Metasurface optical elements, image projection devices, light source devices, imaging devices, optical scanning devices

Abstract

To provide a metasurface optical element with improved refractive index by forming a surface coating on the surface of a microstructure that does not have an absorption band in the near-ultraviolet region. [Solution] The present invention relates to a metasurface optical element in which a plurality of microstructures are continuously formed on a light-transmitting surface, and the refractive index for light is adjusted by the density of the microstructures, wherein the horizontal dimension d of the microstructures with respect to the light-transmitting surface satisfies λ≧d with respect to the wavelength λ of the light, the density of the microstructures of the metasurface optical element is determined by a phase amount corresponding to the phase wavefront of the light, and a surface coating of a material that does not have an absorption band in the near-ultraviolet region is formed on the surface of the microstructures.

Description

Technical Field

[0001] This invention relates to a transmissive metasurface optical element and an image projection device, a light source device, an imaging device, and an optical scanning device using the metasurface optical element.

Background Art

[0002] A metamaterial composed of periodic structures finer than the wavelength is an artificial material that does not exist in nature, and a two-dimensional version of this is called a metasurface. Since the interval or size of the fine periodic structures (meta-atoms) constituting the metasurface depends on the wavelength, in the field of relatively large wavelength regions (Radio Frequency), development has been actively carried out for intelligent reflectors, beam scanning antennas, etc.

[0003] On the other hand, in the optical field using the visible light region, because the wavelength is short, the required size of the meta-atoms is also as small as several hundred nm, and the difficulty of processing has been an obstacle. However, in recent years, by applying semiconductor processing technology, technologies such as precisely engraving glass and dielectrics have advanced, and the development of optical elements using such metasurfaces has also been promoted in the optical field (see, for example, Patent Documents 1, 2, Non-Patent Document 1, etc.).

[0004] Among such metasurfaces using such technology, a metalens is a nanodevice composed of nano-sized minute structures. It has a condensing action like a lens and can be significantly thinned compared to conventional refractive lenses. Although there are several types of its operating principles, basically, it is already known to control the amplitude and phase of light when the metalens transmits or reflects light. Now, the utilization of nanoimprint is being studied as a technology for producing metalenses at a lower cost. However, given the principle that metalens change the refractive index depending on the arrangement of metaatoms, it has been a challenge to realize ultra-high aspect ratio metaatoms when considering mold release properties. On the other hand, it is also known that forming a single layer film on the surface of a metasurface element can provide further functionality by using it as an anti-reflective or protective film (see Patent Document 3, etc.). Furthermore, attempts have been made to solve the challenge of nanoimprint processes, which is that it is difficult to realize ultra-high aspect ratio metaatoms, by making this single layer film a material with a high refractive index, thereby giving it a pseudo-refractive index of a metaatom (see Non-Patent Literature 2). In such cases, TiO2, which has a high refractive index in the visible region and a low extinction coefficient, is often used as a thin-film forming material with a high refractive index. In addition to its aspect as a high-refractive-index material, TiO2 is known as a material with photocatalytic function. Specifically, it extracts electrons from OH groups from water present on the TiO2 surface, becoming hydroxyl radicals with strong oxidizing power, and thus has the effect of decomposing organic matter. In many cases, the materials used in nanoimprint processes are organic materials. This is not a problem in environments where the wavelength range used is limited, such as with lasers and LEDs, but in cases where it is used as a lamp light source or in general environments under natural light, the wavelength range included in the light beam is wide, and ultraviolet light is also included, so there is a risk that the imprint material will self-decompose, which has been a challenge from the perspective of long-term reliability. [Overview of the project] [Problems that the invention aims to solve]

[0005] The present invention aims to solve the above-mentioned problems and to provide a metasurface optical element with improved optical properties through improved refractive index. [Means for solving the problem]

[0006] The present invention provides a metasurface optical element in which a plurality of microstructures are continuously formed on a light-transmitting surface, and the refractive index for light is adjusted by the density of the microstructures, wherein the horizontal dimension d of the microstructures with respect to the light-transmitting surface satisfies λ≧d with respect to the wavelength λ of the light, the density of the microstructures of the metasurface optical element is determined by a phase amount corresponding to the phase wavefront of the light, and a surface coating of a material that does not have an absorption band in the near-ultraviolet region is formed on the surface of the microstructures. [Effects of the Invention]

[0007] According to the present invention, a metasurface optical element with improved refractive index can be provided by forming a surface coating on the surface of a microstructure that does not have an absorption band in the near-ultraviolet region. [Brief explanation of the drawing]

