Large-view-field super-structure lens based on multi-layer structure

Abstract

The invention belongs to the field of optical devices, and particularly relates to a large-view-field super-structure lens based on a multilayer structure, which is characterized in that Ge with high refractive index and ZnSe with low refractive index are alternately stacked to form a super-structure unit with an eight-layer structure, so that excitation of a high-order mode is destroyed, and the super-structure lens only supports a single electric dipole mode; the resonant frequency is broadened by controlling the coupling effect between layers, so that more consistent phase response can be generated under different incident angles; the total height of the superstructure unit of the eight-layer structure is 7 microns, the angle insensitive characteristic of the superstructure unit can be improved, meanwhile, the aspect ratio of the superstructure unit can be reduced, and the superstructure unit evenly and reasonably corresponds to the 2pi phase range within the simulation radius; compared with a super-structure lens composed of a super-structure unit of a single-layer structure, the focusing efficiency of the super-structure lens composed of the super-structure units of an eight-layer structure is improved by about 30% during 50-degree incidence, and the super-structure lens has important significance in optimizing the optical performance of the large-view-field super-structure lens.

Description

Technical Field

[0001] The present invention relates to the field of optical devices, and in particular to a large-field meta-lens based on a multi-layer structure. Background Art

[0002] In recent years, people have made some progress in the design of large-field metalenses, such as double-layer cascaded metalenses, topology-optimized multi-layer metalenses, quadratic phase metalenses, and special meta-atom structures such as moth-eye meta-atom metalenses. However, these methods usually increase the difficulty and precision of manufacturing. For example, double-layer cascaded metalenses require high alignment accuracy, topology optimization and moth-eye meta-atom metalenses require difficult manufacturing processes due to their complex shapes; or introduce spherical aberrations, which reduce the focusing performance, such as the introduction of spherical aberrations in quadratic phase metalenses, which reduce the focusing performance. In addition, meta-atoms based on the transmission phase principle are often used in the design of large-field metalenses because their single-mode waveguides have the angle-insensitive characteristics of phase. In order to cover the 2π phase control range, it is usually necessary to increase the height or refractive index of the meta-atom, which will lead to the emergence of higher-order modes, especially in the case of oblique incidence, which is more likely to cause resonance and cause the phase mutation of the meta-atom, so that the angle-insensitive characteristics of the phase are destroyed, which increases the difficulty of the design of large-field metalenses and the selection of their meta-atoms. Summary of the invention

[0003] In order to solve the above technical problems, the present invention provides a large-field-of-view meta-lens based on a multi-layer structure.

[0004] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0005] One of the technical solutions of the present invention is to provide a large-field-of-view meta-lens based on a multi-layer structure, including a substrate and a meta-unit arrayed on the substrate; the meta-unit is a columnar structure, and is composed of a first Ge layer, a first ZnSe layer, a second Ge layer, a second ZnSe layer, a third Ge layer, a third ZnSe layer, a fourth Ge layer, and a fourth ZnSe layer from bottom to top.

[0006] Furthermore, the height of the superstructure unit is 7 μm.

[0007] Further, the heights of the first Ge layer, the second Ge layer, the third Ge layer and the fourth Ge layer are all 0.65 μm, and the heights of the first ZnSe layer, the second ZnSe layer, the third ZnSe layer and the fourth ZnSe layer are all 1.10 μm.

[0008] Preferably, the material of the substrate is barium fluoride BaF2.

[0009] Compared with the prior art, the present invention selects high-refractive-index germanium and low-refractive-index ZnSe to be alternately stacked to form an eight-layer structured meta-unit, thereby destroying the excitation of high-order modes so that it only supports a single electric dipole mode, and by controlling the coupling effect between layers, its resonant frequency is broadened, so that it can produce a more consistent phase response under different incident angles; the total height of the eight-layer structured meta-unit is 7μm, which can not only increase the angle-insensitive characteristics of the meta-unit, but also reduce the aspect ratio of the meta-unit and make it uniformly and reasonably correspond to the 2π phase range within the simulation radius; compared with the meta-lens composed of a single-layer structured meta-unit, the focusing efficiency of the meta-lens composed of an eight-layer structured meta-unit is increased by about 30% at an incidence of 50°, which is of great significance for optimizing the optical performance of the large-field-of-view meta-lens. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a schematic diagram of the structure of the superstructure unit column of Example 1.

