Optical element and optical device

Abstract

An optical element (1) is provided with: a substrate (2); and a fine structure (3) that is formed on the surface (21) of the substrate (2) and changes the phase of light. The microstructure (3) has a plurality of structural bodies (30) which have a height (H) in a direction perpendicular to the surface (21), have a width (D) in a direction parallel to the surface (21), and are arranged at intervals (P) in the direction parallel to the surface (21). At least one of the height (H), the width (D), and the interval (P) of the structure (30) varies depending on the region (B) in the microstructure (3).

Description

Technical Field

[0001] The present disclosure relates to an optical element having functions such as convergence and diffraction, and an optical device having the optical element. Background Art

[0002] In recent years, a metamaterial in which structures (also referred to as superatoms) smaller than the wavelength of electromagnetic waves such as light are three-dimensionally arranged has been studied. The metamaterial can achieve physical properties that do not exist in nature and has attracted attention.

[0003] In addition, in recent years, a metasurface in which structures are two-dimensionally arranged has also been studied. Compared with the metamaterial having a three-dimensional structure, the metasurface is easy to manufacture. For example, in the case of using light having a relatively long wavelength such as terahertz light, the structure can be made relatively large, and the manufacture of the metasurface becomes easy.

[0004] In addition, as a lens using a metasurface, a metalens in which structures are two-dimensionally arranged on a substrate is known. Different from a lens having a curved surface, the metalens can be configured to be flat and thin. In addition, it can be manufactured by a method suitable for mass production such as semiconductor processing or nanoimprinting.

[0005] The substrate of the metalens is made of, for example, SiO2, Si, TiO2, Ge, or GaN, and a plurality of substrates may be stacked. When the substrate is made of a material having a high refractive index, light is reflected due to the refractive index difference at the interface between the structure and the substrate or at the interface between the substrate and air.

[0006] For example, in Patent Document 1, regarding a lens in which columns as cylindrical structures are two-dimensionally arranged on a substrate, a lens is disclosed in which the height and diameter of the columns are changed for each region, and the upper ends (i.e., the ends farthest from the substrate) of the columns are arranged on the same plane. By changing the height and width of the columns, the phase of light can be controlled.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-99399 (see Figure 4 ) Summary of the Invention

[0010] Problems to be Solved by the Invention

[0011] The diameter of the pillars is determined in such a way that it is below the wavelength of light and can suppress the reflection of light at the interface. For example, for terahertz waves with a wavelength of 125 μm, the diameter of the pillars is controlled in the range of 13 μm to 25 μm. On the other hand, for near-infrared light with a wavelength of 700 to 2500 nm, the diameter of the pillars needs to be controlled in the range of several tens of nm to several hundreds of nm. Therefore, an exposure apparatus with high resolution is required, etc., and the manufacturing cost becomes high.

[0012] The present disclosure has been made in view of the above problems, and an object thereof is to reduce the manufacturing cost of optical elements.

[0013] Means for Solving the Problems

[0014] The optical element of the present disclosure includes: a substrate; and a fine structure formed on the surface of the substrate that causes a phase change of light. The fine structure has a plurality of structures, and the plurality of structures have a height in a direction perpendicular to the surface, a width in a direction parallel to the surface, and are arranged at intervals in a direction parallel to the surface. At least one of the height, width, and interval of the plurality of structures, including the interval, varies according to the region within the fine structure.

[0015] Advantages of the Invention

[0016] In the present disclosure, at least one of the height, width, and interval of the structures in the fine structure varies according to the region. Therefore, for example, even when using an exposure apparatus with relatively low resolution, the phase resolution can be improved. Therefore, the manufacturing cost of the optical element can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a perspective view showing the optical element of Embodiment 1.

[0018] Figure 2 is a side view showing the optical element of Embodiment 1.

[0019] Figure 3 is a view showing the fine structure of the optical element of Embodiment 1.

[0020] Figure 4 is a view conceptually showing the fine structure of Embodiment 1.

[0021] Figure 5 are schematic views (A), (B) for explaining the optical characteristics of the fine structure of Embodiment 1.

[0022] Figure 6 is a cross-sectional view (A) showing the fine structure of Embodiment 1 and a view (B) showing its phase distribution.

[0023] Figure 7It is a cross-sectional view (A) and a top view (B) showing the microstructure of Embodiment 1.

[0024] Figure 8 It is a diagram (A) to (H) showing an example of a method for manufacturing an optical element of Embodiment 1.

[0025] Figure 9 It is a graph (A) to (C) showing the relationship between the height of a structure body, transmittance, and phase when the diameter of the structure body of the microstructure of Embodiment 1 is changed.

[0026] Figure 10 It shows the diameter ratio of the structure body of the microstructure of Embodiment 1 Figure 9 Graph showing the relationship between the height of the structure body, transmittance, and phase when it is larger than (A).

[0027] Figure 11 It shows the pitch ratio of the microstructure of Embodiment 1 Figure 9 Graph showing the relationship between the height of the structure body, transmittance, and phase when it is larger than (A).

[0028] Figure 12 It is a graph showing the relationship between the height and phase of a structure body that obtains a transmittance equal to or higher than a specified value in the microstructure of Embodiment 1.

[0029] Figure 13 It shows the relationship between the ratio of diameter to pitch, transmittance, reflectance, and phase when only the diameter is changed while fixing the height and pitch of the structure body.

[0030] Figure 14 It is a graph (A) showing the relationship between structural parameters, transmittance, and phase when only the height and diameter of the structure body are changed, and a graph (B) showing the relationship between structural parameters, transmittance, and phase when the height, diameter, and pitch of the structure body are changed.

[0031] Figure 15 It is a cross-sectional view (A) of the microstructure of Embodiment 2 and a diagram (B) showing its phase distribution.

[0032] Figure 16 It is a perspective view (A) and a schematic diagram (B) showing an example of an optical device to which the optical elements of Embodiments 1 and 2 can be applied. Detailed Embodiments

[0033] Hereinafter, the optical element of the embodiment will be described with reference to the drawings. The following embodiments are merely examples, and the embodiments can be appropriately changed, and in addition, the embodiments can be appropriately combined.

