Metalens and light-emitting devices

Abstract

This suppresses the increase in the difference in phase delay when two or more light sources with different wavelengths pass through a metalens. [Solution] The metalens comprises a light-transmitting substrate and a plurality of microstructures arranged in a specific region of the substrate. In the metalens, the shape of each of the plurality of microstructures is such that, when viewed in the direction of the central axis of the metalens, the width in a first direction and the width in a second direction different from the first direction are different from each other. This metalens is used, for example, in lighting devices and measuring devices.

Description

Technical Field

[0001] The technology disclosed in this specification relates to a metalens and a light-emitting device.

Background Art

[0002] A metalens (also referred to as a metasurface lens) includes a substrate that transmits light and a plurality of microstructures (also referred to as meta-atoms) disposed on the surface of the substrate. By controlling the structure, shape, orientation of each microstructure and the arrangement of the plurality of microstructures on the substrate, a metalens can refract and reflect light and radio waves in an arbitrary direction, control the phase, or absorb a specific wavelength. For each microstructure in a conventional metalens, the shape when viewed in the direction of the central axis of the metalens is circular. Such a metalens is used as an optical system provided in a measuring device such as LiDAR (light detection and ranging) or a lighting device (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Here, the amount of phase delay when light passes through a metalens depends on the wavelength of the light. Therefore, if the wavelengths of the light incident on the metalens are different, the amount of delayed phase also differs accordingly. For this reason, when two lights having different wavelengths are incident on a conventional metalens, a difference occurs in the amount of phase delay between the two lights. The difference in the amount of phase delay between the two lights may, for example, reduce the condensing property of the two lights.

[0005] This specification discloses a technology capable of solving the above-described problems. [Means for solving the problem]

[0006] The technologies disclosed herein can be implemented, for example, in the following forms:

[0007] (1) The metalenses disclosed herein include a light-transmitting substrate and a plurality of microstructures arranged in a specific region of the substrate. The shape of each of the plurality of microstructures is such that, when viewed in the direction of the central axis of the metalens, the width in a first direction and the width in a second direction different from the first direction are different from each other.

[0008] In this configuration, the shape of each microstructure constituting the metalens is such that its width differs in at least two directions. Therefore, the metalens of this configuration can reduce the difference in phase delay between at least two beams of light with different wavelengths. Specifically, each microstructure generates a phase delay for the optical component of incident light in a polarization direction along a specific direction, corresponding to the wavelength of the light and the width of the microstructure in that specific direction. Furthermore, the longer the wavelength of the incident light, the smaller the positional delay at the metalens. For example, if the width of the microstructure in a first direction is longer than the width in a second direction, the phase delay of the optical component of the first light with a relatively longer wavelength in the first polarization direction along the first direction will be greater than the phase delay of the optical component of the second light with a relatively shorter wavelength in the second polarization direction along the second direction. Therefore, this configuration can reduce the difference in phase delay between at least two beams of light with different wavelengths compared to a configuration in which the microstructure is a perfect circle.

[0009] (2) In the above-mentioned metalens, the first direction and the second direction may be configured to be orthogonal to each other. With this configuration, the shape of the microstructure can be made relatively simple.

[0010] (3) The light-emitting device comprises a metalens, a first light-emitting element that emits first light having a first wavelength and including a light component in a first polarization direction along the first direction toward the metalens, and a second light-emitting element that emits second light having a second wavelength shorter than the first wavelength and including a light component in a second polarization direction along the second direction toward the metalens. The width in the first direction of the microstructure may be longer than the width in the second direction. With this configuration, the light-emitting device can emit at least two lights with different wavelengths while reducing the difference in phase delay between at least two lights.

[0011] (4) In the above-described light-emitting device, the first light emitted by the first light-emitting element may include only the light component in the first polarization direction, and the second light emitted by the second light-emitting element may include only the light component in the second polarization direction. This configuration makes it possible to improve the light-gathering ability of the entire first light emitted by the first light-emitting element and the entire second light emitted by the second light-emitting element.

[0012] (5) A metalens disclosed herein comprises a substrate that allows light transmission and a plurality of microstructures arranged in a specific region of the substrate, wherein the plurality of microstructures have a shape that makes the phase delay amount of at least two lights that differ from each other in both wavelength and polarization state an integer multiple of 2π. This metalens can reduce the difference in phase delay amounts of at least two lights that differ from each other in wavelength.

