Polarization separation element and method for manufacturing same

Abstract

PROBLEM: To expand a separation angle while maintaining high diffraction efficiency in a polarizing grating. SOLUTION: In this polarization separation element, a plurality of wavelength plates 401, 402 having an optical axis on an xy plane are arranged in the z-axis direction in a three-dimensional space x, y, z, and each of the wavelength plates 401, 402 has a plurality of regions repeated in the x-axis direction, with each region being divided into a plurality of belt-like sub-regions in the x-axis direction. The direction of the optical axis of the odd-numbered wavelength plate 401 changes stepwise in one direction in a range in which the angle with respect to the y-axis direction is from 0° to 180° in the region. The direction of the optical axis of the even-numbered wavelength plate 402 changes stepwise in the direction opposite to the one direction in a range in which the angle with respect to the y-axis direction is from 0° to 180° in the region. The polarization separation element expands, by the number of the wavelength plates, the angle at which a clockwise circularly polarized light component and a counterclockwise circularly polarized light component incident in the z-axis or obliquely incident from the z-axis direction or are separated.

Description

Polarization separation element and its manufacturing method

[0001] The present invention relates to an optical element that uses a plurality of polarization gratings in which the axial orientation of half-wave plates changes periodically within a plane, thereby increasing the branching angle, and also to an optical component using the optical element and a method for manufacturing the same.

[0002] In recent years, there has been active development of flat optical elements that exhibit characteristic behavior with respect to polarized light by controlling the axial orientation of the wave plate in-plane. In particular, polarization grating technology, which periodically changes the axial orientation of a half-wave plate to achieve so-called grating-like behavior, not only makes prism-shaped polarization separation elements flat, but also introduces the concept of geometric phase, which can control the phase of circularly polarized light by changing the axial orientation of the wave plate. This technology has attracted considerable attention from the market, which aims to reduce the size and weight of optical devices as a whole (Non-Patent Document 1).

[0003] This section explains in detail the phenomenon known as the geometric phase (Pancharatnam-Bery phase; (Non-Patent Document 2)). Imagine a situation where half-wave plates, which use birefringence to create a phase difference of half the wavelength between vertically polarized light and horizontally polarized light, are lined up, with their orientations differing by an angle θ. When circularly polarized light is incident on these plates and passes through each half-wave plate, the phase of the transmitted light differs by 2θ. This means that by controlling the orientation of the half-wave plates at each location, it is possible to control the phase of light at each location.

[0004] For example, as shown in Figure 1, prepare a wave plate whose orientation changes by 180 degrees with a period P (101 in the figure) and whose retardation is π+2(n-1)π: (n = 1, 2, 3...). Based on the principle of geometric phase, the phase of circularly polarized light passing through this plate changes by 360 degrees at each location with a period P. Therefore, light of wavelength λ is diffracted at an angle Φ given by the following equation.

[0005] In FIG. 1, the 180-degree azimuth change of the wave plate is represented by four divisions. In this case, the wave plate orientation 102 changes in increments of π / 4. As a result, the phase of the output light changes in increments of π / 2. Because phases of 0 and 2π are equivalent, the phase of each region in the lower part of FIG. 1 is always π / 2 ahead of the adjacent region to the left. By increasing the number of divisions, the phase change becomes smoother, ultimately resulting in a straight line with a slope of 2π over a distance P. As a result, as shown in FIG. 2, when left-handed circularly polarized light traveling in the +z direction is incident on the wave plate 201 patterned as shown in FIG. 1, each region is a half-wave plate, so the polarization state of the output light becomes right-handed circularly polarized and is diffracted at an angle 202 in the +x direction. Similarly, when right-handed circularly polarized light traveling in the +z direction is incident on the wave plate 201, the polarization state of the output light becomes left-handed circularly polarized and is diffracted at an angle 203 in the -x direction. In this way, it functions as a diffraction grating with polarization dependency.

[0006] If this period P is constant, the incident plane wave will be emitted as a plane wave with an angle φ. If the period P is not constant but changes continuously, the rate of change of the period will change the slope of the wavefront, and the wavefront can be changed from flat to curved, like a lens.

[0007] D. Lin, P. Fan, E. Hasman and M. Brongersma, “Dielectric gradient metasurface optical elements,” Science, Applied Optics, July 18, 2014, pp. 298-302. S. Pancharatnam, “Generalized theory of interference and its applications,” Proc. Indian Acad. Sci. A 44, 398 (1956). Sharp Technical Journal No. 100, February 2010, p. 10

[0008] Patent No. 3288976

[0009] Polarization gratings diffract light, but not all incident light is diffracted. The efficiency depends on the diffraction angle and the refractive index difference (hereinafter referred to as Δn) between the fast and slow axes of the birefringent material that makes up the wave plate. In birefringent optical crystals or liquid crystals, the refractive index difference is determined by the material and cannot be freely controlled. For typical liquid crystal materials, Δn is approximately 0.1 to 0.2. For example, Figure 3 shows the results of calculating the diffraction efficiency versus diffraction angle for a wavelength of 500 nm and a refractive index difference of 0.1. As shown, as diffraction increases, efficiency rapidly deteriorates. The undiffracted component continues to travel straight. This impairs the polarization grating's ability to bend light and degrades device performance. Furthermore, reducing the straight-traveling component reduces the diffraction angle. For example, to achieve the same separation width, the overall device length increases, negating the advantage of a thin polarization grating.

[0010] Therefore, an object of the present invention is to aim to increase the separation angle while maintaining high diffraction efficiency in a polarization grating.

