Diffractive optical element and manufacturing method for same

Abstract

Provided is a diffractive optical element with which it is possible to suppress light collection noise even when second or higher order diffraction is used, and to achieve a high light concentration degree. A diffractive optical element according to an embodiment of the present invention comprises a first lens portion provided at the center, and a second lens portion provided around the first lens portion, wherein: the diffractive optical element is configured to use second or higher order diffraction of light having a wavelength in the range of 380 to 780 nm, and has a numerical aperture NA of 0.2 or more; the second lens portion includes a plurality of prisms aligned toward the outer periphery; each of the plurality of prisms has a Fresnel surface inclined toward the outer periphery, a rising surface extending in the optical axis direction of the diffractive optical element, and a top portion, which is a connecting portion between the Fresnel surface and the rising surface; and the second lens portion is configured such that the interval between the top portions of adjacent prisms gradually decreases toward the outer peripheral side and the height of the plurality of prisms gradually decreases toward the outer peripheral side.

Description

Diffractive optical element and method for manufacturing the same

[0001] The present invention relates to a diffractive optical element and a method for manufacturing the same.

[0002] Regarding diffractive optical elements, theoretical research has been progressing. Since a diffractive optical element exhibits an optical function by convex portions (e.g., prisms) on the order of the wavelength of light, miniaturization, weight reduction, and thinning of an optical system are possible, and practical applications are expected. As a diffractive optical element, for example, a phase Fresnel lens is known. A Fresnel lens has a function of focusing or diverging light in the same manner as a normal lens. A Fresnel lens has concentric convex portions (e.g., prisms) formed thereon, and each convex portion plays a role of adjusting the phase of light in the focusing or diverging of light. Different from a normal Fresnel lens that is focused by geometric optics, a phase Fresnel lens can deform the wavefront of transmitted light into a specific shape in order to focus on a focal position by precisely controlling the height of the convex portion. Further, since the height of the prism of the phase Fresnel lens can be controlled so as to match the phase of light, unlike a normal prism, flare of light hardly occurs at the prism top, and a sharper image and / or focus can be obtained. In addition to being able to be miniaturized, lightweight, and thinned, the phase Fresnel lens has such excellent optical functions, and thus, for example, a lens that can achieve both thinning and high performance in a smartphone and / or a digital camera, a lens for a special projector that requires a lightweight and compact design, a medical device such as an endoscope that requires small size and high performance, and applications to a head-mounted display are expected.

[0003] However, it is very difficult to manufacture a diffractive optical element such as a phase Fresnel lens by precisely controlling the overall shape and thickness, as well as the height and interval of the convex portions, etc., and it has not yet been widely used. Specifically, currently, the configurations or applications to which a phase Fresnel lens is applied are substantially limited to those using first-order diffraction or those having a low light collection rate (small lens power).

[0004] Japanese Patent Application Laid-Open No. 2023-270418

[0005] The main objective of the present invention is to provide a diffractive optical element that can suppress focusing noise and achieve high focusing accuracy even when using second-order or higher diffraction.

[0006] [1] The diffractive optical element according to an embodiment of the present invention comprises a first lens portion provided in the center and a second lens portion provided around the first lens portion; the diffractive optical element is configured to utilize the second or higher order diffraction of light with a wavelength in the range of 380 nm to 780 nm, and has an numerical aperture NA of 0.2 or more; the second lens portion has a plurality of prisms arranged toward the outer periphery; each of the plurality of prisms has a Fresnel surface inclined toward the outer periphery, a rise surface extending in the direction of the optical axis of the diffractive optical element, and a apex which is the connection point between the Fresnel surface and the rise surface; the second lens portion is configured such that the distance between the apexes of adjacent prisms gradually narrows toward the outer periphery, and the height of the plurality of prisms gradually decreases toward the outer periphery. [2] In [1] above, the diffractive optical element has a radius of 0.1 mm to 100 mm. [3] In [1] or [2] above, the height of the prism adjacent to the first lens portion in the second lens portion is 1.0 μm to 20 μm. [4] In any of [1] to [3] above, each of the plurality of prisms has a refractive index of 1.40 to 2.20. [5] In any of [1] to [4] above, the height reduction profile of the plurality of prisms is a linear reduction with a constant gradient, a curved reduction that is convex upward, or it is constant from the first lens portion side to a predetermined range, and outside the predetermined range it shows a linear reduction with a constant gradient or a curved reduction that is convex upward. [6] In any of [1] to [5] above, the height reduction profile A of the plurality of prisms satisfies the following formula (1): In equation (1), n ​​is the refractive index of the prism, f is the focal length of the diffractive optical element, and x is the distance from the center of the diffractive optical element. [7] In any of [1] to [6] above, the numerical aperture NA is 0.3 to 0.9. [8] In any of [1] to [7] above, the Fresnel surface is inclined linearly. [9] In any of [1] to [7] above, the Fresnel surface is inclined curvedly.

[10] In any of [1] to [7] above, the Fresnel surface is inclined in a stepped manner.

[11] According to another aspect of the present invention, a method for manufacturing any of [1] to

[10] above is provided. A manufacturing method according to one embodiment includes forming the first lens portion and the second lens portion by stereolithography.

[12] A manufacturing method according to another embodiment of the present invention includes forming the first lens portion and the second lens portion by nanoimprinting.

[13] In the above

[12] , the replica mold used in the nanoimprint method is formed by stereolithography.

[0007] According to embodiments of the present invention, a diffractive optical element is provided that can suppress focusing noise and achieve high focusing accuracy even when using second-order or higher diffraction.