[0013] Furthermore, the technologies disclosed herein can be realized in various forms, for example, as metalens, light-emitting devices (e.g., floodlights, measuring devices, and lighting devices) that incorporate metalens as an optical system. [Brief explanation of the drawing]

[0014] [Figure 1] A cross-sectional view schematically showing the configuration of the lighting device of the embodiment. [Figure 2]Schematic diagram showing the exit surface of the metalens of the first embodiment. [Figure 3] A schematic diagram illustrating the process by which yellow light passes through a metaatom. [Figure 4] A schematic diagram illustrating the process by which blue light passes through metaatoms. [Figure 5] Diagram illustrating a modified form of a metaatom. [Modes for carrying out the invention]

[0015] (Embodiment) This embodiment will be described with reference to Figures 1 to 4. Each figure shows mutually orthogonal X, Y, and Z axes for specifying direction. For convenience, in this specification, the X-axis direction will be referred to as the "front-back direction," the negative Y-axis direction as the "left-right direction," and the Z-axis direction as the "up-down direction." However, the lighting device 10 may actually be installed in a direction different from these. The lighting device 10 is an example of a light-emitting device.

[0016] As described later, the lighting device 10 of this embodiment includes a yellow light-emitting element 30Y and a blue light-emitting element 30B. The lighting device 10 mixes the yellow light LY emitted by the yellow light-emitting element 30Y and the blue light LB emitted by the blue light-emitting element 30B to generate white light LW, which is then irradiated to the outside. The lighting device 10 can be mounted on a vehicle as a lamp, or on a lighting fixture, for example.

[0017] As shown in Figure 1, the lighting device 10 includes a metalens 20, a yellow light-emitting element 30Y, a blue light-emitting element 30B, a semiconductor substrate 40, and a reflective member 50.

[0018] The reflective member 50 is a block body in which a through-passage 52 for passing light is formed. The through-passage 52 penetrates the reflective member 50 in a straight line in the front-to-back direction. The through-passage 52 has a shape that opens radially from the entrance 53 toward the exit 55. The inner circumferential surface 54 of the through-passage 52 is a reflective surface that is, for example, mirror-finished or has a reflective film formed on it.

[0019] The yellow light-emitting element 30Y is a light-emitting member that emits yellow light LY. The yellow light LY is visible light having a wavelength in the yellow wavelength range (550 nm or more and 590 nm or less). The yellow light LY contains only the light component of vertical polarization. The yellow light-emitting element 30Y is disposed on the incident port 53 side of the reflection member 50 and emits the yellow light LY toward the through passage 52. The yellow light LY is an example of the first light, the yellow wavelength range is an example of the first wavelength, and the direction of vertical polarization (vertical direction) is an example of the first polarization direction (first direction).

[0020] Specifically, the yellow light-emitting element 30Y includes a yellow LED 31Y and a vertical polarization element 32Y. The yellow LED 31Y is an LED (Light Emitting Diode) that emits unpolarized yellow light LYp (laser light). The vertical polarization element 32Y is an element that converts the yellow light LYp emitted from the yellow LED 31Y into vertical polarized yellow light LY. The vertical polarization element 32Y is configured to include, for example, a polarizer, a wavelength plate (such as a quarter-wave plate), a polarization separation element, a polarization filter, and the like.

[0021] The blue light-emitting element 30B is a light-emitting member that emits blue light LB. The blue light LB is visible light having a wavelength in the blue wavelength range (430 nm or more and 490 nm or less). The blue light LB contains only the light component of horizontal polarization. The blue light-emitting element 30B is disposed on the incident port 53 side of the reflection member 50 and emits the blue light LB toward the through passage 52. The blue light LB is an example of the second light, the blue wavelength range is an example of the second wavelength, and the direction of horizontal polarization (left-right direction) is an example of the second polarization direction.

[0022] Specifically, the blue light-emitting element 30B includes a blue LED 31B and a horizontal polarization element 32B. The blue LED 31B is an LED that emits unpolarized blue light LBp. The horizontal polarization element 32B is an element that converts the blue light LBp emitted from the blue LED 31B into horizontally polarized blue light LB. The horizontal polarization element 32B is configured to include, for example, a polarizer, a wavelength plate (such as a quarter-wave plate), a polarization separation element, a polarization filter, and the like.

[0023] The semiconductor substrate 40 is positioned on the side of the reflective member 50 facing the entrance 53. A yellow light-emitting element 30Y and a blue light-emitting element 30B are mounted on the surface of the semiconductor substrate 40 facing the entrance 53. The semiconductor substrate 40 also has multiple circuit elements mounted on it, for example, for controlling the light emission of the yellow light-emitting element 30Y and the blue light-emitting element 30B.

