Optical element

Abstract

This optical element comprises a substrate, a fine structure part provided on a main surface of the substrate and including a plurality of protruding bodies, a top layer provided to the top portion of the fine structure part, and a bottom layer separated from the top layer and provided to the bottom portion of the fine structure part. The shape of each of the plurality of protruding bodies has a columnar shape, a frustum shape, or a combination thereof. The top layer and the bottom layer contain a material different from the plurality of protruding bodies as a main component. The thickness of the top layer and the thickness of the bottom layer are different.

Description

Optical elements

[0001] The present disclosure relates to optical elements.

[0002] In recent years, research and development of optical elements having fine structures such as metalenses has been progressing (see, for example, Patent Document 1 and Non-Patent Document 1).

[0003] US Patent Application Publication No. 2023 / 0194760

[0004] Huang, Luocheng, et al. "Long wavelength infrared imaging under ambient thermal radiation via an all-silicon metalens.", Optical Materials Express, Sep. 2021, Vol. 11, No. 9, pp. 2907-2914

[0005] Conventional optical elements have room for improvement in terms of transmittance.

[0006] The present disclosure provides optical elements with high transmittance.

[0007] An optical element according to one aspect of the present disclosure comprises a substrate, a microstructure portion provided on a main surface of the substrate, the microstructure portion including a plurality of convex bodies, a first layer provided on the top of the microstructure portion, and a second layer spaced apart from the first layer and provided on the bottom of the microstructure portion, wherein each of the plurality of convex bodies has a shape of a column or a frustum, or a combination thereof, the first layer and the second layer contain a material different from the plurality of convex bodies as a main component, and the thickness of the first layer is different from the thickness of the second layer.

[0008] According to the present disclosure, an optical element with high transmittance can be provided.

[0009] FIG. 1 is a perspective view of an optical element according to an embodiment. FIG. 2 is a perspective view of one unit cell included in the optical element according to the embodiment. FIG. 3 is a side view illustrating the function of the optical element according to the embodiment. FIG. 4 is a cross-sectional view illustrating a first example of one unit cell included in the optical element according to the embodiment. FIG. 5 is a graph illustrating the transmittance of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. FIG. 6 is a graph illustrating the increase in the transmittance of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. FIG. 7 is a cross-sectional view illustrating a second example of one unit cell included in the optical element according to the embodiment. FIG. 8 is a graph illustrating the transmittance of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. FIG. 9 is a graph illustrating the increase in the transmittance of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated.

[0010] (Summary of the present disclosure) An optical element according to a first aspect of the present disclosure comprises a substrate, a microstructure portion provided on a main surface of the substrate, the microstructure portion including a plurality of convex bodies, a first layer provided on the top of the microstructure portion, and a second layer spaced apart from the first layer and provided on the bottom of the microstructure portion, wherein each of the plurality of convex bodies has a shape of a column or a frustum, or a combination thereof, the first layer and the second layer contain a material different from the plurality of convex bodies as a main component, and the thickness of the first layer is different from the thickness of the second layer.

[0011] Thus, by providing the first and second layers, the transmittance of the optical element can be increased compared to when the first and second layers are not provided. Furthermore, by making the thicknesses of the first and second layers different, the transmittance of the optical element can be further increased.

[0012] An optical element according to a second aspect of the present disclosure is the optical element according to the first aspect, wherein the first layer is at least 20% thicker than the second layer.

[0013] This can increase the transmittance of the optical element. For example, when the optical element has a large convex body, the transmittance of the optical element can be effectively increased.

[0014] An optical element according to a third aspect of the present disclosure is the optical element according to the second aspect, wherein the plurality of convex bodies are periodically arranged two-dimensionally on the main surface, and the proportion of the area occupied by one convex body with respect to a unit cell corresponding to one period of the arrangement is 23% or more and 65% or less.

[0015] As a result, when the area ratio of the convex body is large, the transmittance of the optical element can be effectively increased by making the first layer provided at the top 20% or more thicker than the second layer provided at the bottom.

[0016] An optical element according to a fourth aspect of the present disclosure is the optical element according to the first aspect, wherein the first layer is at least 73% thinner than the second layer.

[0017] This can increase the transmittance of the optical element. For example, when the optical element has small convex portions, the transmittance of the optical element can be effectively increased.

[0018] An optical element according to a fifth aspect of the present disclosure is the optical element according to the fourth aspect, wherein the plurality of convex bodies are periodically arranged two-dimensionally on the main surface, and the proportion of the area occupied by one convex body with respect to a unit cell corresponding to one period of the arrangement is 10% or more and 13% or less.

[0019] As a result, when the area ratio of the convex body is small, the transmittance of the optical element can be effectively increased by making the first layer provided at the top 73% or more thinner than the second layer provided at the bottom.

[0020] An optical element according to a sixth aspect of the present disclosure is an optical element according to any one of the first to fifth aspects, wherein the refractive index of each of the first layer and the second layer is greater than 1 and less than 5 and differs from the refractive index of the convex body.

[0021] An optical element according to a seventh aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the first layer and the second layer are each a single-layer film.

[0022] This allows the first and second layers to be both single-layer films, which facilitates film formation, and improves controllability of the thickness of each of the first and second layers, making it easy to realize an optical element with a desired transmittance.