[0008] This is a schematic plan view of a diffractive optical element according to one embodiment of the present invention. This is a schematic cross-sectional view of the diffractive optical element in Figure 1A along the line B-B. This is an enlarged schematic cross-sectional view of the main part of the second lens section (prism) in the diffractive optical element in Figure 1A. This is a schematic diagram for explaining the meaning of numerical aperture NA in the diffractive optical element according to an embodiment of the present invention. This is a schematic cross-sectional view for explaining an example of a configuration in which the height of the prism gradually decreases toward the outer periphery in the diffractive optical element according to an embodiment of the present invention. This is a schematic cross-sectional view for explaining another example of a configuration in which the height of the prism gradually decreases toward the outer periphery in the diffractive optical element according to an embodiment of the present invention. This is a conceptual diagram for explaining the use of second-order or higher diffraction of light in the diffractive optical element according to an embodiment of the present invention. This is an energy distribution diagram of the emitted light from the diffractive optical element of Reference Example 1. This is an energy distribution diagram of the emitted light from the diffractive optical element of Reference Example 2. This is an energy distribution diagram of the emitted light from the diffractive optical element of Reference Example 3. This is an energy distribution diagram of the emitted light from the diffractive optical element of Reference Example 4. This is an energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 1, a profile of the decrease in the height of multiple prisms in the diffractive optical element of Example 1, and an energy distribution diagram of the emitted light from the diffractive optical element of Example 1. This shows the energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 2, the height reduction profile of the multiple prisms in the diffractive optical element of Example 2, and the energy distribution diagram of the emitted light from the diffractive optical element of Example 2. This shows the energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 3, the height reduction profile of the multiple prisms in the diffractive optical element of Example 3, and the energy distribution diagram of the emitted light from the diffractive optical element of Example 3. This shows the energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 4, the height reduction profile of the multiple prisms in the diffractive optical element of Example 4, and the energy distribution diagram of the emitted light from the diffractive optical element of Example 4. This shows the energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 5, the height reduction profile of the multiple prisms in the diffractive optical element of Example 5, and the energy distribution diagram of the emitted light from the diffractive optical element of Example 5.This diagram shows the energy distribution of light emitted by the diffractive optical element of Comparative Example 6, the height reduction profile of the multiple prisms in the diffractive optical element of Example 6-1, and the energy distribution of light emitted by the diffractive optical element of Example 6-1. This diagram shows the energy distribution of light emitted by the diffractive optical element of Comparative Example 6, the height reduction profile of the multiple prisms in the diffractive optical element of Example 6-2, and the energy distribution of light emitted by the diffractive optical element of Example 6-2. This diagram shows the energy distribution of light emitted by the diffractive optical element of Comparative Example 7, the height reduction profile of the multiple prisms in the diffractive optical element of Example 7, and the energy distribution of light emitted by the diffractive optical element of Example 7. This diagram shows the energy distribution of light emitted by the diffractive optical element of Comparative Example 8, the height reduction profile of the multiple prisms in the diffractive optical element of Example 8, and the energy distribution of light emitted by the diffractive optical element of Example 8. This diagram shows the energy distribution of light emitted by the diffractive optical element of Comparative Example 9, the height reduction profile of the multiple prisms in the diffractive optical element of Example 9, and the energy distribution of light emitted by the diffractive optical element of Example 9. This shows the energy distribution diagram of the emitted light from the diffractive optical element of Comparative Example 10, the height reduction profile of the multiple prisms in the diffractive optical element of Example 10, and the energy distribution diagram of the emitted light from the diffractive optical element of Example 10. This is a photograph showing the plan view of the diffractive optical element of Comparative Example 11. This is a photograph showing the plan view of the diffractive optical element of Example 11. This is a schematic diagram showing the height profile along the diameter direction of the diffractive optical element of Comparative Example 11. This is a schematic diagram showing the height profile along the diameter direction of the diffractive optical element of Example 11. These are bar graphs showing the collection ratios of the 11th, 12th, and 13th order diffracted light from the diffractive optical element of Comparative Example 11 and the diffractive optical element of Example 11, respectively.

[0009] Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Specifically, a higher-order phase Fresnel lens will be described as an example of a diffractive optical element, but it will be apparent to those skilled in the art that the technical concept of the present invention is not limited to such specific configurations and can be applied to any optical element that performs a function through diffraction. For example, the technical concept of the present invention can also be applied to cylindrical lenses (one-dimensional focusing) and point focus lenses (two-dimensional focusing).

[0010] For ease of viewing and understanding, the drawings are schematic or conceptual, and the length, width, height, thickness, shape, size, proportions, direction, number, etc., may differ from reality.

[0011] A. Diffractive Optical Element Figure 1A is a schematic plan view of a higher-order phase Fresnel lens, which is an example of a diffractive optical element according to an embodiment of the present invention; Figure 1B is a schematic cross-sectional view of the higher-order phase Fresnel lens of Figure 1A along the line B-B; Figure 1C is an enlarged schematic cross-sectional view of the main part of the second lens portion (prism) in the higher-order phase Fresnel lens of Figure 1A. The illustrated example of the higher-order phase Fresnel lens 100 is circular in plan view. However, the higher-order phase Fresnel lens may have any appropriate plan view shape depending on the purpose. The plan view shape of the higher-order phase Fresnel lens may be, for example, elliptical, rectangular, square, or polygonal.

[0012] The illustrated example of a higher-order phase Fresnel lens 100 comprises a first lens portion 10 provided in the center and a second lens portion 20 provided around the first lens portion 10. In the illustrated example, the first lens portion 10 and the second lens portion 20 are provided on the first main surface 100a side of the higher-order phase Fresnel lens. The first lens portion 10 and the second lens portion 20 may also be provided on the second main surface 100b side of the higher-order phase Fresnel lens, or on both sides of the higher-order phase Fresnel lens. In the illustrated example, the first main surface 100a is typically the exit surface, but it may also be the incident surface.

[0013] The higher-order phase Fresnel lens may be configured as an independent component, as shown in the illustrated example, or it may be formed on any suitable substrate (not shown).

[0014] As described above, the first lens section 10 is located in the center of the higher-order phase Fresnel lens 100. Typically, the optical axis of the first lens section 10 coincides with the optical axis (central axis) 100c of the higher-order phase Fresnel lens 100. Any appropriate surface shape can be adopted for the first lens section depending on the purpose. Examples of surface shapes for the first lens section include a curved surface and an inclined surface. Examples of curved surfaces include the surface shape near the optical axis of a convex lens (illustrated example) and the surface shape near the optical axis of a concave lens. The height of the first lens section can be appropriately set depending on the purpose. For example, the height of the first lens section may be the same as or different from the height of the innermost (i.e., adjacent to the first lens section) prism (described later) among the multiple prisms constituting the second lens section. For example, when the radius of the first lens section is designed to be small, the height of the first lens section may be less than the height of the prism.

[0015] As described above, the second lens section 20 is provided around the first lens section 10. The second lens section 20 has a plurality of prisms 21 arranged toward the outer circumference. Typically, the plurality of prisms 21 are provided coaxially with respect to the optical axis (central axis) 100c of the higher-order phase Fresnel lens 100. In a higher-order phase Fresnel lens that is circular in plan view as shown in the illustrated example, the plurality of prisms 21 are annular. Each of the plurality of prisms 21 has: a Fresnel surface 21a that is inclined toward the outer circumference; a rise surface 21b provided between adjacent Fresnel surfaces 21a, 21a and extending in the direction of the optical axis 100c of the higher-order phase Fresnel lens 100 (vertical direction in the illustrated example); a top portion 21c which is the connection between the upper part of the Fresnel surface 21a and the upper part of the rise surface 21b; and a bottom portion 21d which is the connection between the lower part of the Fresnel surface 21a and the lower part of the rise surface 21b. The Fresnel surface 21a may be inclined linearly, curved, or stepped, as shown in the illustrated example. When the Fresnel surface is inclined curvedly, the curve may be convex upwards, convex downwards, or a combination of these (i.e., a shape with inflection points). When the Fresnel surface is inclined stepped, the number of steps may be, for example, 10 or more, or 20 to 300. The inclination angle of the Fresnel surface (angle with respect to the direction perpendicular to the optical axis 100c of the Fresnel lens: angle with respect to the horizontal in the illustrated example) may be, for example, 1° to 55°, or 1° to 45°. The rise surface 21b may have a flat surface (its cross-section may be linear as shown in the illustrated example), or it may be a curved surface (its cross-section may be curved).

