative to the incident light can be further improved. Since the liquid crystal diffraction element of this embodiment has the above-mentioned configuration, it can obtain excellent diffraction efficiency, and when used in, for example, a short-focus concave mirror, the liquid crystal diffraction element of this embodiment can focus light with high focusing efficiency.

[0111] In this embodiment, the cholesteric liquid crystal layer is Ｒ In at least one region inclined with respect to the normal direction, the direction D Ｒ The absolute value of the optical axis tilt angle φ calculated from the measurement angle θ2, which is the angle between the normal direction and the incident direction, and the average refractive index n of the cholesteric liquid crystal layer using the following formula (1), is preferably 5 to 85°, more preferably 10 to 80°, and even more preferably 15 to 70°: sin|θ2|=n·sinφ (1) Note that the measurement angle θ2 is the angle between the normal direction and the incident direction of the measurement light at which the retardation Re is minimum before the measurement light passes through the air-side interface of the cholesteric liquid crystal layer.

[0112] In the plane of the cholesteric liquid crystal layer, the direction D ＲIt is preferable that the angle θ2 formed between the θ2 and the normal direction to the principal surface of the cholesteric liquid crystal layer gradually changes. In particular, it is more preferable that, in at least a portion (more preferably all) of the plane of the cholesteric liquid crystal layer, the angle θ2 gradually changes as the period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer gradually changes, and it is even more preferable that the angle θ2 increases as the period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer shortens. By having the cholesteric liquid crystal layer have the above configuration, the diffraction efficiency can be improved even in regions where the period of the liquid crystal diffraction element is short, and the amount of reflected light relative to the incident light can be further improved. The gradual change in the angle θ2 may be a continuous change or a stepwise change having regions where the angle θ2 is the same, or a mixture of regions where the angle θ2 changes continuously and regions where it changes stepwise.

[0113] Also, direction D Ｒ is preferably inclined in a direction toward the diffraction direction of reflected light for regular reflection by the liquid crystal diffraction element with respect to the normal direction of the main surface of the cholesteric liquid crystal layer. That is, when the liquid crystal diffraction element 18 (cholesteric liquid crystal layer 36) shown in FIG. 1 acts as a concave mirror that collects light, the direction D Ｒ is preferably inclined in a direction toward the center, i.e., the light-collecting direction, with respect to the normal direction (the direction perpendicular to the paper surface in FIG. 1). In addition, when the liquid crystal diffraction element acts as a convex mirror that diverges reflected light, the direction D Ｒ is preferably inclined from the inside to the outside, that is, in the direction in which light diverges, with respect to the normal direction of the main surface.

[0114] The cholesteric liquid crystal layer has a polarization direction D Ｒ In addition, the cholesteric liquid crystal layer may have a region in which the direction D Ｒ may be configured to be inclined.

[0115] In the cholesteric liquid crystal layer of the liquid crystal diffraction element, the direction D Ｒ The method for measuring the retardation Re will be described below.Ｒ The retardation Re of the cholesteric liquid crystal layer can be detected by measuring the retardation Re of the cholesteric liquid crystal layer by irradiating a measurement light from the normal direction of the principal surface of the cholesteric liquid crystal layer, and then measuring the retardation Re of the cholesteric liquid crystal layer while sequentially changing the incident direction (incidence angle relative to the normal) of the measurement light. The retardation Re refers to the retardation in a plane perpendicular to the direction of incidence of the measurement light. The retardation Re is measured by using a polarized phase difference analyzer Axoscan (manufactured by Axometrics) to calculate the slow axis direction by the above-mentioned method, and then sequentially tilting the measurement light in a plane (slow axis plane) that is perpendicular to the principal surface of the cholesteric liquid crystal layer and includes the slow axis of the cholesteric liquid crystal layer, and in a plane (fast axis plane) that is perpendicular to the principal surface of the cholesteric liquid crystal layer and includes a direction (fast axis) perpendicular to the slow axis of the cholesteric liquid crystal layer in the plane. The measurement light used for measuring the retardation Re is preferably light of a wavelength outside the selective reflection wavelength range of the cholesteric liquid crystal layer, for example, infrared light which is invisible light. In the cholesteric liquid crystal layer of this embodiment, the fast axis plane usually coincides with the direction in which the optical axis continuously rotates in the in-plane direction, i.e., the direction of arrow A, and the slow axis direction coincides with the direction perpendicular to the direction in which the optical axis continuously rotates in the in-plane direction.

[0116] The liquid crystal diffraction element of this embodiment has a direction D Ｒ has a region that is inclined with respect to the normal direction, and the direction D Ｒ The method for producing a cholesteric liquid crystal layer having regions in which the direction D Ｒ The direction D in which the retardation Re in the in-plane region of the cholesteric liquid crystal layer is minimized is determined by the tilt angle and direction of the tilt alignment of the liquid crystal compound. Ｒ The method for producing a cholesteric liquid crystal layer in which the liquid crystal compound is tilt-aligned and the method for adjusting the tilt angle and the like are as described in the first embodiment.

[0117] In this embodiment, it is preferable that at least one surface of the cholesteric liquid crystal layer has a region in which the liquid crystal compound has a tilt angle with respect to the surface of the cholesteric liquid crystal layer, and further, the cholesteric liquid crystal layer has a region in which the tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer varies within the plane. When the cholesteric liquid crystal layer has such a region within the plane, it is possible to further suppress a decrease in diffraction efficiency even in a region in which one period of the liquid crystal diffraction element is short, and it is possible to further improve the amount of reflected light relative to incident light.

[0118] It is preferable that the tilt angle of the liquid crystal compound relative to the surface of the cholesteric liquid crystal layer gradually changes in at least a part (more preferably all) of the plane of the cholesteric liquid crystal layer, and it is more preferable that the tilt angle of the liquid crystal compound gradually changes as one period Λ of the liquid crystal orientation pattern of the cholesteric liquid crystal layer gradually changes in at least a part (more preferably all) of the plane of the cholesteric liquid crystal layer. In particular, considering that in a liquid crystal diffraction element (cholesteric liquid crystal layer), the shorter one period Λ of the liquid crystal orientation pattern is, the lower the diffraction efficiency is, it is even more preferable that the tilt angle of the liquid crystal compound increases as one period Λ becomes shorter in at least a part (more preferably all) of the liquid crystal orientation pattern of the cholesteric liquid crystal layer.

[0119] However, the liquid crystal diffraction element of this embodiment is not limited to the above-mentioned embodiment. The tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer may be constant or may vary within the plane. The tilt angle of the liquid crystal compound with respect to the surface of the cholesteric liquid crystal layer may decrease as one period Λ in the liquid crystal alignment pattern shortens, or the tilt angle of the liquid crystal compound may not be linked to the change in one period Λ in the liquid crystal alignment pattern.

[0120] The liquid crystal diffraction element of this embodiment has the same composition of the cholesteric liquid crystal layer containing the liquid crystal compound, the structure and physical properties (optical and physical) of the cholesteric liquid crystal layer including the tilt angle of the cholesteric liquid crystal compound, and the method for forming the cholesteric liquid crystal layer, including their preferred aspects, as the liquid crystal diffraction element of the first embodiment already described.

[0121] In the cholesteric liquid crystal layer of the liquid crystal diffraction element of this embodiment, the inclination angle of the dark portion with respect to the surface of the cholesteric liquid crystal layer obtained from the cross-sectional SEM image and the above direction D Ｒ and the measurement angle θ2, which is the angle between the normal to the main surface of the cholesteric liquid crystal layer and the dark portion, may be the same or different in the thickness direction of the cholesteric liquid crystal layer. That is, the tilt angle of the dark portion with respect to the surface of the cholesteric liquid crystal layer and the measurement angle θ2 may be the same throughout the thickness direction of the cholesteric liquid crystal layer. Furthermore, there may be a region in the thickness direction of the cholesteric liquid crystal layer where the tilt angle of the dark portion with respect to the surface and the measurement angle θ2 differ, and the tilt angle of the dark portion with respect to the surface and the tilt angle of the liquid crystal compound may be different throughout the thickness direction.