[0034] Embodiment 1

[0035] <Overall Structure of Optical Element>

[0036] First, the optical element 1 of Embodiment 1 will be described. Figure 1 It is a perspective view showing the optical element 1 of Embodiment 1. Figure 2 It is a side view showing the optical element 1.

[0037] The optical element 1 of Embodiment 1 is a lens that converges or collimates light. As Figure 2 shown, on the surface 21 of the flat substrate 2, the optical element 1 is formed with a fine structure 3 that is an uneven structure for changing the phase of light. The fine structure 3 is also called a metasurface. The optical element 1 is also called a metalens.

[0038] The wavelength of the light incident on the optical element 1 ( Figure 2 the incident light 11 shown) is, for example, less than 30 μm. As an example, it is 400 nm to 2000 nm. The light in this wavelength band includes visible light and infrared light.

[0039] The substrate 2 is made of a material that transmits light. For example, the substrate 2 is formed of Si or TiO2. The refractive index of the substrate 2 can be 2 or more. Preferably, the thickness of the substrate 2 is, for example, 0.3 mm or more and 2.0 mm or less. The substrate 2 has a surface 21 and a back surface 22, and the fine structure 3 is formed on the surface 21.

[0040] <Structure of Fine Structure>

[0041] Figure 3 It is a top view showing the fine structure 3 of the optical element 1. As Figure 3 shown, the fine structure 3 has a plurality of concentric annular bands A1, A2, A3... Am. m is an integer of 2 or more. The imaginary line passing through the centers of the concentric circles of the annular bands A1 to Am and extending in the thickness direction of the substrate 2 becomes the optical axis C of the optical element 1. The annular bands A1, A2, A3... Am are arranged in this order from the side closer to the optical axis C.

[0042] The farther away from the optical axis C, the narrower the width of each of the annular bands A1 to Am (i.e., the radial dimension centered on the optical axis C). Each of the annular bands A1 to Am is a band region where the phase change imparted to the light is 0 to 2π. The annular bands A1 to Am are collectively referred to as the annular band A.

[0043] Each annular band A has a plurality of concentric circular regions B1, B2, B3... Bn centered on the optical axis C. n is an integer of 2 or more. The regions B1, B2, B3... Bn are arranged in this order from the side closer to the optical axis C. The regions B1 to Bn are collectively referred to as the region B.

[0044] Figure 4It is a perspective view conceptually showing a structural example of the micro-structure 3. The micro-structure 3 is formed by arranging a plurality of structures 30 two-dimensionally. The structure 30 is, for example, cylindrical and is also referred to as a superatom or a pillar.

[0045] The structure 30 has a height H in the direction perpendicular to the surface 21 of the substrate 2 (i.e., the direction of the optical axis C), and has a width (more specifically, a diameter) D in a plane parallel to the surface 21 of the substrate 2. In addition, in a plane parallel to the surface 21 of the substrate 2, the center-to-center distance between adjacent structures 30 is the pitch P.

[0046] According to the regions B1, B2, B3... Bn ( Figure 3 ), at least one of the height H, diameter D, and pitch P of the structure 30 is different. In particular, at least one of the height H, diameter D, and pitch P including the pitch P is different. The structures included in the same region (for example, region B1) have the same height H, the same diameter D, and the same pitch P.

[0047] In this way, by varying the height H, diameter D, and pitch P of the structure 30 for each region B, it is possible to cause a phase change imparted to light by the structure 30. In addition, the shape of the structure 30 is not limited to cylindrical, and may be, for example, prismatic. The diameter D of the structure 30 is also referred to as the width. In addition, the pitch P of the structure 30 is also referred to as the interval.

[0048] For example, by linearly changing the phase imparted to light by the structure 30, it is possible to deflect (refract) the light incident on the optical element 1. In addition, by changing the phase imparted to light by the structure 30 as in a Fresnel lens, as Figure 2 shown, it is possible to exhibit a function of converging the incident light 11 or collimating the light incident in a divergent state. In addition, it is also possible to exhibit a function of branching or reflecting the incident light 11.

[0049] Figure 5 (A) and (B) of are schematic diagrams for explaining the phase distribution in the case where the optical element 1 is configured as a condenser lens or a collimating lens. In Figure 5 (A) and (B) of, let the distance from the center of the optical element 1, i.e., the optical axis C, be r, and let the phase at the position at a distance r from the center of the optical element 1 be Let the wavelength of the light incident on the optical element 1 be λ, and let the focal length of the lens of the optical element 1 be f.

[0050] Regarding each of the regions B1 to Bn, by changing the height H, diameter D, and pitch P of the micro-structure 3, a phase distribution satisfying the following formula (1) is obtained.

[0051]

Mathematical formula 1

[0052]

[0053] is the phase at the center (r = 0) of the optical element 1. Here, let The phase distribution is such that the phase is wrapped every 2π.

[0054] In the present embodiment, the optical element 1 is configured as a transmissive lens in which light is incident perpendicularly to the surface 21 of the substrate 2. However, even if the optical element 1 is a reflective lens, a diffraction grating, or a reflective element, it can be configured by arranging the structure body 30 according to their respective phase distributions.

[0055] In addition, in Figure 4 and the following Figure 6 (A), Figure 7 (A), etc., the bottom 32 of the structure body 30 ( Figure 4 ) is located on the same plane, and the position of the top 31 of the structure body 30 ( Figure 4 ) is changed. However, the structure is not limited to this, and the top 31 of the structure body 30 may be located on the same plane, and the position of the bottom 32 may be changed.

[0056] Next, a structural example of the fine structure 3 will be described. Figure 6 (A) of is a cross-sectional view along the radial direction of the structure body 30 schematically showing the regions B1 to B10 of the annuli A1 and A2. Figure 6 (B) of shows the phase imparted to light by the structure body 30 in each region B of the annuli A1 and A2 and a graph of the height H of the structure body 30.