[0016] In an embodiment of the present invention, the second lens section 20 is configured such that the spacing between the tops 21c of adjacent prisms 21 (hereinafter sometimes referred to as the prism pitch) gradually narrows toward the outer periphery, and the height of the multiple prisms 21 gradually decreases toward the outer periphery. With such a configuration, even when diffraction of second order or higher is used and the numerical aperture of the lens is large (high light-gathering power), focusing noise can be suppressed.

[0017] A configuration in which the prism pitch gradually narrows toward the outer edge can be appropriately set depending on the purpose. In one embodiment, this configuration may satisfy the following equation (2). That is, with the innermost prism as the first, the prism pitch may gradually narrow toward the outer edge such that the position xi of the top of the i-th prism from the center of the lens satisfies the following equation (2). In equation (2), n is the refractive index of the higher-order phase Fresnel lens (essentially a prism), f is the focal length of the higher-order phase Fresnel lens, and d ０ k is the height of the prism. ０ k is a variable related to the radius of the first lens section, and can be arbitrarily set by the designer within the range of 0 to 1. For example, k ０ If is 0, the height of the first lens is d ０ It is equal to , and as it increases, xi decreases (i.e., the radius of the first lens part decreases), and the height of the first lens part is d ０ It will be lower than that.

[0018] Regarding the height of the prisms, for example, if the height of the innermost prism (i.e., adjacent to the first lens portion) is set to 100, the height of the outermost prism may be, for example, 97 or less, or 60 to 97, or 65 to 96, or 70 to 93, or 72 to 90, or 73 to 85. In one embodiment, the ratio "height of the outermost prism / height of the innermost prism" is expressed by the following formula (3). In equation (3), n is the refractive index of the prism, and NA is the numerical aperture. The numerical aperture NA is an indicator of the lens's focusing power; a larger numerical aperture NA means higher focusing power (higher lens power). The numerical aperture NA can be calculated from the following equation (4). In equation (4), R is the radius of the higher-order phase Fresnel lens, f is the focal length of the higher-order phase Fresnel lens, and θ is the focusing angle (see Figure 2).

[0019] The height of the prism refers to the vertical distance from the bottom 21d to the top 21c of the prism. In the illustrated example, since the rise surface 21b extends vertically, the height of the prism 21 is synonymous with the height of the rise surface 21b. The mode in which the height of the prism gradually decreases toward the outer circumference may be such that the height level of the bottom 21d is constant and the height level of the top 21c gradually decreases toward the outer circumference, as shown in Figure 3, or the height level of the top 21c is constant and the height level of the bottom 21d gradually increases toward the outer circumference, as shown in Figure 4.

[0020] The prism height reduction profile can be appropriately set depending on the purpose. For example, the prism height reduction profile may be a linear reduction with a constant slope, or a curved reduction that is convex upwards, and may be constant from the first lens portion side up to a predetermined range, and outside the predetermined range it may be a linear reduction with a constant slope or a curved reduction that is convex upwards. In one embodiment, the prism height reduction profile A satisfies the following formula (1). With such a configuration, the effects of the embodiment of the present invention may become more pronounced. In equation (1), n ​​is the refractive index of the prism, f is the focal length of the higher-order-phase Fresnel lens, and x is the distance from the center of the higher-order-phase Fresnel lens.

[0021] In embodiments of the present invention, the higher-order phase Fresnel lens is configured to utilize the second-order or higher diffraction of light with wavelengths in the range of 380 nm to 780 nm. A phase Fresnel lens that can utilize the second-order or higher diffraction may have the following advantages. A phase Fresnel lens using first-order diffraction has high wavelength dependence due to the diffraction phenomenon, and its focal length changes significantly with wavelength, thus limiting its applications. In contrast, a phase Fresnel lens using higher-order diffraction, where the diffraction order is 2 or higher in visible light, can function as a diffraction lens with the same or substantially the same integer order of focal length for multiple wavelengths in visible light and near-infrared light. Therefore, it is useful for color displays and / or color optical systems that handle these multiple wavelengths (colors).

[0022] Figure 5 is a conceptual diagram illustrating the case where the second-order diffraction of light is utilized in a higher-order phase Fresnel lens (i.e., a second-order phase Fresnel lens). As shown in Figure 5, incident light (parallel light) of wavelength λ is emitted from each prism at a predetermined angle so as to be directed toward the focusing direction. In the case of the second-order phase Fresnel lens shown in the example, the wavefronts of the emitted light from adjacent prisms are shifted by two wavelengths (2λ). That is, the optical path difference of the emitted light from adjacent prisms is 2λ. Therefore, the phases of the emitted light from adjacent prisms coincide. The fact that the optical path difference of the emitted light from adjacent prisms is 2λ means that, as shown in Figure 5, the optical path difference of the emitted light from all adjacent prisms is 2λ. As a result, the phases of the emitted light from all prisms coincide, and the intensity of the light focused by the phase Fresnel lens becomes very strong. In the case of a third-order phase Fresnel lens, the optical path difference of the emitted light from adjacent prisms is 3λ; in the case of an n-order phase Fresnel lens, the optical path difference of the emitted light from adjacent prisms is nλ. The diffraction order of a higher-order phase Fresnel lens is, as described above, second or higher, preferably third or higher, more preferably fourth or higher, even more preferably fifth or higher, and particularly preferably sixth or higher. The larger the diffraction order, the more pronounced the effects of the embodiments of the present invention may become. On the other hand, the diffraction order may be, for example, 20th or lower, or even 15th or lower. If the diffraction order is too large, the lens portion and / or prism become larger while maintaining the same precision required for size control, making manufacturing more difficult. Furthermore, diffraction orders of 20th or higher result in a lens effect almost equivalent to focusing by geometrical optics, which may cause flare at the top of the prism and result in insufficient lens performance at the contour of the prism.

