Metasurface optical element, projection optical system, image projection apparatus, light source apparatus, imaging apparatus, and optical scanning apparatus

Abstract

In an optical system including a lens and a curved mirror, a metasurface optical element having a mirror function is used to form a curved surface into a flat surface, thereby saving space and suppressing a decrease in image quality due to a temperature change.SOLUTION: According to the present invention, there is provided a single optical element including a metasurface surface in which a plurality of fine structures are continuously formed on a light transmission surface and a refractive index with respect to the light is adjusted by density of the fine structures, and a flat mirror surface that reflects the light passing through the metasurface surface. A dimension d of the fine structure in a horizontal direction with respect to the transmission surface satisfies λ ≥ d with respect to a wavelength λ of the light, and the optical element bends the light in a direction different from an incident direction by reflecting the light bent by the metasurface surface by the mirror surface.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a metasurface optical element, and a projection optical system, an image projection device, a light source device, an imaging device, and an optical scanning device that use a metasurface optical element. [Background technology]

[0002] Metamaterials, which consist of periodic structures smaller than the wavelength, are artificial materials that do not exist in nature, and their two-dimensional form is called a metasurface (see, for example, Non-Patent Document 1). Since the spacing or size of the tiny periodic structures (metaatoms) that make up metasurfaces depends on the wavelength, in the field of relatively large wavelength ranges (Radio Frequency), active development is underway, including intelligent reflectors and beam-scanning antennas.

[0003] In recent years, advances have been made in the application of semiconductor processing technology to precisely engrave glass and dielectrics, and the development of optical elements using such metasurfaces is also progressing in the optical field, which has short wavelength ranges. However, development of metasurfaces in the optical field has focused mainly on lenses, and no proposals have been made for reflective optical elements such as mirrors. Such mirror surfaces are placed at the final stage of the projection optical system, particularly in UST-type projectors, and often have a relatively large surface area within the optical system in order to cover the entire range of incident light beams. Furthermore, in order to achieve the desired performance on the image plane, advances have been made in functionality, such as applying free-form surface correction and providing power, but the volume of the mirror itself has become larger, which has been an issue (see, for example, Patent Documents 1 and 2). On the other hand, if metasurface technology can be used to create a flat optically functional surface with the same functionality as a mirror surface with such complex shapes, it will be possible to significantly reduce the space required compared to optical systems that use conventional concavely curved folding mirrors.

[0004] Furthermore, when the thickness of a conventional concave curved mirror is reduced to make it smaller and lighter, the heat capacity also decreases, which is known to cause problems with changes in the shape of the reflective surface due to temperature changes. This type of shape change is known as temperature drift, which manifests itself in projectors as distortion of the projected image (or out of focus), and this is also a problem (see, for example, Patent Documents 3 and 4). Since metasurface mirrors have a flat shape, they are expected to be effective against such temperature drift. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention is intended to solve the above-mentioned problems, and aims to use a metasurface optical element with mirror function in an optical system that includes a lens and a curved mirror, thereby saving space by turning the curved surface into a flat surface and suppressing the deterioration of image quality due to temperature changes. [Means for solving the problem]

[0006] The metasurface optical element of the present invention is a single optical element having a metasurface surface in which a plurality of microstructures are continuously formed on a light-transmitting surface, and which adjusts the refractive index for the light by the density of the microstructures, and a flat mirror surface that reflects the light that has passed through the metasurface surface, wherein the horizontal dimension d of the microstructures relative to the transmitting surface satisfies λ≧d where λ is the wavelength λ of the light, and the optical element is characterized in that the light bent by the metasurface surface is reflected by the mirror surface, thereby bent the light in a direction different from the incident direction. [Effects of the Invention]

[0007] According to the present invention, by using a metasurface optical element with mirror function in an optical system including a lens and a curved mirror, space can be saved by converting the curved surface into a flat surface.In addition, since the imaging position of light is determined by the diameter and pitch of the pillars that make up the microstructure, the curvature does not change due to expansion and contraction caused by temperature changes, as occurs with curved mirrors.Therefore, the imaging position can be maintained stably even if there are temperature changes, and it is possible to suppress degradation of image quality. In addition, in metasurfaces, by adjusting the distribution of filling rate from on the axis to the periphery of the metamirror, the degree of convergence of light incident on the metamirror and the direction in which the light exits can be separated and designed as desired.For example, in a curved mirror, each light beam portion can be designed to have a positive refractive power to converge light onto the image plane, while the mirror as a whole can be designed to have a negative refractive power to expand the angle of view. This allows for wider angle projection or scanning, which allows the diameter of the metamirror to be smaller, increasing the number of metamirrors that can be cut out from a single wafer and reducing costs. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing a schematic configuration of a metasurface optical element according to a first embodiment of the present invention. FIG. [Figure 2] FIG. 2 is a diagram schematically illustrating an example of the microstructure of the surface of the metasurface optical element shown in FIG. 1. [Figure 3] FIG. 1 is a diagram schematically illustrating refraction by a convex lens. [Figure 4] FIG. 3 is a diagram illustrating a schematic change in refractive index caused by the microstructure shown in FIG. 2. [Figure 5] FIG. 1 is a diagram showing an example of the surface state of a metasurface. [Figure 6] 1A to 1C are diagrams illustrating an example of a method for generating a metasurface optical element. [Figure 7] FIG. 1 is a flow diagram illustrating an example of a method for generating a metasurface optical element. [Figure 8]10A to 10C are diagrams illustrating another example of a method for generating a metasurface optical element. [Figure 9] 9A to 9C are diagrams illustrating an example of the configuration of a metasurface optical element generated by the generation method of FIG. 8. [Figure 10] FIG. 1 is a diagram showing an example of the configuration of a conventional image projection device using a refractive / reflective optical element. [Figure 11] FIG. 1 is a diagram showing an example of the configuration of an image projection device using a metasurface optical element according to the present invention. [Figure 12] FIG. 10 is a diagram showing another example of an image projection device using the metasurface optical element of the present invention. [Figure 13] FIG. 1 is a diagram showing an example of the configuration of an optical device using the metasurface optical element of the present invention. [Figure 14] FIG. 1 is a diagram showing an example of the configuration of an imaging device using the metasurface optical element of the present invention. [Figure 15] FIG. 1 is a diagram showing an example of the configuration of an optical scanning device using the metasurface optical element of the present invention. [Figure 16] 16 is a diagram illustrating an example of the configuration of the optical scanning device illustrated in FIG. 15 in the sub-scanning direction. [Figure 17] 16 is a diagram illustrating an example of the configuration of an optical path in the optical scanning device illustrated in FIG. 15. FIG. [Figure 18] FIG. 16 is a diagram showing an example of the distribution of the filling rate of the metasurface optical element shown in FIG. 15. [Figure 19] 16 is a diagram illustrating an example of the configuration of the optical scanning device illustrated in FIG. 15 in the sub-scanning direction. [Figure 20] FIG. 1 is a diagram showing an example of the configuration of an optical scanning device that uses the metasurface optical element of the present invention and a two-dimensional deflection mirror. [Figure 21] 21A and 21B are diagrams illustrating an example of the operation of the optical scanning device illustrated in FIG. 20 in the main scanning direction. [Figure 22] FIG. 21 is a diagram showing an example of the distribution of the filling rate of the metasurface optical element shown in FIG. 20. [Figure 23] FIG. 23 is a diagram showing an example of the configuration of an enlarged projection device using the metasurface optical element shown in FIG. 22. DETAILED DESCRIPTION OF THE INVENTION