[0011] A first aspect of the present invention relates to a polarization separation element and a lens. The first embodiment relates to a polarization separation element. In the first embodiment, the polarization separation element includes a plurality of wave plates having optical axes in the xy plane and arranged in the z-axis direction in a three-dimensional space x, y, and z. Each of these wave plates has a plurality of regions repeated in the x-axis direction. Each region is divided into a plurality of strip-shaped subregions in the x-axis direction. Here, the direction of the optical axis of the odd-numbered wave plates changes stepwise in one direction within a region, with the angle relative to the y-axis direction ranging from 0° to 180°. Meanwhile, the direction of the optical axis of the even-numbered wave plates changes stepwise in the opposite direction to the one direction within a region, with the angle relative to the y-axis direction ranging from 0° to 180°. The polarization separation element separates right-handed circularly polarized light components and left-handed circularly polarized light components incident in the z-axis direction or obliquely from the z-axis, increasing with each wave plate.

[0012] The second embodiment relates to a polarization separation element. In the second embodiment, the polarization separation element includes a plurality of wave plates, each having an optical axis in the xy plane, arranged in the z-axis direction in a three-dimensional space (x, y, z). Each of these wave plates has a plurality of regions repeated in the x-axis direction. Within each region, the direction of the optical axis of each odd-numbered wave plate continuously changes in one direction at an angle relative to the y-axis direction within a range from 0° to 180°. Meanwhile, within each region, the direction of the optical axis of each even-numbered wave plate continuously changes in the opposite direction from the one direction at an angle relative to the y-axis direction within a range from 0° to 180°. Furthermore, the polarization separation element separates right-handed circularly polarized light components and left-handed circularly polarized light components incident in the z-axis direction or obliquely from the z-axis, increasing with each wave plate.

[0013] The third embodiment relates to a lens. In the third embodiment, in a three-dimensional space x, y, z, multiple wave plates with optical axes on the xy plane are arranged in the z-axis direction. Each wave plate is divided into regions in an annular shape centered on a certain point, and also divided into regions in the circumferential direction. Each region is a wave plate with a uniform orientation, and the orientation is calculated based on the radius of the annulus. In the circumferential direction, the orientation rotates 180 degrees × n (n is an integer greater than or equal to 0) within one revolution, with the centers of the wave plates coinciding. The direction of the optical axis of odd-numbered wave plates changes stepwise in one direction, with the angle ranging from 0° to 180° according to the radius from the center. The direction of the optical axis of subsequent wave plates changes stepwise in the opposite direction to the one direction. This lens is bent so that one of the right-handed and left-handed circularly polarized components incident in the z-axis direction or obliquely from the z-axis converges in the z-axis direction passing through the center, while the other is bent so that it moves away from the z-axis direction passing through the center, and the bending angle increases with the number of wave plates.

[0014] The fourth embodiment relates to a lens. In this fourth embodiment, in a three-dimensional space (x, y, z), multiple wave plates with optical axes on the xy plane are arranged in the z-axis direction. Each wave plate has a center point, and its orientation is calculated based on the radius from the center. The orientation rotates continuously 180 degrees × n (n is an integer greater than or equal to 0) in the circumferential direction throughout a full circle, with the centers of the wave plates coinciding. The direction of the optical axis of odd-numbered wave plates continuously changes in one direction within an angle range of 0° to 180° according to the radius from the center. The direction of the optical axis of even-numbered wave plates continuously changes in the opposite direction. This lens is bent so that one of the right-handed and left-handed circularly polarized components incident in the z-axis direction or obliquely from the z-axis converges toward the z-axis passing through the center, while the other is bent away from the z-axis passing through the center, with the bending angle increasing with the number of wave plates.

[0015] The lens is characterized in that the axial orientation φ of each wave plate is proportional to the square of the radius from the center.

[0016] In the polarization separation elements according to the first and second embodiments or the lenses according to the third and fourth embodiments, the wave plate is preferably a photonic crystal, a metasurface, an alignment film, or a polymerizable liquid crystal.

[0017] In the polarization separation elements according to the first and second embodiments or the lenses according to the third and fourth embodiments, the wave plates may be photonic crystals, and may include a planarizing layer between multiple wave plates formed on the same substrate.

[0018] A second aspect of the present invention relates to a method for manufacturing a polarization separation element or an optical element that bends the wavefront of light at different locations. Specifically, the second aspect is a method for manufacturing the polarization separation element or lens according to the first aspect. The wave plate is made of a polymerizable liquid crystal wave plate. In this case, the method for manufacturing the polarization separation element includes a step of stacking multiple patterned wave plates, in which an alignment film is applied to the surface of a single substrate, a polarization pattern is exposed using a patterned wave plate, and the polarization pattern generated by the patterned wave plate affects the molecular orientation of the alignment film, and a polymerizable liquid crystal is applied thereon and polymerized, resulting in the liquid crystal being oriented depending on the molecular orientation of the alignment film.

[0019] In the second aspect of the present invention, the wave plate may be made of an alignment film. In this case, the method for manufacturing the polarization separation element or lens includes stacking a plurality of patterned wave plates, in which an alignment film is applied to the surface of a single substrate, and a polarization pattern is exposed using a patterned wave plate, so that the polarization pattern produced by the patterned wave plate affects the molecular orientation of the alignment film, and the alignment film itself obtains a desired phase difference.

[0020] Alternatively, it may be realized by repeatedly stacking a thin alignment film, which has the sole purpose of aligning the liquid crystal, on one substrate and, if necessary, a liquid crystal layer.

[0021] A third aspect of the present invention relates to a light guide plate. This light guide plate includes the polarization separation element according to the first aspect. The light guide plate is configured to deflect light incident from the air by the polarization separation element at an angle within the glass, and to efficiently guide the light that has entered from the air into the glass by setting the angle to cause total reflection within the glass.