[0024] The metalens 20 is positioned on the exit port 55 side of the reflecting member 50. As shown in Figure 2, the metalens 20 comprises a substrate 22 and a plurality of metaatoms 24 arranged on the surface of the substrate 22. The metalens 20 can achieve desired optical performance by arranging a large number of metaatoms 24 on the surface of the substrate 22 and appropriately designing the shape and arrangement pattern of these metaatoms 24. In other words, the desired optical performance can be achieved with a single metalens 20. The metaatoms 24 are an example of microstructures (also called subwavelength structures).

[0025] The substrate 22 is a rectangular plate that allows light emitted from the yellow light-emitting element 30Y and the blue light-emitting element 30B to pass through. The surface of the substrate 22 includes a flat exit surface SO (the surface of the metalens) and a flat incident surface SI (the surface of the metalens) located on the opposite side of the exit surface SO. The central portion of the exit surface SO is the effective region Ab. The material of the substrate 22 is, for example, glass, TiO2, SiO2, or synthetic resin. In this embodiment, the substrate 22 is made of glass. The thickness of the substrate 22 may be, for example, 100 nm or more and 3 mm or less. The effective region Ab is an example of a specific region.

[0026] Metaatoms 24 are fine protrusions that project from the surface of the substrate 22. The metaatoms 24 are arranged in a predetermined pattern in the effective region Ab. The arrangement pattern of the metaatoms 24, as well as the shape and size of each metaatom 24, are designed according to the optical function required of the metalens 20. In this embodiment, the metaatoms 24 are designed to function as a focusing lens that concentrates light in the metalens 20. The distance between adjacent metaatoms 24 is set to be sufficiently small compared to the wavelength of light incident on the metalens 20. In this embodiment, the distance between adjacent metaatoms 24 is set to be sufficiently small compared to the wavelength of light emitted from the yellow light-emitting element 30Y and the blue light-emitting element 30B, i.e., the wavelength of yellow light LY and blue light LB. The material of the metaatoms 24 is, for example, silicon (Si), glass, TiO2, or SiO2. In this embodiment, the material of the metaatoms 24 is silicon with a refractive index of about 1.5-4. The height of the metaatom 24 from the exit surface SO may be approximately 1 μm or more and 2 μm or less.

[0027] Figure 3 shows the process of yellow light LY passing through metaatom 24, and Figure 4 shows the process of blue light LB passing through metaatom 24. In Figures 3 and 4, only the shape of metaatom 24 as viewed in the central axis direction (front-to-back direction) of the metalens 20 is shown.

[0028] For each meta-atom 24, the shape of the meta-atom 24 when viewed in the direction of the central axis of the meta-lens 20 (hereinafter sometimes referred to as the "cross-sectional shape") is such that the vertical width H (for example, about 500 nm) is longer than the horizontal width W (for example, about 200 nm). The shape and size of the cross-section of each meta-atom 24 are substantially uniform over the entire length in the direction of the central axis (front-to-back direction) of the meta-lens 20. The ratio of the vertical width H to the horizontal width W of the meta-atom 24 (=H / W) preferably correlates with the wavelength ratio of the wavelength λY of vertically polarized yellow light LY to the wavelength λB of transversely polarized blue light LB. The width ratio may be, for example, 1.5 or more, 2 or more, or 2.5 or more. Specifically, the cross-sectional shape of the meta-atom 24 is an elongated ellipse in the vertical direction (see Figures 3 and 4).

[0029] In this embodiment, the optical axis of the yellow light-emitting element 30Y is parallel to the central axis direction of the metalens 20 (the direction perpendicular to the substrate 22), and the yellow light-emitting element 30Y is positioned to emit yellow light LY toward the metalens 20 along the central axis direction of the metalens 20. The optical axis of the blue light-emitting element 30B is parallel to the central axis direction of the metalens 20, and the blue light-emitting element 30B is positioned to emit blue light LB toward the metalens 20 along the central axis direction of the metalens 20.

[0030] Here, the longer the wavelength of the light incident on the metalens 20 (meta-atom 24), the smaller the phase delay when the light passes through the metalens. The wavelength λY of the yellow light LY (LYp) used in this embodiment is longer than the wavelength λB of the transversely polarized blue light LB (LBp). Therefore, if a conventional metalens with a circular cross-sectional shape of the meta-atom were used instead of the metalens 20, the phase of the blue light LB passing through the metalens would lag behind the phase of the yellow light LY passing through the metalens. As a result, it would not be possible to focus the yellow light LY and the blue light LB to a predetermined position.

