CN114114473A - Phase-change-material-based double-mode simultaneous focusing super-structure lens capable of dynamically tuning polarization at will - Google Patents

Abstract

The invention discloses a phase-change material-based double-mode simultaneous focusing metamaterial capable of realizing random polarization and dynamic tuning, which is characterized by comprising a CaF₂Substrate layer and method for implementing transmission phase modulation using phase change material Ge₂Sb₂Se₄Te₁The prepared super surface layer is formed by a plurality of rectangular nano columns with the same height and 0-degree or 90-degree azimuth angle on the upper surface of the substrate layer in a periodic mannerpEdge ofx‑yThe planar arrangement is formed by rectangular nano-pillar arrays to regulate and control the phase and the intensity of transmitted waves, high-efficiency convergence is realized on incident light in any polarization state, and due to the strong robustness of phase dispersion of a super-surface unit structure, the super-structure lens can realize effective focusing in a medium infrared 4000-4500 nm bandwidth, the focusing efficiency can reach 70%, and the phase change material Ge is regulated and controlled₂Sb₂Se₄Te₁The phase state and focusing effect can realize the dynamic switching of ON and OFF, the excitation wavelength is reasonably set, the invention can also realize the high-efficiency focusing of a reflection mode and a transmission mode simultaneously, and the practicability is stronger.

Description

A dynamically tunable dual-mode simultaneous focusing metalens based on phase-change materials

technical field

The invention relates to a dual-mode simultaneous focusing metal lens based on a phase-change material, which can be arbitrarily polarized and dynamically tunable, and belongs to the field of novel artificial electromagnetic materials and optical devices.

Background technique

Metasurface, also known as two-dimensional metamaterials, is characterized by the use of metasurface unit structure coupled with incident electromagnetic waves to generate phase space changes to control wavefronts. The functional film layer device not only retains the exotic properties of three-dimensional metamaterials, but also overcomes the difficulties faced in the preparation of three-dimensional metamaterials. By selecting appropriate materials and rationally designing parameters such as the shape, size and orientation of the metasurface unit structure, the metasurface can flexibly adjust the isotropy/anisotropy of the nanostructure, and optimize various optical parameters, such as amplitude, phase, and polarization. , frequency and spectrum of nano-functional devices, greatly expanding the research and application of metasurfaces in many fields.

As an important representative of the practical application of metasurfaces, metalens have aroused great interest of researchers due to their outstanding advantages such as super light wave manipulation ability, ultra-compact structure, versatility, and compatibility with semiconductor processes. At present, various functional metalens have been proved theoretically or experimentally, including broadband achromatic metalens, super-resolution metalens, multifocal metalens, and multifunctional metalens. However, the reported metalenses are limited by the design of phase distribution, and it is difficult to realize arbitrary polarization and broadband focusing functions at the same time; and once the structure is determined, its electromagnetic properties cannot be changed, which is greatly limited in terms of flexible modulation of electromagnetic waves.

Germanium Antimony Selenium Tellurium (Ge ₂ Sb ₂ Se ₄ Te ₁ , GSST), as a new class of phase change materials, has received extensive attention in recent studies, especially in the study of dynamically tunable optical devices. Due to its special material properties: Ge ₂ Sb ₂ Se ₄ Te ₁ is amorphous (aGSST) at room temperature, and becomes crystalline (cGSST) after reaching the threshold temperature, and the refractive index difference between the two states is obvious, and the phase transition is reversible. , which enables GSST to provide more degrees of freedom for the reconfigurability and flexibility of the optics. By combining metalens with phase-change material GSST, dynamic tunable metalens can be realized with the help of external stimuli (thermal, optical, electrical, etc.). However, the current research in this area mainly focuses on using the change of the phase state of GSST in a uniform dielectric layer to achieve dynamic regulation of electromagnetic properties. The research on patterning GSST into metaunits and realizing dynamically tunable metalens is still in its infancy. has not received enough attention.

SUMMARY OF THE INVENTION

In view of the above situation, the purpose of the present invention is to provide a metalens based on phase change materials, which can be dynamically tunable with any polarization and can focus simultaneously with dual modes, so as to solve the incident polarization sensitivity and narrow working bandwidth of the existing metalens. , the performance is not easy to dynamically tune and other technical bottlenecks.

This scheme presents the design of a dynamically tunable dual-mode simultaneous focusing metalens based on phase-change materials, using the phase-change material Ge ₂ Sb ₂ Se ₄ Te ₁ between amorphous and crystalline states. The high refractive index contrast between the two can realize the dynamic switching of "ON" and "OFF" for the focusing effect of fixed frequency arbitrary polarized waves.