[0057] Figure 6 The left vertical axis of (B) of represents the value obtained by dividing the phase by 2π, and the right vertical axis represents the value obtained by dividing the height H by the wavelength λ. The horizontal axis represents the distance r from the optical axis C.

[0058] Each annulus A is divided into a plurality of regions B, here 10 regions B. For example, the annulus A1 is composed of the regions B1, B2... B10. The structure body 30 in the first region B1 imparts a phase change of 2π to light. In addition, the phase change imparted to light by the structure body 30 in the tenth region B10 is 0. Here, each annulus A is divided into 10 regions B1 to B10, but the number of divisions is not limited to 10.

[0059] According to each region B (that is, according to the regions B1, B2... B10), at least one of the height H, diameter D, and pitch P of the structure body 30 is changed, thereby changing the phase imparted to light. The height H, pitch P, and diameter D of the structure body 30 are also referred to as structural parameters.

[0060] By means of the structure body 30 in each region B, at least a phase change of 0 to 2π is imparted to light, whereby the design of the optical element 1 can be flexibly carried out. In addition, although the structure body 30 is configured here to impart a phase change of 0 to 2π to light, it may also be configured to impart a phase change exceeding 2π.

[0061] In addition, the height H, diameter D, and pitch P of the structure body 30 may be determined in such a manner as to suppress reflection of light at the interface between the structure body 30 and air (more preferably, no reflection of light occurs). Thereby, even without forming an antireflection film, an optical element 1 with high transmittance can be obtained, and cost reduction can be achieved.

[0062] <Dimensions and Arrangement of the Structure>

[0063] The arrangement of the structure body 30 in the fine structure 3, as well as the height H, pitch P, and diameter D, are determined as follows. In the first step, for example, using FEM (Finite Element Method), the height H, diameter D, and pitch P are each varied within a certain range, and data on phase and transmittance are accumulated.

[0064] In the second step, the ideal phase distribution denoted by reference numeral K1 in (B) of Figure 6 is determined. This is, for example, the phase distribution of a Fresnel lens to be realized by the optical element 1. Based on this ideal phase distribution, as shown by reference numeral K2, discrete phase values are determined for each of the regions B1, B2... B10. That is, the phases of the regions B1, B2... B10 are determined such that the phase of region B1 is T1, the phase of region B2 is T2, and so on.

[0065] In the third step, the height H, diameter D, and pitch P (i.e., the structural parameters) that yield the discrete phase values T1, T2... T10 obtained in the second step from the phase data accumulated in the first step are determined. For example, in the case of the structure body 30 in region B1, the height H, diameter D, and pitch P are determined so as to obtain the phase value T1. The height H determined in this way, as H / λ, is denoted by reference numeral K3 in (B) of Figure 6

[0066] Here, data on phase and transmittance are accumulated using FEM, and the structural parameters are determined based on the values obtained by discretizing the ideal phase distribution. However, the phase of the structure body 30 in each region may also be calculated by analyzing the effective refractive index to determine the structural parameters. However, in the case of a structure body 30 whose diameter is smaller than the wavelength of light, it is difficult to obtain the phase by analyzing the effective refractive index. Therefore, it is preferable to use the above-mentioned FEM or RCWA (Rigorous Coupled-Wave Analysis), etc.​

[0067] Figure 7 (A) and (B) are a top view and a side view conceptually showing an enlarged area B1 - B4 of the annulus A1 in the fine structure 3.

[0068] The structure 30 in area B1 has a height H1, a diameter D1, and a pitch P1. The structure 30 in area B2 has a height H2, a diameter D2, and a pitch P2. The structure 30 in area B3 has a height H3, a diameter D3, and a pitch P3. The structure 30 in area B4 has a height H4, a diameter D4, and a pitch P4.

[0069] In different areas B (such as area B1 and area B2), at least one of the height H, diameter D, and pitch P of the structure 30 may be different. For example, it may also be that in two or more different areas B, the diameter D and pitch P of the structure 30 are the same, and only the height H of the structure 30 is different.

[0070] In Figure 7 In the example shown in (B), in each area B, the structures 30 are arranged in a triangular lattice. However, the arrangement of the structures 30 is not limited to this example. For example, they may also be arranged in a square lattice. However, compared with the case of arranging them in a square lattice, arranging the structures 30 in a triangular lattice can increase the filling rate of the structures 30 and enable the phase to change flexibly.

[0071] As Figure 7 shown in (B), the radial direction of the optical element 1 passing through the center of the optical element 1 (i.e., the optical axis C) is defined as the R direction. In addition, the circumferential direction centered on the optical axis C of the optical element 1 is defined as the L direction. For the convenience of illustration, the L direction as the circumferential direction is shown as a straight line.

[0072] First, a structure 30 is arranged at the center of the optical element 1. This structure 30 is called the first structure 30. The first structure 30 is given a height H, a diameter D, and a pitch P whose phase is closest to the discrete value of the phase at this position obtained from the above - mentioned accumulated data (i.e., the value on the curve K2 shown in (B) of Figure 6 ).

[0073] In Figure 7 the example shown in (B), since the first structure 30 is included in area B1, it is given a height H1, a diameter D1, and a pitch P1.

[0074] Next, a second structure 30 is arranged at a position spaced apart from the first structure 30 by a pitch P1. In Figure 7In the example of (B), the structures 30 are arranged in a triangular lattice. Therefore, the second structure 30 is arranged at a position spaced by a distance P1 in a direction inclined 30 degrees with respect to the R direction from the first structure 30. The second structure 30 is given a height H, a diameter D, and a pitch P such that the phase is closest to the discrete value of the phase at that position obtained from the accumulated data described above.

[0075] In Figure 7 In the example shown in (B), the second structure 30 is also included in the region B1, and thus is given a height H1, a diameter D1, and a pitch P1. A plurality of second structures 30 are arranged at intervals of the given pitch P1 in the circumferential direction, i.e., the L direction.