[0008] [Figure 1] This diagram shows the schematic configuration of a metalens as a comparative example. [Figure 2] Figure 1 is a schematic diagram showing an example of the microstructure of the surface of the metalens shown. [Figure 3] This diagram schematically illustrates the refraction caused by a convex lens. [Figure 4] This figure schematically shows the change in refractive index caused by the microstructure shown in Figure 2. [Figure 5] This figure shows an example of the effect of the height of the microstructure on the surface of a metalens on the phase quantity. [Figure 6] This figure shows an example of the relationship between pillar height and phase quantity. [Figure 7] This figure shows the schematic configuration of the metalens of the present invention. [Figure 8] This is a schematic diagram illustrating the effects of the coating layer of the present invention. [Figure 9] Figure 7 illustrates an example of a method for generating a metasurface. [Figure 10] This diagram illustrates one example of a method for generating metalens. [Figure 11] This figure illustrates another example of the method for generating the metasurface shown in Figure 7. [Figure 12] This diagram illustrates another example of a method for generating metalens. [Figure 13] This figure shows an example of the configuration of a roll-to-roll metasurface manufacturing apparatus. [Figure 14] This figure shows an example of the configuration of an image projection device utilizing the metalens of the present invention. [Figure 15] This figure shows an example of the optical system configuration of a light source device utilizing the metalens of the present invention. [Figure 16] This figure shows an example of the configuration of an imaging device utilizing the metalens of the present invention. [Figure 17] This figure shows an example of the configuration of an optical scanning device utilizing the metalens of the present invention. [Modes for carrying out the invention]

[0009] Figure 1 shows an example of a metalens configuration as an example of a conventional optical element having a metasurface, representing a first embodiment of the present invention. In the figure, the metalens 10 is shown as an example of an optical element in which a substrate portion 11 made of plate-shaped quartz glass and multiple cylindrical pillars 13, which are microstructures, are provided on the light-transmitting surface 12 of the substrate portion 11. In Figure 2, the pillars 13 are specifically described as cylindrical microstructures with a diameter φ and a height H, but the configuration is not limited to this, and for example, their upper surfaces may be polygonal prisms with polygonal shapes such as triangles, squares, or hexagons, or shapes that are combinations of rectangles, or shapes that are extended in one direction.

[0010] In this case, if the spacing between the pillars 13 is a subwavelength interval with respect to the transmitted light, the transmission surface 12 functions as a so-called metasurface. The wavelength of the transmitted light can be arbitrary, but in this embodiment, for example, we will mainly deal with light with a wavelength of λ = 600 nm as an example of light in the visible light region.

[0011] When the diameter φ of pillar 13 and the pitch p between adjacent pillars 13 are used, the functionality of the transmissive surface 12 as a metasurface varies depending on the filling ratio of pillar 13. This is because, for a pitch p with a subwavelength interval, when light waves enter, pillar 13 acts as a meta-atom that inhibits the propagation of light waves. Also, it has been found that by changing the filling ratio of these pillars 13, that is, by changing the coarseness, it is possible to give a refractive index different from that of the material of pillar 13 to the entire transmissive surface 12.

[0012] To explain this point, first, as a simple example, FIG. 3 shows the propagation of light waves in a convex lens together with the phase wavefront. In FIG. 3, the portions where the phases of adjacent parallel light rays coincide are schematically shown as thin lines as the phase wavefront. Needless to say, the normal direction to the phase wavefront is the direction of light propagation. When light enters a convex lens 200 with a refractive index n, the speed of light is reduced accordingly according to the refractive index, so the phase wavefront inclines toward the convex direction of the convex lens 200 with respect to the parallel light rays. Since the direction in which such a series of phase wavefronts progresses is the direction of light propagation, the direction of light propagation becomes the tangent direction of such a series of phase wavefronts and is bent toward the optical axis direction of the convex lens 200 as shown in FIG. 3.

[0013] FIG. 4(a) and FIG. 4(b) schematically show the effect of pillar 13 on the transmissive surface 12 which is a metasurface. As shown in FIG. 4(a), when pillars 13 smaller than wavelength λ stand on the transmissive surface 12, on the transmissive surface 12, light of wavelength λ does not recognize pillar 13 as individual cylinders, but is recognized in the form of the magnitude of the refractive index of the transmissive surface 12 according to the magnitude of the filling ratio.

[0014] In other words, on the transmissive surface 12, which is a metasurface, the pseudo refractive index n2 changes depending on the diameter and height of the pillars 13, or the spacing (pitch) between the upright pillars 13. The magnitude of such pseudo refractive index n2 is roughly shown in Figure 4(a) by color coding based on the intensity of the gradient.