The technical solution adopted by the present invention to solve the technical problem is as follows: It is characterized in that the superstructure lens comprises a _CaF2 substrate layer and _{a superstructure} made of phase change material _Ge2Sb2Se4Te1 for realizing transmission phase modulation and using phase change material _Ge2Sb2Se4Te1 The surface layer, the metasurface layer is formed by a plurality of rectangular nano-pillars with the same height arranged on the upper surface of the substrate layer as a rectangular nano-pillar array with a period p along the xy plane, and the plurality of rectangular nano-pillars with the same height are all high and deep. The aspect ratio of the structural feature and its azimuth angle is 0° or 90°, the period refers to the distance between the geometric centers of two adjacent rectangular nanopillars on the x and y axes, and the number of rectangular nanopillars in any row along the x axis is the same. is 2n, the rectangular nanopillars in each row are point-symmetrically distributed with respect to the row center (x=0), and either side of the center of each row contains w rows of rectangular nanopillars with long axes along the x-axis direction and v long axes along the y-axis direction. A column of rectangular nanopillars in the axial direction, and w+v=n, the length and width of all rectangular nanopillars are the long axis and the short axis, respectively, the short axis length is b and the same, the long axis length is a and is a variable, which is provided by The transmission phase is determined by , in order to obtain 0-2π phase modulation, and realize the focusing function of metalens on incident waves of fixed frequency and arbitrary polarization.

Preferably, the row of rectangular nano-columns refers to the rectangular nano-column azimuth angle β=0°, the column of rectangular nano-columns refers to the rectangular nano-column azimuth angle β=90°, and the azimuth angle β refers to the rectangular nano-column The counterclockwise rotation angle of the long axis a relative to the positive direction of the x-axis.

Preferably, along the x-axis direction of the surface, the steps for solving the long-axis length a(x) of each rectangular nanopillar are:

1) Set the working frequency of the metalens to f ₀ (corresponding to the working wavelength λ ₀ ), and the variation range of the long-axis length a(x) of the rectangular nanopillars is [a _min , a _max ];

2) The azimuth angle β of each rectangular nanocolumn in the metasurface layer is set to 0°, and when the incident wave frequency f ₀ is obtained by simulation, the left (right) circularly polarized light passes through the metasurface unit structure, and the orthogonal polarization transmits The characteristic curve 1 between the wave transmittance T _cross and the long-axis length a(x) parameter of the rectangular nanopillar and the characteristic between the phase _ψcross of the orthogonally polarized transmitted wave and the long-axis length a(x) parameter of the rectangular nanopillar curve 2;

3) Change the azimuth angle β of each rectangular nanocolumn in the metasurface layer to 90°, and also obtain the incident wave frequency f ₀ through simulation simulation, after the left (right) circularly polarized light passes through the metasurface unit structure, the orthogonal polarization transmits The characteristic curve between the wave transmittance T _cross and the long-axis length a(x) parameter of the rectangular nanopillar3 and the phase _ψcross of the orthogonally polarized transmitted wave and the long-axis length a(x) parameter of the rectangular nanopillar curve 4;

计算得到入射波频率为f₀、焦距为F₀的平面超构透镜在其表面沿x轴方向的透射相位分布

4) Using the surface phase distribution formula of spherical lens

The transmission phase distribution along the x-axis direction of the plane metalens with the incident wave frequency f ₀ and the focal length F ₀ is obtained by calculation

F₀表示超构透镜预设焦距，x表示超表面层矩形纳米柱几何中心的位置坐标，可用

来表示，λ₀表示入射电磁波的波长。in,

F ₀ represents the preset focal length of the metalens, and x represents the position coordinates of the geometric center of the rectangular nanopillars of the metasurface layer.

to represent, λ ₀ represents the wavelength of the incident electromagnetic wave.

确定对应任意x位置的矩形纳米柱的方位角及长轴长度a(x)。5) Combine the characteristic curves 2 and 4 between the phase ψ _cross of the orthogonally polarized transmitted wave in steps 2) and 3) and the long-axis length a(x) parameter of the rectangular nanopillar, according to the arbitrary x position calculated in step 4). Transmission phase distribution at

Determine the azimuth angle and major axis length a(x) of the rectangular nanopillars corresponding to any x position.

Preferably, in order to achieve better polarization-independent focusing effect, the number n of the rectangular nanopillars on either side of the center of each row is an integer greater than or equal to 20.