[0011] Figure 2 The simulation results of the super structure unit column of Example 1.

[0012] Figure 3 Schematic diagrams of the structures of the superstructure unit columns of Comparative Examples 1 to 3 and their simulation results.

[0013] Figure 4 This is a simulation diagram of a large-field-of-view metalens.

[0014] Figure 5 The focusing efficiency (left) and full width at half maximum (right) of the meta-lens of Example 1, Comparative Example 3 and Comparative Example 2 at different NAs: (a) NA = 0.4; (b) NA = 0.48; (c) NA = 0.6. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the invention.

[0016] Example 1

[0017] like Figure 1As shown, this embodiment exemplarily shows a large-field meta-lens based on a multi-layer structure, including a substrate and meta-units arrayed on the substrate; wherein the material of the substrate is barium fluoride BaF2; the meta-unit is a columnar structure, which is uniformly recorded as a meta-unit column in the following embodiments, and the meta-unit column is composed of a first Ge layer 1, a first ZnSe layer 2, a second Ge layer 3, a second ZnSe layer 4, a third Ge layer 5, a third ZnSe layer 6, a fourth Ge layer 7, and a fourth ZnSe layer 8 from bottom to top. It should be noted that the preparation method of this embodiment is not the focus of protection of this embodiment, and the preparation methods involved are all known methods in the art; for example, the Ge layer can be processed by conventional ion beam assisted deposition technology in the art, and the ZnSe layer can be processed by conventional magnetron sputtering technology in the art.

[0018] Since ZnSe has good optical transmittance in the spectral range of 0.5-15μm, the refractive index at 10.6μm is 2.39, the extinction coefficient K=0, it has good thermal stability, and the far-infrared band shows low absorption and scattering losses, making it easier to obtain high transmission efficiency. As a typical semiconductor material, Ge has the characteristics of high refractive index, low absorption and low scattering loss in the far-infrared band, which will make the design of meta-lens more flexible. Ge has a refractive index of 4 at 10.6μm and an extinction coefficient K=0. Through the combination of the above two materials, a multilayer meta-unit column consisting of alternating stacks of high-refractive-index Ge and low-refractive-index ZnSe is designed. Considering high transmittance, 2π phase coverage and low angle sensitivity, an eight-layer structure of the meta-unit column is designed. The total height of a single meta-unit column is 7μm; the heights of the first Ge layer, the second Ge layer, the third Ge layer and the fourth Ge layer are all 0.65μm, and the heights of the first ZnSe layer, the second ZnSe layer, the third ZnSe layer and the fourth ZnSe layer are all 1.10μm.

[0019] Comparative Example 1

[0020] The difference from Example 1 is that, see Figure 3 The superstructure unit column is a columnar ZnSe with a height of 7μm.

[0021] Comparative Example 2

[0022] The difference from Example 1 is that, see Figure 3 The superstructure unit column is a columnar ZnSe with a height of 13μm.

[0023] Comparative Example 3

[0024] The difference from Example 1 is that, see Figure 3 The superstructure unit column is a columnar Ge with a height of 5μm.

[0025] Simulation Example 1

[0026] The superstructure unit columns of Example 1 and Comparative Examples 1 to 3 were simulated respectively, and the specific simulation process is as follows:

[0027] 1. Establish a geometric model and define the material properties of the substrate and superstructure unit column (such as refractive index, dielectric constant, conductivity, etc.);

[0028] 2. Set the simulation light source to a linearly polarized plane wave light source and transmit it from the substrate along the z-axis through the structure to infinity, with a wavelength of 10.6μm;

[0029] 3. Set boundary conditions: To simulate the electromagnetic field behavior of the actual environment, set the boundary conditions of the x and y directions of the simulation area to periodic boundary conditions; to minimize unnecessary reflections caused by the transmitted electromagnetic waves reaching the simulation boundary, set the boundary condition in the z direction to a perfectly matched layer;

[0030] 4. Calculate the transmission phase and transmittance of the nanostructure in the simulation area through the S parameter analysis group; wherein the S parameters in the S parameter analysis group are expressed in the form of complex amplitude to represent the reflection and transmission coefficients of the nanostructure unit, and its transmittance can be obtained; perform fast Fourier transform (FFT) on the time domain electric field signal of the monitoring point to obtain the frequency domain electric field E(f), and the frequency domain electric field data contains the amplitude and phase information of the signal. By Fourier transforming the electric field at the transmission point, the transmission phase of each frequency component can be obtained;

[0031] 5. Set the scanning conditions to change the radius of the superstructure unit column and obtain multiple sets of phase and transmittance data.