[0023] An optical element according to an eighth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the first layer and the second layer are each a multilayer film.

[0024] By forming the first and second layers as multilayer films, it is possible to precisely set the transmittance for each wavelength, and for example, it is possible to cut off unnecessary wavelengths.

[0025] An optical element according to a ninth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein each of the first and second layers contains, as a main component, one or a mixture of two or more selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, a fluoride compound, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic.

[0026] This allows the optical element to transmit incident light, which is far-infrared light, with high transmittance. Conventionally, glass cannot be used as a material that transmits light in the far-infrared region, and high-refractive-index materials have generally been used. However, when a high-refractive-index material is used, transmittance is, in principle, low. In contrast, the optical element according to the present disclosure can increase the transmittance of incident light by having a first layer and a second layer with different thicknesses, and is therefore useful as an optical element targeting the far-infrared region. For example, in image sensing using the far-infrared region, noise can be reduced and sensing performance can be improved by using an optical element with high transmittance.

[0027] An optical element according to a tenth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the substrate and the microstructure contain silicon as a main component, and the plane orientation of the main surface of the substrate is any one of (100), (110), and (111).

[0028] This allows the use of general semiconductor processing techniques that can be applied to silicon, thereby improving the accuracy of the shape and arrangement of the multiple microstructures, and thus enabling the realization of optical elements with desired transmittance with high precision.

[0029] An optical element according to an eleventh aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the thickness of each of the first layer and the second layer is 50 nm or more and 5 μm or less.

[0030] This allows the first and second layers to be easily formed, and the controllability of the thickness of each of the first and second layers is improved, making it easy to realize an optical element with a desired transmittance.

[0031] An optical element according to a twelfth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the wavelength of light incident on the optical element is λ, the refractive index of a medium surrounding the optical element is n, and the numerical aperture of the optical element is NA=n sin θ ｆ , the maximum half angle of view of the optical element is θ ｉ In this case, the arrangement period P of the plurality of convex bodies is

[0032] Meet the following.

[0033] This makes it possible to reduce aberration and suppress reduction in light collection efficiency even for light incident obliquely when the optical element has a lens function.

[0034] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0035] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0036] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0037] Furthermore, in this specification, terms indicating the relationship between elements, such as parallel or perpendicular, terms indicating the shape of elements, such as square or cylindrical, and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0038] Furthermore, in this specification, the terms "above" and "below" do not refer to the upward direction (vertically upward) and downward direction (vertically downward) in absolute spatial recognition, but are used as terms defined by the relative positional relationship between the substrate and the convex body. Specifically, the direction in which the convex body protrudes from the main surface of the substrate is considered to be "above," and the opposite direction is considered to be "below." In this case, of the multiple main surfaces of the substrate, the main surface on which the convex body is provided corresponds to the "top surface" of the substrate. Furthermore, the terms "above" and "below" apply not only to cases in which two components are arranged with a gap between them and another component is present between them, but also to cases in which two components are arranged closely together and the two components are in contact with each other.

[0039] In this specification, the term "thickness direction" refers to the thickness direction of the substrate of the optical element, i.e., the direction perpendicular to the main surface of the substrate. Furthermore, unless otherwise specified, the term "plan view" refers to the view from the direction perpendicular to the main surface of the substrate.

[0040] In this specification, the term "major component" refers to the component that is contained in the largest proportion, expressed in mole percent, in a material.

[0041] Furthermore, in this specification, ordinal numbers such as "first" and "second" do not refer to the number or order of components unless otherwise specified, but are used for the purpose of avoiding confusion and distinguishing between components of the same type.

[0042] Furthermore, in this disclosure, the term "light" is not limited to referring to visible light, but is also used to refer to invisible light. Visible light is light with a wavelength of 380 nm or more and 780 nm or less. Invisible light includes ultraviolet light, infrared light, far-infrared light, and radio waves. Ultraviolet light is light with a wavelength of 10 nm or more and 380 nm or less. Infrared light is light with a wavelength of 780 nm or more and 3000 nm (= 3 μm) or less. Far-infrared light is light (electromagnetic waves) with a wavelength of 3 μm or more and 1000 μm (= 1 mm) or less. Radio waves are electromagnetic waves with a wavelength of 1 mm or more.

[0043] (Embodiment) [Configuration] First, the configuration of an optical element according to an embodiment will be described with reference to Fig. 1, Fig. 2, and Fig. 3. Fig. 1 is a diagram showing an optical element 100 according to the present embodiment. Fig. 2 is a perspective view showing an example of the structure of one unit cell 101 included in the optical element 100 according to the present embodiment. Fig. 3 is a diagram showing the function of the optical element 100 according to the present embodiment.

[0044] As shown in Fig. 1, the optical element 100 includes a substrate 110 and a microstructure portion including a plurality of convex bodies 120. As shown in Fig. 3, the optical element 100 also includes a top layer 130, which is an example of a first layer provided on the top of the microstructure portion, and a bottom layer 140, which is an example of a second layer provided on the bottom of the microstructure portion and spaced apart from the top layer 130. For convenience of illustration, the top layer 130 and the bottom layer 140 are omitted from Fig. 1.