[0009] FIG. 1 shows an example of the configuration of a metasurface optical element as a first embodiment of the present invention. In the figure, a metamirror 10 shown as an example of a metasurface optical element is an optical element that has a substrate 11 made of plate-shaped quartz glass, a plurality of cylindrical pillars 13 that are microstructures provided on a light-transmitting surface 12 of the substrate 11, and a reflecting surface 14 on the surface opposite the transmitting surface 12. Note that in Figure 2, the pillars 13 are particularly shown as cylindrical microstructures with a diameter φ, but are not limited to this configuration, and may, for example, have a polygonal prism whose top surface is a polygonal shape such as a triangle, square, or hexagon, or a shape that combines rectangles, or a shape that is an extension of any of these in one direction. In this embodiment, the transmitting surface 12, pillar 13, and substrate portion 11 are all made of quartz glass that transmits light, and the reflecting surface 14 is formed as a flat mirror surface that reflects light on its surface using metal plating or a metal plate.

[0010] In this case, if the interval between the pillars 13 is a sub-wavelength interval for the transmitted light, the transmitting surface 12 functions as a so-called metasurface. The wavelength of the transmitted light may be any wavelength, but in this embodiment, for example, light of λ=532 nm is used as an example of light in the visible light region.

[0011] When the diameter of the pillars 13 is φ and the distance between adjacent pillars 13 is the pitch p, the functionality of the transmitting surface 12 as a metasurface changes depending on the filling rate of the pillars 13. This is because, when a light wave enters the sub-wavelength pitch p, the pillars 13 act as meta-atoms that block the progression of the light wave. It has also been found that by changing the packing ratio of these pillars 13, that is, the density, it is possible to give the entire transmitting surface 12 a refractive index different from the refractive index of the material of the pillars 13.

[0012] To explain this point, first, as a simple example, Figure 3 shows the progression of a light wave through a convex lens, along with the phase wavefront. In Figure 3, the portion where the phases of adjacent parallel light rays match is shown as a phase wavefront by a thin line. Needless to say, the normal direction to the phase wavefront is the direction of light propagation. When light enters convex lens 200 with refractive index n, the speed of light slows down in proportion to the refractive index, and the phase wavefront is tilted relative to the parallel light beam toward the convex direction of convex lens 200. Since the direction in which the series of phase wavefronts travels is the direction in which light travels, the direction in which light travels is tangent to the series of phase wavefronts, and the light is bent toward the optical axis of convex lens 200 as shown in FIG.

[0013] 4(a) and 4(b) are schematic diagrams showing the effect of pillars 13 on a metasurface, i.e., a transmitting surface 12. As shown in Fig. 4(a), when pillars 13 smaller than the wavelength λ are arranged in a forest on the transmitting surface 12, light of wavelength λ does not recognize the pillars 13 as individual cylinders on the transmitting surface 12, but rather recognizes them as the refractive index of the transmitting surface 12, which varies depending on the filling rate.

[0014] That is, on the transmitting 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 standing pillars 13. The magnitude of such pseudo-refractive index n2 is roughly shown in Figure 4(a) by color-coding according to the gradation intensity.

[0015] Therefore, as shown in Figure 4(b), when parallel light rays are incident on the transmitting surface 12 from a direction perpendicular to the transmitting surface 12, the phase wavefront of the light rays incident on the transmitting surface 12 will move more slowly in areas with a larger refractive index, as shown in Figure 4(b). As already mentioned, the magnitude of the refractive index of the transmitting surface 12 depends on the filling rate at which the pillars 13 are formed. In other words, for the transmitted light, a change in the filling rate of the pillars 13 on the transmitting surface 12 is equivalent to a change in the optical path length of 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 at the transmitting surface 12 equal to the phase wavefront that is delayed due to the difference in optical path length caused by the thickness of the lens, it is possible to achieve a refractive effect similar to that of the convex lens 200. In this way, if the pillars 13 are of the subwavelength order with respect to the wavelength λ of light and the refractive index of the transmitting surface 12 is gradient due to their filling rate, as shown in Figure 4(b), the progression of the wavefront will be slowed down by the filling rate of the pillars 13, and the phase wavefront will be tilted to the left as shown in Figure 4(b). When multiple pillars 13 of the subwavelength order are arranged in a regular pattern, this phenomenon plays a role similar to that of atoms for light waves traveling within a molecular crystal lattice, distorting the phase wavefront as if it were refracted. Conversely, if such distortion of the phase wavefront can be artificially created by the regularity of the pillars 13, the transmitting surface 12 will have an optical function equivalent to that of a lens surface with a refractive index n2. This is the simple principle of optically functional surfaces called metasurfaces, and in such optically functional surfaces, microstructures such as pillar 13 are sometimes called meta-atoms, likening them to atoms.