[0022] The present invention makes it possible to increase the diffraction angle of a polarization grating while maintaining high diffraction efficiency, thereby enabling the miniaturization of the entire optical device. Furthermore, when used as a lens, this means that a lens with a shorter focal length can be realized.

[0023] Figure explaining the operating principle of a polarization grating Figure explaining the operating form of a polarization grating Figure of an example of calculated diffraction efficiency of a polarization grating Figure explaining the operation when two polarization gratings are arranged so that the patterns are reversed in the xy plane Figure explaining the operation of a waveguiding structure that confines light within glass using a polarization grating Flow diagram of the process for creating a polarization separation element by realizing a multilayer structure using an alignment film and polymerizable liquid crystal on a single substrate Flow diagram of the process for creating a polarization separation element by realizing a multilayer structure using an alignment film on a single substrate Diagram explaining the concentric pattern that functions as a lens

[0024] The principle of the polarization separation element according to the present invention will now be explained. Specifically, by stacking two identical polarization gratings at angles 180 degrees apart, it is possible to double the in-plane wavenumber component that can be imparted by a single element. (When the angle is small, this is almost equivalent to doubling the angle.) Of course, the two elements do not need to be identical; if they are different, the in-plane wavenumbers that each element can impart to light are simply added together. Furthermore, even if there are multiple elements, rather than just two, as long as the adjacent polarization gratings on top and bottom are 180 degrees apart from each other, the effects will be added together. In this way, it is possible to achieve high efficiency and a large diffraction angle that was not easily achieved with a single polarization grating.

[0025] The principle will be explained with reference to Figure 4. Light traveling in the z direction is reflected in the x direction with a period P １ When the light is incident on the polarization grating 401 having the polarizing grating 402, the right-handed circularly polarized light is split to the right and the left-handed circularly polarized light is split to the left. The angle Φ (402, 403) can be expressed as the following equation (1). n indicates the refractive index of the medium on the exit side, and in the case of air, it may be set to 1. This is because the polarization grating 401 causes the wave numbers ±k ｘ１ This can be interpreted as occurring because kx is given. １ can be expressed as the following equation (2).

[0026] It is assumed that each light beam is incident on a second polarization grating 404, which also has a period P2 in the x direction. The second polarization grating has a pattern that is 180 degrees different in the y direction from the pattern of the first polarization grating. In this case, the right-handed circularly polarized light incident on the first grating becomes left-handed circularly polarized light and is further bent to the right, and the left-handed circularly polarized light incident on the first grating becomes right-handed circularly polarized light and is further bent to the left. In other words, the wave number given on the first grating is further bent in the same direction on the second grating by the wave number k given by equation (3) ｘ２ is added.

[0027] As a result, the wave number k given to light by overlapping two sheets ｘ is k x =k x1 +k x2 and the bending angle increases. Of course, even if the period of the first sheet is different from the period of the second sheet, the fact that the wave numbers are added remains unchanged; only the result of this addition changes. Furthermore, if a third and fourth polarization grating with patterns that are 180 degrees different from each other in the y direction are stacked, the wave numbers given by the third and fourth sheets are added together, increasing the wave number, and as a result the bending angle increases even further.

[0028] The lens according to the present invention will now be described. Consider, for example, a concentric pattern as shown in FIG. 8 . Assume that the orientation 802 of the wave plate changes according to the radius r from the center 801. As explained in paragraph 0003 above, when the orientation of the wave plate changes by θ, the phase of the light emitted from it changes by 2θ. Changing the orientation so that ar^2 is achieved according to the radius r can achieve a phase distribution that changes parabolically from the center. This is physically identical to the function of a spherical lens. Of course, if the pattern orientation is changed according to a different function, it is possible to achieve an effect equivalent to that of an aspherical lens, and it is also possible to realize a lens in which the phase changes in proportion to the radius, such as an axicon lens. Furthermore, as explained in paragraph 0005 above, in principle, the lens functions for either left- or right-handed circularly polarized light. For the opposite circularly polarized light, the lens's sign (convex vs. concave) is reversed. In other words, a lens that focuses light with one circular polarization will diverge light with circular polarization perpendicular to it.

[0029] Furthermore, when two elements such as those shown in Figure 8 are stacked, the rotation direction of the azimuth change according to the distance from the center is reversed. This doubles the in-plane wavenumber component that can be imparted by a single element. (When the angle is small, this is roughly equivalent to doubling the angle.) Of course, the two elements do not need to be identical; if they are different, the in-plane wavenumbers that each element can impart to light are simply added together. Furthermore, even with multiple elements, as long as the radial change in the azimuth of the elements adjacent above and below is in opposite directions, the effects are simply added together. In this way, it becomes possible to achieve high efficiency and a large diffraction angle (light concentration) that was not easily achieved with a single lens.

[0030] Example 1: Realized with photonic crystals An electron beam resist is spin-coated onto a 0.5 mm thick, 30 mm square quartz substrate, and a polarization grating pattern with a period of 2 μm is written using an electron beam writing device. The pattern is then transferred to the quartz by development and dry etching. A multilayer film made of tantalum pentoxide and silicon dioxide is layered on top of this using the autocloning method. Each layer is 40 nm thick, and by layering 50 layers, a multilayer film that functions as a half-wave plate at a wavelength of 500 nm is obtained. The thickness of the two layers on the side of the multilayer film that comes into contact with air is adjusted to prevent reflection at the boundary between the air and the multilayer film.