[0122] In the liquid crystal diffraction element of this embodiment, the direction D in which the retardation Re is minimum Ｒ It is preferable that the measurement angle θ2, which is the angle between the direction D and the normal direction of the principal surface of the cholesteric liquid crystal layer, is close to the traveling direction of light inside the cholesteric liquid crystal layer. Ｒ It is preferable that the angle θ2 formed between the direction of propagation of light inside the cholesteric liquid crystal layer and the normal direction be close to the angle formed between the direction of propagation of light inside the cholesteric liquid crystal layer and the normal direction to the principal surface.

[0123] More specifically, in an optical device having the liquid crystal diffraction element of this embodiment and a light source that inputs light to the liquid crystal diffraction element, when the angle of incident light from the light source to the cholesteric liquid crystal layer (liquid crystal diffraction element) is θin, the refractive index of the cholesteric liquid crystal layer is nG, and the output angle of the primary light reflected by the cholesteric liquid crystal layer (liquid crystal diffraction element) is θm, the direction D ＲIt is preferable that the angle θ2 [°] between the normal direction and the θG [°] satisfies the following formulas (B1) to (B3): (B1) θ2 [°] = (θG [°] - θr [°]) / 2 ± 15 [°] (B2) sin θG = sin θm / nG (B3) sin θr = sin θin / nG The above formula (B1) can be rewritten as the following formula (B4): (B4) (θG [°] - θr [°]) / 2 - 15° ≦ θ2 [°] ≦ (θG [°] - θr [°]) / 2 + 15° By having such a configuration, a liquid crystal diffraction element having excellent diffraction efficiency can be obtained even if one period Λ of the liquid crystal orientation pattern in the cholesteric liquid crystal layer is short. This is particularly preferable when the diffraction angle is large, as it allows for the production of a liquid crystal diffraction element having excellent diffraction efficiency and excellent polarization (high degree of circular polarization). When the liquid crystal diffraction element of this embodiment has a plurality of cholesteric liquid crystal layers, the refractive index nG of the cholesteric liquid crystal layer is the same as that of the first embodiment, and the direction D Ｒ Regarding the angle θ2 between the normal direction and the tilt angle θP [°] of the liquid crystal compound in the first embodiment, the average angle of the multiple cholesteric liquid crystal layers calculated by taking into account the thickness of each layer can be used.

[0124] Furthermore, the members other than the cholesteric liquid crystal layer included in the liquid crystal diffraction element of this embodiment, including their preferred aspects, are the same as those of the liquid crystal diffraction element of the first embodiment already described.

[0125] Hereinafter, the features of the liquid crystal diffraction element of the present invention will be described without distinguishing between the first and second embodiments.

[0126] The liquid crystal diffraction element of the present invention and an optical device having the liquid crystal diffraction element of the present invention, which will be described later, may have an adhesive layer for bonding to other components. In this specification, the term "adhesion" is used to include the concept of "stickiness." Examples of adhesives include water-soluble adhesives, ultraviolet-curable adhesives, emulsion-type adhesives, latex-type adhesives, mastic adhesives, multilayer adhesives, paste-like adhesives, foam-type adhesives, supported film adhesives, thermoplastic adhesives, hot-melt adhesives, heat-setting adhesives, heat-activated adhesives, heat-seal adhesives, thermosetting adhesives, contact adhesives, pressure-sensitive adhesives (i.e., pressure-sensitive adhesives), polymerization-type adhesives, solvent-type adhesives, solvent-activated adhesives, and ceramic adhesives. Specific examples include an aqueous solution of a boron compound, a curable adhesive of an epoxy compound that does not contain an aromatic ring in the molecule, as disclosed in JP 2004-245925 A, an active energy ray-curable adhesive described in JP 2008-174667 A, which contains as essential components a photopolymerization initiator having a molar absorption coefficient of 400 or greater at a wavelength of 360 to 450 nm and an ultraviolet-curable compound, and an active energy ray-curable adhesive described in JP 2008-174667 A, which contains, per 100 parts by mass of the total amount of (meth)acrylic compounds, (a) a (meth)acrylic compound having two or more (meth)acryloyl groups in the molecule, (b) a (meth)acrylic compound having a hydroxyl group in the molecule and only one polymerizable double bond, and (c) a phenol ethylene oxide-modified acrylate or nonylphenol ethylene oxide-modified acrylate. These adhesives may be used alone or, if necessary, may be used in combination.

[0127] From the viewpoint of reducing unnecessary reflection, it is preferable that the difference in refractive index between the adhesive layer and the adjacent layer is small. Specifically, the difference in refractive index between the adjacent layer is preferably 0.05 or less, more preferably 0.01 or less. There are no particular limitations on the method for adjusting the refractive index of the adhesive layer, but known methods can be used, such as adding fine particles such as zirconia-based, silica-based, acrylic-based, acrylic-styrene-based, and melamine-based particles, adjusting the resin refractive index, and the method described in JP-A-11-223712. Furthermore, if the adjacent layers have in-plane refractive index anisotropy, it is preferable that the difference in refractive index between the adjacent layers be 0.05 or less in all directions in the plane. Therefore, the adhesive layer may have in-plane refractive index anisotropy. If the refractive index difference between the interfaces to be bonded is large, the interface reflectance can be reduced by providing a refractive index distribution in the thickness direction of the adhesive layer. Methods for imparting a refractive index distribution in the thickness direction include providing multiple adhesive layers, mixing the interfaces between multiple adhesive layers, and controlling the uneven distribution of materials within the adhesive layers to impart a refractive index distribution.

[0128] The adhesive layer can be provided on one or both of the members to be bonded by any method such as coating, vapor deposition, or transfer. From the viewpoint of increasing adhesive strength, post-treatment such as heat treatment and ultraviolet irradiation can be carried out depending on the type of adhesive. The thickness of the adhesive layer can be adjusted as desired, but is preferably 20 μm or less, and more preferably 0.1 μm or less. Methods for forming an adhesive layer of 0.1 μm or less include silicon oxide (SiO x One example is a method of depositing a ceramic adhesive such as a ceramic adhesive layer (a ceramic adhesive layer) on the bonding surface. The bonding surface of the bonding member can be subjected to surface modification treatments such as plasma treatment, corona treatment, and saponification treatment before bonding, and a primer layer can be applied. Furthermore, when there are multiple bonding surfaces, the type and thickness of the adhesive layer can be adjusted for each bonding surface.

[0129] The laminate produced using the liquid crystal diffraction element of the present invention can be cut to a predetermined size. There are no limitations on the method for cutting the laminate, and various known methods can be used, such as physical cutting using a blade such as a Thomson blade or cutting by irradiating a laser. When using a laser, it is preferable to select the pulse width (nanoseconds, picoseconds, femtoseconds) and wavelength taking into consideration cutting ability and damage to the material. Furthermore, after processing the laminate into a predetermined shape, for example, polishing of the end faces may be performed. From the perspective of improving processability during cutting and suppressing dust generation, cutting can also be performed with a peelable protective film attached. Furthermore, by cutting while observing the liquid crystal orientation pattern, for example, using the method disclosed in JP 2004-141889 A, it is possible to arbitrarily determine the cutting position. In this case, the liquid crystal orientation pattern can be easily observed through a polarizing plate, a retardation film, or the like. Furthermore, when multiple optical elements are provided on a single substrate, it is preferable to cut the multiple optical elements simultaneously.