[0045] The top of the microstructure is the upper surface of each of the plurality of convex bodies 120. The bottom of the microstructure is a portion that corresponds to a so-called valley bottom, and specifically, is a portion of the upper surface of the substrate 110 that is located between the plurality of convex bodies 120 in a plan view. By providing the top layer 130 and the bottom layer 140, it is possible to increase the transmittance compared to when the top layer 130 and the bottom layer 140 are not provided.

[0046] A microstructure including a plurality of convex bodies 120 is provided on the main surface of the substrate 110. Specifically, the plurality of convex bodies 120 are provided so as to stand on the main surface of the substrate 110. Each of the plurality of convex bodies 120 has a columnar shape. A columnar convex body is also called a columnar body or a "pillar." In the optical element 100, a unit element including one convex body 120 is called a "unit cell."

[0047] The optical element 100 is an assembly of unit cells 101 shown in FIG. 2 . The optical element 100 is configured by arranging a plurality of unit cells 101 two-dimensionally. Specifically, the unit cells 101 are unit elements obtained by dividing the main surface of the substrate 110 into a plurality of convex bodies 120, one at a time, in a planar view of the substrate 110, such that the convex body 120 is located at the center of each unit cell 101. For example, the outer shape of the unit cell 101 corresponds to the area enclosed by the perpendicular bisector of the line segment connecting the centers of two adjacent convex bodies 120. When the plurality of convex bodies 120 are arranged in a matrix (i.e., a square lattice) at equal intervals, each of the plurality of unit cells 101 has an outer shape that is a square area of ​​the same size. The length of one side of the unit cell 101 in a planar view corresponds to the distance between the centers of adjacent convex bodies 120. Depending on the arrangement of the convex bodies 120, the shape of the unit cell 101 in plan view may not be a square, but may be another polygon such as a regular hexagon or a regular octagon.

[0048] As shown in FIG. 2 , the unit cell 101 includes a portion of the substrate 110, one convex body 120, a top layer 130, and a portion of the bottom layer 140. The substrate 110 and the bottom layer 140 are provided across the plurality of unit cells 101. The unit cell 101 generates a phase shift in incident light according to the structure of the convex body 120. In this embodiment, the plurality of unit cells 101 have the same structure. The unit cell 101 corresponds to one period of the periodic arrangement of the plurality of convex bodies 120.

[0049] In FIG. 3 , arrows indicate examples of light rays passing through the optical element 100. In this embodiment, the optical element 100 has the function of focusing incident light, similar to a conventional convex lens. Specifically, in the example shown in FIG. 3 , incident light incident on the lower surface of the substrate 110 of the optical element 100 is focused by undergoing different phase changes depending on the incident position by the multiple convex bodies 120. To achieve desired focusing characteristics, the shape, size, orientation, etc. of each convex body 120 are appropriately determined. The structure of each convex body 120 can be appropriately determined based on, for example, data indicating the phase profile to be achieved and the results of electromagnetic field simulation. In this way, an optical element having the function of focusing or diverging light using a microstructure can be called a "metalens." A metalens has multiple convex bodies 120 on its surface that are smaller than the wavelength of the incident light, and achieves lens function through the phase shift caused by the convex bodies 120.

[0050] Each of the multiple convex bodies 120 has a size of a subwavelength shorter than the wavelength of light incident on the optical element 100, and can be arranged at subwavelength intervals or periods. The size of the convex bodies 120 refers to at least one of the maximum width and height of the convex bodies 120 in a planar view. The interval between the convex bodies 120 is the distance between the centers of two adjacent convex bodies 120 when the main surface of the substrate 110 is viewed in a planar view. The interval between the convex bodies 120 is also called the pitch. The multiple convex bodies 120 may be arranged periodically or non-periodically.

[0051] The maximum width D of the convex body 120 in plan view (for example, the diameter in the case of a circle) is, for example, 0.1 μm or more and 10 μm or less. The pitch P of the convex body 120 is, for example, 1 μm or more and 10 μm or less. The height H of the convex body 120 is, for example, 1 μm or more and 50 μm or less. The aspect ratio H / D of the convex body 120 is, for example, 1 or more and 500 or less. Note that these numerical values ​​are merely examples and can be adjusted as appropriate depending on the application or purpose of the optical element 100.

[0052] The optical element 100 can be designed to have desired optical properties for light in a predetermined target wavelength band. The target wavelength band is, for example, a wavelength band defined by specifications. If the lower limit of the target wavelength band is, for example, 1 μm, the size and spacing of the convex bodies 120 can be set to a value shorter than 1 μm. Such microstructures, such as the convex bodies 120 with a nanoscale size smaller than 1 μm, are sometimes called "submicron structures" or "nanostructures." If the target wavelength band is in the infrared region, the size and spacing of the convex bodies 120 may be greater than 1 μm.