[0023] Configurations that can utilize second-order or higher diffraction can typically be achieved by controlling the height of the prism. The height of the prism is d. ０ This can be obtained from the following equation (5). In equation (5), n is the refractive index of the prism, m is the order of diffraction, and λ is the wavelength of light. For example, if the refractive index of the prism is 1.55, the height of the prism that can utilize third-order diffraction for a wavelength of 532 nm is 2902 nm (2.902 μm). Assuming that the refractive index does not change with wavelength, from equation (5), a prism height of 2902 nm can utilize fourth-order diffraction for light with a wavelength of 399 nm and second-order diffraction for light with a wavelength of 798 nm. Thus, according to the embodiment of the present invention, light of multiple wavelengths (colors) can be focused simultaneously. As a result, the higher-order phase Fresnel lens according to the embodiment of the present invention can be suitably applied to a multi-color optical system. By appropriately adjusting the height of the prism, the higher-order phase Fresnel lens can be applied in a wavelength range of, for example, 300 nm (ultraviolet region) to 2000 nm (infrared region).

[0024] In the embodiments of the present invention, as described above, the height of the prism is configured to gradually decrease toward the outer edge. This may have the following technical implications. In conventional high-order phase Fresnel lenses, the height of the prism is constant, as specified by, for example, formula (5) above. Here, the inventors have newly discovered a problem in which, when the height of the prism is constant in this way, and when focusing with a high lens power such as a numerical aperture NA of 0.2 or more, focusing occurs due to diffracted light of an order different from the target order (for example, an order 1 to 3 higher than the target order), resulting in some light being focused at a focal length shorter than the target focal length, which becomes focusing noise and prevents the acquisition of a clear image. The inventors have diligently studied means to solve this problem and have found that by configuring the prism so that the height gradually decreases toward the outer edge as described above, focusing noise can be suppressed (preferably eliminated), and have completed the present invention. Furthermore, the inventors have achieved precise control of the prism height and pitch, which was previously difficult, by employing a predetermined manufacturing method described later in section B. More specifically, the following applies: Fresnel lenses utilizing higher-order diffraction require the same size precision despite having larger prism and / or lens components than first-order diffraction Fresnel lenses. Furthermore, phase Fresnel lenses require more precise machining to control the phase, and the configuration according to the embodiment of the present invention (a configuration in which the height of the prism gradually decreases toward the outer edge) requires even more precise machining. One of the major achievements of the embodiment of the present invention is not only the discovery that focusing noise can be suppressed even when using second-order or higher diffraction and when the numerical aperture of the lens is large (high focusing power), but also the realization of a higher-order phase Fresnel lens with such a configuration.

[0025] In embodiments of the present invention, for example, as described above, the height d of the prism represented by formula (5) is as follows: ０This is reduced by a profile specified by equation (1) above, depending on the position from the center. This makes it possible to suppress (preferably eliminate) focusing noise even when using second-order or higher diffracted light. In such a configuration, the height d of the prism represented by equation (5) above is ０ This can correspond to the height of the innermost prism (i.e., adjacent to the first lens section). The height of the innermost prism can be appropriately set according to the wavelength (color) of the light to be focused, the order of diffraction, the refractive index of the prism, etc. The height of the innermost prism may be, for example, 1.0 μm to 20 μm, or for example, 1.5 μm to 18 μm, or for example, 2.0 μm to 15 μm, or for example, 2.5 μm to 12 μm.

[0026] In embodiments of the present invention, the higher-order phase Fresnel lens has a numerical aperture (NA) of 0.2 or higher, preferably 0.3 or higher. The new problem of focusing noise caused by diffracted light of different orders, as described above, is particularly pronounced when the numerical aperture (NA) is large. According to embodiments of the present invention, even when the numerical aperture (NA) is large (high focusing degree), focusing noise can be suppressed (preferably eliminated). On the other hand, the numerical aperture (NA) is preferably 0.9 or lower, more preferably 0.8 or lower, even more preferably 0.7 or lower, and particularly preferably 0.6 or lower. If the upper limit of the numerical aperture (NA) is within this range, light can be emitted from the inclined portion of the prism to the focal point, and a good lens effect can be obtained. The numerical aperture (NA) can be determined from the above formula (4).

[0027] Higher-order phase Fresnel lenses are typically composed of light-transmitting materials. These materials can be appropriately selected depending on the purpose, desired light transmittance, and applicability to the manufacturing method. Specific examples of light-transmitting materials include (meth)acrylic resins such as polymethyl (meth)acrylate, polycarbonate resins, olefin resins such as polyethylene and polypropylene, and cyclic olefin resins such as polynorbornene.

[0028] The refractive index n of the higher-order phase Fresnel lens (and therefore the refractive index of the first and second lens parts) may be, for example, 1.40 to 2.20, or for example, 1.45 to 2.00, or for example, 1.50 to 1.95, or for example, 1.55 to 1.90.

[0029] The radius R of the higher-order phase Fresnel lens may be, for example, 0.1 mm to 100 mm, or for example, 0.5 mm to 50 mm, or for example, 1 mm to 30 mm. The radius of the higher-order phase Fresnel lens may be, for example, 20 mm or more, or for example, 40 mm or more, or for example, 60 mm or more, or for example, 80 mm or more. According to embodiments of the present invention, even with a large aperture such as a radius of 100 mm, focusing noise caused by diffracted light of different orders can be suppressed (preferably eliminated).

[0030] B. Method for Manufacturing a Diffractive Optical Element According to embodiments of the present invention, the diffractive optical element described in Section A above (for example, a higher-order phase Fresnel lens) can be actually manufactured. Therefore, the method for manufacturing the above-mentioned diffractive optical element (for example, a higher-order phase Fresnel lens) can also be included in embodiments of the present invention.

[0031] In the manufacturing method according to the embodiments of the present invention, the first lens portion and the second lens portion may be formed by stereolithography or by nanoimprint lithography. The stereolithography method and the nanoimprint lithography method will be described below.

[0032] A typical example of a stereolithography method is two-photon polymerization (TPP). Two-photon polymerization (TPP) is a technology used in high-resolution 3D printers. Typically, two-photon polymerization converts liquid photocurable resin into a solid using photopolymerization. Two-photon polymerization is based on the nonlinear interaction of light, and the polymerization reaction occurs only in a very small region around the focal point of the light, making it possible to process extremely fine details on the nanometer scale. The monomer composition used for two-photon polymerization typically includes a radical polymerizable monomer and a two-photon polymerization initiator. Examples of radical polymerizable monomers include bifunctional acrylic monomers, trifunctional acrylic monomers, and polyfunctional acrylic monomers. Examples of trifunctional acrylic monomers include pentaerythritol triacrylate (PETA) and trimethylolpropane triacrylate (TMPTA). Examples of polyfunctional acrylic monomers include dipentaerythritol hexaacrylate (DPHA). Aromatic acrylic monomers (e.g., benzyl acrylate) may be used for refractive index adjustment. Two-photon polymerization initiators are typically photopolymerization initiators that, when activated by two-photon absorption, generate radicals and / or acids, which trigger the photopolymerization reaction. Two-photon polymerization initiators can be appropriately selected depending on the wavelength of light used for two-photon polymerization. Preferably, two-photon polymerization initiators have a large two-photon absorption cross-section. With such a configuration, the reaction occurs only when laser light of a higher intensity than normal photon absorption is used, and as a result, high resolution can be achieved. A typical example of a two-photon polymerization initiator is bis(4-diallylaminobenzylidene)ketone. This compound is highly sensitive and can achieve high resolution.