[0016] Now, it is known that the condition for the phase wavefront to change based on this principle is practically equivalent to conditional formula (1). Here, the wavelength of the transmitted light is λ, and the diameter of the pillar 13 is φ. Furthermore, for shapes other than cylinders, this formula can also be written as λ≧d, where d is the horizontal dimension of the microstructure relative to the transmitting surface 12. For example, if the pillar 13 is a rectangular prism, d can be the length of the long side / short side or the diagonal, or if the pillar 13 is a triangular prism, it can be the length of one side. In either case, the geometrically most typical distance when viewing the pillar 13 from the vertical direction can be treated as the horizontal dimension: d.

[0017]

number

[0018] Extending this concept, it can be seen that since adjacent light waves arrive with a delay, the degree of inclination of the convex lens can be reproduced in a pseudo manner by varying the 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 conditional formula (1), the refractive index n2 at each point on the transmitting surface 12 can be adjusted by adjusting the density of the pillars 13, which are called meta-atoms.By distributing the refractive index n2 of the transmitting surface 12, for example, concentrically, it is possible to obtain a refracted wavefront similar to that of a convex lens 200 simply by passing light through the flat transmitting surface 12.

[0019] As described above, the pseudo refractive index n2 of the transmitting surface 12 can be controlled by the degree of density of the pillars 13, i.e., the area (filling rate) occupied by the pillars 13 in a plane perpendicular to the transmitted light. In this embodiment, the degree of density is adjusted by changing the diameter φ of the pillars 13 to change the filling rate of the pillars 13.

[0020] In other words, if it is possible to create a distribution of the diameters of the pillars 13 such that the phase wavefront of the transmitted light passing through the transmitting surface 12 matches the phase wavefront of the light passing through a convex lens with a refractive index n2, the light passing through such transmitting surface 12 will be the same as light that has passed through a convex lens with a refractive index n2. By applying this, it is possible to realize a functional surface in which the phase wavefront of the outgoing light that passes through the transmitting surface 12, is reflected by the reflecting surface 14, and passes through the transmitting surface 12 again and emerges is the same as that of a free-form concave mirror. Alternatively, although a concave mirror is used here, it is also possible to reproduce a convex mirror with a flat transmitting surface 12 and reflecting surface 14 in the same way.

[0021] In this way, by appropriately changing the filling rate of the pillars 13 formed on the transmitting surface 12 and controlling the phase wavefront of light at various points on the transmitting surface 12, the metamirror 10 can be given various optical lens-like functions in addition to the reflective function of the reflecting surface 14, while remaining in a macroscopically flat shape. Utilizing this, it is possible to impart effects similar to those of a concave or convex mirror to a flat metasurface optical element. FIG. 5 shows an example of the transmitting surface 12 of such a meta-mirror 10 and an enlarged view in which the pillars 13 are visible. The pillars 13 formed on the transmitting surface 12 of this embodiment shown in FIG. 5 are sufficiently small compared to the wavelength of the light to be transmitted.

[0022] Detailed design conditions and manufacturing methods are omitted as they are described in the prior art, but simply using ray simulation software, for example, to quantify the phase wavefront of incident light and find a correspondence between the numerical value and the filling rate results in a value that is like a discretized lens surface, and the range of possible density becomes wide. In addition, the diameter of the pillars 13 formed in the meta-mirror 10 is usually finite, and when the filling rate is controlled by the diameter of the cylindrical pillars 13, the upper surface of the pillars 13 is circular, so the filling rate of the circle relative to the rectangle is approximately 78% at most. The pillars 13 must be arranged so that their diameters are at most equal to or less than the wavelength. Therefore, the metasurface properties of the metamirror 10 cannot be fully utilized if the filling factor is determined solely by a coefficient proportional to the numerical value of the phase wavefront. Therefore, the filling factor of the pillars 13 must be kept within a certain range.

[0023] Therefore, a typical method for making a phase wavefront equivalent to that of a refractive lens using a filling factor is to digitize the phase wavefront, and then calculate the remainder when dividing each value by 2π as the filling factor. The phase wavefront quantified by this calculation method has a filling factor that varies with a period of 2π, and so concentric light and dark lines appear in accordance with this phase period, as shown in Figure 5. Therefore, if the transmitting surface 12 is formed using a method other than these, it is possible to create a meta-mirror 10 that does not exhibit the light intensity variations shown in Figure 5.

[0024] Now, a method for generating such a transmitting surface 12 will be described. Conventionally, methods such as electron beam lithography (EBL) have been used to form multiple microstructures such as those shown in Figure 2. This method has a resolution of less than 10 nm and is therefore widely used to fabricate metasurfaces. However, this method is time-consuming and cost-intensive, so a more efficient manufacturing method has been sought. As a method for manufacturing such a large area and with high efficiency, for example, a method of transferring by nanoimprint using a prototype 81 formed by lithography is known.

