A large-field metalens based on a multi-layer structure

Abstract

The present invention belongs to the field of optical devices, and in particular to a large-field-of-view metalens based on a multi-layer structure. The eight-layer metaunit is composed of alternately stacked high-refractive-index Ge and low-refractive-index ZnSe, thereby destroying the excitation of high-order modes and supporting only a single electric dipole mode. The resonance frequency is broadened by controlling the coupling effect between layers, thereby enabling a more consistent phase response under different incident angles. The total height of the eight-layer metaunit is 7 μm, which not only increases the angle-insensitivity of the metaunit, but also reduces the aspect ratio of the metaunit and makes it uniformly and reasonably correspond to the 2π phase range within the simulation radius. Compared with a metalens composed of single-layer metaunits, the focusing efficiency of the eight-layer metalens is improved by about 30% at an incident angle of 50°, which is of great significance for optimizing the optical performance of the large-field-of-view metalens.

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, researchers have made some progress in designing large-field metalenses, such as double-layer cascaded metalenses, topology-optimized multi-layer metalenses, quadratic phase metalenses, and specialized meta-atomic structures such as moth-eye metalenses. However, these approaches often increase manufacturing difficulty and precision. For example, double-layer cascaded metalenses require high alignment precision, while topology optimization and moth-eye metalenses, due to their complex shapes, require complex manufacturing processes. Alternatively, they introduce spherical aberration, which can degrade focusing performance, as in the case of quadratic phase metalenses. Furthermore, metaatoms based on the transmission phase principle are often used in large-field metalens design due to the angle-insensitivity of their single-mode waveguides. To cover the 2π phase control range, the height or refractive index of the metaatom must be increased, which can lead to the emergence of higher-order modes. This, especially under oblique incidence, can easily induce resonances, leading to phase abrupt shifts in the metaatom, destroying the angle-insensitivity of the phase. This complicates the design of large-field metalenses and the selection of their metaatoms. 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 metalens based on a multi-layer structure, including a substrate and a metaunit arrayed on the substrate; the metaunit is a cylindrical structure, consisting 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 meta-unit is 7 μm.

[0007] Furthermore, 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 meta-unit, thereby destroying the excitation of high-order modes and supporting only a single electric dipole mode. The resonance frequency is broadened by controlling the coupling effect between layers, thereby enabling it to produce a more consistent phase response under different incident angles. The total height of the eight-layer meta-unit is 7 μm, which not only increases the angle-insensitivity of the meta-unit, but also reduces the aspect ratio of the meta-unit and makes it uniformly and reasonably correspond to the 2π phase range within the simulation radius. Compared with the meta-lens composed of single-layer meta-units, the focusing efficiency of the meta-lens composed of eight-layer 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Schematic diagram of the structure of the super-structure 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 (FWHM) (right) of the metalenses of Example 1, Comparative Example 3, and Comparative Example 2 at different NAs are as follows: (a) NA = 0.4; (b) NA = 0.48; (c) NA = 0.6. DETAILED DESCRIPTION

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

[0016] Example 1

[0017] like Figure 1As shown, this embodiment exemplarily demonstrates a large-field metalens based on a multi-layer structure, including a substrate and metaunits arrayed on the substrate; wherein the substrate is made of barium fluoride BaF2; the metaunits are columnar structures, which are uniformly referred to as metaunit columns in the following embodiments. The metaunit columns are composed, from bottom to top, 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. 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 using conventional ion beam assisted deposition technology in the art, and the ZnSe layer can be processed using conventional magnetron sputtering technology in the art.

[0018] Because ZnSe has good optical transmittance in the 0.5-15μm spectral range, a refractive index of 2.39 at 10.6μm, an extinction coefficient K=0, and good thermal stability, it also exhibits low absorption and scattering losses in the far-infrared band, making it easier to achieve high transmission efficiency. Ge, as a typical semiconductor material, has high refractive index, low absorption, and low scattering losses in the far-infrared band, which will make the design of metalenses more flexible. Ge has a refractive index of 4 at 10.6μm and an extinction coefficient K=0. By combining the above two materials, a multilayer metastructure unit column composed of alternating high-refractive-index Ge and low-refractive-index ZnSe was designed. Considering high transmittance, 2π phase coverage, and low angular sensitivity, the metastructure unit column was designed as an eight-layer structure. The total height of a single metastructure unit column is 7μm; the heights of the first, second, third, and fourth Ge layers are all 0.65μm, and the heights of the first, second, third, and fourth ZnSe layers 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 metastructure unit column (such as refractive index, dielectric constant, conductivity, etc.);

[0028] 2. Set the simulated 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 simulation area in the x and y directions to periodic boundary conditions; to minimize unnecessary reflections caused by the transmitted electromagnetic wave 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 using an S-parameter analysis group. The S parameters in the S-parameter analysis group use complex amplitudes to represent the reflection and transmission coefficients of the nanostructure unit, thereby obtaining its transmittance. Perform a fast Fourier transform (FFT) on the time-domain electric field signal at the monitoring point to obtain the frequency-domain electric field E(f). The frequency-domain electric field data contains both 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 metastructure unit column to 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 low 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 metastructure unit column, the minimum height needs to be 13μm, which greatly increases the aspect ratio of the metastructure unit column and increases the difficulty of the actual manufacturing process. Here, the aspect ratio is defined as the ratio of the height of the metastructure 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 metastructure 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 region, and this region 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 metalens. Figure 2 As shown, in this embodiment, the aspect ratio of the multilayer metastructure 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 metastructure 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 the 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 instead of metalenses 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 aberration correction. When correcting aberrations, the goal is not to make the various aberrations tend to zero in value, but to achieve a reasonable match 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 residual aberrations 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 system imaging effect by observing PSF, MTF images, and wavefront diagrams. Based on the analysis results, further adjust the operation parameters and iterate optimization 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 metalens.

[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 to match the unit column to construct the metalens;

[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 prevent electromagnetic waves from being reflected back into 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 based on the field intensity data on the transverse cross section of the focus point. Find the two positions corresponding to 50% of the maximum 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 a 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 a metalens is defined as the ratio of the optical power within 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 for x-polarized light is shown. The focusing efficiency is defined as the ratio of the optical power passing through the circular aperture of the focal plane (radius 3 times the full width at half maximum) to the total incident beam power. Figure 5It can be seen that under different numerical apertures, although the single-layer Ge meta-lens has a lower aspect ratio, its focusing efficiency decreases significantly with increasing angle, indicating that its phase is angularly insensitive. The focusing efficiency of multi-layer meta-lens and single-layer ZnSe meta-lens decreases more slowly, and has higher focusing efficiency at large angles of incidence. In particular, the multi-layer meta-lens 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 metalenses are all close to the theoretical values, which indicates that the large-field metalens of this embodiment has an imaging resolution close to the diffraction limit.

[0051] In summary, the present invention selects high-refractive-index Ge and low-refractive-index ZnSe to be alternately stacked to form an eight-layer meta-unit, thereby destroying the excitation of high-order modes and making it support only 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 meta-unit is 7μm, which not only increases the angle-insensitivity of the meta-unit, but also reduces the aspect ratio of the meta-unit and makes it uniformly and reasonably correspond to the 2π phase range within the simulation radius; compared with the meta-lens composed of single-layer meta-units, the focusing efficiency of the meta-lens composed of eight-layer 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. Any 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 metalens based on a multi-layer structure, comprising a substrate and metacells 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; 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.

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

3. The multi-layered large-field-of-view metalens according to claim 1, characterized in that: The material of the substrate is barium fluoride BaF2.

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