Achromatic superlens array and achromatic light field imaging system

Abstract

The invention discloses an achromatic superlens array and an achromatic light field imaging system, and belongs to the technical field of optical imaging, and the achromatic superlens array comprises a plurality of achromatic superlenses which are arranged in a two-dimensional manner; each achromatic super lens comprises a substrate and a plurality of nano structure units; the plurality of nanostructure units are distributed on the substrate in a quasi-periodic or periodic manner. The imaging system comprises a primary mirror, an achromatic superlens array, an objective lens and a photosensitive element; the achromatic superlens array is located between the objective lens and the primary lens. The device has optical response irrelevant to polarization, four-dimensional light field information including two-dimensional positions and two-dimensional directions can be obtained without optical elements such as polaroids, an imaging system can be miniaturized and lightened, and the device has important research value in the field of integrated optical systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical imaging, and particularly relates to an achromatic metalens array and an achromatic light field imaging system. Background Art

[0002] Light field imaging technology is an imaging technology that can obtain four-dimensional information of an image, including two-dimensional position information and two-dimensional direction information. Compared with traditional amplitude imaging, it can obtain richer image information, including depth information, so it is of great significance in imaging in some special scenarios.

[0003] Traditional light field imaging technology uses means such as microlens arrays, camera arrays or masks to obtain light field information. However, they are bulky, have low resolution or cannot solve the problem of broadband achromatism, which does not conform to the development concept of integration and diversification of current optical systems, and is not conducive to the development of optical imaging systems towards integration, practicality and diversification. In recent years, metasurfaces, which are known for their newness, miniaturization, compactness and ease of integration, have provided a new development direction. Through precise design of two-dimensional planar structural units, flexible control of incident light polarization, amplitude and phase can be achieved, and they have been widely used in various fields of optical imaging. In 2019, the prior art proposed a light field imaging system operating in the visible light band. The use of an achromatic metalens array makes the system have full-color characteristics. However, the polarization-related characteristics of the system greatly limit it in some imaging scenarios. Later, a scheme for realizing light field imaging by using a virtual moving metalens array was proposed, but this method introduces additional mechanical devices. Therefore, there is a need for a polarization-independent achromatic light field imaging system with a wider range of application scenarios. Summary of the Invention

[0004] The purpose of the present invention is to address the above deficiencies in the prior art and provide an achromatic metalens array and an achromatic light field imaging system to solve the problems of large volume, low resolution or inability to solve broadband achromatism in existing light field imaging technology.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] In a first aspect, an achromatic metalens array includes a plurality of achromatic metalenses arranged in a two-dimensional manner; each of the achromatic metalenses includes a substrate and a plurality of nanostructure units; the plurality of nanostructure units are distributed on the substrate in a quasi-periodic or periodic manner.

[0007] Further, the achromatic metalens array realizes polarization-independent achromatic focusing in a continuous band range, and its phase needs to satisfy:

[0008]

[0009] In the formula, is the basic phase term; is the phase shift term; is the additional phase compensation term; r is the distance between a certain position on the lens and the center; λ is the working wavelength; λ0 is the basic wavelength.

[0010] Furthermore, the basic phase term is expressed as:

[0011]

[0012] The phase shift term is expressed as:

[0013]

[0014] The additional phase compensation term is expressed as:

[0015]

[0016] In the formula, f is the focal length of the lens; R is the radius of the lens.

[0017] Furthermore, each achromatic metalens is a sub-lens, and the sub-lens has a centrosymmetric shape, which is a circle, a square or a regular hexagon.

[0018] Furthermore, the nanostructure unit includes at least one nanostructure column.

[0019] Furthermore, the cross-sectional shape of the nanostructure column is an anisotropic pattern, specifically an ellipse or a rectangle.

[0020] Furthermore, the nanostructure unit is made of a low-loss material with a high dielectric constant, and the low-loss material with a high dielectric constant is gallium nitride, hafnium dioxide, titanium dioxide, silicon nitride, silicon or germanium.

[0021] Furthermore, the material of the substrate is quartz glass, sapphire, calcium fluoride, barium fluoride or infrared chalcogenide glass.

[0022] In a second aspect, an achromatic light field imaging system includes a primary mirror, an achromatic metalens array, an objective lens and a photosensitive element; the primary mirror, the objective lens and the photosensitive element are arranged in sequence along the optical path; the achromatic metalens array is located between the objective lens and the primary mirror.

[0023] The achromatic metalens array and the achromatic light field imaging system provided by the present invention have the following beneficial effects:

[0024] 1. The imaging system of the present invention has a polarization-independent optical response and can achieve the function of light field imaging without additional polarization elements. Therefore, it has the characteristics of a simple system and a small volume. In addition, the adoption of anisotropic structural units provides a wider degree of freedom in the selection of nanostructures, which helps to realize large-bandwidth and high-efficiency achromatic devices and has important value in imaging-related applications.