[0015] Therefore, as shown in Figure 4(b), when parallel light rays are incident on the transmission surface 12 from a direction perpendicular to it, considering the light rays incident on the transmission surface 12, the phase wavefront, as shown in Figure 4(b), propagates more slowly in areas with a larger refractive index. At this time, as already mentioned, the magnitude of the refractive index of the transmission surface 12 is the magnitude of the filling density in which the pillars 13 are formed. In other words, for transmitted light, a change in the filling density of the pillars 13 on the transmission surface 12 is equivalent to a change in the optical path length in that area. Therefore, similar to the refractive index of the convex lens 200 shown in Figure 3, by providing a delay due to the refractive index on the transmission surface 12 by the same amount as the delay caused by the difference in optical path length due to the lens thickness, it is possible to obtain a refractive effect similar to that of the convex lens 200. Thus, when the pillar 13 is on the order of subwavelengths relative to the wavelength λ of light, and when the refractive index of the transmission surface 12 is given a gradient by its packing density as shown in Figure 4(b), the wavefront propagation is slowed down by the packing density of the pillar 13, so the phase wavefront tilts to the left in the figure as shown in Figure 4(b). If multiple pillars 13 of the subwavelength order are arranged regularly, this phenomenon acts as if they were atoms for light waves traveling through a molecular crystal lattice, causing the phase wavefront to be distorted as if it were refracted. Conversely, if such a distortion of the phase wavefront can be artificially created by the regularity of the pillar 13, then the transmission surface 12 will have optical functions equivalent to a lens surface with refractive index n2. This is a simplified principle of an optically functional surface called a metasurface, and in such an optically functional surface, microstructures like pillar 13 are likened to atoms and called metaatoms.

[0016] Now, it is known that the conditions under which the phase wavefront changes according to this principle are practically about the extent of condition (1). Here, the wavelength of the transmitted light is λ and the diameter of the pillar 13 is φ. Furthermore, if the shape is not a cylinder, for example, this equation can also be written as λ≧d for the horizontal dimension d with respect to the transmission surface 12 of the microstructure. Such d may be, for example, the ratio of the long side / short side or the length of the diagonal if the pillar 13 is a rectangular prism, or the length of one side if it is a triangular prism. In any case, the most geometrically typical distance when viewing the pillar 13 from the vertical can be treated as the horizontal dimension d.

[0017]

number

[0018] Extending this idea, we can see that because adjacent light waves arrive with a delay, the degree of tilt of the convex lens can be simulated by the change in density of the pillars 13 on the surface. In other words, if the surface pillars 13 can be appropriately positioned using an optical element that satisfies condition (1), the refractive index n2 at various points on the transmission surface 12 can be adjusted by varying the density of the pillars 13, called metaatoms. By distributing the refractive index n2 of the transmission surface 12, for example, in a concentric pattern, it becomes possible to obtain a refracted wavefront similar to that of a convex lens 200 simply by passing light through the planar transmission surface 12.

[0019] As described above, the pseudo refractive index n2 at the transmission surface 12 can be controlled by the degree of density of the pillars 13, that is, the area occupied by the pillars 13 in a plane perpendicular to the transmitted light (filling density). In this embodiment, this degree of density is adjusted by changing the filling density by the pillars 13 by changing the diameter φ of the pillars 13. However, there is no problem with using methods other than controlling the diameter φ, and as mentioned above, the shape of the pillars 13 can be any polygon.

[0020] In other words, if a distribution of pillar diameters 13 can be created such that the phase wavefront of the transmitted light passing through the transmission surface 12 coincides with the phase wavefront of the light passing through the convex lens with refractive index n2, then the light passing through such a transmission surface 12 will be the same as the light that has passed through the convex lens with refractive index n2.

[0021] In this way, the distribution of the diameter of pillar 13 allows the metasurface optical element to reproduce a convex lens with refractive index n2. However, the diameter of the pillars 13 formed on the metalens 10 is finite, and the reproducible refractive index n2 is ultimately difficult to keep within a range greater than the ratio of the maximum and minimum values ​​of the filling density of the pillars 13. For example, in the case of cylinders, even if they are filled so closely that they touch each other, there is an upper limit to the generation, which is about 78%, the filling density of circles relative to a rectangle. Also, in areas where there are no pillars 13, the refractive index n1 of the substrate portion 11 naturally becomes the lower limit. Furthermore, increasing the filling density of the pillars 13 directly leads to increased difficulty and cost in molding the pillars 13 themselves, which is undesirable. This means that no matter how the phase wavefront can be controlled, the ratio of the refractive index n1 of the substrate portion 11 in the metalens 10 to the refractive index n2 that can be reproduced by the metalens 10 will have an upper limit determined by the shape or material of the pillar 13.