[0033] Two-photon polymerization can typically be carried out by irradiating the above monomer composition with laser light. Preferably, the laser light has a high energy density. Such a configuration allows for good two-photon absorption. Examples of lasers with high energy density include femtosecond lasers. Typical wavelengths of the laser light can be in the near-infrared region (e.g., around 800 nm). Laser irradiation devices of such wavelengths can be high-powered and inexpensive.

[0034] A specific procedure for two-photon polymerization may be as follows: Using a 780 nm femtosecond laser (pulse width = 100 femtoseconds), focusing it with a high-lens-power lens with a numerical aperture NA = 1.4, and scanning with a laser output of 25 mW, a scan speed of 80 mm / sec, an in-plane spacing of 0.2 μm between scan lines, and a thickness-direction spacing of 0.4 μm between scan lines, a microstructure with controllable phase can be fabricated. The specific configuration of the microstructure is as described in section A above. After polymerization (fabrication), the monomers can be removed from the fabricated object by immersing it in a solvent (e.g., ketones, esters) that easily dissolves or disperses unpolymerized monomers for, for example, 5 to 120 minutes. Next, the fabricated object is immersed in a solvent with low surface tension (e.g., alcohol, hexane, methyl nonaflubutyl ether) for, for example, 15 minutes and dried to obtain the final photopolymerized object (e.g., a high-order phase Fresnel lens) or a master mold or replica mold for a high-order phase Fresnel lens.

[0035] The above describes a method for converting a liquid photocurable resin into a solid using photopolymerization. However, it is also possible to change the solubility of the resin in a solvent by a chemical reaction caused by light, and then create a microstructure by washing with the solvent. In this case, only the parts that reacted with light may be dissolved, or only the parts that reacted with light may be left intact.

[0036] The nanoimprint method typically involves using a replica mold with a shape corresponding to the final photomolded object (e.g., a high-order phase Fresnel lens) and pressing it onto a resin or resin precursor applied on a substrate to transfer (stamp) the shape. The replica mold may be fabricated using a replication transfer technique as described below or by two-photon polymerization photomolding as described above.

[0037] Fabrication of a replica mold by a replication transfer technique can be performed, for example, according to the following procedure. A final photomolded object (high-order phase Fresnel lens) is formed on an arbitrary suitable substrate, and then a conductive thin film of a metal (e.g., nickel) is formed on the surface of the photomolded object and the surrounding substrate surface by sputtering. The thickness of the conductive thin film can be, for example, 10 nm to 500 nm. Next, electrolytic metal plating is performed using the conductive thin film as a seed film. The metal used for electrolytic metal plating is preferably the same as that of the conductive thin film. By electrolytic metal plating, a metal replica mold with a thickness of 0.5 mm or more is formed, and finally, the photomolded object is removed with a solvent or chemical. In this way, a replica mold can be obtained.

[0038] Nanoimprint can be either photonic nanoimprint or thermal nanoimprint. Photonic nanoimprint typically involves pressing the pattern of a replica mold onto a photo (typically ultraviolet light) curable resin and irradiating ultraviolet light to cure the resin to replicate the pattern. Photonic nanoimprint enables work at room temperature and thus has high pattern reproduction accuracy. Thermal nanoimprint typically involves pressing the pattern of a replica mold onto a thermosetting resin or a thermoplastic resin and replicating the pattern by heating. When using a thermosetting resin, the pattern is replicated by heating in a state where the replica mold is pressed onto the uncured (liquid) thermosetting resin to cure the thermosetting resin. When using a thermoplastic resin, the pattern of the replica mold is pressed onto a thermoplastic resin film, and the pattern is replicated by heating and then cooling. Thermal nanoimprint can be directly processed as long as the material is cured or softened by heating.

[0039] A photocurable resin (resin composition) typically contains a curing component and a photoinitiator. As the curing component, typically, acrylate monomers or oligomers are mentioned. Specific examples of the curing component include dipropylene glycol diacrylate (DPGDA), hexanediol diacrylate (HDDA), and tripropylene glycol diacrylate (TPGDA). The photoinitiator typically generates radicals upon irradiation with ultraviolet light and initiates the polymerization of monomers and / or oligomers. Examples of photoinitiators include benzoin ether-based initiators, acetophenone-based initiators, and oxime ester-based initiators. The photoinitiator can be appropriately selected according to the type of monomers and / or oligomers used, the wavelength of ultraviolet light, etc. The photocurable resin (resin composition) may contain a release agent and / or a viscosity modifier. As the release agent, typically, fluorine compounds and silicone-based compounds are mentioned. By using a release agent, the releasability from the replica mold can be improved. As the viscosity modifier, typically, monomers and low molecular weight oligomers are mentioned. By using a viscosity modifier, the filling property and transferability to the replica mold can be improved.

[0040] Examples of thermoplastic resins include polymethyl methacrylate (PMMA), polycarbonate (PC), and polyethylene terephthalate (PET).

[0041] Examples of thermosetting resins include epoxy resins and phenolic resins.

[0042] In the above-mentioned photocurable resin, thermoplastic resin, and thermosetting resin, inorganic nanoparticles may be blended (substantially dispersed) in order to increase the refractive index of the obtained molded body (for example, to 1.60 or more). Examples of the material constituting the inorganic nanoparticles include titanium oxide, niobium oxide, silicon nitride, and zirconium oxide. The particle size of the inorganic nanoparticles can be, for example, 3 nm to 50 nm. The blending amount of the inorganic nanoparticles can be, for example, 10 parts by weight to 80 parts by weight with respect to 100 parts by weight of the resin.

[0043] An example of a specific procedure for nanoimprinting is described below. Here, optical nanoimprinting is used as an example. First, an uncured resin is uniformly applied to a replica mold or substrate. Examples of application methods include spin coating and dip coating. Next, the applied resin is sandwiched between the replica mold and the substrate, and uniform pressure is applied. By applying pressure, the resin fills the microstructure of the replica mold. Heat is applied as needed to increase the fluidity of the resin and promote filling. With the resin sandwiched between the replica mold and the substrate, the resin is cured by irradiation with ultraviolet light, for example. The curing process may vary depending on the type of monomer, light irradiation conditions, etc. Light irradiation is preferably continued until the resin no longer hardens. For example, when irradiating with ultraviolet light, typical light sources include high-pressure mercury lamps, metal halide lamps, and UV-LEDs. High-pressure mercury lamps have a peak wavelength of 365 nm and can provide high output over a wide range of wavelengths. Metal halide lamps can irradiate over a wider wavelength range. UV-LEDs have high output at specific wavelengths. The light source can be appropriately selected depending on the type of resin, etc. Finally, the cured resin is peeled off the replica mold. A peeling roll may be used if necessary. By using a peeling roll, the problem of cured material residue remaining on the replica mold can be suppressed.