[0025] An example of a method for manufacturing an optical element including such a meta-mirror 10 will be described with reference to FIGS. As shown in Figure 6(a), synthetic quartz glass 88 is first formed on a quartz substrate 86, which will be the material that will form the substrate portion 11 of the meta-mirror 10 (step S101 in Figure 6). Note that the reflective surface 14 may be formed on the non-processed surface of the substrate 86 from the beginning, but here we will describe a manufacturing method in which the reflective surface 14 is formed as a reflective thin film by vacuum deposition or the like in the final step. Next, a resist layer 89 made of a photosensitive resin is formed in a layer shape on the synthetic quartz glass 88 (step S102). A pattern prototype 81 having cylindrical or polygonal prism-shaped voids formed therein according to processing data having a shape that is the reverse of the desired pillar 13 in terms of concavity and convexity is pressed against a resist layer 89 formed on synthetic quartz glass 88, and a layer having the same shape as the pillar 13 is formed (step S103). Step S103 is a mask formation process in which a mask pattern is formed using the resist layer 89. In the mask formation process, a pattern is formed on the surface of the synthetic quartz glass 88 using the resist layer 89 so as to resemble the shape of the pillars 13 to be manufactured. In this mask formation process, it is desirable that the height from the bottom to the top of the pillar shape formed by the resist layer 89, in other words, the thickness of the resist layer 89, be approximately the same as the thickness of the synthetic silica glass 88 layer. In addition, in this embodiment, the mask is formed by nanoimprint using the prototype 81, but it may also be formed by photolithography or the like.

[0026] Next, a dry etching process such as ECR plasma etching or RIE etching is performed using an etching gas that is a mixture of oxygen gas for etching photosensitive resin and fluorocarbon gas for etching synthetic quartz glass (step S104). In the etching process of step S104, the resist layer 89 and the synthetic quartz glass 88 are etched in a state in which the shape of the resist layer 89 is transferred onto the synthetic quartz glass 88, as shown in FIG. 6(c).

[0027] When the etching process is continued until the resist layer 89 is removed, the shape of the resist layer 89 is transferred to the synthetic quartz glass 88, and pillars 13 are formed as shown in FIG. 6(d). This method can be used in a similar manner even if the substrate 86 and the synthetic quartz glass 88 are made of the same material, and may also be used for a substrate formed of multiple layers. In this embodiment, the thickness of the resist layer 89 and the thickness of the synthetic quartz glass 88 are approximately the same, and the etching rates are also approximately the same, so when the resist layer 89 is removed, the synthetic quartz glass 88 forms pillars 13 that are spaced apart from each other. In this case, if a material for the substrate 86 is selected that is not or is difficult to etch by the etching gas, pillars 13 can be formed in the same manner even if the thickness of the resist layer 89 is different from the thickness of the synthetic quartz glass 88. Similarly, the etching rate may be controlled in accordance with the layer thickness by changing the mixture ratio of the etching gas or the materials of the substrate 86 and synthetic quartz glass 88 .

[0028] As shown in FIG. 6(e), after the pillars 13 are formed on the transmitting surface 12, a metal reflective layer 87 is formed on the surface opposite to the surface on which the pillars 13 are formed (step S105). This metal reflective layer 87 forms the reflective surface 14 when viewed from the transmitting surface 12. As already mentioned, it is also possible to use a member on which the metal reflective layer 87 is formed as a mirror from the beginning on the substrate 86. Such a metal reflective layer 87 is formed using a thin film formation method such as vapor deposition, but other methods capable of forming a metal layer may also be used. The metal reflective layer 87 may be a metal thin film made of aluminum, silver, or the like, or a dielectric multilayer film.

[0029] Alternatively, as another example, as shown in Fig. 8, a metal reflective layer 87 may be provided on the upper surface of the substrate 86 between the substrate and the synthetic quartz glass 88. In that case, in the etching step shown in S104 of Fig. 7, the end layer of the etching becomes the metal reflective layer 87 as shown in Fig. 8(d). If this manufacturing method is used, the layered structure of meta-mirror 10 will be one in which reflective surface 14 is located between transmitting surface 12 and substrate 11, as shown in Figure 9. In this case, pillars 13 still function as meta-atoms and have a refractive effect, so meta-mirror 10 can be manufactured without significantly changing its functionality.

[0030] Now, some examples of how the meta-mirror 10 thus formed can be used will be described. First, as the most typical conventional example, an image projection device 110 that projects an image onto a screen 119, which is a projection surface, as shown in FIG. 10, will be described.

[0031] The image projection device 110 has a light source 111, an image display element 112 for displaying image information to be projected onto a screen 119, a refractive optical system 113 consisting of multiple lenses LN, and a refractive reflective optical element 114 arranged at the final stage of the refractive optical system 113. The refractive / reflective optical element 114 is composed of a reflecting surface member 115 and a refractive medium portion 116. The reflecting surface member 115 forms a reflecting surface, and the refractive medium portion 116 is provided in close contact with the reflecting surface, and the reflecting surface member 115 and the refractive medium portion 116 are integrally composed as a "single optical element."

[0032] According to this conventional configuration, light is refracted by the refractive medium portion 116 of the refractive / reflective optical element 114 and reflected by the reflective surface member 115, which shortens the distance between the image projection device 110 and the screen 119 while improving the controllability of the light rays and making it easier to meet the required optical design values. On the other hand, as is clear from Figure 10, in such a configuration, the volume of the refractive / reflective optical element 114 tends to become large as a result of incorporating the necessary optical design values, which may become an obstacle to achieving miniaturization.