[0031] Two of these polarization gratings are fabricated and arranged so that their directions are opposite to each other, as shown in Figure 4. When light with a wavelength of 500 nm is incident on them, one polarization grating separates the light into right-handed and left-handed circularly polarized light at an angle of 14.5 degrees. Figure 3 shows that the diffraction efficiency is 96%. When two gratings are stacked, the separation angle is 30 degrees, and the diffraction efficiency is 96% x 96% = 92%.

[0032] If one lens were to split the light at the same angle (30 degrees), the diffraction efficiency would be approximately 66%, as shown in Figure 3, and the remaining light would travel straight. In this way, using two lenses can achieve a significant improvement in diffraction efficiency.

[0033] Example 2: Realization by Optical Alignment First, a photonic crystal that functions as a polarization mask is prepared. Electron beam resist is spin-coated onto a 0.5 mm thick, 30 mm square quartz substrate, and a polarization grating pattern with a period of 6 μm, as shown in Figure 1, is written using an electron beam lithography system. The pattern is then transferred to the quartz by development and dry etching. A multilayer film made of tantalum pentoxide and silicon dioxide is then layered on top of this using the autocloning method. Each layer is 20 nm thick, and by layering 80 layers, a multilayer film that functions as a half-wave plate at a wavelength of 313 nm is obtained. The thickness of the two layers on the air-contact side of the multilayer film is adjusted to prevent reflection at the boundary between the air and the multilayer film.

[0034] [A] Next, an alignment film is prepared. A 0.5 mm thick, 30 mm square quartz plate is coated with an alignment film to a thickness of 100 nm by spin coating. The substrate is heated at 80°C for 5 minutes to evaporate the solvent. Note that the temperature and time for these processes may need to be adjusted depending on the material.

[0035] The polarizing mask fabricated in [A] is placed on top of the alignment film so that the top surface of the photonic crystal faces the alignment film. The polarizing mask and alignment film can be in direct contact or there can be some distance between them. However, it is important that the distance is such that the light beams emitted from the polarizing masks can interfere with each other.

[0036] Linearly polarized light with a wavelength of 313 nm is incident from the photonic crystal side. A mercury lamp is used as the light source for the incident light, and the light emitted from the lamp is converted into parallel light using a lens. The parallel light is then transmitted through a bandpass filter with a central wavelength of 313 nm and a bandwidth of 10 nm, and a linear polarizer to form linearly polarized light. This linear polarizer may be a wire grid polarizer or a polarizer using a photonic crystal (Patent Document 1).

[0037] Light emitted from a photonic crystal is divided into left-handed and right-handed circularly polarized components by the polarization grating, and these components are diffracted in opposite directions at an angle θ from the perpendicular, parallel to the direction of the polarization grating's period. The diffracted light components interfere with each other. As a result, an electric field distribution can be created in a plane parallel to the photonic crystal, in which the polarization direction changes from 0 to 180 degrees over half the period of the polarization grating. If an alignment film is present in this space, the molecules there sense this electric field distribution and exhibit an anisotropic reaction. For example, photodecomposition occurs when polyimide is irradiated with ultraviolet light, but it is known that anisotropy occurs in this photodecomposition when polarized light is irradiated (Non-Patent Document 3).

[0038] By irradiating the alignment film with patterned polarized light in this way, a reaction occurs within the molecules according to the polarization pattern. After that, by raising the alignment film to near its glass transition temperature, the molecules rotate due to thermal motion, but due to the influence of the molecular direction caused by the polarized light irradiation, they become preferentially aligned in one direction.

[0039] In this way, the alignment film can be made anisotropic. For example, by applying a polymerizable liquid crystal onto the film and then heating and cooling it, the liquid crystal molecules will be aligned in a direction that follows the anisotropy of the alignment film. The birefringence difference Δn of the aligned liquid crystal molecules is determined by the material, so for example, if the wavelength you ultimately want to use is λ, then a half-wave plate can be realized by achieving a thickness of λ / 2 / Δn.

[0040] Alternatively, since the alignment film itself has birefringence, if a film is prepared in advance so that the thickness is λ / 2 / Δn according to the Δ of the alignment film, a half-wave plate can be realized using only the alignment film.

[0041] [B] When the Δn of the liquid crystal is 0.1, the diffraction angle is 16.1 degrees and the diffraction efficiency is expected to be approximately 93% when the operating wavelength is 500 nm and the polarization grating period is 1.8 μm, as shown in Figure 2. If the period is halved to 0.9 μm, the diffraction angle becomes 33.7 degrees, but the diffraction efficiency is approximately 50%.

[0042] Attempting to achieve such a large diffraction angle would not result in high diffraction efficiency. However, as shown in the present invention, two liquid crystal films with polarization grating patterns are prepared and bonded together so that the polarization grating patterns are oriented in opposite directions.

[0043] It is desirable to inject a liquid with a refractive index matching that of the liquid crystal into the bonded surfaces, otherwise reflection will occur at the interface with air, resulting in loss.

[0044] When light is incident on such a polarization grating, the light that is deflected by the first grating is further deflected in the same direction by the second grating.

[0045] For example, by stacking two polarization gratings with a period of 1.8 μm as described in [B], a diffraction angle of 33.7 degrees can be achieved, but the diffraction efficiency is 0.93×0.93=0.86, which is a high diffraction efficiency of 86%.

[0046] When three lenses are stacked, the diffraction angle is about 56 degrees and the diffraction efficiency is 0.93 x 0.93 x 0.93 = 0.80, which is an expected diffraction efficiency of 80%. If you try to achieve the same angle with one lens, the diffraction efficiency will be about 20%.

[0047] When there are three or more polarizing gratings, they cannot be stuck together facing each other, but the phenomenon does not change even if the distance between the polarizing gratings increases.