[0130] The laminate having the liquid crystal diffraction element of the present invention can be provided with marks of any shape as needed to accurately install the laminate in a device, improve the accuracy of the cutting axis and cutting position, etc. The type of mark can be selected as desired, and methods such as physical marking using a laser or inkjet method, partial modification of the liquid crystal alignment state, and partial decolorization or dyeing can be selected. Furthermore, to protect the liquid crystal layer, a protective layer (such as a gas barrier layer, a moisture-blocking layer, an ultraviolet-absorbing layer, or a scratch-resistant layer) can be provided as needed. The protective layer can be formed directly on the liquid crystal layer or via an adhesive layer or another optical film. An anti-reflection layer (such as a low reflection (LR) layer, an anti-reflective (AR) layer, or a moth-eye layer) can be provided to reduce the surface reflectance. Various protective layers can be appropriately selected from known materials. When a gas barrier layer is provided, polyvinyl alcohol is preferred. Polyvinyl alcohol can also function as a polarizer. The ultraviolet absorbing layer is a layer containing an ultraviolet absorber, and from the viewpoint of good display properties, it is preferable to use an ultraviolet absorber that has excellent absorption ability for ultraviolet rays with wavelengths of 370 nm or less and little absorption of visible light with wavelengths of 400 nm or more. Only one ultraviolet absorber may be used, or two or more ultraviolet absorbers may be used in combination. Examples of ultraviolet absorbers include those described in JP-A No. 2001-072782 and JP-A No. 2002-543265. Specific examples of ultraviolet absorbers include oxybenzophenone-based compounds, benzotriazole-based compounds, salicylic acid ester-based compounds, benzophenone-based compounds, cyanoacrylate-based compounds, and nickel complex salt-based compounds.

[0131] [Uses of Liquid Crystal Diffraction Element] The liquid crystal diffraction element of the present invention can be used as an optical unit in combination with various members. Furthermore, the liquid crystal diffraction element of the present invention and an optical unit including the liquid crystal diffraction element of the present invention can be used as an optical module in combination with various members. Furthermore, the liquid crystal diffraction element of the present invention, the optical unit (optical element) including the liquid crystal diffraction element of the present invention, and the optical module including the liquid crystal diffraction element of the present invention can be used in various optical devices.

[0132] In particular, the liquid crystal diffraction element of the present invention is preferably used in an optical device having the liquid crystal diffraction element of the present invention and a light source that inputs light to the liquid crystal diffraction element. Examples of optical devices having the liquid crystal diffraction element of the present invention and a light source include a head-mounted display, a VR (Virtual Reality) display device, a sensor, and a communication device.

[0133] In the optical device of the present invention, there are no limitations on the light source, and various known light sources can be used. Therefore, the light source may be one that emits white light, or one that emits monochromatic light such as red light, green light, or blue light, or may be various image display elements such as liquid crystal displays and organic electroluminescence displays. Since the liquid crystal diffraction element of the present invention can be suitably used as a reflective liquid crystal diffraction element in VR systems such as head-mounted displays, various image display elements are suitable examples of the light source. Below are application examples of the optical device of the present invention.

[0134] [Application Example of Optical Device] Fig. 16 conceptually shows an example of an image display device having the optical device of the present invention. The image display device (virtual reality display device) 200 shown in Fig. 16 has, in this order, an image display element 202, a circular polarizer 204, and an optical unit 210. The optical unit 210 has a first partially reflective element 211 and a second partially reflective element 213.

[0135] The image display element 202 is a known display. Examples of the image display element 202 include a liquid crystal display element (LCD (Liquid Crystal Display)), an organic electroluminescence display element (OLED (Organic Light Emitting Diode)), a CRT (Cathode-ray tube), a plasma display element, electronic paper, an LED (Light Emitting Diode) display element, a micro LED display element, a DLP (Digital Light Processing), and a MEMS (Micro-Electro-Mechanical Systems) display element. In the present invention, the liquid crystal display element includes LCOS (Liquid Crystal On Silicon) and the like. The image display element may also be a transparent display that can transmit light.

[0136] The image display element may be one that displays a monochrome image, one that displays a two-color image, or one that displays a color image.

[0137] Furthermore, the light emitted by the image display element may be unpolarized, linearly polarized, or circularly polarized. Furthermore, the display surface (viewing surface) side of the image display element may have an element (e.g., a linear polarizer or a circular polarizing plate) that converts the polarization state of light. In the example shown in Fig. 16, a circular polarizing plate 204 is provided on the display surface side of the image display element 202. The circular polarizing plate 204 has, for example, a linear polarizer and a λ / 4 retardation plate.

[0138] There are no limitations on the linear polarizer. Therefore, the linear polarizer may be a reflective polarizer or an absorptive polarizer, and various known linear polarizers can be used, such as an iodine-based polarizer, a dye-based polarizer using a dichroic dye, a polyene-based polarizer, a wire-grid polarizer, and a film obtained by stretching a dielectric multilayer film as described in JP 2011-053705 A, etc. Furthermore, there are no limitations on the λ / 4 retardation plate. Therefore, various known λ / 4 retardation plates can be used, such as a stretched polycarbonate film, a stretched norbornene-based polymer film, a transparent film containing and oriented inorganic particles having birefringence such as strontium carbonate, a thin film obtained by obliquely depositing an inorganic dielectric on a support, a film obtained by uniaxially aligning and fixing a polymerizable liquid crystal compound, and a film obtained by uniaxially aligning and fixing a liquid crystal compound.

[0139] 16 , a first partial reflecting element 211 and a second partial reflecting element 213 are arranged in this order on the surface of circular polarizer 204 opposite to image display element 202. First partial reflecting element 211 and second partial reflecting element 213 constitute optical unit 210. Optical unit 210 can increase the optical path length in a limited space by causing light to travel back and forth between first partial reflecting element 211 and second partial reflecting element 213, thereby contributing to the miniaturization of the image display unit.

[0140] In the image display device 200, either the first partially reflective element 211 or the second partially reflective element 213 is a liquid crystal diffraction element having a cholesteric liquid crystal layer according to the present invention. A liquid crystal diffraction element (partially reflective element) having a cholesteric liquid crystal layer reflects one circularly polarized light component of incident light and transmits the other circularly polarized light component, and diffracts the reflected light. Therefore, it can function as a concave mirror while maintaining its flat shape, making it possible to further reduce the thickness of the optical device (image display device).

[0141] As an example, in the example shown in FIG. 16 , the first partial reflecting element 211 is a liquid crystal diffraction element of the present invention, and the second partial reflecting element 213 is a partial reflecting element without diffractive properties (lens properties), such as a general half mirror. In this case, as shown in FIG. 16 , light irradiated from the image display element 202 and passing through the circular polarizer 204 passes through the first partial reflecting element 211 and reaches the second partial reflecting element 213. The second partial reflecting element 213 reflects a portion of the light toward the first partial reflecting element 211. The first partial reflecting element 211 reflects the light reflected by the second partial reflecting element 213 toward the second partial reflecting element 213. In this case, the first partial reflecting element 211 acts as a concave mirror, diffracting (bending) light at a larger angle toward the end so that the reflected light is concentrated. A portion of the light reflected by the first partial reflecting element 211 passes through the second partial reflecting element 213 and is visually recognized as an image by the user U.

[0142] As shown in Figure 16, the first partially reflective element 211 acts as a concave mirror, diffracting (bending) light more significantly in the edge regions than in the central region. In conventional partially reflective elements, the larger the diffraction angle, the lower the diffraction efficiency. This resulted in a problem where the brightness of the image displayed by the image display device was high in the center and decreased toward the edges, resulting in significant brightness unevenness across the surface. In contrast, in the image display device 200 shown in Figure 16, the cholesteric liquid crystal layer in one of the partially reflective elements has the above-described configuration. This shortens the period Λ of the liquid crystal orientation pattern, increasing the diffraction efficiency even at the edges where the diffraction angle is large, resulting in more uniform in-plane diffraction efficiency. As a result, brightness unevenness in the displayed image can be reduced.