[0032] The simulation results are as follows Figure 3 As shown, Figure 3 As shown in (a) of the figure, a ZnSe atom with a height of 7 μm is simulated. Although the ZnSe superstructure unit column has high transmittance at different angles within the simulation radius, due to the lower refractive index, it is impossible to obtain a 2π phase control within this radius range, and can only achieve a phase control of about 1π. Figure 3 As shown in (b), in order to achieve 2π phase control of the ZnSe superstructure unit column, the minimum height needs to be 13μm, which greatly increases the aspect ratio of the superstructure unit column and increases the difficulty of the actual manufacturing process. The aspect ratio is defined here as the ratio of the height of the superstructure unit column to the minimum diameter. For a single layer of Ge with a higher refractive index, a stronger guided mode resonance will be excited, resulting in a larger phase coverage within the same height and radius. At the same time, the resonance will also cause a phase mutation. Therefore, the Ge superstructure unit column can achieve 2π phase control with a lower aspect ratio, corresponding to Figure 3The height of the Ge metastructure unit column in (c) is only 5μm. However, due to the multiple phase mutations caused by resonance, the phase angle in this area is sensitive, and it corresponds to a region with low transmittance. At the same time, it can be found that the phase range of π-2π corresponds to a smaller radius area, and this area becomes narrower as the incident angle increases, which means that the radius difference in the large phase range is too small, which increases the manufacturing precision requirements and is therefore not conducive to the manufacture of metalenses. Figure 2 As shown, in this embodiment, the aspect ratio of the multilayer superstructure unit column composed of alternating stacking of high-refractive-index Ge and low-refractive-index ZnSe can be reduced to less than 46% of that of the single-layer ZnSe superstructure unit column, and has a more reasonable phase distribution.

[0033] Simulation Example 2

[0034] The performance of the diffractive optical element at different incident angles was analyzed using Zemax's analytical model based on Kirchhoff diffraction integral, and the required phase profile was calculated. The meta-lenses prepared in Example 1 and Comparative Examples 1 to 3 were simulated using the finite difference time domain method (FDTD). Figure 4 As shown, the simulated meta-lens operates at a wavelength of 10.6 μm.

[0035] 1) The specific process of phase calculation is as follows:

[0036] 1. Set the basic parameters of the optical system such as system aperture, field of view, wavelength, etc.

[0037] 2. Build an optical system, place optical components, use binary optical surfaces to replace meta-lenses in the system, and set lens parameters such as focal length, aperture, and material;

[0038] 3. Set the phase coefficients A of the binary optical surface n As variables, iterative optimization is performed to achieve the correction of aberrations. When correcting aberrations, it is not to make various aberrations tend to 0 in value, but to achieve reasonable matching and balance between them in value; in order to achieve the best correction state of aberrations, the aberration linear equation composed of the weighted square sum of the aberration residuals is often used as the merit function; the optimization process solves the constructed aberration linear equation through the damped least squares method;

[0039] 4. Evaluate image quality and analyze the imaging effect of the system by observing PSF, MTF images, and wavefront diagrams; further adjust the operation number and iterate optimization based on the analysis results to ensure that the optical system meets the design requirements;

[0040] 5. Obtain the phase coefficient of the final binary optical surface, and derive the phase of the designed meta-lens as:

[0041]

[0042] in is the radial radius along the metalens plane, A n is the optimization coefficient that minimizes the focal spot size (RMS spot size), and R is the normalized radius of the meta-lens.

