Metasurface filter, metasurface optical element, and optical system using metasurface optical element

Abstract

To provide a metasurface filter capable of correcting aberration to be improved without changing the focal length of an optical system of a preceding model or the configuration of a lens system when developing an optical product of a new model.SOLUTION: The metasurface filter MS has a thin film shape and is used by being attached to a flat surface of the optical element 10, a metasurface pattern is formed on a surface opposite to an attachment surface by distribution of metaatoms, and distribution of a phase change caused by the metasurface pattern to act on a transmitted light flux has a predetermined aberration correction function.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a metasurface filter, a metasurface optical element, and an optical system using a metasurface optical element. [Background technology]

[0002] As is well known, a "fine periodic structure with a period shorter than the wavelength of light" formed on the surface of an optical material such as glass is called a metasurface.

[0003] Metasurfaces have the ability to "change the phase" of light that passes through them, and by adjusting the pattern of the metasurface, which is composed of a two-dimensional arrangement of tiny units (called "metaatoms") that make up the metasurface, the refraction angle of the transmitted light beam can be adjusted positionally.

[0004] In other words, the metasurface can have an effect on the transmitted light beam equivalent to that of a refractive surface. For example, Patent Document 1 discloses an invention that achieves a collimating function using a metasurface.

[0005] By using the functions of such metasurfaces, it is possible to realize the functions of multiple lenses introduced for aberration correction, for example, with a single lens.

[0006] In recent years, advances in AI have led to increased development of factory automation (FA), augmented reality (AR), virtual reality (VR), and other technologies, and the demand for light-based products has diversified.

[0007] The performance of the developed product is evaluated in the market as sales performance, and the evaluation results are fed back to the development team. This feedback provides a guide for areas to improve in the development of the next product model.

[0008] In recent years, there has been a growing demand for optical products with high resolution and compact size, which requires advanced optical design technology for development and improvement.

[0009] For example, when trying to improve aberration compared to current models, problems arise such as an increase in the number of lenses, which increases the space required and affects the focal length, an increase or decrease in the number of lenses due to the use of aspherical lenses, and a re-examination of the mechanical configuration, and it is possible that developing the next model will cost the same as designing a new model from scratch. Therefore, if changes are required to the optical design of the next model, there is a problem of huge costs. Summary of the Invention [Problem to be solved by the invention]

[0010] The objective of this invention is to realize a metasurface filter that can correct the aberrations that need to be improved when improving the aberrations of subsequent optical products without making any changes to the focal length or lens system configuration of the optical system of the previous model. [Means for solving the problem]

[0011] The metasurface filter of this invention is in the form of a thin film that is attached to the flat surface of an optical element. A metasurface pattern is formed on the surface opposite to the attached surface by the distribution of metaatoms, and the distribution of phase changes that the metasurface pattern exerts on the transmitted light beam has a predetermined aberration correction function. [Effects of the Invention]

[0012] The metasurface filter of this invention is a thin filter that is attached to the plane of an optical element, making it possible to improve correction of aberrations without increasing the space required for subsequent models. [Brief explanation of the drawings]

[0013] [Figure 1] This figure shows a single lens whose spherical aberration is effectively corrected by a metasurface filter and its imaging ray. [Figure 2] FIG. 2 is a lateral aberration diagram showing the spherical aberration of the single lens provided with the metasurface filter shown in FIG. 1. [Figure 3]FIG. 1 shows a plano-convex single lens that attempts to effectively correct spherical aberration and its imaging rays. [Figure 4] FIG. 4 is a lateral aberration diagram showing spherical aberration of the single lens shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] As described above, the "metasurface filter" of this invention is in the form of a thin film and is used by being attached to the flat surface of an optical element.

[0015] A thin film metasurface filter can be configured, for example, as a "thin resin film" and adhered to an optical element with an adhesive.

[0016] Alternatively, a metasurface filter can be created by coating a transparent resin material onto the flat surface of an optical element as a thin resin layer and then forming a metasurface on the free surface. In this way, "adhesion" also includes the formation of a coating layer by painting or the like.