[0048] Example 3: Application as a light guide plate It is assumed that the polarization grating produced in Example 2 is used as a light guide plate that guides light to a glass substrate. As shown in FIG. 4, ０ , the refractive index of the glass 402 is n ｇ In this case, the angle θ of the light ray entering the glass through the polarization grating 401 with a period P is １ can be expressed by the following formula:

[0049] In order to make the glass function as a light guide plate by total reflection, the angle θ １ must satisfy the following formula:

[0050] If the refractive index of glass is 1.8, the minimum θ １ is 33 degrees. For example, θ ０ Light of wavelength 500 nm incident on the polarization grating at angle n = 0 is ｇ The angle θ １ In order to diffract light at λ, the period P must be equal to the wavelength λ. When the Δn of the polarization grating is 0.1, the diffraction efficiency is 20% or less.

[0051] However, when the same diffraction angle is achieved by stacking two lenses using the present invention, the diffraction angle required per lens becomes smaller, and the diffraction efficiency per lens becomes approximately 55%. As a result, a total diffraction efficiency of 55% x 55% ≒ 30% can be expected.

[0052] Example 4 Multilayer Structure (with Liquid Crystal) A manufacturing method for forming a multilayer structure of polarization gratings of the present invention on a single substrate will be described with reference to FIG.

[0053] An alignment film 602 is applied to a 0.5 mm thick, 30 mm square glass plate 601 by spin coating to a thickness of 100 nm. It is then heated at 80°C for 5 minutes to remove the solvent. A polarization pattern is then exposed to the plate using the patterned photonic crystal waveplate used in Example 2 at a wavelength of 313 nm, and the plate is then heated to 130°C to align the film. Polymerizable liquid crystal 603 is then applied to the plate by spin coating, and the solvent is evaporated at 80°C. This alignment causes the liquid crystal molecules to be influenced by the underlying alignment film and align in the same pattern. Light with a wavelength of 365 nm is then irradiated onto the plate. This progresses the polymerization reaction, solidifying the alignment of the liquid crystal molecules. A UV absorber 604, used in the manufacturing process of LCD televisions, is then applied, and the plate is heated at 80°C to remove the solvent. An alignment film 605 is then applied on top of the plate, and the polarization pattern is again exposed using a photonic crystal waveplate. The pattern orientation is changed by 180° from the first exposure. During this process, the UV absorber in the lower layer prevents UV light from penetrating below that temperature, leaving the alignment film and liquid crystal exposed the first time unaffected. The substrate is then heated to 130°C to align it. Polymerizable liquid crystal 606 is then applied on top of it by spin coating, and the solvent is evaporated at 80°C. This alignment causes the liquid crystal molecules to be influenced by the underlying alignment film and align in the same pattern. Light with a wavelength of 365 nm is then irradiated onto the substrate. This progresses the polymerization reaction, fixing the alignment of the liquid crystal molecules. In this way, liquid crystals oriented in different directions can be stacked on a single substrate.

[0054] The photonic crystals may have different periods in the first and second passes. It is essential that the pattern direction differs by 180 degrees. It is clear that the same process can be repeated a third and fourth time. The UV absorbing layer is not necessary if the fixed liquid crystal 603 is not affected by UV light in subsequent processes. Since the alignment film also has patterned birefringence, it is essential that the required phase difference is generated when combined with the liquid crystal above it. Therefore, the thickness of the alignment film and the liquid crystal layer can be selected appropriately depending on the process. A UV absorbing layer is necessary when utilizing the phase difference of the alignment film. Note that the temperature and other parameters used must be selected appropriately depending on the material. This example is merely an example.

[0055] Example 5 Multilayered Structure (without Liquid Crystal) A manufacturing method for forming a multilayered structure of polarization gratings of the present invention on a single substrate will be described with reference to FIG.

[0056] An alignment film 702 is applied to a 0.5 mm thick, 30 mm square glass plate 701 by spin coating to a thickness of 100 nm. It is then heated at 80°C for 5 minutes to remove the solvent. A polarization pattern is then exposed to the alignment film at a wavelength of 313 nm using the patterned photonic crystal waveplate used in Example 2, and the film is then heated to 130°C to align the alignment film. The alignment film has birefringence. Therefore, it is possible to increase the thickness of the alignment film to a thickness that results in a half-wave plate as the phase difference of the alignment film. A UV absorber 703, which is used in the manufacturing process of LCD televisions, is then applied, and heated at 80°C to remove the solvent. An alignment film 704 is then applied on top of this, and the polarization pattern is again exposed using a photonic crystal waveplate. This time, the pattern orientation is changed by 180° from the first exposure. The UV absorber in the lower layer prevents UV light from penetrating below this point, preventing any effect on the alignment film or liquid crystal exposed in the first exposure. This is then heated to 130°C to align the alignment film. In this way, liquid crystals aligned in different directions can be stacked on one substrate. Compared to Example 4, the process of forming the liquid crystal layer is unnecessary, and therefore costs can be expected to be reduced.

[0057] Here too, the photonic crystal can have a different period for the first and second times. It is important that the pattern direction differs by 180 degrees. It is clear that the same process can be repeated a third and fourth time. Note that the parameters used, such as the temperature, must be selected appropriately depending on the material, etc. This is just an example.

[0058] Example 6: Realizing a lens using a photonic crystal An electron beam resist is applied by spin coating to a quartz substrate 0.5 mm thick and 30 mm square, and the substrate is divided into annular regions with a width of 2 μm. A pattern oriented at an angle θ is written inside the region using an electron beam lithography device. For example, assuming operation at a wavelength of 500 nm, the pattern filling the annular region is a line and space pattern with a period of 200 nm. Note that this period is preferably 1 / 2 or less of the operating wavelength.