[0143] In the above examples, one partial reflection element is a liquid crystal diffraction element that acts as a concave mirror, and the other partial reflection element is a half mirror that does not have a general lens function, but this is not limited to this, and the other partial reflection element may be one that acts as a concave mirror or one that acts as a convex mirror. Furthermore, if the other partial reflection element made of a half mirror, a reflective volume hologram, or the like acts as a concave mirror, one partial reflection element made of a liquid crystal diffraction element may be one that acts as a convex mirror.

[0144] Fig. 17 conceptually shows another example of an image display device having an optical device of the present invention. The image display device 110 shown in Fig. 17 has an image projection element 112, a retardation plate 114, a transparent substrate 116, and a liquid crystal diffraction element 118 of the present invention. The image display device shown in Fig. 17 is an image display device that displays augmented reality by superimposing a virtual image A on a real scene R.

[0145] The image display device 110 in the illustrated example is, for example, AR glasses. Fig. 17 is a view of a user U wearing the AR glasses as viewed from above (the upside down). In such an image display device 110, the image projection element 112 is, for example, attached to the temples of the AR glasses.

[0146] In the image display device 110, the image projection element 112 projects (displays) a virtual image A. In other words, the image projection element 112 projects an image that becomes the virtual image A. There are no limitations on the image projection element 112, and various known projection elements (display elements, projectors) used in AR glasses and the like can be used. Examples of the image projection element 112 include a scanning projection element that uses a laser light source and a spatial light modulator (SLM) to two-dimensionally scan a light beam modulated according to an image, and a known display exemplified as the image display element 202 described above.

[0147] Examples of spatial light modulation elements that can be used include MEMS (Micro Electro Mechanical Systems) type spatial light modulation elements, optical elements (PLZT elements) that modulate transmitted light by electro-optical effects, liquid crystal shutter arrays such as liquid crystal shutters (FLC), and known optical deflection elements. The spatial light modulation elements may be either reflective or transmissive. The MEMS type spatial light modulation element refers to a spatial light modulation element that is driven by electromechanical operation utilizing electrostatic force, and any of the well-known MEMS (optical) scanners, MEMS optical deflectors, MEMS mirrors, and DMDs (Digital Micromirror Devices) that deflect (deflection scan) light by oscillating a mirror using a piezoelectric actuator or the like, such as the MEMS optical deflection element described in JP 2012-208352 A, the MEMS optical deflection element described in JP 2014-134642 A, and the MEMS optical deflection element described in JP 2015-22064 A, can be used.

[0148] The retarder 114 converts the linearly polarized virtual image A projected by the image projection element 112 into a predetermined circularly polarized virtual image A corresponding to the liquid crystal diffraction element 118. In the image display device 110 shown in the figure, the retarder 114 converts the linearly polarized virtual image A into a right-handed circularly polarized virtual image A, for example. The retarder 114 is preferably a λ / 4 plate (quarter-wave plate). By using a λ / 4 plate as the retarder 114, the linearly polarized virtual image A can be suitably converted into a right-handed circularly polarized virtual image A, thereby improving the utilization efficiency of the virtual image A projected by the image projection element 112. A known retarder can be used as the retarder 114, and the λ / 4 retarder that constitutes the above-mentioned circular polarizer can be used.

[0149] The retarder 114 is preferably a laminate of multiple retarders that effectively achieves the desired effect. In the case of a λ / 4 plate, it is also preferable to use a laminate of multiple retarders that effectively functions as a λ / 4 plate. For example, a λ / 4 plate, which is a broadband plate combining a λ / 2 plate and a λ / 4 plate as described in International Publication No. 2013 / 137464, can accommodate incident light with a wide wavelength range and is therefore preferably used. Furthermore, the retarder 114 preferably has reverse wavelength dispersion. The reverse wavelength dispersion of the retarder 114 allows it to accommodate incident light with a wide wavelength range. The retarder 114 is positioned such that the direction of its slow axis is adjusted to convert linearly polarized light of the image projected by the image projection element 112 into circularly polarized light with the desired rotation direction, depending on the polarization direction of the linearly polarized light.

[0150] The transparent substrate 116 is not limited, and a substrate made of various known materials can be used as long as it has sufficient transparency for observing the real scene R and can support the liquid crystal diffraction element 118. The transparent substrate 116 can be, for example, the transparent support for the substrate 32. The transparent substrate 116 may also be an eyeglass lens for AR glasses.

[0151] In the image display device 110, the real scene R passes through the transparent substrate 116 and the liquid crystal diffraction element 118 and is observed by the user U. Meanwhile, the virtual image A (projected image) projected by the image projection element 112 is converted into predetermined circularly polarized light by the retardation plate 114, for example, and diffracted by the liquid crystal diffraction element 118, reflected toward the user U, and observed by the user U. The user U of the image display device 110 can thereby observe an augmented reality in which the virtual image A is superimposed on the real scene R.

[0152] More specifically, when the image projection element 112 of the image display device 110 projects a linearly polarized image of green light as virtual image A (the image that becomes virtual image A), the linearly polarized virtual image A projected by the image projection element 112 is converted to right-handed circularly polarized light by the retardation plate 114. The right-handed circularly polarized virtual image A converted by the retardation plate 114 is irradiated onto the observation position of the user U by the cholesteric liquid crystal layer of the liquid crystal diffraction element 118, which acts as a concave mirror. Meanwhile, in the image display device 110, the real scene R passes through the transparent substrate 116 and the liquid crystal diffraction element 118, and is observed by the user U. This allows the user U of the image display device 110 to observe an augmented reality in which the virtual image A is superimposed on the real scene R. If the cholesteric liquid crystal layer of the liquid crystal diffraction element 118 is a reflective polarization diffraction element that reflects only right-handed circularly polarized green light and transmits all other light, only right-handed circularly polarized green light is reflected by the liquid crystal diffraction element 118, while all other light passes through the liquid crystal diffraction element 118 and reaches the observation position of the user U. Furthermore, if the liquid crystal diffraction element 118 has three cholesteric liquid crystal layers that reflect red, green, and blue light, respectively, circularly polarized light with a rotation direction opposite to the circularly polarized light reflected by each cholesteric liquid crystal layer passes through the liquid crystal diffraction element 118. Therefore, with the image display device 110, the user U can observe an augmented reality in which a virtual image A is superimposed on a bright real scene R. In such an image display device 110, the cholesteric liquid crystal layer included in the liquid crystal diffraction element 118 has the above-described configuration, which increases the diffraction efficiency of reflected light even at the end of the liquid crystal diffraction element 118, where the period Λ of the liquid crystal orientation pattern is short and the diffraction angle is large, thereby achieving more uniform in-plane diffraction efficiency. This makes it possible to reduce unevenness in brightness of the virtual image A observed by the user U.

[0153] 18 to 20 conceptually show other examples of image display devices having the optical device of the present invention. In Fig. 18 to 20, the same components as those shown in Fig. 17 are given the same reference numerals. Components given the same reference numerals have the same functions, so their explanation will be omitted.

[0154] The image display device 110A shown in Fig. 18 includes an image projection element 112, a transparent substrate 116, and a liquid crystal diffraction element 118 of the present invention. The image projection element 112 shown in Fig. 18 is a spatial light modulator (SLM) that converts a light beam. As indicated by the arrow in Fig. 18, a virtual image A projected by the image projection element 112 is reflected by a cholesteric liquid crystal layer (not shown) of the liquid crystal diffraction element 118 and is irradiated onto the observation position of a user U.

[0155] The image display device 110B shown in Figure 19 includes an image projection element 112, a MEMS mirror 120, a transparent substrate 116, and a liquid crystal diffraction element 118 of the present invention. The MEMS mirror 120 is a MEMS-type spatial light modulation element that deflects (deflection-scans) light by oscillating a mirror using a piezoelectric actuator. As indicated by the arrows in Figure 19, a virtual image A projected by the image projection element 112 is reflected by the MEMS mirror 120, then reflected by a cholesteric liquid crystal layer (not shown) of the liquid crystal diffraction element 118, and irradiated onto the observation position of a user U.