[0033] Furthermore, due to its large size, the reflective surface member 115 is required to be as thin as possible. Furthermore, due to the demand for lighter weight, materials such as resin are often used. However, simply making the reflective surface member 115 thin and light naturally reduces the heat capacity of the reflective surface member 115, and the influence of heat-induced distortion of the reflective surface member 115 may not be negligible, particularly when a high-intensity light source 111 is used or depending on the temperature of the operating environment. Such distortion is likely to result in anisotropic expansion if the curved surface of the reflective surface member 115 is a free-form surface, which has a significant impact on the image quality of the image projection device 110. Furthermore, in order to suppress the effects of temperature changes on aberration performance, etc., a commonly known method is to combine materials with different temperature characteristics to design lenses so as to offset the changes in characteristics caused by temperature changes. However, combining different materials also means that there are limitations on the materials that can be used. It has been difficult to increase the ambient temperature of use while suppressing the effects of temperature changes on optical materials.

[0034] Therefore, the image projection device 100 shown in FIG. 11 is designed to replace such a refractive / reflective optical element 114 with a meta-mirror 10 having a metasurface. Of the components shown in FIG. 11, the light source 111, the image display element 112 for displaying image information to be projected onto the screen 119, and the refractive optical system 113 consisting of multiple lenses LN are common to the image projection device 110, and therefore the same reference numerals are used and their explanations are omitted. In this embodiment, the refractive reflective optical element 114 is replaced by a meta-mirror 10 which has the same function.

[0035] 1, 2, or 9, the metamirror 10 in this configuration has a metasurface in which multiple pillars 13 are formed continuously as a light transmitting surface 12, and the refractive index of the light is adjusted by the density of the pillars 13. It also has a reflective surface 14, which is a flat mirror surface that reflects the light that has passed through the transmitting surface 12, and both the transmitting surface 12 and the reflective surface 14 are within a single optical element. As shown in FIG. 11, the meta-mirror 10 bends light in a direction different from the incident direction by having the light bent by the transmitting surface 12 reflected by the reflecting surface 14. At this time, the filling rate of the pillars 13 formed on the transmitting surface 12 of the meta-mirror 10 is adjusted so that the phase wavefront reproduces the lens shape of the refractive medium portion 116 of the refractive / reflective optical element 114 shown in FIG.

[0036] A filling factor distribution in which the phase wavefront at the time of emission from the refractive / reflective optical element 114 coincides with the phase wavefront at the time of emission from the meta-mirror 10 can be reproduced by, for example, ray simulation. By providing a meta-mirror 10 that replicates the refractive / reflective optical element 114, the size of the optical element can be reduced, as shown in FIG.

[0037] In this way, by using the meta-mirror 10 as a substitute for the existing refractive / reflective optical element 114, it is possible to further reduce the size and weight of the folding mirror optical system by using a flat optical element that combines refractive and reflective properties. Furthermore, in this embodiment, the pillars 13 and the substrate portion 11 are both made of quartz glass, and there is no difference in their thermal expansion coefficients. In addition, the refractive power of the meta-mirror 10 is determined by the fill factor of the pillars 13, unlike the refractive / reflective optical element 114. Furthermore, because the meta-mirror 10 is a flat optical element, it is highly likely to expand isotropically even when thermal expansion occurs. In other words, even if thermal expansion occurs due to a high-intensity light source 111 or other heat source, unlike the reflective surface member 115 which uses a free-form surface, all of the components of the meta-mirror 10 will expand and contract equally, and this will not disrupt the filling relationship of the meta-atoms that controls the reflected wavefront.

[0038] For the reasons described above, the use of the meta-mirror 10 can reduce the influence of heat more effectively than the use of the refractive / reflective optical element 114, which is an optical element with a large refractive power and a free-form surface.

[0039] Furthermore, the direction of light reflection can be arbitrarily controlled by controlling the fill factor of meta-mirror 10. That is, while in Fig. 11 reflective surface 14 is placed so as to be inclined with respect to the incident direction of the light beam from refractive optical system 113, if the fill factor is further controlled to define the reflection direction as shown in Fig. 12 , the length of the housing of image projection device 100 can be shortened, which is effective in further reducing the size and weight.

[0040] In FIG. 12, the meta-mirror 10 is arranged upright in a direction perpendicular to the optical axis of the lens LN of the refractive optical system 113. This configuration reduces the dead space of the entire image projection device 100, which is effective in further reducing the size and weight.

[0041] The configuration of the meta-mirror 10 can also be used in other optical devices. 13 shows a light source device 130 having a light source optical system 131 including a meta-mirror 10 and a light source 111 that emits light. The light source optical system of such light source device 130 may use a mirror with power, and meta-mirror 10 can be used as a substitute for such a mirror. According to this configuration, the use of the flat meta-mirror 10 can contribute to space saving compared to a configuration using a concave mirror as the mirror.

[0042] Also, in Figure 14, camera 140, which is an imaging device, has a lens system 141, which is an imaging optical system that transmits light from a subject and forms an image on an imaging surface, a plurality of lenses LN that make up lens system 141, and an imaging element 142, which is a light receiving element that receives light that reaches the imaging surface.

[0043] The camera 140 also has a viewfinder 143 for checking the field of view, a metamirror 10 for deflecting light from a mirror 144 to the viewfinder 143, a shutter 145 for adjusting the exposure time, and a control unit 146 for controlling these components.

[0044] In such a camera 140, light passes through a lens system 141, passes through a shutter 145, and forms an image on an image sensor 142, thereby making it possible to capture an image. Conventionally, a prism or the like has been used to deflect light from mirror 144 to viewfinder 143. However, since meta-mirror 10 makes it possible to change the reflection direction of light rays by changing its packing ratio, using flat meta-mirror 10 can contribute to space savings compared to configurations that use prisms as mirrors. Furthermore, space can be saved by replacing a mirror with power or a plane mirror and lens configuration with the meta-mirror 10 in the optical path separating the image sensor 142 and the viewfinder 143 of the camera 140.