[0059] The pattern is then transferred to quartz by development and dry etching. A multilayer film made of tantalum pentoxide and silicon dioxide is then laminated on top of it using the autocloning method. Each layer is 40 nm thick, and by laminating 50 layers, a multilayer film is obtained that functions as a half-wave plate at a wavelength of 500 nm. The thickness of the two layers on the side of the multilayer film that comes into contact with air is adjusted to prevent reflection at the boundary between the air and the multilayer film. For example, the orientation φ [rad] of the pattern at a radius r [mm] is given by φ = r where f [mm] is the focal length of the lens to be realized and λ [mm] is the operating wavelength. ２ / (2f)×2π / λ.

[0060] In reality, the region is divided into rings of finite width, so the radius also has a range. For example, it is necessary to determine the value for that region within a range where the error can be ignored, such as by using the average value of the minimum and maximum radii of the rings, and then determine the orientation of the pattern. When light enters the lens pattern, one circularly polarized light is diffracted at an angle at each location and directed toward the focal point. The other circularly polarized light is diffracted in the opposite direction and diverges. Calculating the diffraction efficiency of a lens is not easy because the orientation of the wave plate is not constant. However, as shown in Figure 3, as the diffraction angle increases, the diffraction efficiency rapidly deteriorates. Therefore, as in Example 1, stacking two lenses with small diffraction angles results in higher efficiency than achieving a large diffraction angle with a single lens.

[0061] Example 7: Realizing a lens by optical alignment First, a photonic crystal that functions as a polarization mask is prepared. Electron beam resist is spin-coated onto a 0.5 mm thick, 30 mm square quartz substrate, which is then divided into annular regions with a width of 2 μm. A pattern oriented at an angle θ is written inside each region using an electron beam lithography system.

[0062] The pattern is then transferred to quartz by development and dry etching. A multilayer film made of tantalum pentoxide and silicon dioxide is then laminated on top of it using the autocloning method. The thickness of each layer is 20 nm, and by laminating 80 layers, a multilayer film is created that functions as a half-wave plate at a wavelength of 313 nm. The thickness of the two layers on the side of the multilayer film that comes into contact with air is adjusted to prevent reflection at the boundary between the air and the multilayer film. For example, the orientation φ [rad] of the pattern at a radius r [mm] is given by φ = r where f [mm] is the focal length of the lens to be realized and λ [mm] is the operating wavelength of the lens. ２ / (2f)×2π / λ / 2.

[0063] [A] Next, an alignment film is prepared. A 0.5 mm thick, 30 mm square quartz plate is coated with an alignment film to a thickness of 100 nm by spin coating. The plate is heated at 80°C for 5 minutes to evaporate the solvent. The temperature and time of these processes may need to be adjusted depending on the material. The alignment film is then placed so that it faces the top surface of the photonic crystal of the polarizing mask prepared in [A]. The polarizing mask and the alignment film may be in direct contact, or there may be some distance between them.

[0064] Linearly polarized light with a wavelength of 313 nm is incident from the photonic crystal side. A mercury lamp is used as the light source for the incident light, and the light emitted from the lamp is converted into parallel light using a lens. The parallel light is then transmitted through a bandpass filter with a central wavelength of 313 nm and a bandwidth of 10 nm, and a linear polarizer to form linearly polarized light. This linear polarizer may be a wire grid polarizer or a polarizer using a photonic crystal (Patent Document 1).

[0065] The polarization direction of light emitted from the photonic crystal is rotated by the pattern of the half-wave plate. The polarization direction of light incident on a wave plate at an angle θ to the incident polarization direction is 2θ. This polarized light is irradiated onto an alignment film, and the molecules in the alignment film sense this electric field distribution and exhibit an anisotropic reaction. For example, photodecomposition occurs when light in the ultraviolet wavelength range is irradiated onto polyimide, but it is known that anisotropy occurs in the photodecomposition when polarized light is irradiated (Non-Patent Document 3).

[0066] By irradiating the alignment film with patterned polarized light in this way, a reaction occurs within the molecules according to the polarization pattern. After that, by raising the alignment film to near its glass transition temperature, the molecules rotate due to thermal motion, but due to the influence of the molecular direction caused by the polarized light irradiation, they become preferentially aligned in one direction.

[0067] In this way, the alignment film can be made anisotropic. For example, by applying a polymerizable liquid crystal onto the film and then heating and cooling it, the liquid crystal molecules will align in a direction that follows the anisotropy of the alignment film. The birefringence difference Δn of aligned liquid crystal molecules is determined by the material, so if the final desired operating wavelength is λ, for example, a half-wave plate can be realized by achieving a thickness of λ / 2 / Δn. Alternatively, since the alignment film itself has birefringence, if a film is prepared in advance so that it has a thickness λ / 2 / Δn corresponding to the Δ of the alignment film, a half-wave plate can be realized using only the alignment film.

[0068] As shown in this invention, two liquid crystal films with lens patterns are prepared and bonded together so that they face each other. It is desirable to inject a liquid whose refractive index matches that of the liquid crystal into the bonded surfaces. If not, reflection occurs at the interface with air, resulting in loss. When light is introduced into such an element, the light that is subjected to the lens action of the first film is further subjected to the lens action of the second film in the same direction, resulting in a stronger concentration or divergence than when there is only one film.

[0069] Example 8: Multilayer structure (with liquid crystal) A manufacturing method for forming a multilayered lens structure of the present invention on a single substrate will be described with reference to Fig. 6. Although the pattern is different from that of Example 4, the process is the same, so the same diagram will be used.