[0053] Here, the "target wavelength band" refers to the wavelength band of light in which the optical element 100 is intended to be used, and can be determined based on the specifications of the optical element 100 or the specifications of a device in which the optical element 100 is mounted. The target wavelength band may include, for example, at least a portion of the visible light wavelength band (approximately 400 nm to approximately 700 nm). Alternatively, the target wavelength band may include at least a portion of the ultraviolet wavelength band (approximately 10 nm to approximately 400 nm). Alternatively, the target wavelength band may include at least a portion of the infrared wavelength band (approximately 700 nm to approximately 1 mm). Alternatively, the target wavelength band may include at least a portion of the radio wave wavelength band (wavelengths of approximately 1 mm to approximately 1 m). In this disclosure, the term "wavelength" refers to the wavelength in free space unless otherwise specified.

[0054] In one example, the target wavelength band may include at least a portion of the infrared wavelength band from 2.5 μm to 25 μm. The wavelength band from 2.5 μm to 25 μm is used in infrared sensing devices, such as LiDAR (Light Detection and Ranging) sensors or infrared cameras. The target wavelength band may also include the far-infrared wavelength band from 5 μm to 14 μm or the far-infrared wavelength band from 8 μm to 12 μm. The far-infrared wavelength band is used for thermal imaging, etc. The optical element 100 may be used in combination with an image sensor in, for example, an imaging device. The optical element 100 may also be used in telescopes, microscopes, or scanning optical devices. However, the optical element 100 is not limited to these applications.

[0055] The number of convex bodies 120 provided on the main surface of the optical element 100 is determined to an appropriate number depending on the lens characteristics to be achieved. The number of convex bodies 120 is, for example, in the range of 100 to 10,000, and may be less than 100 or more than 10,000 in some cases.

[0056] Here, an example of a design method for an optical element 100 having a lens function will be briefly described. In the optical element 100, a plurality of convex bodies 120 are arranged at intervals shorter than the wavelength of the incident light. A desired phase profile is realized by adjusting the orientation or size of each convex body 120. To increase the focusing efficiency, the pitch P of the convex bodies 120, i.e., the size of the unit cell 101, can be designed to satisfy the Nyquist criterion for sampling (P<λ / 2NA). Here, λ is the design wavelength of the optical element 100, and NA is the numerical aperture of the optical element 100.

[0057] Furthermore, the optical element 100 may be designed to obtain high focusing efficiency even for light that is obliquely incident on the optical element 100. For example, when the refractive index n ｓ The optical element 100 is irradiated with a light beam from a medium (for example, air) having a refractive index n at an incident angle θ ｉ and wave number k ｉ Let k be the wave number corresponding to the shortest wavelength λ in the target wavelength band. ｔ(=2π·n / λ), and the numerical aperture of the optical element 100 is NA=n sin θ ｆ The incident angle θ ｉ is the maximum half angle of view of the optical element 100 (i.e., the maximum angle of incidence of light that can be used in a device including the optical element 100). The maximum angle of incidence of light here may be, for example, the maximum angle of view of a device such as an imaging device, telescope, or microscope that includes the optical element 100, or the maximum scanning angle of a scanning optical device that includes the optical element 100. The maximum half angle of view of the optical element 100 is expressed as θ ｉ The plurality of convex bodies 120 has a maximum of K １ The optical element is formed so as to give the incident light a wave number component (i.e., a spatial frequency component) of .gtoreq.1.

[0058] The maximum spatial frequency component K in the unit cell 101 １ The minimum sampling interval P required to provide the above is determined by the sampling theorem so as to satisfy the following inequality (1):

[0059] Therefore, the interval P between the multiple convex bodies 120 is determined so as to satisfy the following formula (2).

[0060] By arranging the multiple convex bodies 120 so as to satisfy formula (2), the sampling theorem can be satisfied even for obliquely incident light, making it easier to reproduce an ideal phase. This makes it possible to reduce aberrations and suppress reductions in light collection efficiency. Note that it is not essential for the optical element 100 to have a lens function, so it is not necessary for the optical element 100 to satisfy formula (2) above.

[0061] In this embodiment, the substrate 110 and the plurality of convex bodies 120 are integrally formed using the same material. The substrate 110 and each convex body 120 are made of a material containing silicon as a main component with a (100) crystal plane orientation. The silicon crystal plane orientation may be (110) or (111). The substrate 110 and the convex bodies 120 may be made of a material other than silicon.

[0062] The thickness of the substrate 110 is, for example, 500 μm. The shape of the substrate 110 is square, as shown in FIG. 1 , and its size is 8 mm × 8 mm. A plurality of convex bodies 120 are arranged within a circular region with a diameter of 8 mm on the surface of the substrate 110. Note that the thickness and shape of the substrate 110 and the shape and size of the arrangement region of the convex bodies 120 are not particularly limited.

[0063] The optical element 100 according to this embodiment is provided with a top layer 130 and a bottom layer 140 having different thicknesses. The top layer 130 is provided in contact with the top of the microstructure, specifically, with the upper surfaces of each of the plurality of convex bodies 120. The bottom layer 140 is provided in contact with the bottom of the microstructure, specifically, with the portions of the main surface of the substrate 110 that are located between the plurality of convex bodies 120 in a planar view. By appropriately adjusting the thickness, material, number of layers, etc. of each of the top layer 130 and the bottom layer 140, it is possible to increase the transmittance of incident light. The transmittance is expressed as the ratio of the intensity of light emitted from the optical element 100 to the intensity of light incident on the optical element 100.