[0076] Similarly, the third structure 30 is arranged at a position spaced by a distance P1 from the second structure 30. The third structure 30 is given a height H, a diameter D, and a pitch P such that the phase is closest to the discrete value of the phase at that position obtained from the accumulated data described above. The third structure 30 is arranged at a position spaced by a distance P1 in a direction inclined 30 degrees with respect to the R direction from the second structure 30.

[0077] In Figure 7 In the example shown in (B), the third structure 30 is also included in the region B1, and thus is given a height H1, a diameter D1, and a pitch P1. A plurality of third structures 30 are arranged at intervals of the given pitch P in the circumferential direction, i.e., the L direction.

[0078] In this way, all the structures 30 included in the regions B1 to B10 included in the annular belt A1 are arranged, and a height H, a diameter D, and a pitch P (i.e., structural parameters) that give a phase closest to the discrete value of the phase at that position are given. Similarly, for the annular belts A2 to Am, all the structures 30 are arranged and the structural parameters are given respectively. Thus, the fine structure 3 in which the structures 30 are arranged on the substrate 2 is obtained.

[0079] In addition, in Figure 7 In the example shown in (B), the structures 30 are arranged in a triangular lattice, but the structures 30 may also be arranged in a square lattice. In this case, the second structure 30 is arranged at a position spaced by a distance P1 in the R direction from the first structure 30.

[0080] <Manufacturing process of the optical element>

[0081] Next, the manufacturing process of the optical element 1 will be described. Figure 8 (A) to (H) are diagrams showing an example of the manufacturing process of the optical element 1. The fine structure 3 can be produced, for example, using electron beam lithography technology or deep ultraviolet lithography technology.

[0082] First, asFigure 8 As shown in (A) of FIG.

[0083] Next, as shown in Figure 8 (B) of FIG.

[0084] Next, as shown in Figure 8 (C) of FIG.

[0085] Next, as shown in Figure 8 (D) of FIG.

[0086] Next, as shown in Figure 8 (E) of FIG. Figure 8 the developed resist 5 is used as an etching mask, and the metal mask layer 4 is patterned by dry etching or the like. Thus, the metal layer 40 remains on the resist 5. Then, as shown in

[0087] Next, as shown in Figure 8 (G) of FIG.

[0088] Then, as shown in Figure 8 (H) of FIG.

[0089] As a method for manufacturing the structures 30 having different heights H, for example, an exposure method using a gray-scale mask, a method of drawing while modulating the intensity in electron beam exposure, a method based on laser focused scanning, a method of repeated photolithography, etc. can be used.

[0090] <Function>

[0091] The optical element 1 is used, for example, as a lens of a camera or a sensor of an optical device (refer to (A) and (B) of FIG. Figure 16 described later). A camera generally has a lens group in which a plurality of lenses are combined, but the lens group can be replaced by one optical element 1 having the micro-structure 3.

[0092] In a camera, aspherical lenses are sometimes used to form a lens group with fewer lenses. However, in the case of a wide-angle lens, an optical system with a high NA (Numerical Aperture) is required, the thickness of the lens becomes thick, and the manufacturing cost increases.

[0093] In addition, it is difficult to suppress aberration by using a single aspherical lens. For example, in the case of using a lens with an NA of 0.7 and an opening diameter of 12.5 mm, even if a lens is formed using Si (silicon) with a high refractive index, the thickness becomes 4.5 mm or more. In a glass with a low refractive index, the thickness becomes more than twice, and in addition, it sometimes becomes a shape that is difficult to manufacture.

[0094] By using the optical element 1 having the fine structure 3, a lens with a thickness of 2.0 mm or less (more preferably 1.0 mm or less) can be manufactured, and miniaturization and weight reduction of the optical device equipped with the lens can be achieved.

[0095] In addition, in order to manufacture a high-performance lens with good converging characteristics, it is necessary to improve the resolution of the phase of light that changes in the fine structure 3. In Patent Document 1, the height and diameter of the structure body are determined so that the phase becomes a desired value. In order to improve the resolution of the phase distribution, it is necessary to precisely control the width of the structure body, and an exposure device with high resolution is required.

[0096] For example, in the case of using an electron beam exposure device or an exposure device based on laser scanning, in order to improve the resolution, it is necessary to reduce the converging point. The smaller the converging point, the longer the time required for the overall exposure of the lens surface, and in addition, an expensive exposure device is required, so the manufacturing cost increases.

[0097] In the present embodiment, since the height H, diameter D, and pitch P of the structure body 30 are changed, even if the height H, diameter D, and pitch P are coarsely controlled, the resolution of the phase of light that changes in the fine structure 3 can be improved. Therefore, even if an exposure device with relatively low resolution is used, a high-performance lens can be manufactured.

[0098] <Design method of the structure body>

[0099] Here, a design method of the structure body 30 based on numerical simulation will be further described. Figure 9 (A) shows the transmittance T and the phase difference when the height H is changed while fixing the diameter D and the pitch P of the structure body 30, respectively. Figure 9 The left vertical axis of (A) represents the transmittance T, and the right vertical axis represents the phase difference divided by 2π. The horizontal axis represents the height H of the structure body 30.

[0100] According to Figure 9 From the curve graph of (A), it can be seen that the larger the height H of the structure 30, the larger the phase difference . In addition, the transmittance T changes periodically according to the height H. Therefore, it can be known that there is a height H at which the transmittance T becomes high.

[0101] Figure 9 (B) shows the relationship between the height H of the structure 30, the transmittance T, and the phase difference Figure 9 when the diameter D is smaller than that in (A) of Figure 9 at the same pitch P as in (A) of .