[0022] As shown in Figures 2 and 4, the metalens 10 generates a pseudo refractive index due to the phase difference between the light transmitted through the pillar 13 and the light transmitted through the air layer. In other words, for the transmission surface 12 to be treated as a surface with refractive index n2, it is desirable that the height H of the pillar 13 in the metalens 10 creates an optical path length difference of about one period of the wavelength of light compared to the air layer.

[0023] In other words, the greater the difference in refractive index between the pillar 13 and the layer of air between the pillars 13, the stronger the function of the transmitting surface 12 in delaying the phase of light waves. Until now, in order to produce a metamirror 10 with high refractive power, it has been necessary to produce pillars 13 made of high refractive index material.

[0024] For example, taking commonly used quartz glass as an example, the relative refractive index of quartz glass at around 600 nm is approximately n = 1.45 to 1.46, compared to the refractive index of air which is about 1.

[0025] Here, Figure 5 shows the estimated packing density and phase amount of the metalens 10, using a meta-atom with a refractive index of 1.45 as an example. In Figure 5, the change in phase amount with respect to light with a wavelength of 600 nm is plotted on the vertical axis, and the packing density is plotted on the horizontal axis. Each plot shows the case for pillar 13 heights H = 1.5 μm, 2.5 μm, and 3.5 μm. In other words, Figure 5 can be said to show the optical path difference when the packing density is changed. Furthermore, Figure 6 shows a graph plotting the amount of phase change with respect to the height of pillar 13 for each filling density. As already mentioned, the packing density of pillar 13 is limited to approximately 78% when forming an actual metalens 10. To increase the phase change of the metalens 10, as is clear from Figure 5, a larger height H results in a better slope of the phase change with respect to the packing density. Naturally, given the principle of the metalens 10, which reproduces the optical path difference or phase difference by the packing density of pillar 13, it is advantageous for the metalens to be able to achieve a wide range of phase changes, such as 90 to 450 degrees, when the packing density is varied in the range of 10% to 70%. On the other hand, as is clear from Figure 6, if the height H of pillar 13 is set to approximately 1.5 μm, it becomes difficult to secure a sufficient phase change regardless of how much the packing density is changed. Thus, in the metalensor 10, a larger pillar height H is advantageous because it allows for a larger range of parameter variation in controlling the phase difference of light.

[0026] In particular, when calculating with a meta-atom with a refractive index of approximately 1.45, similar to that of quartz glass, if we want to shift the phase of the light passing through pillar 13 by up to about one wavelength relative to the phase of the light passing through the air layer, then, based on the phase amount shown in Figure 6, the minimum height H required for pillar 13 is approximately 2.5 μm.

[0027] However, microfabrication technology in the field of optics is still under development, and there are many technically difficult aspects. In particular, when producing the metalens 10 using methods such as nanoimprint, the unlimited increase in the height H of the pillars 13 leads to a decrease in release properties due to the anchoring effect, and there is also a high probability of damage or breakage of the pillars 13 when separating them from the mold. Therefore, in the manufacturing of the metalens 10 by nanoimprint, it has been considered difficult to make the height H of the pillar 13, which is such a microstructure, large relative to the lateral length φ of the pillar 13. This is the technical challenge that makes it difficult to realize ultra-high aspect ratio metaatoms using the nanoimprint process. In this embodiment, the ratio of the diameter φ of the pillar 13 to the height H of the pillar 13 is expressed as the aspect ratio. However, there are various definitions of such an aspect ratio, and it is also acceptable to express the aspect ratio as the ratio of the pitch p to the height H of the pillar 13.

[0028] Furthermore, even if a method other than nanoimprinting, such as lithography, is used, the height H of the pillar 13 leads to an extension of the etching time, so the problem of difficulty in improving productivity remains the same.

[0029] Thus, for manufacturing reasons, there is a need for a method that minimizes the physical length required to excavate the substrate portion 11 in order to form the pillar 13.

[0030] As mentioned above, in the metalens 10, increasing the height H of the pillar 13 is required to secure the phase amount, but in reality, it is difficult to increase the height H indefinitely. Therefore, a method was needed to ensure a sufficient amount of light phase change while keeping the height of pillar 13 low. In other words, this was a technical challenge to maintain the optical distance of pillar 13 while minimizing its physical distance.