[0045] 11, etc., the configuration used as a projection system can be used as an imaging optical system by replacing the image display element 112 with the imaging element 142. In such a case, too, space can be saved by replacing the concave mirror with the meta-mirror 10. Furthermore, optical elements such as Fθ mirrors or a combination of a plane mirror and a lens may also be used in scanning optical systems. Therefore, if the meta-mirror 10 of the present invention is substituted for an Fθ mirror or the like, it will be possible to save space in scanning optical systems or optical scanning devices equipped with such scanning optical systems. FIG. 15 shows an embodiment of an optical scanning device 150 using the meta-mirror 10 as an Fθ mirror.

[0046] The optical scanning device 150 has a semiconductor laser 151 as a light source, a coupling lens 152, and a polygon mirror 153 as a deflector that reflects incident light with a rotating mirror surface to produce scanning light. The optical scanning device 150 also irradiates the light beam emitted from the polygon mirror 153 via the meta-mirror 10 onto a photosensitive member 155 , which is the surface to be scanned, and forms an image of the emitted light on the surface of the photosensitive member 155 . At this time, the photosensitive member 155 in this embodiment is a surface to be scanned that is irradiated with the scanning light from the polygon mirror 153, and also functions as an image forming surface.

[0047] The semiconductor laser 151 is a laser light source that emits light based on an image signal sent from another control unit or a reading unit of the image forming apparatus, and in this embodiment, the center wavelength is 780 nm. The coupling lens 152 is an optical element for converting an incident divergent light beam into a parallel light beam, and changes the incident light so that it becomes parallel in the X direction, which is the main scanning direction, and converges on the surface of the polygon mirror 153 in the Y direction, which is the sub-scanning direction, as shown in FIG. 16.

[0048] Light emitted from semiconductor laser 151 and converted into a parallel beam by coupling lens 152 is incident on polygon mirror 153, where it is refracted while continuously changing its angle in the main scanning direction, and forms a beam spot with a diameter of approximately 70 μm on the scanned surface of photosensitive member 155. In Figure 15, for the sake of simply showing the optical path from meta-mirror 10 to the surface of photosensitive body 155, the light is depicted as passing through meta-mirror 10 and exiting toward photosensitive body 155; however, in reality, as shown in Figure 17, the light refracted by meta-mirror 10 is irradiated onto the surface of photosensitive body 155.

[0049] At this time, as shown in FIG. 15, the off-axis light is incident on the meta-mirror 10 while the incident angle θ1 with respect to the normal to the transmitting surface 12 fluctuates in accordance with the deflection of the polygon mirror 153. Therefore, the transmitting surface 12 of the meta-mirror 10 is designed so that the distribution of the filling rate of the pillars 13 from on the axis to the periphery corresponds to the incident height H of the light. The distribution of this packing factor is expressed as a gradation of density at each point, with the densest parts in black and the sparsest parts in white, and the change in the packing factor around the optical axis at each cross section as a function of the incident height H of light in both the X and Y directions is shown in Figure 18. As is clear from Figure 18, in meta-mirror 10, the distribution of the pillar packing factor from on the optical axis to the periphery is designed according to the incident height H of light, so that the bias in the packing factor distribution changes sequentially according to the incident height H for the incident off-axis light flux.

[0050] This configuration determines the focusing position and emission direction of off-axis light, and the emission angle θ2, which is the scanning angle, is determined so that the image height H reaching the image plane is proportional to the incident angle θ1, which is determined by the deflection angle of the polygon mirror 153, i.e., so that scanning is performed at a constant speed. In FIG. 15, the angle of emergence from the meta-mirror 10 with respect to the light beam having an incident angle θ1 is shown as θ2.

[0051] To scan light over a wide angle of view, it is desirable to control the emission direction so that θ2, which is the scanning angle of the off-axis light beam emitted by meta-mirror 10, is enlarged relative to incident angle θ1, which is determined by the deflection angle of polygon mirror 153. In other words, it is desirable to control the emission direction so that θ2 / θ1≧1. Furthermore, in this embodiment, as shown in FIG. 18, there is an inflection point midway from on the axis to the periphery in the main scanning direction. By adjusting the filling rate of the pillars in this way, it is possible to simultaneously achieve both a positive refractive power for focusing light onto the surface to be scanned and a negative refractive power for widening the angle of view. Because the filling rate distribution has an inflection point Q in this way, the filling rate of the pillars 13 of the meta-mirror 10 changes so that it becomes lower as you move toward the periphery along the annular zone extending from the axis to the periphery, and the positive refractive power becomes larger as you move toward the periphery on the peripheral side of the inflection point. With this configuration, the meta-mirror 10 focuses each beam of light with a positive refractive power, while controlling the distribution of the filling factor of the pillars 13 so that the image plane curvature is such that the exit angle θ2, which is the angle of the exit light relative to the incident angle θ1, is larger. With this configuration, the exit angle θ2 is larger than the incident angle θ1, while the beam diameter converges, so that both the positive refractive power for focusing light on the scanned surface and the negative refractive power for expanding the angle of view are simultaneously satisfied.

[0052] In the embodiment, a single metamirror 10 is used to show a metasurface that is asymmetric with respect to the optical axis, where the change in the distribution of filling rate from on the optical axis to the periphery is different in the main scanning direction and the sub-scanning direction, in other words, a curved mirror, an example of a toroidal surface with different curvatures in the main scanning direction and the sub-scanning direction, but the present invention is not limited to such a configuration, and the metamirror 10 may also have a metasurface structure that is symmetric with respect to the optical axis. For example, as shown in Figure 19, if a long cylindrical lens 154 with curvature in the sub-scanning direction is combined and placed in the optical path between the meta-mirror 10 and the photosensitive body 155, and the function of correcting the surface tilt of the polygon mirror 153 is also shared by the long cylindrical lens 154, even if the meta-mirror 10 is an axially symmetric metasurface that follows the design of the filling rate in the main scanning direction, the focusing position in the sub-scanning direction is determined by the long cylindrical lens, so the focusing position in the sub-scanning direction can be aligned with the focusing position in the main scanning direction.