[0070] An alignment film 602 is applied to a 0.5 mm thick, 30 mm square glass plate 601 by spin coating to a thickness of 100 nm. It is then heated at 80°C for 5 minutes to remove the solvent. A polarization pattern is then exposed to the plate using the patterned photonic crystal waveplate used in Example 7 at a wavelength of 313 nm, and the plate is then heated to 130°C to align the film. Polymerizable liquid crystal 603 is then applied on top of the plate by spin coating, and the solvent is evaporated at 80°C. This alignment causes the liquid crystal molecules to be influenced by the underlying alignment film and align in the same pattern. Light with a wavelength of 365 nm is then irradiated onto the plate. This progresses the polymerization reaction, solidifying the alignment of the liquid crystal molecules. A UV absorber 604, used in the manufacturing process of LCD televisions, is then applied, and the plate is heated at 80°C to remove the solvent. An alignment film 605 is then applied on top of the plate, and a polarization pattern is again exposed using a photonic crystal waveplate. It is important to align the center and ensure that the pattern orientation changes in the opposite direction from the first time as it moves from the center to the periphery. The UV absorber in the lower layer prevents UV light from penetrating below this point, leaving the alignment film and liquid crystal exposed in the first exposure unaffected. The substrate is then heated to 130°C to align it. Polymerizable liquid crystal 606 is then applied on top of it by spin coating, and the solvent is evaporated at 80°C. This alignment causes the liquid crystal molecules to be influenced by the underlying alignment film and align in the same pattern. Light with a wavelength of 365 nm is then irradiated onto the substrate. This progresses the polymerization reaction, solidifying the alignment of the liquid crystal molecules. In this way, liquid crystals oriented in different directions can be stacked on a single substrate.

[0071] The photonic crystal may have a pattern with different focal lengths for the first and second passes. It is important that the pattern changes in the opposite direction from the first when moving from the center to the periphery. It is clear that the same process can be repeated a third and fourth time. If the fixed liquid crystal 603 is not affected by UV light in subsequent processes, a UV absorbing layer is not necessary. Because the alignment film also has patterned birefringence, it is essential that the required phase difference is generated when combined with the liquid crystal above it. Therefore, the thickness of the alignment film and the liquid crystal layer can be selected appropriately depending on the process. A UV absorbing layer is necessary when utilizing the phase difference of the alignment film. Note that the temperature and other parameters used must be selected appropriately depending on the material. This example is merely an example.

[0072] Example 9: Multilayer structure (no liquid crystal) A manufacturing method for forming a multilayer structure of polarization gratings of the present invention on a single substrate will be described with reference to Fig. 7. Although the pattern is different from that of Example 5, the process is the same, so the same diagram will be used.

[0073] An alignment film 702 is applied to a 0.5 mm thick, 30 mm square glass plate 701 by spin coating to a thickness of 100 nm. It is then heated at 80°C for 5 minutes to remove the solvent. A polarization pattern is then exposed to the plate using the patterned photonic crystal waveplate used in Example 7 at a wavelength of 313 nm, and the plate is then heated to 130°C to align the alignment film. The alignment film has birefringence. Therefore, it is possible to increase the thickness of the alignment film and set it to a thickness that provides a half-wave plate as the phase difference of the alignment film. A UV absorber 703, which is used in the manufacturing process of LCD televisions, is then applied, and heated at 80°C to remove the solvent. An alignment film 704 is then applied on top of this, and a polarization pattern is again exposed using a photonic crystal waveplate. This time, the exposure is performed with a pattern that changes in the opposite direction from the first exposure, moving from the center to the periphery. The UV absorber in the lower layer prevents UV light from penetrating below this point, preventing any effect on the alignment film or liquid crystal exposed in the first exposure. The liquid crystal is then heated to 130 degrees to orient it. In this way, liquid crystals oriented in different directions can be stacked on a single substrate. Compared to Example 8, this method is expected to reduce costs because it does not require the process of forming a liquid crystal layer. The photonic crystal may be patterned with different focal lengths for the first and second times. It is important that the pattern changes in the opposite direction from the first sheet when moving from the center to the periphery. It is clear that the same process can be repeated a third and fourth time. Note that the temperature and other parameters used must be selected appropriately depending on the material, etc. This is merely an example.

[0074] In the above, in order to express the contents of the present invention, the present specification has described the embodiments and examples of the present invention with reference to the drawings. However, the present invention is not limited to the above embodiments and examples, and includes modifications and improvements that are obvious to those skilled in the art based on the matters described in the present specification.

[0075] 101 Period of polarization grating 102 Axial orientation of wave plate constituting polarization grating 201 Polarization grating 202 Diffraction angle of right-handed circularly polarized light diffracted by polarization grating 203 Diffraction angle of left-handed circularly polarized light diffracted by polarization grating 401 Polarization grating 402 Diffraction angle of right-handed circularly polarized light diffracted by polarization grating 403 Diffraction angle of left-handed circularly polarized light diffracted by polarization grating 404 Polarization grating 501 Polarization grating 502 Glass 601 Glass plate 602 Orientation film 603 Polymerizable liquid crystal 604 UV absorbing film 605 Orientation film 606 Polymerizable liquid crystal 701 Glass plate 702 Orientation film 703 UV absorbing film 704 Orientation film 801 Center of concentric circle pattern 802 Axial orientation of wave plate of wave plate constituting lens

Claims

1. A polarization separation element in which, in a three-dimensional space x, y, and z, a plurality of wave plates having optical axes in the xy plane are arranged in the z-axis direction, each of the wave plates has a plurality of regions that are repeated in the x-axis direction, each of the regions is divided into a plurality of strip-shaped sub-regions in the x-axis direction, the direction of the optical axis of odd-numbered wave plates changes stepwise in one direction at an angle with respect to the y-axis direction within a range of 0° to 180° within the region, and the direction of the optical axis of even-numbered wave plates changes stepwise in the opposite direction to the one direction at an angle with respect to the y-axis direction within a range of 0° to 180° within the region, and the angle at which right-handed circularly polarized light components and left-handed circularly polarized light components that are incident in the z-axis direction or obliquely from the z-axis increases with each addition of wave plates.