[0044] In the case of thermal nanoimprinting, the same procedure can be adopted, except that ultraviolet light irradiation is replaced with heating. The heating temperature can be appropriately set depending on the type of resin, etc. For example, in the case of thermoplastic resins, the heating temperature can be set to a temperature 20°C or more higher than the glass transition temperature. Furthermore, delamination can be performed after the resin, replica mold, and substrate have cooled.

[0045] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0046] <Reference Example 1> A first-order phase Fresnel lens is fabricated by two-photon polymerization photolithography. Specifically, a first-order phase Fresnel lens is fabricated by subjecting a monomer composition containing a radically polymerizable monomer and a photopolymerization initiator (Nanoscribe, product name "IP-Dip2") to two-photon polymerization using a high-resolution 3D printer (Nanoscribe, product name "QuantumX"). The conditions for two-photon polymerization are as follows: Laser light used: femtosecond laser (wavelength 780 nm) Focusing: Focused using a high-lens-power lens with a numerical aperture NA = 1.4 Laser output: 25 mW Scanning speed: 80 mm / sec In-plane spacing of scan lines: 0.08 μm Thickness-direction spacing of scan lines: 0.08 μm

[0047] The configuration (design) of the first-order phase Fresnel lens is as follows: Refractive index: 1.55 Prism height: 0.95 μm (constant height) Prism pitch: Gradually narrows towards the outer edge Lens numerical aperture NA: 0.3 Lens radius: 150 μm The energy distribution of light was simulated for this first-order phase Fresnel lens. Specifically, using COMSOL's simulation software "Multiphysics® 6.0", a two-dimensional model of wave optics and electromagnetic wave (frequency domain) interface (solving the time-harmonic wave equation for the electric field) was used, with a maximum mesh size of 1 / 5 of the wavelength. Under the condition that light (wavelength 532 nm) vibrating out of the plane with a uniform electric field intensity of 1 [V / m] from the flat surface of the lens was incident, the energy distribution of the emitted light was determined by FEM (finite element method) simulation. The energy distribution diagram is shown in Figure 6. As is clear from Figure 6, no focusing noise was observed for the first-order phase Fresnel lens.

[0048] <Reference Example 2> A first-order phase Fresnel lens was designed in the same manner as in Reference Example 1, except that the numerical aperture (NA) was set to 0.5. This Fresnel lens was subjected to the same evaluation as in Reference Example 1. The results are shown in Figure 7. As is clear from Figure 7, no focusing noise was observed for the first-order phase Fresnel lens.

[0049] <Reference Example 3> A first-order phase Fresnel lens was designed in the same manner as in Reference Example 1, except that the numerical aperture (NA) was set to 0.6. This Fresnel lens was subjected to the same evaluation as in Reference Example 1. The results are shown in Figure 8. As is clear from Figure 8, no focusing noise was observed for the first-order phase Fresnel lens.

[0050] <Reference Example 4> A third-order phase Fresnel lens was designed in the same manner as in Reference Example 1, except that the diffraction order was set to third order (prism height set to 2.902 μm) and the numerical aperture NA was set to 0.1. This Fresnel lens was subjected to the same evaluation as in Reference Example 1. The results are shown in Figure 9. As is clear from Figure 9, no focusing noise was observed for the third-order phase Fresnel lens with a small numerical aperture NA.

[0051] <Comparative Example 1> A third-order phase Fresnel lens was designed in the same manner as in Reference Example 1, except that the diffraction order was set to third-order and the numerical aperture (NA) was set to 0.2. This Fresnel lens was subjected to the same evaluation as in Reference Example 1. The results are shown in Figure 10, along with the results of Example 1. As is clear from Figure 10, focusing noise was observed in the third-order phase Fresnel lens with a constant prism height and a large numerical aperture (NA). Furthermore, the focusing intensity was calculated for this third-order phase Fresnel lens. Specifically, using the COMSOL simulation software "Multiphysics® 6.0", a two-dimensional model of wave optics and electromagnetic wave (frequency domain) interface (solving the time-harmonic wave equation for the electric field) was used. With a maximum mesh size of 1 / 5 of the wavelength, light (wavelength 532 nm) with a uniform electric field intensity of 1 [V / m] from the flat surface of the lens and vibrating out of the plane was incident. The square of the electric field norm [unit: V / m] at the position where the light is most concentrated after passing through the lens (= focal position) was defined as the focusing intensity. The focusing intensity was 309. The results, along with those of Example 1, are shown in Table 1.

[0052] <Example 1> A third-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 10, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 96.3. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results are shown in Figure 10, along with the results of Comparative Example 1. As is clear from Figure 10, no focusing noise was observed even with a third-order phase Fresnel lens with a large numerical aperture NA. Furthermore, the focusing intensity was 315. The results are shown in Table 1.

[0053] <Comparative Example 2> A third-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the numerical aperture (NA) was set to 0.3. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 2, are shown in Figure 11 and Table 1. As is clear from Figure 11, focusing noise was observed in the third-order phase Fresnel lens with a constant prism height and a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 433.

[0054] <Example 2> A third-order phase Fresnel lens was designed in the same manner as in Comparative Example 2, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 11, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 92. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Comparative Example 2, are shown in Figure 11 and Table 1. As is clear from Figure 11, no focusing noise was observed even with a third-order phase Fresnel lens with a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 449.

[0055] <Comparative Example 3> A third-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the numerical aperture (NA) was set to 0.6. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 3, are shown in Figure 12 and Table 1. As is clear from Figure 12, focusing noise was observed in the third-order phase Fresnel lens with a constant prism height and a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 361.

[0056] <Example 3> A third-order phase Fresnel lens was designed in the same manner as in Comparative Example 3, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 12, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 74. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Comparative Example 3, are shown in Figure 12 and Table 1. As is clear from Figure 12, no focusing noise was observed even with a third-order phase Fresnel lens with a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 687, which is almost twice the focusing intensity of Comparative Example 3.

[0057] <Comparative Example 4> A 6th-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the diffraction order was set to 6th order (prism height set to 5.804 μm) and the radius was set to 200 μm. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 4, are shown in Figure 13 and Table 1. As is clear from Figure 13, focusing noise was observed in the 6th-order phase Fresnel lens with a constant prism height and a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 412.