Preferably, the randomly polarized incident electromagnetic wave includes left-handed circularly polarized light, right-handed circularly polarized light and linearly polarized light with different linearly polarized angles, and the linearly polarized angle of the linearly polarized light ranges from [0°, 180° ], with a step size of 5°.

Preferably, the wavelength range of the mid-infrared electromagnetic wave is [3950nm, 4500nm], and the step size is 50nm.

Preferably, the metasurface layer is in an amorphous state at room temperature, and changes to a crystalline state after being heated to a threshold value. In the crystalline state, the incident wavelengths were set to λ=4000 nm, 4200 nm and 4700 nm, respectively.

The beneficial effects of the present invention are as follows: Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-columns with high aspect ratio are arranged on the upper surface of the substrate layer at an azimuth angle of 0° or 90°, and the length of the long axis is adjusted to control the transmission wave. The phase and intensity can achieve efficient convergence for incident light in any polarization state; in addition, thanks to the strong robustness of the phase dispersion of the metasurface unit structure, the metalens of the present invention can achieve effective focusing in the mid-infrared 4000-4500 nm bandwidth, The focusing efficiency can reach 70%, which is more suitable for the application of practical scenarios; and by adjusting the phase state of the phase change material Ge ₂ Sb ₂ Se ₄ Te ₁ , the focusing function of the metal lens of the present invention at a fixed frequency can achieve "ON" and "ON" OFF” dynamic switching; in addition, by setting the excitation wavelength reasonably, the metal lens of the present invention can also achieve simultaneous efficient focusing of “reflection” and “transmission” dual modes, and is more practical.

Description of drawings

1 is a schematic structural diagram of an embodiment of a phase-change material-based dynamically tunable dual-mode simultaneous focusing metalens embodiment;

Among them, 1 is the CaF ₂ substrate layer, and 2 is the metasurface layer composed of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars;

Fig. 2-(a) is a schematic diagram of a metasurface layer based on phase change material Ge ₂ Sb ₂ Se ₄ Te ₁ of the present invention; Fig. 2-(b) is a perspective view of the unit structure of the metalens embodiment shown in Fig. 1 Fig. 2-(c) is the top view of the unit structure of the metal lens embodiment shown in Fig. 1;

where a is the long-axis length of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-pillar, b is the short-axis length of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-pillar, h is the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-pillar height, p is the period of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-column matrix, β is the azimuth angle, which refers to the counterclockwise rotation angle of the long axis of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-column relative to the positive direction of the x-axis;

Each row of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars along the x-axis has the same short-axis length, and the long-axis length is determined by the transmission phase it provides, and each row of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars is about x= 0 is symmetrically distributed, and the number of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars on both sides of x=0 is n;

Fig. 3-(a) shows the unit structure of the metal lens embodiment shown in Fig. 1 under the incidence of right-handed circularly polarized wave with wavelength λ ₀ =4200nm, the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanocolumn satisfies b=600nm, h = 2800nm, p=3000nm, β=0° and β=90°, the characteristic diagram between the transmission efficiency of the transmitted left-handed circularly polarized wave and the long-axis length a of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillar;

Fig. 3-(b) shows the unit structure of the metal lens embodiment shown in Fig. 1 under the incidence of right-handed circularly polarized wave with wavelength λ ₀ =4200nm, the rectangular nanocolumn of Ge ₂ Sb ₂ Se ₄ Te ₁ satisfies b=600nm, h = 2800nm, p=3000nm, β=0° and β=90°, the characteristic diagram between the phase of the transmitted left-handed circularly polarized wave and the long-axis length a of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillar;

Among them, the length of the long axis a varies from 860 nm to 2900 nm, R0 represents the case of β=0°, R90 represents the case of β=90°, and the unit structure corresponding to the length of the long axis marked with the character “meter” on the graph is the present invention. All optimized cell structures selected for the Metalens Example.

Fig. 4 shows that when Ge ₂ Sb ₂ Se ₄ Te ₁ is in an amorphous state, when a right-hand circularly polarized wave with a working wavelength of λ ₀ =4200 nm is incident from the side of the CaF ₂ substrate layer, the metal lens of the present invention transmits the left-hand circular polarization The focusing diagram of the wave; (a) is the focused electric field intensity distribution of the left-handed circularly polarized transmitted wave in the xz plane; (b) is the electric field intensity of the left-handed circularly polarized transmitted wave in the xz plane and along the z-axis when x=0 Distribution curve; (c) is the electric field intensity distribution curve along the x-axis of the left-handed circularly polarized transmitted wave in the xz plane when z=102 μm (the center of the focal spot);

Fig. 5 is the focusing diagram of the transmitted wave of the metalens of the present invention under the incident of wavelength λ ₀ =4200nm and different polarized light when Ge ₂ Sb ₂ Se ₄ Te ₁ is in an amorphous state; (a) is the metalens in the The peak light intensity, FWHM and diffraction limit of focusing under different polarized incident light; (b) is the focusing efficiency of metalens under different polarized incident light.