[0156] The image display device 110C shown in Figure 20 includes a light guide plate 122, a transparent substrate 116, and a liquid crystal diffraction element 118 of the present invention. The light guide plate 122 is a component that functions to propagate light (virtual image) emitted by an image projection element (not shown) within the light guide plate 122. A liquid crystal diffraction element 118 is disposed on the surface of the light guide plate 122 opposite the user U side. In the image display device 110C shown in Figure 20, as indicated by the arrow, a virtual image A projected by an image projection element (not shown) propagates within the light guide plate 122, is reflected by a cholesteric liquid crystal layer (not shown) of the liquid crystal diffraction element 118, and is irradiated onto the observation position of the user U.

[0157] 18 to 20, the cholesteric liquid crystal layer of the liquid crystal diffraction element 118 has the above-mentioned predetermined liquid crystal orientation pattern. As a result, as indicated by the arrows in each drawing, the virtual image A projected by the image projection element can be properly projected onto the observation position of the user U over the entire surface of the polarization diffraction element, and the diffraction efficiency of reflected light can be increased even at the end of the liquid crystal diffraction element, where the period Λ of the liquid crystal orientation pattern is short and the diffraction angle is large, making the diffraction efficiency more uniform within the plane.

[0158] 21 to 23 conceptually show other examples of image display devices having the optical device of the present invention. The image display device 310 shown in FIGS. 21 to 23 has a display element 312 and a light guide element 314. The light guide element 314 has a light guide plate 316, and an incident diffraction element 318, an intermediate diffraction element 320, and an exit diffraction element 324 provided on the light guide plate 316. At least one of the incident diffraction element 318, the intermediate diffraction element 320, and the exit diffraction element 324 of the image display device 310 is the liquid crystal diffraction element of the present invention. Note that the display element 312 is not shown in FIG. 21.

[0159] In the image display device 310 shown in FIGS. 21 to 23, the incident diffraction element 318, the intermediate diffraction element 320, and the output diffraction element 324 are arranged at different positions in the planar direction of the main surface of the light guide plate 316. In the example shown in FIG. 21, the intermediate diffraction element 320 is arranged to the left of the incident diffraction element 318 in FIG. 21, and the output diffraction element 324 is arranged below the intermediate diffraction element 320 in FIG. 21. In the image display device 310, the image (light corresponding to the image) displayed by the display element 312 is diffracted by the incident diffraction element 318 and incident on the light guide plate 316. In this case, the incident diffraction element 318 diffracts light in a direction in which the diffracted light travels toward the intermediate diffraction element 320. In the example shown in FIG. 21, the incident diffraction element 318 diffracts the incident light toward the left in FIG. 21.

[0160] The diffracted light by the input diffraction element 318 propagates through the light guide plate 316 while undergoing total reflection, and is incident on the intermediate diffraction element 320. The intermediate diffraction element 320 diffracts the incident light so that the traveling direction of the incident light is toward the output diffraction element 324. In the example shown in FIG. 21 , the intermediate diffraction element 320 diffracts the incident light downward in FIG. 21 . The light diffracted by the intermediate diffraction element 320 propagates through the light guide plate 316 while undergoing total reflection, and is incident on the output diffraction element 324. The output diffraction element 324 diffracts the incident light so that the angle deviates from the angle at which the light is totally reflected within the light guide plate 316. In the example shown in FIG. 21 , the output diffraction element 324 diffracts the incident light in a direction perpendicular to the plane of the paper in FIG. 21 . That is, as shown in FIG. 22 , the output diffraction element 324 diffracts the incident light in a direction approximately perpendicular to the main surface of the light guide plate 316. The light diffracted by the output diffraction element 324 is output from the light guide plate 316 and directed toward the user U. This allows the image display device 310 to display the image projected by the display element 312. Because the light guide element 314 has the intermediate diffraction element 320, when light is diffracted by the intermediate diffraction element 320, the exit pupil can be enlarged by diffracting part of the light at multiple points on the intermediate diffraction element.

[0161] 21 to 23, at least one of the incident diffraction element 318, the intermediate diffraction element 320, and the output diffraction element 324 (preferably all of the diffraction elements) is a liquid crystal diffraction element of the present invention, and is provided with a cholesteric liquid crystal layer having the above-described configuration. This allows reflected light to be diffracted in the appropriate direction, and also increases the diffraction efficiency of reflected light even at the end of the diffraction element where the period Λ of the liquid crystal orientation pattern is short and the diffraction angle is large, making the diffraction efficiency more uniform within the plane of the diffraction element.

[0162] The liquid crystal diffraction element and optical device of the present invention have been described in detail above, but the present invention is not limited to the above examples, and various improvements and modifications may be made within the scope of the present invention.

[0163] The features of the present invention will be explained in more detail below with reference to examples. The materials, reagents, amounts used, amounts of substances, ratios, treatment details, and treatment procedures shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0164] Comparative Example 1 <Preparation of Liquid Crystal Diffraction Element> (Support) A glass substrate was prepared as a support.

[0165] (Formation of Orientation Film) The following coating solution for forming an alignment film was applied to a support by spin coating. The support on which the coating film of the coating solution for forming an alignment film had been formed was dried on a hot plate at 60° C. for 60 seconds to form an alignment film.

[0166] Coating liquid for forming alignment film ----------------------------------- Photoalignment material A 1.00 parts by mass Water 16.00 parts by mass Butoxyethanol 42.00 parts by mass Propylene glycol monomethyl ether 42.00 parts by mass ---------------------------------------------------

[0167] -Photo alignment material A-

[0168]

[0169] (Exposure of Alignment Film) The alignment film was exposed using the exposure device shown in Figure 15 to form an alignment film P-1 having a concentric alignment pattern. The exposure device used was one that emitted laser light with a wavelength of 355 nm. The exposure dose by the interference light was 1000 mJ / cm. ２ It was decided.

[0170] (Formation of Cholesteric Liquid Crystal Layer) The following composition G-1 was prepared as a liquid crystal composition for forming the cholesteric liquid crystal layer G1.

[0171] Composition G-1 ----------------------------------- Rod-shaped liquid crystal compound L-1 90.00 parts by mass Rod-shaped liquid crystal compound L-2 10.00 parts by mass Polymerization initiator (Omnirad (registered trademark) 819, manufactured by BASF) 3.00 parts by mass Chiral agent Ch-1 5.20 parts by mass Leveling agent T-1 0.20 parts by mass Methyl ethyl ketone 126.7 parts by mass Cyclopentanone 126.7 parts by mass

[0172] Rod-shaped liquid crystal compound L-1

[0173]

[0174] Rod-shaped liquid crystal compound L-2

[0175]

[0176] Chiral agent Ch-1

[0177]

[0178] Leveling agent T-1

[0179]

[0180] The cholesteric liquid crystal layer G1 was formed by applying composition G-1 in multiple layers onto a photo-alignment film. "Multi-layer application" refers to first applying composition G-1 as a first layer onto the alignment film, heating it, and then curing it with UV light to form a liquid crystal fixation layer, and then repeatedly applying layers from the first layer on top of the first layer, heating it, and then curing it with UV light in the same manner.

[0181] First, for the first layer, composition G-1 was applied by spin coating onto alignment film P-1, and the coating was heated on a hot plate at 90°C for 120 seconds. Thereafter, ultraviolet light with a wavelength of 365 nm was applied at 300 mJ / cm using a high-pressure mercury lamp in a nitrogen atmosphere on a hot plate at 90°C. 2The orientation of the liquid crystal compound was fixed by irradiating the coating film with an irradiation dose of 1000 μm. The second and subsequent layers were coated on the liquid crystal fixation layer prepared immediately before, heated under the same conditions as the first layer, and then cured with ultraviolet light to prepare liquid crystal fixation layers. In this way, the coating process was repeated until the desired total thickness was obtained, thereby forming a cholesteric liquid crystal layer G1 (reflective liquid crystal diffraction element G1).