[0058] <Example 4> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 4, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 13, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 96.5. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Comparative Example 4, are shown in Figure 13 and Table 1. As is clear from Figure 13, no focusing noise was observed even with a sixth-order phase Fresnel lens with a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 424.

[0059] <Comparative Example 5> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 4, except that the numerical aperture NA was set to 0.3 and the radius was set to 150 μm. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 5, are shown in Figure 14 and Table 1. As is clear from Figure 14, focusing noise was observed in the sixth-order phase Fresnel lens with a constant prism height and a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 395.

[0060] <Example 5> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 5, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 14, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 92. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 5, are shown in Figure 14 and Table 1. As is clear from Figure 14, no focusing noise was observed even with a sixth-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 462.

[0061] <Comparative Example 6> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 4, except that the numerical aperture (NA) was set to 0.6 and the radius to 150 μm. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 6-1, are shown in Figure 15 and Table 1. As is clear from Figure 15, focusing noise was observed in the sixth-order phase Fresnel lens with a constant prism height and a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 217.

[0062] <Example 6-1> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 6, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 15, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 73. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 6, are shown in Figure 15 and Table 1. As is clear from Figure 15, no focusing noise was observed even with a sixth-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 710, which is more than three times the focusing intensity of Comparative Example 6.

[0063] <Example 6-2> A sixth-order phase Fresnel lens was designed in the same manner as in Example 6-1, except that it was configured as shown in Figures 1A, 1B, and 4, that is, the height level of the top of the prism was constant and the height level of the bottom gradually increased towards the outer circumference. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 73. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 6, are shown in Figure 16 and Table 1. As is clear from Figure 16, no focusing noise was observed even with a sixth-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 619, which is nearly three times the focusing intensity of Comparative Example 6.

[0064] <Comparative Example 7> A 6th-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the refractive index was set to 1.90, the diffraction order to 6th, and the numerical aperture NA to 0.6. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 7, are shown in Figure 17 and Table 1. As is clear from Figure 17, focusing noise was observed in the 6th-order phase Fresnel lens with a constant prism height and a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 401.

[0065] <Example 7> A sixth-order phase Fresnel lens was designed in the same manner as in Comparative Example 7, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 17, as shown in Figures 1A, 1B, and 4. That is, the height level of the top of the prism was constant, and the height level of the bottom gradually increased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 82. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 7, are shown in Figure 17 and Table 1. As is clear from Figure 17, no focusing noise was observed even with a sixth-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 922, which is more than twice the focusing intensity of Comparative Example 7.

[0066] <Comparative Example 8> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 1, except that the diffraction order was set to 12th order (prism height set to 11.607 μm) and the radius was set to 300 μm. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 8, are shown in Figure 18 and Table 1. As is clear from Figure 18, focusing noise was observed in the 12th-order phase Fresnel lens with a constant prism height and a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 563.

[0067] <Example 8> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 8, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 18, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 96.5. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 8, are shown in Figure 18 and Table 1. As is clear from Figure 18, no focusing noise was observed even with a 12th-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 638.

[0068] <Comparative Example 9> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 8, except that the numerical aperture NA was set to 0.3. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 9, are shown in Figure 19 and Table 1. As is clear from Figure 19, focusing noise was observed in the 12th-order phase Fresnel lens with a constant prism height and a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 543.

[0069] <Example 9> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 9, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 19, as shown in Figures 1A, 1B, and 3. That is, the height level at the bottom of the prism was constant, and the height level at the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 92. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Comparative Example 9, are shown in Figure 19 and Table 1. As is clear from Figure 19, no focusing noise was observed even with a 12th-order phase Fresnel lens with a large numerical aperture (NA). Furthermore, as shown in Table 1, the focusing intensity was 923, which is nearly twice the focusing intensity of Comparative Example 9.

[0070] <Comparative Example 10> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 8, except that the numerical aperture NA was set to 0.6 and the radius was set to 200 μm. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with those of Example 10, are shown in Figure 20 and Table 1. As is clear from Figure 20, focusing noise was observed in the 12th-order phase Fresnel lens with a constant prism height and a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 152.

[0071] <Example 10> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 10, except that the prism height was gradually reduced toward the outer edge in the profile shown in Figure 20, as shown in Figures 1A, 1B, and 3. That is, the height level of the bottom of the prism was constant, and the height level of the top gradually decreased toward the outer edge. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 73. This Fresnel lens was subjected to the same evaluation as in Comparative Example 1. The results, along with the results of Comparative Example 10, are shown in Figure 20 and Table 1. As is clear from Figure 20, no focusing noise was observed even with a 12th-order phase Fresnel lens with a large numerical aperture NA. Furthermore, as shown in Table 1, the focusing intensity was 926, which is more than six times the focusing intensity of Comparative Example 10.

[0072]

[0073] <Evaluation> As is clear from Figures 6 to 20 and Table 1, according to the embodiments of the present invention, even when using second-order or higher diffraction and when the focusing intensity is high (large numerical aperture NA), focusing noise is suppressed. Furthermore, it can be seen that the focusing intensity is increased with the configuration corresponding to the comparative example. This effect is particularly pronounced when the numerical aperture NA is large.

[0074] <Comparative Example 11> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 10, except that the numerical aperture NA was set to 0.5 and the radius was set to 2500 μm. According to this design, a Fresnel lens was fabricated by subjecting a monomer composition (Nanoscribe, product name "IP-S") containing a radically polymerizable monomer and a photopolymerization initiator to two-photon polymerization using a high-resolution 3D printer (Nanoscribe, product name "QuantumX"). The refractive index of the obtained Fresnel lens (essentially IP-S after polymerization) was 1.515. The conditions for two-photon polymerization were as follows. Laser light used: femtosecond laser (wavelength 780 nm) Focusing: Focusing with a lens with medium lens power (numerical aperture NA = 0.8) Substrate: Fused silica wafer (thickness 0.5 mm) Laser output: 50 mW (maximum) Scanning speed: 200 mm / sec In-plane spacing of scan lines: 0.2 μm Thickness-direction spacing of scan lines: 1.0 μm Furthermore, printing was performed using a printing mode (2GL mode) that reduced the laser output according to the height design, so that the thickness-direction resolution was higher than 1.0 μm. The time required for printing was 6 hours. After printing, the fused silica wafer was immersed in propylene glycol monoethyl ether acetate (PGMEA) for 15 minutes, then immersed in isopropyl alcohol (IPA) for 1 minute, and the IPA was dried to remove the unpolymerized monomer composition from the surface of the phase Fresnel lens, finally obtaining a phase Fresnel lens. An overview of the obtained phase Fresnel lens is shown in Table 2.