[0031] The phase change is caused by the difference in optical path length between light passing through the air layer and light passing through the pillar 13, which is a meta-atom. As mentioned earlier, it changes depending on the height H of the pillar 13, and can also be ensured by the refractive index of the material that makes up the pillar 13.

[0032] For example, if a high-refractive index material of about 1.7 can be used for pillar 13, then, performing the same reasoning as used in Figures 5 and 6, the ratio of refractive index 1.7 to refractive index 1.45 indicates that the height H required to secure the necessary optical path length (≒phase amount) is 1.7 μm, thus shortening the physical length required to excavate the substrate portion 11 to form pillar 13. However, developing high-refractive index materials is difficult, and a challenge is anticipated: for example, when processing an optical material with a refractive index of 1.7 as the substrate 11, high costs are unavoidable.

[0033] Various methods are being considered to solve these problems. One such method is the example shown in Figure 7, which is a metasurface optical element, the metalens 20, in which a coating layer 24 is formed on the surface of the transmissive surface 22 on which the pillars 23 are formed, as a surface coating that does not have an absorption band in the ultraviolet range. In the metalens 20, the substrate portion 21 and the pillars 23 are made of resin for optical lenses. However, the configuration is not limited to this, and like the substrate portion 11, it may also be made of quartz glass. In this embodiment, it is particularly preferable to use either tantalum pentoxide (Ta2O5) or niobium pentoxide (Nb2O5) as the coating material for the coating layer 24. This is because it is preferable to use a material with a higher refractive index than the optical lens resin constituting the pillar 23 in order to improve the refractive index, and furthermore, compared to photocatalytic materials such as titanium dioxide (TiO2) that simply have a high refractive index, it does not have an absorption band in the near-ultraviolet region around 300 nm to 400 nm, thus preventing deterioration of light transmission characteristics originating from the coating material.

[0034] Furthermore, in order to ensure high productivity, production of Metalens 20 by nanoimprinting is being considered, as will be discussed later. However, the materials used in nanoimprinting are generally organic materials, and if titanium dioxide is used as a coating material, there is a concern that the photocatalytic effect of titanium dioxide due to absorption in the near-ultraviolet range may cause degradation of the imprint material. In this regard, by using materials such as tantalum pentoxide or niobium pentoxide, which have a higher refractive index than quartz glass and do not have an absorption band in the ultraviolet region, as the coating material, it is possible to form the coating layer 24 without causing deterioration of the imprint material. In other words, this improves the freedom of material selection for the substrate portion 21. In addition to the effect of improving the refractive index by providing the coating layer 24, it also makes it easier to use organic materials such as optical lens resin for the substrate portion 21, thus contributing to improved productivity of the metalens 20.

[0035] For example, research in Non-Patent Document 2 reveals a concept to enhance the functionality of meta-atoms in metasurface optical elements using low refractive index materials by depositing a high refractive index coating material on a microstructure. Thus, it has become clear that by forming the coating layer 24 using a coating material with a higher refractive index than the constituent material of the pillar 23, the function as a metaatom is enhanced not only by the filling rate of the pillar 23, but also by the refractive index of the coating material.

[0036] For example, as schematically shown in Figure 8(a), while a height of 2.5 μm was required for pillar 13 with a refractive index n2 = 1.45 for quartz glass, by providing the pillar 23 and coating layer 24 of the present invention, when viewing pillar 23 as a composite material, the thickness of the coating layer 24 and the diameter of pillar 23 can be determined so that the refractive index becomes pseudo-1.7, thereby achieving the same optical path length as in the case of quartz glass with a shorter physical length, as shown in Figure 8(b). For example, if we assume that pillar 23 has a refractive index n3 = 1.7 for the optical path length n2·H of pillar 13, then for incident light L with a wavelength of 600 nm, H = 1.7 μm is sufficient. Therefore, it is possible to form the pillar 23 necessary as a meta-atom without unnecessarily increasing the aspect ratio on the transmission surface. Therefore, by improving the refractive index, the optical properties can be enhanced, and productivity can be improved by enabling the manufacture of the metalens 20 using nanoimprint. Furthermore, the refractive index n3 of the pillar 23 as a composite material can be controlled to some extent by the thickness of the coating layer 24 of niobium pentoxide, which has a refractive index of 2.3. Also, as described in Figures 5 and 6, the refractive index of the metalens 20 as a whole is determined by appropriately controlling the phase amount of transmitted light by the filling density of the pillar 23. Therefore, the refractive index n3 of the pillar 23 when viewed as a composite material, depending on the thickness, is a parameter that can be set to some extent from the refractive index of the metalens 20 as a whole.