[0043] 2) The specific process of FDTD simulation is as follows:

[0044] 1. Establish a geometric model, use ZEMAX to obtain the phase and simulate the unit column of the meta-structure unit column to match and construct the meta-lens;

[0045] 2. Set the simulated light source to a linearly polarized total-field scattered-field (TFSF) light source with a wavelength of 10.6 μm;

[0046] 3. Set the boundary conditions in the x, y and z directions of the simulation area to perfectly matched layer boundary conditions to avoid electromagnetic waves reflecting back to the calculation area;

[0047] 4. Calculate the FWHM value: obtain the electric field intensity distribution data at the focal plane position through the monitor in the simulation area, obtain the lateral distribution curve of the light spot through the field intensity data on the lateral section of the focus point, and find the two positions corresponding to 50% of the maximum value of the field intensity, i.e. the half-height points; the distance between the two half-height points is the FWHM value of the light spot;

[0048] 5. Calculate the focusing efficiency: Obtain the far-field intensity distribution of the metalens through the monitor in the simulation area and calculate its power density; Integrate the power density in the circular area with a radius of three times the FWHM of the focal spot at the focusing plane to obtain the power P in the focusing area. focus ; Integrate the power density at the incident end of the lens to obtain the total incident power P incident The focusing efficiency of the metalens is defined as the ratio of the optical power in a circular area with a radius of three times the FWHM of the focal spot at the focal plane to the incident optical power. The calculation formula is:

[0049]

[0050] Figure 5 The focusing performance of three groups of metalenses with different numerical apertures when x-polarized light is incident is shown. The focusing efficiency is defined as the ratio of the optical power passing through the circular aperture of the focal plane (radius is 3 times the half-maximum full width) to the total incident beam power. Figure 5It can be seen that under different numerical apertures, although the single-layer Ge metalens has a lower aspect ratio, its focusing efficiency decreases significantly with increasing angle, indicating its angle-insensitive phase. The focusing efficiency of multi-layer metalens and single-layer ZnSe metalens decreases more slowly, and has higher focusing efficiency at large angles of incidence. In particular, the multi-layer metalens has an average efficiency improvement of 30% at 50°. In addition, the theoretical values ​​of the full width at half maximum (λ / 2NA) are 13.25μm, 11.04μm, and 8.83μm at numerical apertures of 0.4, 0.48, and 0.6, respectively. It can be observed that Figure 5 The full width at half maximum of the three groups of meta-lenses are all close to the theoretical values, which indicates that the large-field meta-lens of this embodiment has an imaging resolution close to the diffraction limit.

[0051] In summary, the present invention selects Ge with a high refractive index and ZnSe with a low refractive index to be alternately stacked to form an eight-layer structured meta-unit, thereby destroying the excitation of high-order modes so that it only supports a single electric dipole mode, and by controlling the coupling effect between layers, its resonant frequency is broadened, so that it can produce a more consistent phase response under different incident angles; the total height of the eight-layer structured meta-unit is 7μm, which can not only increase the angle-insensitive characteristics of the meta-unit, but also reduce the aspect ratio of the meta-unit and make it uniformly and reasonably correspond to the 2π phase range within the simulation radius; compared with the meta-lens composed of single-layer structured meta-units, the focusing efficiency of the meta-lens composed of eight-layer structured meta-units is increased by about 30% at 50° incidence, which is of great significance for optimizing the optical performance of large-field-of-view meta-lens.

[0052] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A large-field-of-view meta-lens based on a multi-layer structure, comprising a substrate and meta-units arrayed on the substrate; characterized in that: The superstructure unit is a columnar structure, and is composed of a first Ge layer, a first ZnSe layer, a second Ge layer, a second ZnSe layer, a third Ge layer, a third ZnSe layer, a fourth Ge layer, and a fourth ZnSe layer from bottom to top.

2. The large-field-of-view meta-lens based on a multi-layer structure according to claim 1, characterized in that: The height of the superunit is 7 μm.

3. The large-field-of-view meta-lens based on a multi-layer structure according to claim 1, characterized in that: The heights of the first Ge layer, the second Ge layer, the third Ge layer and the fourth Ge layer are all 0.65 μm, and the heights of the first ZnSe layer, the second ZnSe layer, the third ZnSe layer and the fourth ZnSe layer are all 1.10 μm.

4. The large-field-of-view meta-lens based on a multi-layer structure according to claim 1, characterized in that: The material of the substrate is barium fluoride BaF2.

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