[0031] In contrast, this embodiment utilizes the fact that, with respect to the light component of the light incident on the metalens 20 (meta-atom 24) along a specific polarization direction, the narrower the width of the meta-atom 24 in that specific direction, the smaller the phase delay when the light passes through the metalens. That is, in the metalens 20 of this embodiment, the cross-sectional shape of the meta-atom 24 is such that the vertical width H is longer than the horizontal width W. Therefore, the meta-atom 24 generates a phase delay for the light components of the incident light along the vertical and horizontal polarization directions, respectively, corresponding to the wavelength of the light and the vertical and horizontal widths of the meta-atom 24.

[0032] As shown in Figure 3, vertically polarized yellow light LY is transmitted through the meta-atom 24. The phase delay of the yellow light LY is relatively large depending on the vertical width H of the meta-atom 24. As shown in Figure 4, horizontally polarized blue light LB is transmitted through the meta-atom 24. The phase delay of the blue light LB is relatively small depending on the horizontal width W of the meta-atom 24. In short, the phase delay due to the difference in wavelength between the yellow light LY and the blue light LB is mitigated by the difference in the widths (vertical width H and horizontal width W) of the meta-atom 24 in the polarization direction of the yellow light LY and the polarization direction of the blue light LB. In this embodiment, the ratio of the vertical width H and horizontal width W of the meta-lens 20 is adjusted so that both the phase delay of the yellow light LY and the phase delay of the blue light LB are 2π. As a result, the meta-atom 24 can mix the yellow light LY and the blue light LB to generate white light LW and focus it at a predetermined position.

[0033] The relationship between the phase delay amount Φ and the width of the meta-atom 24 (vertical width H, horizontal width W) is as follows. From this equation, the width of the meta-atom 24 can be determined in order to set the phase delay amount Φ to a desired value. Φ = {β(w) - k0}·h =(2π / λ)·{n eff (w)-1}·h β(w): Phase modulation amount per unit length w: Width of the cross-section of metaatom 24 in a predetermined direction k0: wave vector in air n eff Effective refractive index λ: Wavelength of incident light h: Protrusion length of the metaatom 24 from the substrate 22

[0034] (modified version) The technologies disclosed herein are not limited to the embodiments described above and can be modified in various forms without departing from their essence, for example, the following modifications are possible.

[0035] In the above embodiment, a metalensor 20 having a light-gathering function was exemplified as the metalensor, but the metalensor may also have other functions, such as a diffusion function or a collimating function. In the above embodiment, the microstructure was arranged on the exit surface of the substrate, but the microstructure may also be arranged on the ingress surface of the substrate.

[0036] In the above embodiment, both the incident surface SI and the exit surface SO of the substrate 22 were flat surfaces. However, one of the incident surface SI and the exit surface SO may be flat, a portion of the incident surface SI may be flat, a portion of the exit surface SO may be flat, and the entire incident surface SI and the exit surface SO do not have to be flat. The specific region is not limited to the central region of the substrate 22, but may be a region offset from the center. The specific region is not limited to a portion of the surface of the substrate 22, but may be the entire surface. Among the multiple metaatoms 24 arranged on the substrate 22, the cross-section of only some of the metaatoms 24 may be elongated in a predetermined direction.

[0037] In the above embodiment, the first light was yellow light LY containing only the vertically polarized light component, but it may also be light containing other polarization components in addition to the vertically polarized light component (for example, unpolarized light). The second light was blue light LB containing only the transversely polarized light component, but it may also be light containing other polarization components in addition to the transversely polarized light component (for example, unpolarized light). The first and second wavelengths may be in the wavelength range of color combinations other than yellow and blue (for example, a combination of red, green, and blue). The first and second polarization directions are not limited to the up-and-down and left-and-right directions, but may also be directions that are inclined with respect to the up-and-down direction, for example. The first and second polarization directions are not limited to directions that are orthogonal to each other, but may be directions that intersect each other. The combination of polarization of the first and second light is not limited to a combination of vertical and transverse polarization, but may also include combinations of linear polarization, random polarization, circular polarization, or elliptic polarization in other directions, for example.

[0038] In the above embodiment, the light-emitting element had an LED configuration, but it is not limited to this, and may also have other semiconductor light-emitting elements or a light source such as a light bulb.