[0037] Furthermore, in the present invention, the coating layer 24 is formed of a material that does not have an absorption band in the near-ultraviolet region among the light rays that can transmit through the metalens 20, such as tantalum pentoxide (Ta2O5) and / or niobium pentoxide (Nb2O5). With this configuration, by providing the coating layer 24, it is possible to improve the pseudo refractive index of the pillar 23 while preventing deterioration of optical properties. Furthermore, in this embodiment, a photocatalytic material such as TiO2 is not used in the coating layer 24. With this configuration, the risk of organic materials used in the nanoimprint process self-decomposing due to hydroxyl radicals generated by the coating layer 24 is suppressed, which allows for a wider range of material selection for the substrate portion 21 and pillar 23, contributing to improved productivity.

[0038] Now, let me explain how to achieve such a transparent surface 22. Conventionally, methods such as electron beam lithography (EBL) were used to form multiple microstructures like those shown in Figure 2. Because this method has a resolution of less than 10 nm, it is widely used for fabricating metasurfaces. However, it is time-consuming and costly, so there has been a demand for more efficient manufacturing methods. As a method for manufacturing such a large area and with high efficiency, for example, a method of transferring a prototype 81 formed using lithography by nanoimprint is known.

[0039] An example of a method for manufacturing an optical element including a metalens 20 will be explained with reference to Figure 9. As shown in Figure 9(a), first, a substrate 86 is formed, which will be the material that constitutes the substrate portion 21 of the metalens 20 (step S101 in Figure 10). A prototype pattern 81 with cylindrical or polygonal prism-shaped voids formed according to processing data for a shape where the irregularities are reversed compared to the pillar 23 that is ultimately to be obtained is pressed onto the substrate 86, and a layer with the same shape as the pillar 23 is formed (step S102). Step S102 is a metaatom formation process in which a plurality of pillars 23 are formed on the surface corresponding to the transparent layer 22 by molding the substrate 86 with the prototype 81. In the metaatom formation process, a pattern is formed on the surface of the substrate 86 that mimics the shape of the pillars 23 manufactured by the prototype 81. In addition, in the metaatom formation process, it is desirable that the height H from the lower end to the upper end of the formed pillar shape be determined by working backward from the height of the pillar 23 that satisfies the required height as described above. Furthermore, in this embodiment, metaatoms were formed by nanoimprinting using the prototype 81, but they may also be formed by photolithography or the like. Furthermore, as shown in Figures 11 and 12, it is also possible to form the pillar 23 and the substrate 86 from different materials by using quartz glass for the substrate 86 and forming a resin-forming layer 89 on the substrate 86. In that case, if a photosensitive resin film formation step (step S201) for forming the resin-forming layer 89 is provided after the substrate formation step S101, the pillar 23 will be formed on the uppermost resin-forming layer 89 during the metaatom formation step. The same process is also carried out for the coating process S103.

[0040] In this way, pillars 23 are formed on the transmissive surface 22 of the metalens 20. Subsequently, as shown in Figure 9(c), a coating layer 24 is formed on the upper surface of the pillar 23 by a method such as atomic layer deposition (step S103). This step is a coating step in which a surface film of a material that does not have an absorption band in the near-ultraviolet region is formed on the surface of the microstructure. The coating layer 24 formed by ALD has a thickness of approximately 20 to 50 nm and is uniformly deposited even on micro-surfaces with complex shapes, such as pillars 23. Furthermore, in this embodiment, the coating layer 24 uses a coating material that does not have an absorption band in the near-ultraviolet region, such as tantalum pentoxide or niobium pentoxide. Therefore, the coating layer 24 improves the pseudo refractive index of the meta-atom, making it possible to satisfy the optical performance (optical path length) required for a meta-atom without increasing the aspect ratio of the meta-lens 20, and also suppressing the deterioration of optical properties due to the presence of the coating layer 24. Furthermore, by using such a coating layer 24 material, it becomes possible to use organic materials in the substrate portion 21, which was difficult to select with conventional coatings such as titanium dioxide, thus contributing to improved productivity.

[0041] In addition, the metalens 20 can also be manufactured using a roll-to-roll type manufacturing apparatus 800, for example, as shown in Figure 13, in which a metaatom is formed by a prototype 81 in the same manner as shown in Figures 9 and 11 on a sheet-like optical material F wound between a feed roller 85 and a winding roller 86, and a coating layer 24 is formed by a film deposition apparatus 82 located downstream of the prototype 81 in the transport direction.