[0102] Figure 9 (C) shows the relationship between the height H of the structure 30, the transmittance T, and the phase difference Figure 9 when the diameter D is smaller than that in (B) of Figure 9 at the same pitch P as in (A) of . In (B) and (C), both the vertical axis and the horizontal axis are the same as those in (A) of Figure 9 . Figure 9

[0103] In (B) and (C), compared with (A) of Figure 9 , the change amount of the phase difference Figure 9 with respect to the change in the height H is small. In addition, the transmittance T changes periodically with respect to the change in the height H, but the overall transmittance T is lower than that in (A) of . Figure 9

[0104] Figure 10 ( ) shows the relationship between the height H, the transmittance T, and the phase difference Figure 9 when the diameter D is larger than that in (A) of Figure 9 at the same pitch P as in (A) of . The vertical axis and the horizontal axis are the same as those in (A) of Figure 9 . In Figure 10 , the change in the transmittance T with respect to the change in the height H is not periodic. In addition, the transmittance T changes sharply, and there is a part where the transmittance is 0.1 or less.

[0105] Figure 11 ( ) shows the relationship between the height H, the transmittance T, and the phase difference Figure 9 when the pitch P is larger than that in (A) of Figure 9 at the same diameter D as in (A) of . The vertical axis and the horizontal axis are the same as those in (A) of Figure 9 . The larger the pitch P, the larger the effective refractive index. Therefore, compared with​​Figure 9 The phase difference with respect to the change in height H is smaller than that in (A). The amount of change becomes smaller.

[0106] To facilitate the design of the optical element 1, it is preferable to avoid combinations of the diameter D and the pitch P that cause a sharp change in the transmittance T due to the change in height H (see Figure 10 ). A combination of the height H, the diameter D, and the pitch P is adopted in which the height H is changed among a plurality of combinations of the diameter D and the pitch P to increase the transmittance T. Thereby, the reflected light can be suppressed and the phase can be changed.

[0107] Figure 12 FIG. is a graph showing the relationship between the height H of the structure 30 in which the transmittance becomes 96% or more when the wavelength of the light to be targeted is 685 nm and the optical element 1 is designed using a TiO2 substrate and the phase difference therebetween. The relationship between the phase difference θ / (2π) of the structure 30 in which the transmittance becomes 85% or more when the wavelength of the light to be targeted is 905 nm and the optical element 1 is designed using an Si substrate and the height H is also shown. Figure 12 The vertical axis of represents the value obtained by dividing the phase difference by 2π, and the horizontal axis represents the value obtained by dividing the height H by the wavelength λ.

[0108] In addition, in Figure 12 , the data in the case where the wavelength of the light is 685 nm and a TiO2 substrate is used are represented by the plots of "+", and the data in the case where the wavelength of the light is 905 nm and an Si substrate is used are represented by the plots of "-". From the plots of "+" and "-" in Figure 12 , it is understood that the data in the case where the wavelength of the light is 685 nm and a TiO2 substrate is used and the data in the case where the wavelength of the light is 905 nm and an Si substrate is used are both concentrated on the common straight lines F(1) to F(5).

[0109] There are multiple combinations of the diameter D and the height H when the transmittance T increases. At the points where the transmittance T periodically increases, the relationship with H / λ is as shown in the following formula (2).

[0110]

Mathematical formula 2

[0111]

[0112] In formula (2), C is a constant, and here c = 0.23 ± 1. λ is the wavelength of the light. Figure 12 The straight lines F(1) to F(5) shown correspond to the straight lines in the case where n is changed to 1, 2, 3, 4, 5 and c = 0.23 in formula (2).

[0113] When the ratio of the diameter D to the pitch P (D / P) is too small, as in Figure 9 (C), the transmittance T becomes low. When D / P is too large, as in Figure 10 shown, the variation in the transmittance T becomes large.

[0114] Therefore, it is preferable to set the diameter D and the pitch P of the structure 30 in such a way that the decrease in the transmittance T can be suppressed and the variation can be suppressed. Specifically, for example, when manufacturing the optical element 1 for a wavelength of 685 nm using a TiO2 substrate, it is preferable that the diameter D of the structure 30 is in the range of 150 nm to 250 nm, and the pitch P is in the range of 300 nm to 450 nm.

[0115] The above ranges of the diameter D and the pitch P can be generalized when manufacturing the optical element 1 using a substrate 2 with a refractive index n for light with a wavelength λ. That is, let the refractive index of TiO2 be n0, and let the wavelength of light be λ0 = 685 nm. In addition, let α = (λ / λ0) / (n / n0). It is preferable that the diameter D of the structure 30 is set in the range of α × 150 nm to α × 250 nm, and it is preferable that the pitch P is set in the range of α × 300 nm to α × 450 nm.

[0116] In addition, α × 150 nm, which is the lower limit value of the range of the diameter D, can be expressed as 150 × (n0 / λ0) × (λ / n), and α × 250 nm, which is the upper limit value, can be expressed as 250 × (n0 / λ0) × (λ / n). When the wavelength of light is λ0 = 685 nm and the refractive index of TiO2 is n0 = 2.58, the range of the diameter D of the structure 30 can be expressed as 0.56 × λ / n to 0.95 × λ / n. Similarly, the range of the pitch P of the structure 30 can be expressed as 1.1 × λ / n to 1.7 × λ / n.

[0117] In addition, although the diameter D and the pitch P of the structure 30 differ for each region B, it is preferable that in all regions B1 to Bn ( Figure 3 ), the diameter D of the structure 30 is in the range of 0.56 × λ / n to 0.95 × λ / n, and the pitch P of the structure 30 is in the range of 1.1 × λ / n to 1.7 × λ / n.

[0118] In Equation (2), the relationship between the height H / λ of the structure 30 and the phase difference is shown, but there are manufacturing limitations. That is, for example, when the ratio of the diameter D of the structure 30 to the pitch P exceeds 0.8 (i.e., when D / P > 0.8), the proportion of the area of the structure 30 in the surface area of the substrate 2 becomes large, the aspect ratio becomes high, and etching becomes difficult.

[0119] Therefore, the phase difference can also be determined based on the phase of the structure 30 with D / P = 0.8 and the effective refractive index n eff obtained to determine the upper limit of the phase difference .