[0039] In the above embodiment, the cross-sectional shape of the metaatom 24 was such that the vertical width H was longer than the horizontal width W, but for example, the horizontal width W may be longer than the vertical width H. Furthermore, the cross-sectional shape of the metaatom 24 is not limited to shapes where the widths are different in mutually orthogonal directions, but may be shapes where the widths are different in mutually intersecting directions.

[0040] In the above embodiment, the phase delay amounts of the yellow light LY and the blue light LB were both 2π, but they may be values ​​other than 2π, or they may be different values ​​from each other. In short, in the above embodiment, compared to a configuration in which the shape of the metaatom 24 is a perfect circle, it is sufficient to reduce the difference between the phase delay amounts of the yellow light LY and the blue light LB.

[0041] The cross-sectional shape of the microstructure is not limited to an ellipse; for example, it may be the shape shown in Figure 5. As shown in Figure 5(A), the cross-sectional shape of the microstructure may be a shape in which a relatively long straight section (H1) and a relatively short straight section (W1) intersect. The intersection angle of the two straight sections is not limited to 90 degrees; for example, it may be 45 degrees or 60 degrees. With a microstructure of this shape, it is possible to suppress the increase in the difference in phase delay when two light rays with different wavelengths pass through the metalens.

[0042] As shown in Figure 5(B), the cross-sectional shape of the microstructure may be a roughly rhombic shape with two diagonal lengths (H2, W2) being different. In this case, each side of the cross-sectional shape of the microstructure may be straight, concave, or convex. With a microstructure of this shape, it is possible to suppress the increase in the difference in phase delay when two light rays with different wavelengths pass through the metalens.

[0043] As shown in Figure 5(C), the cross-sectional shape of the microstructure may be a shape in which three or more straight sections of different lengths (H3, W3, W4) and extending in different directions intersect. With a microstructure of this shape, it is possible to suppress the increase in the difference in phase delay when light passes through the metalens for three or more light of different wavelengths.

[0044] In the above embodiment, the focusing ability of two or more beams of light was improved by focusing on propagation phase delay, but the invention is not limited to this, and a configuration that improves the focusing ability of two or more beams of light by focusing on resonance phase delay is also possible. In short, the microstructure may have a shape that makes the phase delay amount of at least two beams of light, each having different wavelengths and polarization states, an integer multiple of 2π.

[0045] In the above embodiment, the lighting device 10 was exemplified as the light-emitting device, but the light-emitting device may be, for example, a LiDAR used in an in-vehicle autonomous driving system (AD) or advanced driver assistance system (ADAS), or a measuring device used in a system other than AD or ADAS. The metalens may be a lens provided in a device or component other than the measuring device or lighting device, for example, a lens provided in an in-vehicle camera. The metalens may be a lens provided in a component for a different purpose than that of a vehicle, for example, a lens provided in a security camera or a lens provided in a smartphone camera. [Explanation of Symbols]

[0046] 10: Lighting device 20: Metalens 22: Substrate 24: Metaatom 30B: Blue light-emitting element 30Y: Yellow light-emitting element 31B: Blue LED 31Y: Yellow LED 32B: Transverse polarizing element 32Y: Vertical polarizing element 40: Semiconductor substrate 50: Reflective element 52: Through-hole 53: Entrance opening 54: Inner surface 55: Exit opening Ab: Effective area SI: Entrance surface SO: Exit surface

Claims

1. A light-transmitting substrate, A metalens comprising a plurality of microstructures arranged in a specific region of the substrate, The shape of each of the plurality of microstructures is such that, when viewed in the direction of the central axis of the metallens, the width in the first direction and the width in a second direction different from the first direction are different from each other. Metalens.

2. A metal lens according to claim 1, The first direction and the second direction are orthogonal to each other. Metalens.

3. A metal lens according to claim 1 or claim 2, A first light-emitting element emits first light having a first wavelength, including a light component in a first polarization direction along the first direction, toward the metalens, A second light-emitting element emits a second light toward the metalens, the second light having a second wavelength shorter than the first wavelength and containing a light component in a second polarization direction along the second direction. A light-emitting device comprising, The width in the first direction of the microstructure is longer than the width in the second direction. Light-emitting device.

4. A light-emitting device according to claim 3, The first light emitted by the first light-emitting element includes only the light component in the first polarization direction. The second light emitted by the second light-emitting element includes only the light component in the second polarization direction. Light-emitting device.

5. A substrate that allows light transmission, A metalens comprising a plurality of microstructures arranged in a specific region of the substrate, The plurality of microstructures have a shape that makes the phase delay amount of at least two lights, each with a different wavelength and polarization state, an integer multiple of 2π. Metalens.

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