[0042] The use of the metalens 20 generated in this way will also be described. Figure 14 shows an image projection device 110 as one embodiment of the present invention, which includes a metalens 20 and a laser light source 111 that emits light, and projects at least a portion of the light transmitted through the metalens 20 onto a screen 114. The image projection device 100 also includes an image display element 112 that imparts an image to the transmitted light, and a projection optical system 113 composed of multiple optical elements such as lenses LN, with at least one of which includes the metalens 20. Although Figure 14 shows a configuration with mirrors, the present invention is not limited to this configuration. By using such an image projection device 110, it is possible to reduce the thickness of the lens by using a metasurface optical element, the metalens 20, in the projection optical system, while minimizing the influence of the absorption band caused by the coating layer 24 and realizing an image projection device using a metasurface optical element with an improved refractive index.

[0043] Figure 15 also shows a light source device 120 as another embodiment of the present invention, which includes a light source optical system 122 including a metalens 20 and a laser light source 121 that emits light. In this embodiment as well, a coating layer 24, such as tantalum pentoxide or niobium pentoxide, is formed on the surface of the pillar 23 of the metalens 20. The coating layer 24 allows for an even higher refractive index than that of a simple pillar 13-forming metalens 10, and also minimizes the influence of the absorption band caused by the coating layer 24, thus enabling the realization of a light source optical system with superior optical properties.

[0044] Figure 16 also shows an imaging device 130 equipped with an optical system 132 including a metalens 20, as another embodiment of the present invention. In this configuration, the optical system 132 is an imaging optical system for forming an image of incident light on the image sensor 131, and is composed of a plurality of lenses LN or metalenses 20. In this embodiment as well, a coating layer 24, such as tantalum pentoxide or niobium pentoxide, is formed on the surface of the pillar 23 of the metalens 20. The coating layer 24 allows for an even higher refractive index than that of a simple pillar 13-forming metalens 10, and also minimizes the influence of the absorption band caused by the coating layer 24, thus enabling the realization of an optical system with better optical properties.

[0045] Figure 17 also shows an optical scanning device 140, which comprises a laser light source 141 and a scanning optical system 142 including a metalens 20, as another embodiment of the present invention. In this configuration, the scanning optical system 142 includes, along with various optical elements such as a coupling lens 143, an aperture 144, a cylinder lens 145, and a folding mirror 146, a polygon mirror 147 that rotates to reflect light off a side mirror and scan the reflected light, and a metalens 20 that functions as an fθ lens. With this configuration, the rotation of the polygon mirror 147 causes light to be irradiated onto the surface of the photoreceptor 149, which is the target of illumination, along the main scanning direction and the sub-scanning direction. In this embodiment as well, a coating layer 24, such as tantalum pentoxide or niobium pentoxide, is formed on the surface of the pillar 23 of the metalens 20. The coating layer 24 allows for an even higher refractive index than that of a simple pillar 13-forming metalens 10, and also minimizes the influence of the absorption band caused by the coating layer 24, thus enabling the realization of an optical system with better optical properties. Furthermore, in an optical scanning device 140 that utilizes monochromatic light such as a laser light source 141, it is easy to create a coating layer 24 that does not have an absorption band for the light being used by appropriately setting the material of the coating layer 24. This configuration makes it possible to realize an optical scanning device with better optical characteristics.

[0046] The embodiments of the present invention are as follows. [1] The metalens 20 of the present invention is a transmissive metalens that adjusts the refractive index for light by the density of the pillars 23, which are microstructures, and the horizontal dimension d of the microstructure with respect to the transmissive surface 22 satisfies λ≧d with respect to the wavelength λ of light. Furthermore, the metalens 20 is characterized in that the density of the pillars 23 is determined by a phase quantity corresponding to the phase wavefront of light, and a coating layer 24 of a material that does not have an absorption band in the near-ultraviolet region is formed on the surface of the pillars 23. With this configuration, the refractive index of a metasurface optical element can be improved by forming a surface coating on the surface of the microstructure that does not have an absorption band in the near-ultraviolet region.

[0047] [2] The metalens 20 of the present invention is a metasurface optical element as described in [1], characterized in that the surface film constituting the coating layer 24 is formed of Ta2O5 and / or Nb2O5. With this configuration, the refractive index of a metasurface optical element can be improved by forming a surface coating on the surface of the microstructure that does not have an absorption band in the near-ultraviolet region and has a high refractive index.

[0048] [3] In addition to the configuration described in [1] or [2], the metalens 20 of the present invention is characterized in that the coating layer 24 of the metalens 20 has a refractive index greater than that of the material constituting the pillar 23. With this configuration, the refractive index can be made even higher than that of a simple metalens 10 with pillars 13, and the influence of the absorption band by the coating layer 24 can be kept small, making it possible to realize a metalens 20 with better optical properties.