[0064] The thickness of each of the top layer 130 and the bottom layer 140 is 50 nm or more and 5 μm or less. The thickness of at least one of the top layer 130 and the bottom layer 140 may be 100 nm or more, 300 nm or more, or 500 nm or more. The thickness of at least one of the top layer 130 and the bottom layer 140 may be 3 μm or less, 2 μm or less, 1.5 μm or less, or 1 μm or less. The top layer 130 and the bottom layer 140 are each a single-layer film. The refractive index of each of the top layer 130 and the bottom layer 140 differs from the refractive index of the convex body 120 by, for example, a value greater than 1 and less than 5.

[0065] The top layer 130 and the bottom layer 140 contain, as a main component, a material different from that of the plurality of convex bodies 120. Each of the top layer 130 and the bottom layer 140 contains, as a main component, one or a mixture of two or more selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, a fluoride compound, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic. The plastic is, for example, polyethylene.

[0066] Below, a case where the top layer 130 is thicker than the bottom layer 140 and a case where the top layer 130 is thinner than the bottom layer 140 will be described based on specific simulation results.

[0067] [First Example (Top Layer>Bottom Layer)] FIG. 4 is a cross-sectional view showing a first example of one unit cell 101 included in the optical element 100 according to this embodiment.

[0068] In the example shown in FIG. 4, the top layer 130 is thicker than the bottom layer 140. That is, the thickness of the top layer 130 is T Ｔ and the thickness of the bottom layer 140 is T Ｂ In this case, T Ｔ >T Ｂ is.

[0069] 5 is a graph showing the transmittance of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. In FIG. 5, the horizontal axis represents the thickness T of the top layer 130. Ｔ The vertical axis represents the thickness T of the bottom layer 140. Ｂ represents.

[0070] In the transmittance simulation, the shape, height H, and pitch P of the convex bodies 120 are fixed. Also, the composition and refractive index of each of the convex bodies 120, the top layer 130, and the bottom layer 140 are fixed. The thickness T of the top layer 130 Ｔ , the thickness T of the bottom layer 140 Ｂ and the width D of the convex body 120 are modulated.

[0071] Specific simulation conditions are as follows: the target wavelength is 10 μm; the convex bodies 120 are made of silicon; the top layer 130 and the bottom layer 140 are single-layer films of zinc sulfide (ZnS); and the convex bodies 120 are cylindrical in shape; the height H of each convex body 120 is 30 μm; and the pitch P of the convex bodies 120, i.e., the size of the unit cell 101, is 2.8 μm.

[0072] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ are modulated in increments of 0.1 μm within a range of 0 μm or more and 1.5 μm or less. The width (diameter) D of each convex body 120 is modulated in increments of 0.07 μm within a range of 1.51 μm or more and 2.56 μm or less. The width D of the convex body 120 can be converted into the area occupied by one convex body 120 relative to the unit cell 101 (i.e., the area ratio). When the width D is 1.51 μm, the area ratio is approximately 23%, and when the width D is 2.56 μm, the area ratio is approximately 65%.

[0073] The transmittance shown in FIG. 5 is T Ｔ and T Ｂ Under the condition above, the width D of each convex body 120 is varied in increments of 0.07 μm within the range of 1.51 μm to 2.56 μm, and the transmittance obtained is the average value according to the value of the width D.

[0074] In FIG. Ｔ =T Ｂ The range to the right of the line (positive direction of the horizontal axis) is T Ｔ >T Ｂ As can be seen from FIG. Ｔ >T Ｂ It can be seen that the transmittance tends to be high when the thickness T Ｂ When the thickness T of the top layer 130 is fixed, Ｔ It can be seen that the larger the value, the higher the transmittance.

[0075] 6 is a graph showing the increase in transmittance of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. Ｔ =T Ｂ Compared to the case where T Ｔ >T Ｂ Specifically, the increase in transmittance shown in FIG. 6 is calculated by comparing the transmittance and T Ｔ =T Ｂ The transmittance is the difference between the maximum value of the transmittance on the line where . In the example shown in FIG. 6, the increase in transmittance was approximately 3% pt at most. Note that "% pt" is an abbreviation for percentage point. A percentage point is a unit that represents the difference between two values ​​expressed in percentage (unit: %), and is sometimes simply called a point (abbreviated as pt).

[0076] Although only the case where the design wavelength is 10 μm is illustrated here, similar simulations were performed for incident light having a certain intensity or higher over the entire range of design wavelengths from 5 μm to 14 μm and from 8 μm to 12 μm, and the results confirmed an improvement in transmittance similar to that for 10 μm. Specifically, the increase in transmittance in the range of 5 μm to 14 μm was approximately 0.22% pt, and the increase in transmittance in the range of 8 μm to 12 μm was approximately 2.2% pt.

[0077] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ Difference T Ｔ -T Ｂ is, for example, the thickness T Ｔ This allows the transmittance to be increased even more. Alternatively, the difference T Ｔ -T Ｂ is, for example, the thickness T Ｔ It may be 10% or more, 20% or more, or 30% or more of the difference T Ｔ -T Ｂ is, for example, the thickness TＴ It may be 80% or less, 70% or less, or 60% or less.