[0017] Alternatively, a metasurface filter can be created by forming a metasurface pattern with a minute periodic structure on one side of a resin or glass plate of 1 mm or more in thickness, and then polishing the other side.

[0018] The thickness of the filter (including the coating layer) is about 15 μm, of which the metasurface layer with power (the layer on which the metasurface pattern is formed by a fine periodic structure) is about 1 to 2 μm, and the remaining part, 13 to 14 μm, serves as the base layer that supports the metasurface layer. With the normal size of an optical system, this can be treated as an error range for the design thickness.

[0019] The optical element to which the metasurface filter is attached has at least a flat surface (adhesion surface). If the optical system to which the metasurface filter is attached is a lens, one side of the lens is flat.

[0020] That is, the optical element in this case is a plano-convex lens or a plano-concave lens, and the lens surface that is not planar is a convex or concave surface, and these surfaces can also be aspherical.

[0021] The "metasurface optical element" of this invention is a light-transmitting optical element with at least one flat surface and a metasurface filter attached (coated) to the flat surface of the element. The side of the metasurface optical element with the metasurface filter attached is the side from which the transmitted light beam emerges. Forming a metasurface on the exit side reduces the angle of incidence of the light beam to the metasurface layer, and higher efficiency can be expected than if the metasurface is attached to the entrance side.

[0022] In other words, since the meta-atoms (such as the micro-pillars described below) that make up the fine periodic structure of the metasurface layer also function as waveguides, when light is incident from the meta-atom side, light rays with a large angle of incidence leak out of the meta-atom without being totally reflected by the meta-atom's surface, reducing the light propagation efficiency.

[0023] When the metasurface layer is placed on the emission side, the incident light on the metasurface layer passes through the "material layer (base layer) with a higher refractive index than air" on the optical element side of the metasurface filter, reducing the angle of incidence on the metasurface layer, allowing the light to propagate and be emitted without reducing propagation efficiency.

[0024] Furthermore, the "optical system" of this invention is an optical system that uses the above-mentioned metasurface optical element, has one or more lenses, the surface closest to the image side of the optical system is flat, and a metasurface filter is attached (coated) to the flat surface, and the metasurface filter has an aberration correction function that "improves and corrects" the aberration of the "lens system composed of one or more lenses (including a lens with a metasurface filter attached)" on the object side.

[0025] "Improved correction of aberrations" means correction that improves the aberrations of the optical system compared to when the metasurface filter is not used.

[0026] In the above-mentioned "optical system using a metasurface optical element," the metasurface optical element with a metasurface filter attached (coated) can be positioned closest to the image side and can be a "lens with a flat image side surface."

[0027] An "optical system using a metasurface optical element" can also be an optical system in which a metasurface optical element with a metasurface filter attached thereto is placed on the image side of a lens system and is a "cover glass with flat surfaces on both sides," and the metasurface filter is attached to one of the flat surfaces of the cover glass (preferably the surface on the image side).

[0028] The surface to which the metasurface filter is attached (coated) is the lens surface closest to the image in the optical system, or the surface of the cover glass on the image side of the lens system. Therefore, when the lens system is composed of multiple lenses, the "accumulation of aberrations" caused by these multiple lenses can be effectively corrected.

[0029] The "metasurface pattern formed by the distribution of metaatoms" formed on the metasurface filter is formed so as to effectively correct the aberrations of one or more lenses located on the object side of the metasurface filter in the above-mentioned optical system.

[0030] Such metasurface patterns can be determined experimentally depending on the "aberration to be improved in the optical system," or they can be determined by computer calculation based on known data that defines the "correspondence between the two-dimensional distribution of meta-atoms and the phase change due to this distribution."

[0031] As a method for forming the metasurface pattern, for example, a nanoimprint method can be used.