[0120] The diameter D and the pitch P of the structure 30 can also be set to a certain fixed width (for example, in units of 10 nm). When the widths of the diameter D and the pitch P are set to be small, a high-resolution exposure apparatus must be used in manufacturing the fine structure 3. When designing the structure 30, there are more combinations of the height H, the diameter D, and the pitch P to be considered, and it is not easy to obtain the relationship with the phase.

[0121] For example, by setting the diameter D and the pitch P in units of a specified value (for example, in units of 10 nm), the resolution of the exposure apparatus can be reduced, the manufacturing cost can be reduced, and in addition, the design of the structure 30 can be made easier. The diameter D and the pitch P can also be set in units that are integer multiples of 5 nm, such as 5 nm or 15 nm. In addition, not limited to integer multiples of 5 nm, they can also be set in units of several nm to several tens of nm. In addition, not only the diameter D and the pitch P, but also the height H can be set in units of a specified value.

[0122] The relationship between the phase of the structure 30 and the height H has been described above. Next, the relationship between the phase of the structure 30 and the diameter D and the pitch P will be described.

[0123] Let the wavelength of the light to be the object be λ, let the refractive index of air be n a , and let the effective refractive index of the structure 30 be n eff . In addition, let the phase at the center position (i.e., the optical axis C) of the optical element 1 be . The phase change based on the structure 30 is expressed by the following equation (3).

[0124]

Equation 3

[0125]

[0126] Here, regarding the effective refractive index n eff , using the refractive index n of air a , the refractive index n of the substrate 2 before processing the fine structure 3 (i.e., the substrate 2 shown in (A) of Figure 8 ), and the diameter D and the pitch P of the structure 30, it is expressed by the following equation (4) as a simple approximate equation. s

[0127]

Equation 4

[0128] ​

[0129] If the micro-structure 3 is a structure such as a simple diffraction grating, the effective refractive index n eff can be well approximated by Equation (4). However, in the case of a structure such as the structure body 30 in the present embodiment where the diameter P is smaller than the wavelength and the height H is low, the actual effective refractive index is smaller than the effective refractive index calculated by Equation (4).

[0130] When the height of the diffraction grating is set as h, the effective refractive index n(h) of a known diffraction grating eff is expressed by the following Equation (5). n(0) eff is the effective refractive index of a diffraction grating with a height of 0. In addition, n L is the effective refractive index when the height h of the diffraction grating is regarded as infinite, and is obtained by the above Equation (4). (dn(h) 2 eff ) / (d(hk)) represents the second derivative of n(h) 2 eff The differential equation is solved numerically, and thereby the effective refractive index is calculated.

[0131]

Mathematical formula 5

[0132]

[0133] The height h of the diffraction grating in Equation (5) can be replaced with the height H of the structure body 30 in the present embodiment.

[0134] The smaller the ratio of the diameter D of the structure body 30 to the pitch P, the smaller the effective refractive index. Therefore, as described in the existing literature 1, when designing the height H of the structure body 30 using a simply approximated effective refractive index, the height deviates from the height for obtaining a high transmittance, and the transmittance of the optical element 1 becomes low.

[0135] The strict effective refractive index can be obtained by numerical simulations such as RCWA, FEM, and FDTD (Finite-Difference-Time-Domain) method. The above-mentioned Figure 9 of (A) to Figure 12 shows the results of obtaining the phase and the transmittance T when the structural parameters of the structure body 30 are changed by using a numerical simulation of FEM.

[0136] Next, the effects achieved by changing the height H, diameter D, and pitch P of the structure body 30 will be further described. Figure 13Shows the transmittance T, reflectance R, and phase difference when the wavelength of the light set as the object is 685 nm and the optical element 1 is made of TiO2, with the height H and pitch P of the fixed structure 30 kept constant and only the diameter D being changed. Analysis results.

[0137] Figure 13 The vertical axis on the left represents the transmittance T and the reflectance R, and the vertical axis on the right represents the phase difference . The horizontal axis represents the value (D / P) obtained by dividing the diameter D by the pitch P.

[0138] Due to the reflection at the interface caused by the refractive index difference between the optical element 1 (here a lens) and air, the overall transmittance of the optical element 1 is low, about 80%. The effective refractive index n s of the substrate 2 of the optical element 1 and the refractive index n a of air, the reflectance R F based on the Fresnel reflection at the interface can be expressed by the following equation (6).

[0139]

Equation 6

[0140]

[0141] The reflectance R of the structure 30 is smaller than the reflectance R F based on the Fresnel reflection. In addition, at the resonance point where the transmittance changes sharply in Figure 13 , the transmittance approximately becomes 0%.

[0142] Figure 14 Figure (A) shows the relationship between the combination of structural parameters (height H, diameter D, pitch P) for which the transmittance T becomes 99.5% or more and the transmittance T and phase difference when the pitch P is fixed, the diameter D is changed in units of 10 nm, and the height H is changed in units of 20 nm. The vertical axis on the left represents the transmittance T, and the vertical axis on the right represents the phase difference , and the horizontal axis represents the combination number (group number) i.

[0143] Figure 14 Figure (B) shows the relationship between the combination of structural parameters (height H, diameter D, pitch P) for which the transmittance T becomes 99.5% or more and the transmittance T and phase difference when the diameter D and pitch P are each changed in units of 10 nm and the height H is changed in units of 20 nm. The vertical axis on the left represents the transmittance T, and the vertical axis on the right represents the phase difference , and the horizontal axis represents the combination number (group number) i.

[0144] InFigure 14 In (A), only the height H and the diameter D are varied. Therefore, the number (i) of combinations of structural parameters is small and the phase distribution is sparse. Thus, in Figure 14 of (A), there is no phase difference in the range of 0.6 to 0.7 for the combinations of structural parameters. In other words, regarding the phase difference , defects in the range of 0.1 or more are generated. Therefore, the accuracy of the phase distribution imparted to the light by the optical element 1 becomes low.