Wherein, the illustration shows the transmitted orthogonal polarizations of the metallens of the present invention when the polarization angles are 0°, 45°, 90°, 135°, and 180°, respectively, when linearly polarized light and left- and right-handed circularly polarized light are incident. The distribution of the focused electric field intensity of the wave in the x-z plane;

6 is a focusing diagram of the metalens of the present invention when the right-handed circularly polarized light of different wavelengths (marked in the figure) is incident when Ge ₂ Sb ₂ Se ₄ Te ₁ is in an amorphous state; (a) is the metalens The focused electric field intensity distribution of the left-handed circularly polarized transmitted wave in the xz plane under the incidence of right-handed circularly polarized light of different wavelengths; (b) is the left-handed circularly polarized transmitted wave when the right-handed circularly polarized light of different wavelengths is incident. Focused focal length position, focal length offset and focal depth; (c) is the peak light intensity, half-width FWHM, diffraction limit and Focus on efficiency.

7 is a focusing diagram of the metalens of the present invention when right-handed circularly polarized light is incident at different specified wavelengths (marked in the figure) when Ge ₂ Sb ₂ Se ₄ Te ₁ is transformed into a crystalline state; (a) is a meta-structure The focused electric field intensity distribution diagram of the left-handed circularly polarized transmitted wave in the xz plane under the incidence of right-handed circularly polarized light of different specific wavelengths of the lens; (b) is the wavelength of 4000nm, and the left-handed circularly polarized light component is reflected at |z|=104μm Realized and achieved phase distribution (Required) curves of the selected optimized 30 cell structures on either side of the center of each row along the x-axis that make up the metalens of the present invention when focusing simultaneously with transmission. (c) is the electric field intensity distribution along the x-axis when the wavelength λ ₀ =4200nm, the left-handed circularly polarized transmitted wave is in the xz plane, when z=102μm (the center of the focal spot); (d) is the wavelength of 4700nm, the left-handed circularly polarized light When the reflected wave achieves reflection focusing at z=90 μm, the phase distribution (Realized) and the phase distribution (Realized) required by the selected optimized 30 unit structures along either side of the center of each row of the metal lens of the present invention Required) curve, wherein the required phase distribution (Required) curve can be obtained by formula (1).

Detailed ways

Hereinafter, the present invention will be described with reference to the accompanying drawings 1-7 by taking the embodiment as an example.

Fig. 1 is a schematic diagram of the structure of a dynamically tunable dual-mode simultaneous focusing metalens based on phase-change material Ge ₂ Sb ₂ Se ₄ Te ₁ of the present invention, including a CaF ₂ substrate layer 1 and a metalens for realizing Phase-modulated Ge ₂ Sb ₂ Se ₄ Te ₁ metasurface layer 2;

Figure _2- (a) shows _a schematic diagram of the Ge ₂ Sb ₂ Se ₄ Te _{1 metasurface} layer used for phase modulation in the present invention _. The nano-pillars are arranged on the upper surface of the substrate layer in a matrix with an azimuth angle of 0° or 90° to control the phase and intensity of the transmitted wave to achieve the focusing effect, and the focusing performance can achieve convergence for incident light of any polarization state; Fig. 2-(b) and FIG. 2-(c) are schematic diagrams of the structural units of the present embodiment shown in FIG. 1, wherein the period of the rectangular nanopillar matrix is p, the length of the long axis of the rectangular nanopillars is a, which is a variable, and the short The axis length is b, the height is h, and the azimuth angle of the rectangular nanopillar is β;

The number of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars in any row along the x-axis is 2n. Preferably, n is an integer greater than or equal to 20. The rectangular nanopillars are point-symmetrically distributed about the center of each row, and the rectangular nanopillars on either side of the center of each row contain w rows of rectangular nanopillars with long axes along the x-axis direction and v columns of rectangular nanopillars with long axes along the y-axis direction. column, and w+v=n, the short-axis length of all rectangular nanopillars is the same, which is set as b, and the long-axis length a is a variable, which is determined by the transmission phase provided by it, so as to obtain 0-2π phase modulation and realize ultra-high The focusing function of the lens structure on the incident wave of fixed frequency and arbitrary polarization; the length of the long axis and the short axis of the rectangular nanocolumn along the y-axis direction satisfies the column equality.