[0078] Second Example (Top Layer<Bottom Layer) FIG. 7 is a cross-sectional view showing a second example of one unit cell 101 included in the optical element 100 according to the present embodiment.

[0079] In the example shown in Figure 7, the top layer 130 is thinner than the bottom layer 140. That is, the thickness of the top layer 130 is T Ｔ and the thickness of the bottom layer 140 is T Ｂ In this case, T Ｔ <T Ｂ is.

[0080] 8 is a graph showing the transmittance of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. In FIG. 8, the horizontal axis represents the thickness T of the top layer 130. Ｔ The vertical axis represents the thickness T of the bottom layer 140. Ｂ represents.

[0081] In the transmittance simulation, the shape, height H, and pitch P of the convex bodies 120 are fixed. Also, the composition and refractive index of each of the convex bodies 120, the top layer 130, and the bottom layer 140 are fixed. These fixed values ​​are the same as those in the first example. The thickness T of the top layer 130 Ｔ , the thickness T of the bottom layer 140 Ｂ and the width D of the convex body 120 are modulated.

[0082] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ are modulated in increments of 0.1 μm within a range of 0 μm or more and 1.5 μm or less. The width (diameter) D of each convex body 120 is modulated in increments of 0.07 μm within a range of 1.01 μm or more and 1.15 μm or less. The width D of the convex body 120 can be converted into the area occupied by one convex body 120 relative to the unit cell 101 (i.e., the area ratio). When the width D is 1.01 μm, the area ratio is approximately 10%, and when the width D is 1.15 μm, the area ratio is approximately 13%.

[0083] The transmittance shown in FIG. 8 is T Ｔ and T Ｂ Under the condition above, the width D of each convex body 120 is varied in increments of 0.07 μm within the range of 1.01 μm to 1.15 μm, and the transmittance obtained is the average value according to the value of the width D.

[0084] In FIG. Ｔ =T Ｂ The range to the left of the line (negative direction of the horizontal axis) is T Ｔ <T Ｂ As can be seen from FIG. Ｔ <T Ｂ It can be seen that the transmittance tends to be high when the thickness T Ｂ When the thickness T of the top layer 130 is fixed, Ｔ It can be seen that the smaller the value, the higher the transmittance.

[0085] 9 is a graph showing the increase in transmittance of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. Ｔ =T Ｂ Compared to the case where Ｔ <T Ｂ Specifically, the increase in transmittance shown in FIG. 9 is calculated by comparing the transmittance and T Ｔ =T Ｂ In the example shown in Fig. 9, the increase in transmittance was approximately 1.3% pt at most.

[0086] Although only the case where the design wavelength is 10 μm is illustrated here, similar simulations were performed for incident light having a certain intensity or higher over the entire range of design wavelengths from 5 μm to 14 μm and from 8 μm to 12 μm, and the results confirmed an improvement in transmittance similar to that for 10 μm. Specifically, the increase in transmittance in the range of 5 μm to 14 μm was approximately 1.4% pt, and the increase in transmittance in the range of 8 μm to 12 μm was approximately 0.8% pt.

[0087] The thickness T of the bottom layer 140 Ｂ and the thickness T of the top layer 130 Ｔ Difference T Ｂ -T Ｔ is, for example, the thickness T Ｂ This allows the transmittance to be increased even more. Alternatively, the difference T Ｂ -T Ｔ is, for example, the thickness T Ｂ It may be 10% or more, 20% or more, or 30% or more of the difference T Ｂ -T Ｔ is, for example, the thickness T Ｂ It may be 80% or less, 70% or less, or 60% or less.

[0088] [Other Examples] Table 1 shows the thickness T of the top layer 130. Ｔ、 The thickness T of the bottom layer 140 Ｂ , and the height H of the convex body 120 is set to a specific value. Ｔ >T Ｂ Examples 7 and 8 show the case of T Ｔ <T Ｂ The left arrow in the table indicates that the content is the same as the column on the left.

[0089] In Example 3, the thickness T of the top layer 130 Ｔ is 1.45 μm, and the thickness of the bottom layer 140 T Ｂ is 1.20 μm. In this case, T Ｔ / T Ｂ= 1.20, and T Ｔ is T Ｂ The concave bodies 120 are 20% thicker than the unit cell 101. The pitch P of the concave bodies 120 is 2.80 μm, and the height H of the convex bodies 120 is 7.0 μm. The width (diameter) D of the convex bodies 120 is modulated in the range of 1.52 μm to 2.54 μm. When the width D of the convex bodies 120 is 1.52 μm, the area ratio of one convex body 120 to the unit cell 101 is approximately 23%, and when the width D is 2.54 μm, the area ratio is approximately 65%.

[0090] The substrate 110 and the wall surface of the protrusion 120 are made of silicon, and the top layer 130 and the bottom layer 140 are made of zinc sulfide (ZnS).

[0091] Under the above conditions, when the width D of each convex body 120 is modulated in the range of 1.52 μm to 2.54 μm, the maximum value of the transmittance of the optical element 100 for incident light with a wavelength of 10 μm is 94.1%. Ｔ =T Ｂ When the other conditions are the same, the maximum transmittance is 93.0%. When no film is formed, i.e., when the top layer 130 and the bottom layer 140 are not provided, and the other conditions are the same, the maximum transmittance is 84.5%.