[0182] The cholesteric liquid crystal layer G1 was confirmed with a polarizing microscope to have a periodic orientation pattern as shown in FIG. 5 . When the cross section of the coating layer was examined with an SEM, the liquid crystal orientation pattern of the cholesteric liquid crystal layer G1 had a period Λ where the optical axis of the liquid crystal compound rotated 180°. The period Λ was 5.3 μm at a distance of 2.5 mm from the center and 0.8 μm at a distance of 20 mm from the center, and the period became shorter toward the outside. Furthermore, in the obtained cholesteric liquid crystal layer G1, the tilt angle of the liquid crystal compound was 0° at a distance of 2.5 mm from the center and also 0° at a distance of 20 mm from the center. The length of one helical pitch (helical pitch P) in the cholesteric liquid crystal layer G1 was 327 nm. Using the above-described method, the direction D in which the retardation Re was minimized at a distance of 2.5 mm and a distance of 20 mm from the center of the cholesteric liquid crystal layer G1 was determined. Ｒ As a result, the cholesteric liquid crystal layer G1 was measured in the direction D where the retardation was minimum at a distance of 2.5 mm from the center and at a distance of 20 mm from the center. Ｒ was not tilted with respect to the normal direction of the principal surface of the cholesteric liquid crystal layer. Therefore, the cholesteric liquid crystal layer G1 was tilted in the direction D Ｒ was the same within the plane.

[0183] Example 1 <Preparation of Liquid Crystal Diffraction Element> (Formation of Alignment Film) In the same manner as in Comparative Example 1, an alignment film was formed on a support.

[0184] (Exposure of Alignment Film) The formed alignment film was irradiated with unpolarized ultraviolet light having a wavelength of 365 nm using an LED-UV exposure device. At this time, the UV irradiation amount and irradiation angle were changed within the plane to irradiate the coating film. Specifically, irradiation of the alignment film was performed by changing the irradiation amount within the plane so that the irradiation amount increased from the center to the outside. Furthermore, when the normal direction of the glass substrate was 0° and the in-plane direction of the glass substrate was 90°, irradiation of the alignment film was performed by changing the irradiation angle within the plane so that the irradiation angle decreased from the center to the outside. Irradiation of the alignment film with unpolarized ultraviolet light was performed concentrically.

[0185] Next, in the same manner as in Comparative Example 1, the alignment film was exposed using the exposure device shown in FIG. 15 to form an alignment film P-2 having a concentric alignment pattern.

[0186] (Formation of Cholesteric Liquid Crystal Layer) The following composition G-2 was prepared as a liquid crystal composition for forming the cholesteric liquid crystal layer G2.

[0187] Composition G-2 ----------------------------------- Rod-shaped liquid crystal compound L-1 90.00 parts by mass Rod-shaped liquid crystal compound L-2 10.00 parts by mass Polymerization initiator (Omnirad (registered trademark) 819, manufactured by BASF) 3.00 parts by mass Chiral agent Ch-1 5.20 parts by mass Leveling agent T-2 0.03 parts by mass Leveling agent T-3 0.10 parts by mass Methyl ethyl ketone 126.7 parts by mass Cyclopentanone 126.7 parts by mass

[0188] Leveling Agent T-2

[0189]

[0190] Leveling Agent T-3

[0191]

[0192] The cholesteric liquid crystal layer G2 was formed by applying composition G-2 onto the alignment film. Specifically, composition G-2 was applied to the alignment film P-2 by spin coating, and the coating was heated on a hot plate at 90°C for 120 seconds. Thereafter, ultraviolet light with a wavelength of 365 nm was applied at 300 mJ / cm using a high-pressure mercury lamp under a nitrogen atmosphere on a hot plate at 90°C. 2 The coating film was irradiated with light at an irradiation dose of 1000 nm to fix the orientation of the liquid crystal compound, thereby forming a cholesteric liquid crystal layer G2 (reflective liquid crystal diffraction element G2).

[0193] The cholesteric liquid crystal layer G2 was confirmed to have a periodic orientation pattern as shown in FIG. 5 using a polarizing microscope. When the cross section of the coating layer was examined using an SEM, the liquid crystal orientation pattern of the cholesteric liquid crystal layer G2 showed that the period Λ, in which the optical axis of the liquid crystal compound rotated 180°, was 5.3 μm at a distance of 2.5 mm from the center of the liquid crystal orientation pattern and 0.8 μm at a distance of 20 mm from the center of the liquid crystal orientation pattern, with the period shortening toward the outside. Furthermore, in this cholesteric liquid crystal layer G2, the tilt angle of the liquid crystal compound was 2° at a distance of 2.5 mm from the center of the liquid crystal orientation pattern and 12° at a distance of 20 mm from the center of the liquid crystal orientation pattern. The length of one helical pitch (helical pitch P) in the cholesteric liquid crystal layer G2 was 327 nm. The direction D of the cholesteric liquid crystal layer G2 was also determined using the above-mentioned method. Ｒ As a result of measuring the liquid crystal orientation pattern, at a distance of 2.5 mm from the center of the liquid crystal orientation pattern, Ｒ The angle θ2 between the direction normal to the principal surface of the cholesteric liquid crystal layer G2 and the direction perpendicular to the liquid crystal alignment pattern was 2°, and at a distance of 20 mm from the center of the liquid crystal alignment pattern, the angle θ2 was 12°. Ｒ was different within the plane.

[0194] [Evaluation] The light intensity of the emitted light was evaluated when light was incident on the fabricated optical element from the front (at an angle of 0° relative to the normal). Specifically, laser light with an output central wavelength of 532 nm was irradiated from a light source and perpendicularly incident on the fabricated liquid crystal diffraction element. The laser light was circularly polarized by perpendicularly incident on a circular polarizer corresponding to the wavelength of the laser light, and then incident on the fabricated liquid crystal diffraction element. Of the light emitted from the liquid crystal diffraction element, the light intensity of diffracted light (first-order light) diffracted in the desired direction from the liquid crystal diffraction element and zero-order light (emitted in the same direction as the incident light) emitted in the other direction was measured using a photodetector. The diffracted light intensity of the first-order light was evaluated using the following formula: Diffracted light intensity = first-order light / (first-order light + zero-order light)

[0195] The liquid crystal diffraction elements produced in Comparative Example 1 and Example 1 had approximately the same diffracted light intensity of first-order light at a wavelength of 532 nm at a position 2.5 mm from the center. On the other hand, at a position 20 mm from the center, the liquid crystal diffraction element of Example 1 had an improved diffracted light intensity of first-order light at a wavelength of 532 nm compared to the liquid crystal diffraction element of Comparative Example 1.

[0196] Comparative Example 2 <Production of Liquid Crystal Diffraction Element> (Formation of Alignment Film) In the same manner as in Comparative Example 1, an alignment film was formed on a support.

[0197] (Exposure of Alignment Film) An alignment film was exposed using the exposure device shown in Figure 24 to form an alignment film P-3 having an alignment pattern. The exposure device 60 shown in Figure 24 was equipped with a light source 64 equipped with a laser 62 and a λ / 2 plate (not shown), a polarizing beam splitter 68 that splits the laser light M emitted by the light source 64 into two light beams MA and MB, mirrors 70A and 70B arranged on the optical paths of the two split light beams MA and MB, and λ / 4 plates 72A and 72B. The polarization direction of the laser light M emitted by the laser 62 was changed by the λ / 2 plate to form linearly polarized light P ０ The λ / 4 plates 72A and 72B have optical axes parallel to each other. The λ / 4 plate 72A is a linearly polarized light P ０ (ray MA) is right circularly polarized P Ｒ The λ / 4 plate 72B is a linearly polarized light P ０ (Light ray MB) is polarized by left-handed circular polarization PＬ The substrate 32 having the alignment film 34 before the alignment pattern was placed in the exposure section, and two light beams MA and MB were made to intersect and interfere on the alignment film 34, and the alignment film 34 was exposed to the interference light, thereby obtaining an alignment film having an alignment pattern in which the alignment state changed periodically. In the exposure device 60, a laser 62 emitting laser light with a wavelength (355 nm) was used. The exposure dose by the interference light was 1000 mJ / cm. ２ In addition, one period (the length of 180° rotation of the optical axis) of the alignment pattern formed on the alignment film 34 by the interference of the two laser beams was controlled by changing the crossing angle α of the two beams MA and MB.