[0053] As shown in FIGS. 20 and 21, the optical scanning device 150 may use, instead of the polygon mirror 153, a two-dimensional deflection mirror 156 having mutually orthogonal rotation axes. By using a two-dimensional deflection mirror 156, a two-dimensional image can be projected onto a screen 157, which is the surface to be scanned, by scanning the light beam from the semiconductor laser 151 back and forth in the main scanning direction while sequentially moving the scanning position in the sub-scanning direction. Here, when the two-dimensional deflection mirror 156 is used, there is no need to focus light in the sub-scanning direction, so the mirror surface can be designed with even greater freedom than when a polygon mirror is simply used. For example, if the filling rate design from on-axis to the periphery to increase the scanning angle θ2 relative to the deflection angle θ1, as already explained, is also applied to the sub-scanning direction, the off-axis light emitted from the meta-mirror 10 can be magnified in both the main scanning direction and the sub-scanning direction, and can be imaged on the scanned surface. That is, with this configuration, the optical scanning device 150 can be used not only for projection onto the screen 157 but also as a scanning type enlarged projection device such as a head-up display, a head-mounted display, or a portable projector.

[0054] FIG. 22 shows an example of the distribution of pillar filling rates from on-axis to the periphery in a meta-mirror 10 used in such an enlarged projection device. Similar to the example of the toroidal surface with different curvatures in the main scanning direction and the sub-scanning direction described in Figure 18, in this example too, the meta-mirror 10 is designed with a pillar filling rate distribution from on-axis to the periphery according to the incident height H of light. Specifically, the meta-mirror 10 is a concave curved mirror with a positive refractive power, and in order to focus light on the screen 157, the filling rate is distributed so that it decreases from on the optical axis to the periphery, and the filling rate is designed so that the refractive power of off-axis light, which is farther from the imaging position, is lower than that of on-axis light.

[0055] Furthermore, as mentioned above, by designing the filling rate distribution to change gradually depending on the incident height H for incident off-axis light, the emission direction can be adjusted so that the light reaches a predetermined position within the screen 157 surface. That is, similarly to the embodiments already described, by adjusting the bias in the distribution of the filling rate from on the axis to the periphery, it is possible to simultaneously satisfy both the positive refractive power for focusing light onto the surface of the screen 157 and the negative refractive power for widening the angle of view, thereby enabling an enlarged image to be projected onto the screen 157 with a wide angle of view.

[0056] Furthermore, in the projection optical system 162 used in the projector 160, which is an ultra-short focus type image projection device as shown in Figure 23, an optical layout is often used in which on-axis light and off-axis light intersect midway between the meta-mirror 10 and the screen surface 164. In such a layout, the closer the on-axis light and off-axis light intersect at a position, the higher the magnification ratio on the screen surface 164. Therefore, by using the meta-mirror 10 to adjust the bias in the distribution of the filling rate so that the on-axis light and off-axis light intersect at a position close to each other, close-range projection is possible even when the distance between the screen surface 164 and the meta-mirror 10 is short. Therefore, by using the meta-mirror 10 in the projection optical system 162, it is possible to individually set the positive refractive power for focusing light onto the projection surface and the positive refractive power for widening the angle of view depending on the filling factor, and it is possible to achieve on a flat surface the same effect as the surface that was previously formed on a concave mirror by free-form surface processing. This flattening makes it easier to handle in terms of layout and also contributes greatly to miniaturization. 23 shows a configuration in which light rays are crossed as shown in the figure to achieve a wide angle of view and light beam convergence through positive refractive power, but the present invention is not limited to such a configuration. Although the configuration differs from the light ray diagram shown in FIG. 23, for example, the mirror surface to be reproduced by meta-mirror 10 may be a convex mirror.

[0057] The aspects of the present invention are as follows. [1] The meta-mirror 10 of the present invention is a single optical element having a plurality of pillars 13 formed continuously on a light-transmitting surface, a transmitting surface 12 that adjusts the refractive index of light depending on the density of the pillars 13, and a flat reflecting surface 14 that reflects light that has passed through the transmitting surface 12.The horizontal dimension d of the pillars 13 relative to the transmitting surface 12 satisfies λ≧d where λ is the wavelength of light, and the meta-mirror 10 is characterized in that the light bent by the transmitting surface 12 is reflected by the reflecting surface 14, thereby bending the light in a direction different from the incident direction. According to this configuration, it is possible to realize further miniaturization and weight reduction of the folding mirror optical system by using a flat optical element that has both refractive and reflective properties.

[0058] [2] The metamirror 10 of the present invention is a metasurface optical element as described in [1], characterized in that the reflecting surface 14 is formed on the surface opposite the transmitting surface 12. With this configuration, light passing through the transmitting surface 12, which is a metasurface, is refracted, and the light is reflected by the reflecting surface 14, thereby realizing a metamirror 10 that has optical functions similar to those of a powered concave mirror, while also being able to keep the spatial volume small.

[0059] [3] In addition to the configuration described in [1] or [2], meta-mirror 10 is characterized in that light incident perpendicularly to transmitting surface 12 is bent and emitted in a direction different from the perpendicular direction. With this configuration, the direction of reflection of light rays by the reflecting surface 14 can be changed to any direction, so that the meta-mirror 10 can be positioned perpendicular to the optical system of the incident light, thereby further eliminating dead space and contributing to miniaturization and weight reduction.