[0049] [4] Furthermore, the present invention is an image projection device 110 having a metalens 20 described in any one of the configurations described in [1] to [3] and a light source 111 that emits light, and projecting at least a portion of the light that has passed through the metalens 10 onto a screen 114 which is a projection surface. This configuration makes it possible to realize a refractive metalens 20 with higher performance and transmission capabilities, thereby reducing the thickness of the optical elements in the image projection device 110, contributing to space saving and improved optical characteristics.

[0050] [5] Furthermore, the present invention is a light source device having a metalens 20 described in any one of the configurations described in [1] to [3] and a light source 121 that emits light. With this configuration, the refractive index can be made even larger than that of a simple metalens 10 with pillars 13, and the influence of the absorption band by the coating layer 24 can be kept small, making it possible to realize a light source optical system with better optical properties.

[0051] [6] Furthermore, the present invention is an imaging device 130 equipped with an optical system 132 including a metalens 20 described in any one of the configurations described in [1] to [3]. With this configuration, the refractive index of the metalens 20 can be made even higher than that of the metalens 10 which simply has pillars 13 formed on it, and the influence of the absorption band by the coating layer 24 can be kept small, making it possible to realize a light source optical system with better optical properties.

[0052] [7] Furthermore, the present invention is an optical scanning device 140 equipped with an optical system including a metalens 20 described in any one of the configurations described in [1] to [3]. With this configuration, the refractive index can be made even larger than that of a simple metalens 10 with pillars 13, and the influence of the absorption band by the coating layer 24 can be kept small, making it possible to realize a light source optical system with better optical properties.

[0053] Although preferred embodiments of the present invention have been described above, the present invention is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the spirit of the invention as described in the claims, unless otherwise specifically limited in the above description. The effects described in the embodiments of the present invention are merely illustrative of the most preferred effects that may arise from the present invention, and the effects of the present invention are not limited to those described in the embodiments. [Explanation of symbols]

[0054] 10. Metasurface optical elements (metalens) 12...Transparent surface 13. Microstructures (pillars) 20. Metasurface optical elements (metalens) 21... Circuit board section 22...Transparent surface 23. Microstructures (pillars) 24. Coating layer 100...optical device 101...Light source optical system (optical system) 110...Image projection device 114...Projection surface 130... Imaging device 140... Optical scanning device d...dimensions n1, n2... refractive index λ...Wavelength [Prior art documents] [License]

[0055] [License 1] Special Announcement No. 2022-502715 [License 2] Special Announcement No. 2024-45435 [License 3] Special Announcement No. 2024-000520 [Non-licensed literature]

[0056] [Non-licensed Document 1] ACS Photonics 2024, 11(3), 816-865 Publication Date:February 27, 2024 [Non-licensed Document 2] Scalable manufacturing of high-index atomic layer-polymer hybrid metasurfaces for metaphotonics in the visible Nature Materials volume 22, pages474-481 (2023)

Claims

1. A metasurface optical element in which a plurality of microstructures are continuously formed on the light-transmitting surface, and the refractive index for light is adjusted by the density of the microstructures, The dimension d of the microstructure in the horizontal direction with respect to the transmission surface satisfies λ ≥ d with respect to the wavelength λ of the light. The metasurface optical element is characterized in that the density of the microstructure is determined by a phase quantity corresponding to the phase wavefront of the light, and a surface coating of a material that does not have an absorption band in the near-ultraviolet region is formed on the surface of the microstructure.

2. A metasurface optical element according to claim 1, The metasurface optical element is characterized in that the surface coating is formed of Ta2O5 and / or Nb2O5.

3. A metasurface optical element according to claim 1, The surface coating is characterized by having a refractive index greater than that of the material constituting the microstructure.

4. A metasurface optical element according to claim 1, The aforementioned microstructure is a metasurface optical element characterized by being formed of quartz glass.

5. A metasurface optical element according to claim 1, The aforementioned microstructure is a metasurface optical element characterized by being formed from an organic material.

6. A metasurface optical element according to any one of claims 1 to 5, The light source emits the aforementioned light, An image projection device that projects at least a portion of the light transmitted through the metasurface optical element onto a projection surface.

7. A light source optical system including a metasurface optical element according to any one of claims 1 to 5, A light source device having a light source that emits the aforementioned light.

8. An imaging apparatus comprising an optical system including a metasurface optical element according to any one of claims 1 to 5.

9. An optical scanning apparatus comprising an optical system including a metasurface optical element according to any one of claims 1 to 5.

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