[0032] The nanoimprinting mold is formed using the well-known "electron writing and anisotropic etching" technique, and the resulting mold is pressed against a softened film or coating layer, allowing it to solidify and form a metasurface pattern.

[0033] As mentioned above, a "metasurface" is composed of a two-dimensional arrangement of tiny "protrusions" called "metaatoms." An example of a metaatom is a "micropillar." "Pillar" means a pillar, and a micropillar is a tiny pillar.

[0034] The micropillars can have a variety of columnar shapes, including cylindrical, elliptical, and rectangular shapes such as square pillars. The thickness of the pillars can be the same for all micropillars, or multiple types of micropillars with different thicknesses can be used. Furthermore, two-dimensional arrangements of meta-atoms such as micro-pillars can take various forms, such as a square matrix, concentric circles, or elliptical arrangements. However, when formed on the flat lens surface of a lens, as in the metasurface optical element of this invention, if the lens is optically symmetric, the arrangement will be concentric circles centered on the optical axis.

[0035] The pitch of the micropillars is approximately half the wavelength. If the optical system is for light in the visible range, it is preferable that the pitch be approximately 200 nm for a blue light wavelength of 400 nm. The height of the micropillars is preferably approximately twice the incident wavelength. For the visible light range, a height of approximately 1.5 μm is required, with some margin from 633 nm, which is generally referred to as red. If the height is insufficient, a phase shift of 2π cannot be generated, resulting in diffraction and reduced light collection efficiency. Therefore, the thickness of the filter (including the coating layer) can be approximately 2 μm, including the above height of 1.5 μm.

[0036] As described above, the surface to which the metasurface filter of this invention is attached (coated) is the lens surface closest to the image in the optical system, or the surface of the cover glass on the image side of the lens system.Therefore, when the lens system is composed of multiple lenses, the accumulation of aberrations caused by these multiple lenses can be effectively corrected.

[0037] In other words, an existing optical system that requires improved correction of aberrations can be used as the lens system on the incident side of the attached metasurface, and improved correction of aberrations can be achieved with the same configuration as the existing optical system, with almost no change to the focal length or space of the existing optical system. As an example, the case of "effectively correcting the spherical aberration of a plano-convex lens" will be explained.

[0038] Figure 3 shows a plano-convex single lens that attempts to effectively correct spherical aberration and its imaging rays. The single lens 10 has an effective lens diameter of 20 mm, a focal length of 10 mm, and is made of BK7 manufactured by Schott.

[0039] In FIG. 3, the left side of the drawing is the object side, that is, the incident side, and the symbol IS indicates the image plane.

[0040] The single lens 10 is a plano-convex lens, with a convex surface 10A on the entrance side and a flat surface 10B on the image side, i.e., the exit side. The symbol AX denotes the optical axis.

[0041] The symbols L0, L1, and L2 are imaging rays. The imaging ray L0 is a ray of a light beam whose chief ray coincides with the optical axis, and the symbols L1 and L2 are imaging rays of an imaging light beam whose chief ray is tilted with respect to the optical axis AX. Ideally, a light beam including ray L0 should form an image at the intersection of the optical axis AX and the image surface IS, but due to spherical aberration, the image is formed at a position slightly shifted from the image surface IS in the direction of the optical axis.

[0042] Ray L2 is the ray of the imaging light beam whose chief ray is most inclined with respect to the optical axis AX and is directed to a position farthest from the optical axis AX. Ray L1 is the ray of the light beam whose chief ray passes midway between rays L0 and L2 and is directed to a position between rays L0 and L2.

[0043] If the spherical aberration of the single lens 10 were perfectly corrected, the image-forming rays L0, L1, and L2 would be focused at the correct image-forming position on the image surface IS. However, in reality, due to the existence of spherical aberration, they are not focused on the image surface IS, as shown in the figure.

[0044] FIG. 4 shows the spherical aberration present in the single lens 10 as lateral aberration. In FIG. 4, the vertical axis represents the relative lateral aberration, and the horizontal axis represents the distance from the optical axis AX on the image plane.