[0060] [4] The projection optical system including the meta-mirror 10 is a projection optical system that enlarges and projects an image displayed on the planar image display surface of the image display element 112 onto a single planar screen 119 as a projection image, and is configured by arranging, in order from the image display surface side toward the screen 119 side, a refractive optical system 113 and a meta-mirror 10, the refractive optical system 113 being composed of multiple lenses LN. The meta-mirror 10 is configured as a single optical element having a single flat reflective surface 14 and a transmitting surface 12 on which multiple pillars 13 are formed in succession and which adjusts the refractive index for light depending on the density of the pillars 13. In addition, the projection optical system including the meta-mirror 10 causes the imaging light beam emitted from the refractive optical system 113 to enter the meta-mirror 10 from the transmitting surface 12, be reflected by the reflecting surface 14, and then exit from the transmitting surface 12 to form an enlarged image on the screen 119. According to this configuration, it is possible to realize further miniaturization and weight reduction of the folding mirror optical system by using a flat optical element that has both refractive and reflective properties.

[0061] [5] In addition to the configuration described in any one of [1] to [3] or [4], the image projection device 100 of the present invention is characterized by comprising a meta-mirror 10, a light source 111, and an image display element 112. According to this configuration, it is possible to realize further miniaturization and weight reduction of the folding mirror optical system by using a flat optical element that has both refractive and reflective properties.

[0062] [6] Furthermore, the light source device including the meta-mirror 10 of the present invention is characterized in that, in addition to the configuration described in any one of [1] to [3] or [4], it has a light source optical system 131 including the meta-mirror 10 and a light source 111 that emits light. According to this configuration, it is possible to realize further miniaturization and weight reduction of the folding mirror optical system by using a flat optical element that has both refractive and reflective properties.

[0063] [7] The camera 140, which is an imaging device of the present invention, is characterized by including the meta-mirror 10 having the configuration described in any one of [1] to [3] or [4]. According to this configuration, the meta-mirror 10 allows the thickness of the optical elements in the optical device to be reduced, contributing to space saving.

[0064] [8] The optical scanning device 150 of the present invention is characterized by including a meta-mirror 10 having the configuration described in any one of [1] to [3]. According to this configuration, the meta-mirror 10 can perform wider-angle projection or scanning, so the diameter of the meta-mirror 10 can be made smaller, the number of meta-mirrors 10 that can be cut out from one wafer increases, and costs can be reduced.

[0065] [9] The optical scanning device 150 of the present invention is characterized by including a meta-mirror 10, a light source 151, and a polygon mirror 153 in addition to the configuration described in any one of [1] to [3]. According to this configuration, the function of the scanning optical system can be integrated into the folding mirror using a flat optical element that has both refractive and reflective properties, thereby realizing a smaller and lighter optical scanning device.

[0066] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to such specific embodiments, and unless otherwise specifically limited in the above description, various modifications and changes are possible within the scope of the spirit of the present invention as described in the claims. The effects described in the embodiments of the present invention are merely examples of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments of the present invention. [Explanation of symbols]

[0067] 10...Metasurface Optical Element (Metasurface Optical Element) 12...Transparent surface 13. Microstructure (pillar) 14. Reflective surface (mirror surface) 100 Image projection device 110 Image projection device 111...Light source 113...Refractive Optical Systems 119 Projection surface (screen) 130...Light source device 140 Imaging device 150 Optical scanning device 160···Projector d...dimension n1, n2... refractive index p... pitch φ Pillar diameter [Prior art documents] [Patent documents]

[0068] [Patent Document 1] Patent No. 6993251 [Patent Document 2] Patent No. 6534802 [Patent Document 3] Patent No. 5280831 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-151640 [Non-patent literature]

[0069]

Non-licensed literature 1

Claims

1. A metasurface surface in which a plurality of microstructures are continuously formed on a light transmitting surface, and a refractive index for the light is adjusted depending on the density of the microstructures; A single optical element having a flat mirror surface that reflects the light that has passed through the metasurface surface, a dimension d of the microstructure in a horizontal direction relative to the transmission surface satisfies λ≧d where λ is the wavelength of the light; The optical element is a metasurface optical element characterized in that the light bent by the metasurface surface is reflected by the mirror surface, thereby bending the light in a direction different from the incident direction.

2. The metasurface optical element according to claim 1, A metasurface optical element characterized in that the mirror surface is formed on a surface opposite to the metasurface surface.

3. The metasurface optical element according to claim 1, A metasurface optical element characterized in that light incident from a direction perpendicular to the transmitting surface is bent and emitted in a direction different from the perpendicular direction.

4. The metasurface optical element according to claim 1, A metasurface optical element characterized in that the distribution of refractive index for light incident from a direction tilted with respect to the normal direction of the transmitting surface is adjusted so that the metasurface surface emits the light at an angle different from the angle of incidence.

5. The metasurface optical element according to claim 4, A metasurface optical element characterized in that the distribution of refractive index for light incident from a direction tilted with respect to the normal direction of the transmitting surface is adjusted so that the metasurface surface emits the light at an angle larger than the angle of incidence.

6. A projection optical system that enlarges and projects an image displayed on a flat image display surface of an image display element onto a single flat projection surface as a projection image, a refractive optical system and a refractive / reflective optical element are arranged in this order from the image display surface side to the projection surface side, the refractive optical system is composed of a plurality of lenses, The refractive reflective optical element is configured as a single optical element having a flat single mirror surface and a metasurface surface on which a plurality of microstructures are continuously formed on a light transmitting surface, and which adjusts the refractive index for the light depending on the density of the microstructures; A projection optical system in which the imaging light beam emitted from the refractive optical system is incident on the refractive reflective optical element from the metasurface surface, reflected by the mirror surface, and emitted from the metasurface surface to form the enlarged image on the projection surface.

7. An image projection device comprising a metasurface optical element according to any one of claims 1 to 5, a light source, and an image display element.

8. A light source optical system including the metasurface optical element according to any one of claims 1 to 5; a light source that emits the light.

9. An imaging device comprising an optical system including the metasurface optical element according to any one of claims 1 to 5.

10. An optical scanning device comprising an optical system including the metasurface optical element according to any one of claims 1 to 5.

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