2. A polarization separation element in which, in a three-dimensional space x, y, and z, a plurality of wave plates having optical axes in the xy plane are arranged in the z-axis direction, each of the wave plates has a plurality of regions repeated in the x-axis direction, the direction of the optical axis of odd-numbered wave plates changes continuously in one direction within said region at an angle with respect to the y-axis direction within a range of 0° to 180°, and the direction of the optical axis of even-numbered wave plates changes continuously in the opposite direction to said one direction within said region at an angle with respect to the y-axis direction within a range of 0° to 180°, and the angle at which right-handed circularly polarized components and left-handed circularly polarized components incident in the z-axis direction or obliquely from the z-axis increases with each addition of wave plates.

3. A polarization separation element according to claim 1 or 2, wherein the wave plate is a photonic crystal, a metasurface, an alignment film, or a polymerizable liquid crystal.

4. A polarization separation element according to claim 1 or 2, wherein the wave plate is a photonic crystal, and a planarizing layer is included between a plurality of wave plates formed on the same substrate.

5. A method for manufacturing a polarization separation element according to claim 1 or 2, wherein the wave plate is made of a polymerizable liquid crystal wave plate, and the method comprises: coating an alignment film on the surface of one substrate; exposing a polarization pattern using a patterned wave plate; causing the polarization pattern produced by the patterned wave plate to affect the molecular orientation of the alignment film; and coating and polymerizing a polymerizable liquid crystal on top of the patterned wave plate, thereby orienting the liquid crystal depending on the molecular orientation of the alignment film.

6. A method for manufacturing a polarization separation element according to claim 1 or claim 2, wherein the wave plate is made of an alignment film, and the method comprises stacking a plurality of patterned wave plates, wherein the alignment film is applied to the surface of a single substrate, and a polarization pattern is exposed using a patterned wave plate, so that the polarization pattern produced by the patterned wave plate affects the molecular orientation of the alignment film, and the alignment film itself obtains the desired phase difference.

7. A method for manufacturing a polarized light separating element according to claim 5 or 6, wherein alignment films and, if necessary, liquid crystal layers are repeatedly stacked on one substrate, and a UV absorbing layer is inserted between them if necessary.

8. A light guide plate comprising the polarized light separation element according to claim 1 or 2, wherein the polarized light separation element deflects light incident from the air at an angle within the glass, and the angle is set to an angle at which the light is totally reflected within the glass, thereby efficiently guiding the light that has entered from the air into the glass.

9. In a three-dimensional space x, y, z, a plurality of wave plates having optical axes in the xy plane are arranged in the z-axis direction, each of the wave plates is divided into regions in an annular shape centered on a certain point and also divided into regions in the circumferential direction, each region is a wave plate with a uniform orientation, the orientation is calculated based on the radius of the annulus, and the orientation in the circumferential direction rotates 180 degrees x n (n is an integer greater than or equal to 0) in one revolution, the centers of the wave plates are coincident, the direction of the optical axis of odd-numbered wave plates changes stepwise in one direction at an angle ranging from 0° to 180° according to the radius from the center, and the direction of the optical axis of even-numbered wave plates changes stepwise in the opposite direction to the one direction, A lens in which one of the right-handed and left-handed circularly polarized components incident in the z-axis direction or obliquely from the z-axis is bent so that it converges in the z-axis direction passing through the center, and the other is bent so that it moves away from the z-axis direction passing through the center, and the bending angle increases with the number of wave plates.

10. A lens in which, in a three-dimensional space x, y, and z, multiple wave plates with optical axes in the xy plane are arranged in the z-axis direction, each of the wave plates has a center point, and its orientation is calculated based on the radius from the center, and in the circumferential direction, its orientation rotates continuously by 180 degrees x n (n is an integer greater than or equal to 0) throughout one revolution, the centers of the wave plates are coincident, the direction of the optical axis of odd-numbered wave plates changes continuously in one direction within an angle range of 0° to 180° according to the radius from the center, and the direction of the optical axis of even-numbered wave plates changes continuously in the opposite direction to the one direction, and one of the right-handed circularly polarized component and left-handed circularly polarized component incident in the z-axis direction or obliquely from the z-axis is bent to converge in the z-axis direction passing through the center, and the other is bent away from the z-axis direction passing through the center, and the bending angle increases with each wave plate.

11. A lens according to claim 9 or 10, characterized in that the axial direction φ of each wave plate is proportional to the square of the radius from the center.

12. A method for manufacturing a lens as defined in claim 9 or 10, wherein the wave plate is made of a polymerizable liquid crystal wave plate, and the method includes stacking a plurality of patterned wave plates, wherein an alignment film is applied to the surface of a single substrate, and a patterned wave plate is used to expose a polarization pattern, so that the polarization pattern produced by the patterned wave plate affects the molecular orientation of the alignment film, and polymerizable liquid crystal is applied thereon and polymerized, thereby orienting the liquid crystal depending on the molecular orientation of the alignment film.

13. A method for manufacturing a lens according to claim 9 or 10, wherein the wave plate is made of an alignment film, and the method includes stacking a plurality of patterned wave plates, which are realized by applying an alignment film to the surface of a single substrate, exposing a polarization pattern using a patterned wave plate, so that the polarization pattern produced by the patterned wave plate affects the molecular orientation of the alignment film, and the alignment film itself obtains the desired phase difference.

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