[0045] This figure shows the difference from the minimum value of lateral aberration, with the minimum value set to 0, and the symbol R indicates that it relates to red light, the symbol G to green light, and the symbol B to blue light.

[0046] As can be seen from FIG. 4, the "relative transverse aberration" of the spherical aberration decreases monotonically from the optical axis to the outermost periphery, and for blue light LB, 310.593 μm occurs at the outermost periphery.

[0047] In the above examples, metasurface filters are not used.

[0048] Figure 1 shows an example in which a metasurface filter MF is formed on the image-side plane 10 of a single lens 10, thereby improving correction of spherical aberration. Symbols L0, L1, and L2 denote the same imaging rays as in Figure 3.

[0049] FIG. 2 is a diagram similar to FIG. 4 showing the transverse aberration of the spherical aberration when the spherical aberration is corrected by the metasurface filter MF.

[0050] As shown in Figure 2, the relative lateral aberration of spherical aberration decreases monotonically from the optical axis to the outermost periphery, but at the outermost periphery where the lateral aberration is maximum, it is improved to 123.930 μm for blue light LB.

[0051] The metasurface filter MS is formed as a very thin film and is adhered to the image-side flat lens surface of the single lens 10 with an adhesive.

[0052] The combined thickness of the metasurface filter MS and adhesive is approximately 15 μm, so the impact on the focal length is limited to that extent, and the impact on the optical function of the single lens 10 can essentially be within the error range of the designed thickness.

[0053] Furthermore, the transverse spherical aberration is improved to less than half of the 310.593 μm observed when the metasurface filter MS in Figures 3 and 4 is not used.

[0054] In this way, by using the metasurface filter of this invention, it is possible to improve the aberrations that occur in existing optical systems using the same optical system configuration, so that old products can be reused as they are.

[0055] Furthermore, the process for fabricating the optical system only requires the addition of one step: attaching the metasurface filter, so the process for fabricating existing optical systems can be reused almost as is, and since existing optical systems can be reused as is, this also leads to a reduction in discarded parts.

[0056] Although the preferred embodiment of the invention has been described above, the invention is not limited to the specific embodiment described above, and unless otherwise specifically limited in the above description, various modifications and changes are possible within the spirit of the invention as described in the claims. The effects described in the embodiments of the present invention are merely a list of preferred effects resulting from the invention, and the effects of the invention are not limited to "those described in the embodiments." [Explanation of symbols]

[0057] 10 Single Lens 10A: Convex surface on the entrance side of the single lens 10 10B: Plane on the exit side of the single lens 10 MF metasurface filter IS image plane L0, L1, L2: Light rays formed by the single lens 10 [Prior art documents] [Patent documents]

[0058] [Patent Document 1] Special table 2018-537804 publication

Claims

1. A metasurface filter in the form of a thin film that is attached to the flat surface of an optical element. A metasurface pattern is formed on the surface opposite to the attached surface by the distribution of meta-atoms, and the distribution of phase changes that the metasurface pattern exerts on the transmitted light beam has a predetermined aberration correction function.

2. A metasurface optical element is an optically transparent element having at least one flat surface and a metasurface filter attached to the flat surface.

3. An optical system using the metasurface optical element according to claim 2, An optical system using a metasurface optical element, which has one or more lenses, the surface closest to the image side being a flat surface, the metasurface filter being attached to the flat surface, and the metasurface filter having an aberration correction function that effectively corrects aberrations in the lens system on the object side.

4. An optical system using the metasurface optical element according to claim 3, The metasurface optical element to which the metasurface filter is attached is an optical system that uses a metasurface optical element, which is a lens that is positioned closest to the image side and has a flat image-side surface.

5. An optical system using the metasurface optical element according to claim 3, An optical system using a metasurface optical element in which a metasurface filter is attached is a cover glass that is placed on the image side of the lens system and has flat surfaces on both sides, and the metasurface filter is attached to one of the flat surfaces of the cover glass.

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