[0145] On the other hand, in Figure 14 of (B), the height H, the diameter D, and the pitch P are varied. Therefore, the number (i) of combinations of structural parameters is large. Thus, there are structural parameters that correspond to the phase difference with high precision. That is, high transmittance can be maintained and a smooth phase distribution can be obtained.

[0146] <Effect of Embodiment 1>

[0147] As described above, the optical element 1 of the present Embodiment 1 has a fine structure 3 on the surface of the substrate 2 that changes the phase of light, and the fine structure 3 has a plurality of structural bodies 30. The height H, the diameter (i.e., width) D, and the pitch (i.e., interval) P of the structural bodies 30 differ according to the region B within the fine structure 3. Therefore, without increasing the resolution of the exposure apparatus, the phase distribution can be controlled with high precision, and the manufacturing cost of the optical element 1 can be reduced.

[0148] In addition, when the wavelength of light is λ and the refractive index of the substrate 2 is n, by setting the pitch P of the structural bodies 30 to 1.1×λ / n to 1.7×λ / n, the transmittance of the optical element 1 can be increased, and in addition, the deviation of the transmittance can be suppressed.

[0149] In addition, by setting the diameter D of the structural bodies 30 to 0.56×λ / n to 0.95×λ / n, the transmittance of the optical element 1 can be increased, and in addition, the deviation of the transmittance can be suppressed.

[0150] In addition, according to the region B within the fine structure 3, the pitch P of the structural bodies 30 is set in units of a specified value (for example, in units of 10 nm), and the diameter width D of the structural bodies 30 is set in units of a specified value (for example, in units of 10 nm). Therefore, without increasing the resolution of the exposure apparatus, the phase distribution can be controlled with high precision.

[0151] In addition, the structural bodies 30 have a height H, a diameter D, and a pitch P that suppress the reflection of light at the interface between the structural bodies 30 and air. Therefore, even without providing an antireflection film or the like, an optical element 1 with high transmittance can be obtained, and the manufacturing cost can be further increased.

[0152] In addition, the height H of the structure 30 is determined in the manner represented by the above formula (2). Therefore, even if the wavelength of light or the material of the substrate 2 is changed, the optical element 1 with high transmittance can be obtained.

[0153] In addition, if in two or more of the multiple regions B of the micro-structure 3, only the height H of the structure 30 is made different while the diameter D and the pitch P of the structure 30 are the same, then for the two or more regions B, the phase distribution can be controlled only by changing the etching depth.

[0154] In addition, since light with a wavelength less than 30 μm is targeted, the optical element 1 that can be used for visible light and infrared light can be obtained.

[0155] In addition, the micro-structure 3 is divided into a plurality of concentric annular bands A1 to Am with a phase difference of every 2π. Each annular band A is divided into a plurality of concentric regions B1 to Bn. The height H, the diameter D, and the pitch P of the multiple structures 30 included in each region B are the same. Therefore, the optical element 1 having, for example, the same function as a Fresnel lens can be realized.

[0156] In addition, the reflectance of light in the optical element 1 is smaller than the reflectance calculated in Fresnel diffraction based on the refractive index n of the substrate 2 and the refractive index of air. Therefore, the reflection and absorption of light at the interface between the optical element 1 and air can be reduced.

[0157] In addition, by making the thickness of the optical element 1 2 mm or less, it can be mounted on an optical device 8 with a relatively thin thickness, which can contribute to the miniaturization and lightening of the optical device 8.

[0158] <Embodiment 2>

[0159] Next, the optical element 1A of Embodiment 2 will be described. In the following description, the structural elements corresponding to those of the optical element 1 of Embodiment 1 are denoted by the same reference numerals for description.

[0160] In the micro-structure 3 of the optical element 1 of the above Embodiment 1, the structures 30 are arranged in a manner of an approximate ideal phase distribution. In contrast, in the micro-structure 3A of the optical element 1A of Embodiment 2, the structures 30 are arranged in a manner of an approximate ideal phase distribution and the lower the phase given to the light, the lower the height of the structure 30.

[0161] As in Embodiment 1, refer to Figure 3As described, the micro-structure 3A has a plurality of concentric annular bands A1, A2, A3... Am. Each annular band A has a plurality of concentric regions B1, B2, B3... Bn. Here, the annular band A is divided into six regions B1 to B6, but the number of divisions is not limited to six and is arbitrary. A plurality of structures 30 are arranged in each region B.

[0162] Figure 15 (A) of is a cross-sectional view along the radial direction of the structure 30 of each region B of the annular bands A1 and A2, schematically showing. Figure 15 (B) of shows the phase imparted to light by the structure 30 of each region B of the annular bands A1 and A2 and a graph of the height H of the structure 30. The vertical axis and the horizontal axis are the same as Figure 6 (B) of.

[0163] In the micro-structure 3A of the second embodiment, the structures 30 of the regions B1 to B6 are arranged such that the height H of the structure 30 of each region B is below the height H of the structure 30 of the region B closer to the center (optical axis C) of the optical element 1.

[0164] In Figure 15 the examples shown in (A) and (B) of, the height H of the structure 30 of the region B2 is lower than the height H of the structure 30 of the region B1. In addition, the height H of the structure 30 of the region B3 is lower than the height H of the structure 30 of the region B2. The same applies to the structures 30 of the regions B4 to B6.

[0165] The manufacturing process of the optical element 1A of the second embodiment is substantially the same as the manufacturing process of the optical element 1 of the first embodiment ( Figure 8 (A) to (H) of ). In the optical element 1A of the second embodiment, since the abrupt change in the height of the structure 30 is suppressed, the workability in the etching process of the substrate 2 ( Figure 8 (G) of ) is improved. As a result, the manufacturing process can be simplified and the manufacturing cost can be reduced.

[0166] When designing the optical element 1A of the second embodiment, similar to the first embodiment, the height H, diameter D, and pitch P of each structure 30 are determined such that the phase in each structure 30 becomes the value closest to the value obtained by discretizing the ideal phase distribution ( Figure 15 reference numeral K1 in (B) of ) ( Figure 15 reference numeral K2 in (B) of ). However, in the second embodiment, it is determined such that the height H of the structure 30 of each region B is below the height H of the structure 30 of the region B closer to the center of the optical element 1A than that region B.