[0092] The above has been described for Example 3, but the same applies to Examples 4 to 8. However, in Example 6, the top layer 130 and the bottom layer 140 are made of silicon dioxide (SiO ２ ) Also, T Ｔ＝ 0.30 μm, T Ｂ The transmittance values ​​when .rho.=0.25 and when no film is formed are values ​​when the wavelength of incident light is 1.55 .mu.m.

[0093]

[0094] [Effects, etc.] As described above, in the optical element 100 according to the present embodiment, by making the thicknesses of the top layer 130 and the bottom layer 140 different, it is possible to increase the transmittance compared to when the thicknesses of the top layer 130 and the bottom layer 140 are equal. When the proportion of the area of ​​the convex body 120 in the unit cell 101 in plan view is greater than a predetermined value, a configuration in which the top layer 130 is thicker than the bottom layer 140 is advantageous in terms of improving the transmittance. Furthermore, when the proportion of the area of ​​the convex body 120 in the unit cell 101 in plan view is smaller than a predetermined value, a configuration in which the top layer 130 is thinner than the bottom layer 140 is advantageous in terms of improving the transmittance. The predetermined value here is, for example, 13% or more and 23% or less.

[0095] Furthermore, because the top layer 130 and the bottom layer 140 have different thicknesses, the conditions for controlling the thickness of each layer in the film formation process are relaxed. That is, the thicknesses of the top layer 130 and the bottom layer 140 can be made different more easily than when the top layer 130 and the bottom layer 140 have the same thickness. In particular, when the aspect ratio of the convex body 120 is large, it is difficult to make the top layer 130 and the bottom layer 140 have the same thickness. For example, when the aspect ratio of the convex body 120 is 1 or greater, an optical element 100 including a top layer 130 and a bottom layer 140 with different thicknesses is useful from the perspective of ease of manufacture.

[0096] Although the first and second examples use a combination of silicon and zinc sulfide, the present invention is not limited to this. By performing a similar simulation when other material combinations are used, the thicknesses of the top layer 130 and the bottom layer 140 and the area ratio of the convex body 120 can be set within appropriate ranges.

[0097] Furthermore, the shape of each of the multiple convex bodies 120 is not limited to a cylindrical shape. Each of the multiple convex bodies 120 may have a shape other than a cylindrical shape, a truncated cone shape, or a combination thereof. The cylindrical shape is a cylindrical or elliptical cylinder, or a rectangular prism shape such as a square prism, a hexagonal prism, or an octagonal prism. The frustum shape is a circular or elliptical frustum shape, or a truncated pyramid shape such as a square frustum, a hexagonal frustum, or an octagonal frustum. In the case of a frustum shape, the area of ​​the bottom surface is larger than the area of ​​the top surface, but the reverse may also be true. Furthermore, a combination of a cylindrical shape and a frustum shape is, for example, a configuration in which a cylindrical body and a frustum-shaped body are stacked in the height direction. In other words, the convex body 120 may have a shape in which the top surface of one of a cylindrical body and a frustum-shaped body is in contact with the bottom surface of the other. The number of combined cylindrical bodies and frustum-shaped bodies may be three or more.

[0098] [Manufacturing Method] The optical element 100 can be manufactured using a general semiconductor manufacturing technique such as lithography, etc. For example, the optical element 100 can be manufactured by the following method.

[0099] First, a silicon substrate having a (100) crystal plane orientation on its surface is prepared as the substrate 110. Note that the "surface" refers to one of the main surfaces of the silicon substrate. When a microstructure including multiple convex bodies 120 is integrally formed with the substrate 110, the "surface" of the silicon substrate becomes the tip surface of the top of the microstructure.

[0100] Next, a positive resist is applied to the surface of the silicon substrate by a method such as spin coating. Subsequently, light or an electron beam is irradiated to desired locations, and then a development process is performed. As a result, the resist is removed from the locations irradiated with light or an electron beam. This silicon substrate is then subjected to SF ６ The etching is performed using a reactive ion etching technique or the like using an etching gas such as a gas. This etches the surface of the silicon substrate where the resist has been removed. Then, a wet process using a resist remover or the like is performed. ２Residual resist on the surface of the silicon substrate is removed by a dry process such as ashing, thereby forming a microstructure including a plurality of convex bodies 120 on the main surface of the substrate 110. The microstructure corresponds to the upper layer portion of the prepared silicon substrate.

[0101] Furthermore, a film is formed using a material different from that of the protrusions 120 so as to cover the microstructure including the plurality of protrusions 120. For example, zinc sulfide (ZnS) is formed on each of the top and bottom of the microstructure. The film formation is performed by EB (Electron Beam) evaporation, sputtering, CVD (Chemical Vapor Deposition), or the like. The thicknesses of the top layer 130 and the bottom layer 140 can be adjusted by adjusting the film formation conditions. For example, the bottom layer 140 can be made thicker than the top layer 130 by applying a bias voltage perpendicular to the substrate 110 during film formation.

[0102] Through these steps, an optical element 100 can be fabricated, which includes a substrate 110, a plurality of convex bodies 120, and a top layer 130 and a bottom layer 140 having different thicknesses.