[0025] 2. Compared with the existing achromatic schemes, the scheme proposed by the present invention can utilize anisotropic unit structures to achieve polarization-independent achromatic functions, which provides greater flexibility in the selection of nano-unit structures and helps to realize large-bandwidth and high-efficiency achromatic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the Si nano-unit structure used in Example 2 of the present invention, where h = 600 nm, Px = Py = 600 nm.

[0027] Figure 2 Dispersion curves of the 4 adopted structural units in Example 2 of the present invention.

[0028] Figure 3 Distribution diagrams of the light field intensity in the x-z plane at each wavelength of the achromatic metalens simulated in Example 2 of the present invention under the incidence of left-handed and right-handed circularly polarized light respectively.

[0029] Figure 4 Optical photograph of the achromatic metalens array in Example 2 of the present invention, including 30×30 sub-lenses.

[0030] Figure 5 Distribution diagrams of the light field intensity in the x-z plane at each wavelength and the normalized intensity distribution curve of the measured achromatic metalens in Example 2 of the present invention under the incidence of linearly polarized light.

[0031] Figure 6 Distribution diagram of the light field intensity at the focal plane at the central wavelength (1250 nm) of the partial area (including 4×10 sub-lenses) of the measured achromatic metalens array in Example 2 of the present invention under the incidence of linearly polarized light.

[0032] Figure 7 Optical path diagram of the light field imaging system in Example 3 of the present invention;

[0033] Wherein:

[0034] 1. Imaging object; 2. Primary mirror; 3. Achromatic metalens array; 4. Objective lens; 5. Photosensitive element. DETAILED DESCRIPTION OF THE INVENTION

[0035] The specific embodiments of the present invention will be described below to facilitate the understanding of those skilled in the art of the present technology. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

[0036] Example 1

[0037] This example provides an achromatic metalens array, which includes:

[0038] A plurality of achromatic metalenses arranged in a two-dimensional array, each achromatic metalens includes a substrate and a plurality of nanostructure units; the plurality of nanostructure units are distributed on the substrate in a quasi-periodic or periodic manner.

[0039] The achromatic metalens array in this example realizes polarization-independent achromatic focusing in a continuous wavelength range, and its phase needs to satisfy:

[0040]

[0041] In the formula, is the basic phase term, which is independent of the working wavelength λ. Any wavelength in the achromatic wavelength range (λ min –λ max ) can be selected as the basic wavelength λ0; is the phase shift term; is the additional phase compensation term; r is the distance between a certain position on the lens and the center; λ is the working wavelength.

[0042] Among them, the basic phase term is expressed as:

[0043]

[0044] The phase shift term is expressed as:

[0045]

[0046] The additional phase compensation term is expressed as:

[0047]

[0048] In the formula, f is the focal length of the lens; R is the radius of the lens.

[0049] As a preference of this example, a single achromatic metalens is a sub-lens, and the sub-lens is a centrosymmetric shape, and the centrosymmetric shape is a circle, a square or a regular hexagon.

[0050] As a preference of this embodiment, the nanostructure units are arranged in a quasi-periodic or periodic manner, and at least one nanostructure column is included on the nanostructure units; the cross-sectional shape of the nanostructure column is an anisotropic pattern, specifically an ellipse or a rectangle. The nanostructure units are made of a low-loss material with a high dielectric constant, and the low-loss material with a high dielectric constant is gallium nitride, hafnium dioxide, titanium dioxide, silicon nitride, silicon or germanium.

[0051] As a preference of this embodiment, the material of the substrate is quartz glass, sapphire, calcium fluoride, barium fluoride or infrared chalcogenide glass.

[0052] The superlens of this embodiment operates in the transmission mode. The nanostructure units are anisotropic structures. By changing the characteristic size, the basic phase required for focusing and the phase dispersion required for chromatic aberration correction within the working wavelength range are provided at the same time. There are only two choices for the angle, 0° and 90°, so as to achieve polarization-independent achromatic focusing in the continuous wavelength range.

[0053] Embodiment 2

[0054] This embodiment is a further preference for the achromatic superlens array solution in Embodiment 1, and specifically includes the following content:

[0055] In this embodiment, the substrate material is Al2O3, and single-crystal Si elliptical nanocolumns are used as the unit structure, thereby realizing an achromatic superlens array operating in the second near-infrared region (1000 nm to 1500 nm).

[0056] More specifically, in this embodiment, each sub-lens is composed of nanocolumns with a square lattice, as Figure 1 shown. The substrate is selected as alumina, and the upper-layer nanocolumns are selected as single-crystalline silicon. In order to provide a phase coverage of 0-2π and sufficient phase dispersion, the column height h = 600 nm is designed. Considering that each unit structure should satisfy both the required phase dispersion and the required focusing phase, multiple structures may need to be arranged in one period, so the period P = 600 nm is designed. Finally, considering the phase and phase dispersion comprehensively, 6×8 = 48 structures are selected as candidates. These anisotropic structures can provide the basic phase required for focusing and the phase dispersion required for achromatic aberration at the same time. The dispersion curves of 4 utilized structures are as Figure 2 shown, and it can be seen that the phase dispersion curves are approximately linearly distributed, which is consistent with formula (3).