[0167] Except for the above aspects, the optical element 1A of Embodiment 2 is configured in the same manner as the optical element 1 of Embodiment 1.

[0168] As described above, in the optical element 1A of Embodiment 2, the height of the structure body 30 in each region B of the fine structure 3A is less than or equal to the height H of the structure body 30 in the region B closer to the center of the optical element 1A than this region B. Therefore, a sharp change in the height of the structure body 30 is suppressed. Thereby, the workability of the etching process of the substrate 2 can be improved, and the manufacturing cost can be reduced.

[0169] A method for analyzing the shape of the fine structures 3, 3A (i.e., metasurfaces) of the optical elements 1, 1A of Embodiments 1 and 2 will be supplemented. As a method for analyzing a three-dimensional structure such as a sphere or an ellipsoid, a method using scattering based on a Mie resonator is known. However, this method is difficult to apply to a fine structure having a structure body with an arbitrary shape (i.e., a meta-atom). As a method for analyzing a fine structure 3, 3A with a complex shape, electromagnetic field simulation is preferred. Specifically, the above-mentioned FDTD, RCWA, FEM, and BPM (Beam Propagation Method) are preferred.

[0170] The optical elements 1, 1A of Embodiments 1 and 2 can be applied to an optical device 8 such as a camera device. For example, Figure 16 (A) shows a perspective view of the appearance of the optical device 8 having the optical element 1, Figure 16 and (B) shows a schematic diagram of the internal structure of the optical device 8. As shown in Figure 16 (A) and (B) of, the optical element 1 is used as a lens of the optical device 8.

[0171] The optical device 8 includes an optical element 1, a light receiving element 9 disposed on the optical axis C of the optical element 1, and a housing 80 covering them. On the front surface of the housing 80, an opening 81 is formed so as to face the optical element 1. In addition, a switch 82 and the like used during imaging are provided. The optical element 1 forms an image of a subject on the light receiving element 9, and the light receiving element 9 converts the received optical signal into an electrical signal.

[0172] A camera generally has a lens group in which a plurality of lenses are combined, but the lens group can be replaced with the optical element 1. The optical element 1 has a thin thickness and can be set to 2 mm or less, for example. Therefore, a thin and compact optical device 8 can be realized. The optical element 1 is not limited to a camera and can also be realized as a sensor or other optical device.

[0173] The preferred embodiments have been specifically described above. However, the present disclosure is not limited to the above embodiments, and various improvements or modifications can be made.

[0174] Description of Reference Numerals

[0175] 1, 1A: Optical elements; 2: Substrate; 3, 3A: Microstructures; 4: Metal mask; 5: Resist; 8: Optical device; 9: Light-receiving element; 21: Surface; 22: Back surface; 30: Structure body; 40: Metal layer; A1 to Am: Ring bodies; B1 to Bn: Regions; C: Optical axis; K1: Ideal phase distribution; K2: Value after discretizing the ideal phase distribution.

Claims

1. An optical element, characterized in that, The optical element has: a substrate; and a microstructure formed on the surface of the substrate to change the phase of light, the microstructure having a plurality of structural bodies, each of the plurality of structural bodies having a height in a direction perpendicular to the surface, a width in a direction parallel to the surface, and being arranged at intervals in a direction parallel to the surface, at least one of the height, width, and interval of the structural body being different according to the region within the microstructure.

2. The optical element according to claim 1, wherein when the wavelength of the light is λ and the refractive index of the substrate is n, the interval of the structural body is 1.1×λ / n or more and 1.7×λ / n or less.

3. The optical element according to claim 1 or 2, wherein when the wavelength of the light is λ and the refractive index of the substrate is n, the width of the structural body is 0.56×λ / n or more and 0.95×λ / n or less.

4. The optical element according to any one of claims 1 to 3, wherein in a plurality of regions of the microstructure, the interval of the structural body is set in units of a specified value, and the width of the structural body is set in units of a specified value.

5. The optical element according to claim 4, wherein in a plurality of regions of the microstructure, the height of the structural body is set in units of a specified value.

6. The optical element according to any one of claims 1 to 5, wherein the structural body has a height, width, and interval that do not cause reflection of the light at the interface between the structural body and air.

7. The optical element according to any one of claims 1 to 6, wherein When the wavelength of the light is set to λ and the phase is where n is an integer and c is a constant 0.23, the height H of the structural body is represented by the following formula 【Mathematical formula 7】 8. The optical element according to any one of claims 1 to 7, wherein the microstructure has two or more regions in which the heights of the structural bodies are different and the widths and intervals of the structural bodies are the same.

9. The optical element according to any one of claims 1 to 8, wherein the wavelength of the light is less than 30 μm.

10. The optical element according to any one of claims 1 to 9, wherein the microstructure is divided into a plurality of concentric annular bands at intervals of 2π in phase difference, each annular band is divided into a plurality of concentric regions, and the heights, widths, and intervals of the plurality of structural bodies included in each region are the same.

11. The optical element according to claim 10, wherein the height of the structural body in each region of the microstructure is less than or equal to the height of the structural body in the region closer to the center of the concentric circles than that region.

12. The optical element according to any one of claims 1 to 11, wherein the reflectance of the light in the optical element is smaller than the reflectance calculated in Fresnel diffraction based on the refractive index n of the substrate and the refractive index of air.

13. The optical element according to any one of claims 1 to 12, wherein The thickness of the optical element is 0.3 mm or more and 2 mm or less.

14. The optical element according to any one of claims 1 to 13, characterized in that the structure is columnar and extends in a direction perpendicular to the surface.

15. An optical device, characterized in that, The optical device includes: the optical element according to any one of claims 1 to 14; and a light receiving element that receives light that has passed through or been reflected by the optical element.

Images