[0103] When the substrate 110 and the convex body 120 are formed using different materials, a film containing a material different from that of the substrate 110 may be formed on the main surface of the substrate 110 and patterned to form the convex body 120. The film formation may be performed by an epitaxial growth method, a vapor deposition method, a sputtering method, a CVD method, a coating method, or the like.

[0104] Other Embodiments While optical elements according to one or more aspects have been described above based on embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by those skilled in the art to the present embodiments and embodiments constructed by combining components of different embodiments are also included within the scope of the present disclosure.

[0105] For example, in the above embodiment, an example has been described in which the multiple unit cells 101 included in the optical element 100 have the same structure, but the multiple unit cells 101 may include unit cells having a structure different from the other unit cells 101. For example, the multiple unit cells 101 may include unit cells that differ from the other unit cells 101 in at least one of the size, shape, and material of the convex body 120. Alternatively, the multiple unit cells 101 may include unit cells that differ from the other unit cells 101 in at least one of the thickness, material, and number of layers of at least one of the top layer 130 and the bottom layer 140.

[0106] Furthermore, in the above embodiment, an example was shown in which the bottom of the microstructure is part of the main surface of the substrate 110, but this is not limiting. For example, the microstructure may include a plurality of convex bodies 120 and a base portion that connects and supports the plurality of convex bodies 120 to each other. The base portion is laminated on the main surface of the substrate 110. The base portion is formed using the same material as the plurality of convex bodies 120. In other words, the refractive index of the base portion is the same as the refractive index of each of the plurality of convex bodies 120. In this case, the upper surface of the base portion (the main surface opposite to the substrate 110) becomes the bottom of the microstructure.

[0107] Furthermore, for example, the top layer 130 and the bottom layer 140 may each be a multilayer film. In this case, the top layer 130 and the bottom layer 140 have, for example, the same film configuration. The thickness of each of the multiple films included in the top layer 130 may be different from the thickness of the corresponding film included in the bottom layer 140. Alternatively, the thickness of at least one of the multiple films included in the top layer 130 may be different from the thickness of the corresponding film included in the bottom layer 140, and the thickness of at least one other of the multiple films included in the top layer 130 may be the same as the thickness of the corresponding film included in the bottom layer 140.

[0108] Furthermore, the top layer 130 and the bottom layer 140 may contain different materials as their main components.

[0109] Furthermore, a layer containing the same material as the top layer 130 or the bottom layer 140 as a main component may be provided on the side surface of the convex body 120 .

[0110] Furthermore, various modifications, substitutions, additions, omissions, etc. can be made to the above-described embodiments within the scope of the claims or their equivalents.

[0111] The present disclosure can be used in devices having optical elements such as lenses, such as cameras, LiDAR sensors, projectors, AR (Augmented Reality) displays, telescopes, microscopes, and scanning optical devices.

[0112] 100 Optical element 101 Unit cell 110 Substrate 120 Convex body 130 Top layer 140 Bottom layer

Claims

1. An optical element, comprising: a substrate; a microstructure portion provided on a main surface of the substrate and including a plurality of convex bodies; a first layer provided on top of the microstructure portion; and a second layer provided at a distance from the first layer and at the bottom of the microstructure portion, wherein each of the plurality of convex bodies has a columnar shape, a frustum shape, or a combination thereof; the first layer and the second layer mainly contain materials different from those of the plurality of convex bodies; and the thickness of the first layer is different from the thickness of the second layer.

2. The optical element according to claim 1, wherein the first layer is 20% or more thicker than the second layer.

3. The optical element according to claim 2, wherein the plurality of convex bodies are two-dimensionally and periodically arranged on the main surface, and the ratio of the area occupied by one of the convex bodies to a unit cell corresponding to one period of the arrangement is 23% or more and 65% or less.

4. The optical element according to claim 1, wherein the first layer is 73% or more thinner than the second layer.

5. The optical element according to claim 4, wherein the plurality of convex bodies are two-dimensionally and periodically arranged on the main surface, and the ratio of the area occupied by one of the convex bodies to a unit cell corresponding to one period of the arrangement is 10% or more and 13% or less.

6. The optical element according to any one of claims 1 to 5, wherein the refractive index of each of the first layer and the second layer is greater than 1 and different from the refractive index of the convex bodies within a range of less than 5.

7. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer is a single-layer film.

8. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer is a multilayer film.

9. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer mainly contains one selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, fluoride compound, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic, or a mixture of two or more thereof.

10. The optical element according to any one of claims 1 to 5, wherein the substrate and the microstructure portion mainly contain silicon, and the plane orientation of the main surface of the substrate is any one of (100), (110), and (111).

11. The thickness of each of the first layer and the second layer is 50 nm or more and 5 μm or less. The optical element according to any one of claims 1 to 5.

12. Let the wavelength of the light incident on the optical element be λ, the refractive index of the medium surrounding the optical element be n, and the numerical aperture NA of the optical element be NA = nsinθ ｆ , where θ is the maximum half-angle of the optical element ｉ . When this is the case, the array period P of the plurality of convex bodies satisfies . The optical element according to any one of claims 1 to 5

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