[0198] (Formation of Cholesteric Liquid Crystal Layer) A cholesteric liquid crystal layer (G3) was formed in the same manner as in Comparative Example 1, except that the above-mentioned alignment film P-3 was used.

[0199] It was confirmed by a polarizing microscope that the cholesteric liquid crystal layer G3 had a periodic orientation pattern as shown in FIG. 5 . When the cross section of the coating layer was examined by SEM, it was found that in the liquid crystal orientation pattern of the cholesteric liquid crystal layer G3, one period Λ, in which the optical axis of the liquid crystal compound rotated 180°, was 0.44 μm. Furthermore, in this cholesteric liquid crystal layer G3, the tilt angle of the liquid crystal compound was 0° at a distance of 5 mm from the left end and also 0° at a distance of 20 mm from the left end. The length of one helical pitch (helical pitch P) in the cholesteric liquid crystal layer G3 was 327 nm. Furthermore, using the above-mentioned method, the cholesteric liquid crystal layer G3 was oriented in the direction D at a distance of 5 mm from the left end and at a distance of 20 mm from the left end. Ｒ As a result, the retardation of the cholesteric liquid crystal layer G1 was minimized in the direction D at a distance of 5 mm from the left end and at a distance of 20 mm from the left end. Ｒ was not tilted with respect to the normal direction of the principal surface of the cholesteric liquid crystal layer. Therefore, the cholesteric liquid crystal layer G1 was tilted in the direction D Ｒ was the same within the plane.

[0200] Example 2 <Production of Liquid Crystal Diffraction Element> (Formation of Alignment Film) In the same manner as in Comparative Example 1, an alignment film was formed on a support.

[0201] (Exposure of Alignment Film) The formed alignment film was irradiated with unpolarized ultraviolet light having a wavelength of 365 nm using an LED-UV exposure device. At this time, the ultraviolet light irradiation amount and irradiation angle were changed within the plane to irradiate the coating film. Specifically, the irradiation amount was changed within the plane so that the irradiation amount increased from the left side to the right side of the alignment film. Furthermore, when the normal direction of the glass substrate was 0° and the plane direction of the glass substrate was 90°, the irradiation angle was changed within the plane so that the irradiation angle decreased from the left side to the right side of the alignment film.

[0202] Next, in the same manner as in Comparative Example 2, the alignment film was exposed using the exposure device shown in FIG. 24 to form an alignment film P-4 having an alignment pattern.

[0203] (Formation of Cholesteric Liquid Crystal Layer) A cholesteric liquid crystal layer (G4) was formed in the same manner as in Example 1, except that the above-mentioned alignment film P-4 was used.

[0204] It was confirmed by a polarizing microscope that the cholesteric liquid crystal layer G4 had a periodic orientation pattern as shown in FIG. 5. When the cross section of the coating layer was examined by SEM, it was found that in the liquid crystal orientation pattern of the cholesteric liquid crystal layer G4, one period Λ, in which the optical axis of the liquid crystal compound rotates by 180°, was 0.44 μm. Furthermore, in this cholesteric liquid crystal layer G4, the tilt angle of the liquid crystal compound was 2° at a distance of 5 mm from the left end of the liquid crystal orientation pattern, and was also 22° at a distance of 20 mm from the left end of the liquid crystal orientation pattern. The length of one helical pitch (helical pitch P) in the cholesteric liquid crystal layer G4 was 327 nm. Furthermore, the direction D of the cholesteric liquid crystal layer G4 was measured by the above-mentioned method. Ｒ As a result of measuring the liquid crystal orientation pattern, at a distance of 5 mm from the left end of the liquid crystal orientation pattern, the direction D ＲThe angle θ2 between the direction normal to the principal surface of the cholesteric liquid crystal layer G4 and the direction perpendicular to the liquid crystal alignment pattern was 2°, and at a distance of 20 mm from the left end of the liquid crystal alignment pattern, the angle θ2 was 22°. Ｒ was different within the plane.

[0205] [Evaluation] The light intensity of the light emitted from the liquid crystal diffraction element fabricated in each example was measured using the following method. As shown in FIG. 25 , the liquid crystal diffraction element 400 was placed on the surface of a Dove prism 410. A glass Dove prism with a refractive index of 1.5 was used as the Dove prism 410. The glass substrate was peeled off from the liquid crystal diffraction element fabricated in each example, and the resulting cholesteric liquid crystal layer and the Dove prism were bonded together using a heat-sensitive adhesive. As shown in FIG. 25 , the liquid crystal diffraction element 400 was placed on top of the Dove prism 410. A laser (not shown) was positioned opposite the inclined surface of the Dove prism 410, and a linear polarizer 412 and a λ / 4 plate 414 were placed between the laser and the Dove prism 410.

[0206] Laser light Li emitted from the laser passes through linear polarizer 412 and λ / 4 plate 414, and enters Dove prism 410 as circularly polarized light. It propagates through Dove prism 410 and then enters the cholesteric liquid crystal layer of liquid crystal diffraction element 400. The diffracted light reflected and diffracted by the cholesteric liquid crystal layer propagates through Dove prism 410 in the direction opposite to the surface on which the cholesteric liquid crystal layer is disposed. The light propagated through Dove prism 410 reaches the lower surface of Dove prism 410 and is emitted as output light Lo.

[0207] The left edge of the cholesteric liquid crystal layer was defined as 0 mm, and laser light Li was incident on the cholesteric liquid crystal layer at positions 5 mm and 20 mm away, and the intensity of the output light Lo at each position was measured. The wavelength of the laser light Li was 532 nm, and the angle of incidence of the laser light Li on the cholesteric liquid crystal layer was set to 54° with respect to the normal direction of the cholesteric liquid crystal layer. The intensity of the output light Lo, which was reflected and diffracted by the cholesteric liquid crystal layer and emitted in the normal direction to the main surface of the cholesteric liquid crystal layer (the normal direction to the lower surface of the Dove prism 410), was measured.

[0208] The diffraction efficiency Deff of the fabricated cholesteric liquid crystal layer is calculated by multiplying the light intensity of the laser light Li incident on the Dove prism 410 by I ｉｎ The light intensity of the diffracted light (first-order light, outgoing light Li) diffracted in the desired direction by the cholesteric liquid crystal layer and emitted from the Dove prism 410 is expressed as I ｏｕｔ When the diffraction efficiency is 1 / 100, the diffraction efficiency is calculated by the following formula: ｏｕｔ / I ｉｎ When calculating the diffraction efficiency, the loss of transmittance at the interface when light is incident on the Dove prism 410 and when it is emitted is excluded.

[0209] In the liquid crystal diffraction elements prepared in Comparative Example 2 and Example 2, the diffraction efficiency of first-order light at a wavelength of 532 nm was approximately the same at a position 5 mm from the left end of the liquid crystal orientation pattern. On the other hand, at a position 20 mm from the left end of the liquid crystal orientation pattern, the diffraction efficiency of first-order light at a wavelength of 532 nm of the liquid crystal diffraction element of Example 2 was improved compared to the liquid crystal diffraction element of Comparative Example 2. Furthermore, in the liquid crystal diffraction element of Example 2, a higher diffraction efficiency was obtained at a position 20 mm from the left end of the liquid crystal orientation pattern than at a position 5 mm from the left end of the liquid crystal orientation pattern.