[0057] Next, an achromatic superlens (d = 48 μm) in the working wavelength range of (1000 nm to 1500 nm) was simulated. Figure 3It shows the light intensity distribution diagrams of the output light field in the x-z plane when the incident light is right-handed and left-handed circularly polarized light respectively. Its focal length almost remains at the same focal plane (f = 190 μm), which is consistent with the expected design. Considering that other polarized lights can be regarded as the superposition of left-handed and right-handed circularly polarized lights, therefore, it can be considered that the achromatic function can still be demonstrated under the incidence of other polarized lights.

[0058] The optical picture of the actually processed achromatic metalens array 3 is as Figure 4 shown. This array contains 30×30 sub-lenses. Each sub-lens is circular, with a diameter of 48 μm and a focal length of 190 μm. The adjacent sub-lenses are tangent to each other. Figure 5 It shows the actually measured results of a single sub-lens. Under the incidence of linearly polarized light, the light intensity distribution diagrams and the normalized intensity distribution curves of each wavelength in the x-z plane. The actually measured focal length f = 196 μm, which is approximately consistent with the simulation result f = 190 μm. Figure 6 It shows the light intensity distribution diagram at the focal plane (z = 190 μm) of a partial area (including 4×10 sub-lenses) in the achromatic metalens array 3 at the central wavelength of 1250 nm. It is not difficult to see that the uniformity of the whole device is good.

[0059] Example 3

[0060] Based on the solution in Example 2, this example proposes an achromatic light field imaging system. The imaging system in this example has a polarization-independent optical response and can achieve the required performance without additional optical elements such as polarizers. Therefore, the system is more concise and smaller in volume. It specifically includes:

[0061] The primary mirror 2, the achromatic metalens array 3, the objective lens 4 and the photosensitive element 5;

[0062] Among them, the primary mirror 2, the objective lens 4 and the photosensitive element 5 are arranged in sequence along the optical path. The achromatic metalens array 3 is located between the objective lens 4 and the primary mirror. The photosensitive element 5 is used to receive the image formed by the system.

[0063] Refer to Figure 7 , after the imaging object 1 is illuminated by the tungsten halogen lamp, it passes through the primary mirror 2, the achromatic metalens array 3 and the objective lens 4 in sequence, and then is imaged on the photosensitive element 5. The photosensitive element 5 records the image light field information, including two-dimensional intensity and two-dimensional direction information.

[0064] Although the specific implementation manners of the invention have been described in detail in conjunction with the accompanying drawings, it should not be construed as a limitation on the protection scope of this patent. Within the scope described in the claims, various modifications and deformations that can be made by those skilled in the art without creative work still fall within the protection scope of this patent.

Claims

1. An achromatic metalens array, characterized in that, It includes a plurality of achromatic metalenses arranged in a two-dimensional pattern; each of the achromatic metalenses includes a substrate and a plurality of nanostructure units; the plurality of nanostructure units are distributed on the substrate in a quasi-periodic or periodic manner.

2. The achromatic metalens array according to claim 1, wherein The achromatic metalens array achieves polarization-independent achromatic focusing in a continuous band range, and its phase needs to satisfy: In the formula, is the basic phase term; is the phase shift term; is the additional phase compensation term; r is the distance between a certain position on the lens and the center; λ is the working wavelength; λ0 is the basic wavelength.

3. The achromatic metalens array according to claim 1, wherein The basic phase term is expressed as: The phase shift term is expressed as: The additional phase compensation term is expressed as: In the formula, f is the focal length of the lens; R is the radius of the lens.

4. The achromatic metalens array according to claim 1, wherein: A single achromatic metalens is a sub-lens, and the sub-lens has a centrosymmetric shape, which is a circle, a square, or a regular hexagon.

5. The achromatic metalens array according to claim 1, wherein: At least one nanostructure pillar is included on the nanostructure unit.

6. The achromatic metalens array according to claim 5, wherein: The cross-sectional shape of the nanostructure pillar is an anisotropic pattern, specifically an ellipse or a rectangle.

7. The achromatic metalens array according to claim 5, characterized in that: The nanostructure unit is made of a low-loss material with a high dielectric constant, and the low-loss material with a high dielectric constant is gallium nitride, hafnium dioxide, titanium dioxide, silicon nitride, silicon, or germanium.

8. The achromatic metalens array according to claim 1, wherein: The material of the substrate is fused quartz, sapphire, calcium fluoride, barium fluoride, or an infrared chalcogenide glass.

9. An achromatic light field imaging system, characterized in that: It includes a primary mirror, the achromatic metalens array according to any one of claims 1 to 8, an objective lens, and a photosensitive element; the primary mirror, the objective lens, and the photosensitive element are arranged in sequence along the optical path; the achromatic metalens array is located between the objective lens and the primary mirror.

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