[0075] <Example 11> A 12th-order phase Fresnel lens was designed in the same manner as in Comparative Example 11, except that the height of the prism was gradually reduced toward the outer circumference, as shown in Figures 1A, 1B, and 3. That is, the height level of the bottom of the prism was constant, and the height level of the top gradually decreased toward the outer circumference. In this Fresnel lens, when the height of the innermost prism was set to 100, the height of the outermost prism was 79. The time required to print this Fresnel lens was 6 hours. An overview of the obtained phase Fresnel lens is shown in Table 2.

[0076]

[0077] <Appearance in Plane View> The phase Fresnel lenses of Comparative Example 11 and Example 11 were photographed in a plan view using a confocal laser microscope manufactured by Keyence Corporation (device name: VK-X3000 (with auto XY stage)) under objective lens magnification of 20x. The results of the photographs for Comparative Example 11 are shown in Figure 21A, and the results of the photographs for Example 11 are shown in Figure 21B. As is clear from Figures 21A and 21B, no difference in appearance was observed between Comparative Example 11 and Example 11.

[0078] <Height Profile> The height profiles along the diameter of the phase Fresnel lenses of Comparative Example 11 and Example 11 were measured using the confocal measurement mode of the confocal laser microscope described above. The measurement conditions were as follows: Objective lens: 150x Height measurement resolution: 0.08 μm Measurement length: 5100 μm (Measured by combining the measurement results of 65 points using the Stitching measurement mode) The results for Comparative Example 11 are shown in Figure 22. The vertical axis of (A) is height (unit: μm), and the vertical axis of (B) is the relative height ratio to the height of the center. The Fresnel lens of Comparative Example 11 has a height of 12.7 μm near the center, and it can be seen that the height along the diameter is maintained within ±5% of the height near the center. The results for Example 11 are shown in Figure 23. The vertical axis of (A) is height (unit: μm), and the vertical axis of (B) is the relative height ratio to the height of the center. The Fresnel lens of Example 11 had a height of 12.7 μm near the center, similar to Comparative Example 11. The height of the prism gradually decreased towards the outer edge, with the height of the outer edge being 79% of the height of the center.

[0079] <Collection ratio of 12th-order diffracted light> For the phase Fresnel lenses of Comparative Example 11 and Example 11, the collection ratios of 11th, 12th, and 13th-order diffracted light were measured by the following method. Light source: Laser wavelength: 532 nm Objective lens: Olympus lens "1-U2C377" (numerical aperture NA = 0.6, focal length: 4.5 mm) Photodetector: THORLABS 2D CMOS sensor "CS165MU" Imaging lens: Sigma Koki lens "DLB-50-100PM" (focal length: 100 mm) Reference: Optical Fresnel zone plate flat lenses made entirely of colored photoresist through an i-line stepper. Following the reference, parallel light was introduced into the lens with the same diameter as the lens, the objective lens was moved every 20 μm relative to the focal direction, and an image magnified 22.22 times, calculated from the focal lengths of the objective lens and imaging lens, was captured by the photodetector. The intensity within 10 μm from the focal center at the focal length corresponding to each diffraction order was integrated and used as the collected intensity at that diffraction order. Furthermore, no focusing corresponding to diffraction of order 10 or lower, or order 14 or higher, was observed in either Comparative Example 11 or Example 11. The focusing ratios of the phase Fresnel lenses of Comparative Example 11 and Example 11 are shown in Figure 24. As is clear from Figure 24, the phase Fresnel lens of Example 11 has lower focusing ratios for both order 11 and order 13 diffraction light compared to the phase Fresnel lens of Comparative Example 11, and as a result, the focusing ratio for order 12 diffraction light is very high (82%, compared to 52% for Comparative Example 11). In other words, it can be seen that a focusing lens for high-order diffraction with suppressed focusing of unwanted diffraction orders can actually be fabricated using a two-photon polymerization 3D printer.

[0080] The diffractive optical elements according to embodiments of the present invention are expected to have applications in lenses for optical systems where miniaturization, lightness, and thinness are required, as well as in medical devices such as endoscopes.

[0081] 10 First lens section 20 Second lens section 21 Prism 21a Fresnel surface 21b Rise surface 21c Top 21d Bottom 100 Diffractive optical element (higher-order phase Fresnel lens) 100a First principal surface 100b Second principal surface 100c Optical axis (central axis)

Claims

1. A diffractive optical element comprising a first lens portion provided in the center and a second lens portion provided around the first lens portion, wherein the diffractive optical element is configured to utilize the second or higher order diffraction of light with a wavelength in the range of 380 nm to 780 nm, and has an numerical aperture NA of 0.2 or more, the second lens portion has a plurality of prisms arranged toward the outer periphery, each of the plurality of prisms has a Fresnel surface inclined toward the outer periphery, a rise surface extending in the direction of the optical axis of the diffractive optical element, and a vertex that is a connection point between the Fresnel surface and the rise surface, the second lens portion is configured such that the distance between the vertices of adjacent prisms gradually narrows toward the outer periphery, and the height of the plurality of prisms gradually decreases toward the outer periphery.

2. The diffractive optical element according to claim 1, wherein the radius is 0.1 mm to 100 mm.

3. The diffractive optical element according to claim 1, wherein the height of the prism adjacent to the first lens portion in the second lens portion is 1.0 μm to 20 μm.

4. The diffractive optical element according to claim 1, wherein each of the plurality of prisms has a refractive index of 1.40 to 2.

20.

5. The diffractive optical element according to claim 1, wherein the height reduction profile of the plurality of prisms is a linear reduction with a constant gradient, a curved reduction that is convex upward, or constant up to a predetermined range from the first lens portion, and outside the predetermined range, exhibits a linear reduction with a constant gradient or a curved reduction that is convex upward.

6. The diffractive optical element according to claim 1, wherein the height reduction profile A of the plurality of prisms satisfies the following formula (1): In equation (1), n ​​is the refractive index of the prism, f is the focal length of the diffractive optical element, and x is the distance from the center of the diffractive optical element.

7. The diffractive optical element according to claim 1, wherein the numerical aperture NA is 0.3 to 0.

9.

8. The diffractive optical element according to claim 1, wherein the Fresnel surface is inclined in a straight line.

9. The diffractive optical element according to claim 1, wherein the Fresnel surface is inclined in a curved manner.

10. The diffractive optical element according to claim 1, wherein the Fresnel surface is inclined in a stepped manner.

11. A method for manufacturing a diffractive optical element according to any one of claims 1 to 10, comprising forming the first lens portion and the second lens portion by stereolithography.

12. A method for manufacturing a diffractive optical element according to any one of claims 1 to 10, comprising forming the first lens portion and the second lens portion by a nanoimprint method.

13. The manufacturing method according to claim 12, wherein the replica mold used in the nanoimprint method is formed by stereolithography.

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