[0210] For example, in AR glasses using a light guide plate equipped with a diffraction element, it is preferable that at least a portion (more preferably all) of the output diffraction element that emits light (images) to the user has a diffraction element whose diffraction efficiency increases from one side to the other. By having such a structure, when light propagating within the light guide plate is diffracted by the diffraction element and emitted from the light guide plate, the brightness of the emitted light can be made uniform. Therefore, compared to Comparative Example 2, the liquid crystal diffraction element of Example 2 is suitable as a diffraction element that uniforms the brightness of the light emitted from the light guide plate as described above. From the above results, the effects of the present invention are clear.

[0211] The liquid crystal diffraction element of the present invention can be suitably used in head-mounted displays and the like.

[0212] 18, 18A, 18B, 400 Liquid crystal diffraction element / 32 Substrate / 34 Alignment film / 36, 36A, 36B, 36C, 36a, 36b, 36c Cholesteric liquid crystal layer / 38 Liquid crystal compound / 38A Optical axis / 40, 64, 84 Light source / 42 Light area / 44 Dark area / 50 Optical device / 60, 80 Exposure device / 62, 82 Laser / 68, 86, 94 Polarizing beam splitter / 70A, 70B, 90A, 90B Mirror / 92 Lens / 72A, 72B, 96, 414 λ / 4 plate / 110, 110A, 110B, 110C, 200, 310 Image display device / 112, 312 Image projection element / 114 Retardation plate / 116 Transparent substrate / 118 Liquid crystal diffraction element / 120 MEMS mirror / 122, 316 Light guide plate / 202 Image display element / 204 Circular polarizer / 210 Optical unit / 211 First partial reflection element / 213 Second partial reflection element / 314 Light guide element / 318 Incident diffraction element / 320 Intermediate diffraction element / 324 Exit diffraction element / 410 Dove prism / 412 Linear polarizer.

Claims

1. A liquid crystal diffraction element comprising a cholesteric liquid crystal layer, the cholesteric liquid crystal layer having a liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while rotating continuously along at least one direction in a plane, at least one surface of the cholesteric liquid crystal layer has a region in which the liquid crystal compound has a tilt angle relative to the surface of the cholesteric liquid crystal layer, and the cholesteric liquid crystal layer has regions in which the tilt angle of the liquid crystal compound relative to the surface of the cholesteric liquid crystal layer varies within the plane.

2. The liquid crystal diffraction element according to claim 1, wherein, when the length of one period is defined as the length of the optical axis direction originating from the liquid crystal compound rotating 180° in the plane, the length of one period in the liquid crystal orientation pattern has regions that differ in the plane.

3. The liquid crystal diffraction element according to claim 2, wherein the length of one period in the liquid crystal orientation pattern varies gradually along the one direction, and the tilt angle of the liquid crystal compound varies gradually along the one direction.

4. The liquid crystal diffraction element according to claim 3, wherein as the length of one period in the liquid crystal orientation pattern becomes shorter along the one direction, the tilt angle of the liquid crystal compound becomes larger along the one direction.

5. The liquid crystal diffraction element described in claim 1, wherein in a cross-sectional image obtained by observing a cross section cut along said one direction and the thickness direction of the cholesteric liquid crystal layer with a scanning electron microscope, the cholesteric liquid crystal layer has light and dark areas extending from one surface to the other surface, and there is a region in the thickness direction of the cholesteric liquid crystal layer where the inclination angle of the dark areas relative to said one surface differs from the tilt angle of the liquid crystal compound.

6. A liquid crystal diffraction element as described in claim 1, comprising at least two of the cholesteric liquid crystal layers, each of the at least two cholesteric liquid crystal layers having light and dark areas extending from one surface to the other surface in a cross-sectional image obtained by observing a cross section cut along the one direction and a thickness direction of the cholesteric liquid crystal layer with a scanning electron microscope, and the inclination angles of the dark areas of the at least two cholesteric liquid crystal layers are different from each other.

7. A liquid crystal diffraction element comprising a cholesteric liquid crystal layer, the cholesteric liquid crystal layer having a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound changes while rotating continuously along at least one direction in a plane, the cholesteric liquid crystal layer has a region in which, when retardation is measured from a normal direction to a main surface of the cholesteric liquid crystal layer and a direction inclined to the normal, the direction in which retardation is minimum is inclined to the normal direction, and the cholesteric liquid crystal layer has regions in which the direction in which retardation is minimum differs within the plane.

8. A liquid crystal diffraction element as described in claim 7, wherein, when the length of one period is defined as the length of the optical axis direction originating from the liquid crystal compound rotating 180° in a plane, the length of one period in the liquid crystal orientation pattern has regions that differ in the plane.

9. A liquid crystal diffraction element as described in claim 8, wherein the length of one period in the liquid crystal orientation pattern gradually changes along the one direction, and the angle from the normal direction to the principal surface of the cholesteric liquid crystal layer of the direction in which the retardation of the cholesteric liquid crystal layer is minimum gradually changes along the one direction.

10. A liquid crystal diffraction element as described in claim 8, wherein in the liquid crystal orientation pattern, as the length of one period becomes shorter, the angle of the direction in which the retardation of the cholesteric liquid crystal layer is minimum from the normal direction of the principal surface of the cholesteric liquid crystal layer becomes larger.

11. The liquid crystal diffraction element described in claim 8, wherein, in a cross-sectional image obtained by observing a cross section cut along the one direction and the thickness direction with a scanning electron microscope, the cholesteric liquid crystal layer has light and dark areas extending from one surface to the other surface, and there is a region in the thickness direction of the cholesteric liquid crystal layer where the inclination angle of the dark areas with respect to the one surface differs from the angle formed by the direction in which the retardation of the cholesteric liquid crystal layer is at a minimum and the normal direction to the main surface of the cholesteric liquid crystal layer.

12. A liquid crystal diffraction element as described in claim 11, comprising at least two of the cholesteric liquid crystal layers, each of the at least two cholesteric liquid crystal layers having light and dark areas extending from one surface to the other surface in a cross-sectional image obtained by observing a cross section cut in the thickness direction along the one direction with a scanning electron microscope, and the inclination angles of the dark areas of each of the at least two cholesteric liquid crystal layers with respect to the one surface are different from each other.

13. An optical device comprising the liquid crystal diffraction element according to any one of claims 1 to 12 and a light source that inputs light to the liquid crystal diffraction element.

14. An optical device comprising the liquid crystal diffraction element according to any one of claims 1 to 6 and a light source that irradiates light onto the liquid crystal diffraction element, wherein the tilt angle θP [°] of the liquid crystal compound satisfies the following formulas (A1) to (A3), where the angle of incident light from the light source to the liquid crystal diffraction element is θin, the exit angle of primary light reflected by the liquid crystal diffraction element is θm, and the refractive index of the cholesteric liquid crystal layer is nG: (A1) θP [°] = (θG [°] - θr [°]) / 2 ± 15 [°] (A2) sin θG = sin θm / nG (A3) sin θr = sin θin / nG 15. A liquid crystal diffraction element according to any one of claims 7 to 12, and a light source that inputs light to the liquid crystal diffraction element, wherein the retardation of the cholesteric liquid crystal layer is minimized in a direction D in which the angle of the incident light from the light source to the liquid crystal diffraction element is θin, the angle of the first-order light output from the liquid crystal diffraction element is θm, and the refractive index of the cholesteric liquid crystal layer is nG. Ｒ and a normal direction to a principal surface of the cholesteric liquid crystal layer, the angle θ2 [°] between them satisfies the following formulas (B1) to (B3): (B1) θ2 [°] = (θG [°] - θr [°]) / 2 ± 15 [°] (B2) sin θG = sin θm / nG (B3) sin θr = sin θin / nG