Preferably, the short-axis length of the rectangular nanopillar is b=600nm, and the long-axis length a=860~2900nm;

The preferred thickness of the CaF ₂ substrate layer is t=2 μm;

The period of the rectangular nano-pillar array is a regular quadrilateral array, and the preferred plane period is p=3000 nm.

In the mid-infrared band, the phase change material Ge ₂ Sb ₂ Se ₄ Te ₁ has low optical loss, and can realize the phase transition from amorphous to crystalline state under external stimuli such as light, electricity and force. are reversible and nonvolatile. Compared with the common phase change material Ge ₂ Sb ₂ Te ₅ , the refractive index contrast of Ge ₂ Sb ₂ Se ₄ Te ₁ before and after the phase transition is larger, which makes it more advantageous in constructing dynamically tunable metasurfaces with electromagnetic properties. Metalens based on phase-change material Ge ₂ Sb ₂ Se ₄ Te ₁ can overcome the technical bottlenecks faced by existing metalens, such as incident polarization sensitivity, narrow operating bandwidth, and difficult dynamic tuning of performance. In the field of new electromagnetic wave devices and electromagnetic wave technology, Such as optical communication, optical encryption and advanced imaging have huge potential application value and prospects.

The technical solutions of the present invention are specifically explained and described below with examples.

Example: The meta-lens preset focal length F ₀ =100 μm, the radius is 90 μm, and the numerical aperture NA is 0.669. The metalens are composed of Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-columns of different sizes and high aspect ratios arranged in a matrix with an azimuth angle of 0° or 90°. The meta-surface layer is fixed on the upper surface of the CaF ₂ substrate layer. In the embodiment, the period p=3000 nm of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-pillar matrix constituting the metasurface layer. The number of rectangular nanopillars in any row along the x-axis direction is 2n, and the n rectangular nanopillars arranged from the center of each row to the left and right include w rows of rectangular nanopillars and v long axes along the x-axis direction. Columns of rectangular nanopillars along the y-axis direction, and w+v=n=30, where w=18, v=12; the n rectangular nanopillars on the left side of the center of each row are related to the n rectangular nanopillars on the right side of the center of each row The centers of each row are distributed point-symmetrically. The short-axis length of all rectangular nanopillars is the same, b=600 nm, and the long-axis length a is a variable ranging from 860 to 2900 nm, which is determined by the transmission phase it provides to obtain 0 to 2π phase modulation.

Here, first set the azimuth angle of each rectangular nanocolumn to 0°, and simulate the relationship between the orthogonal polarization transmission phase and the long axis length a of the rectangular nanocolumn by simulation, and then set the long axis of each rectangular nanocolumn around z The axis is rotated counterclockwise by 90°, and the relationship between the orthogonal polarization transmission phase and the long axis length a of the rectangular nanocolumn is also simulated _; From the above phase distribution curve, all the optimized unit structures constituting the metal lens of the present invention are selected.

The simulation calculation was performed using the wave optics module of the finite element electromagnetic field simulation software Comsol Multiphysics (Comsol Inc.). In the mid-infrared band studied, the CaF ₂ substrate layer is a lossless medium, the refractive index is set as a constant 1.47, and the refractive index of Ge ₂ Sb ₂ Se ₄ Te ₁ changes with the wavelength.

3 shows the transmission efficiency and phase of the transmitted left-handed circularly polarized wave and the length of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nano-column under the incident of right-handed circularly polarized wave with wavelength λ ₀ =4200 nm in the unit structure in the embodiment of the present invention The characteristic diagram between the axis lengths a, showing the transmission spectrum of right-handed circularly polarized wave incident on the "Ge ₂ Sb ₂ Se ₄ Te ₁ metasurface-dielectric layer" unit structure, here the Ge ₂ Sb of the metalens unit structure ₂ Se ₄ Te ₁ rectangular nanocolumns satisfy b=600 nm, h=2800 nm, and p=3000 nm. Under the incidence of right-handed circularly polarized wave with wavelength λ ₀ =4200 nm, when the azimuth angle β of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanocolumn is equal to 0°, and the long axis length a gradually increases from 860 nm to 2500 nm, in the transmitted wave The transmittance of the left-handed circularly polarized wave is relatively high, which is maintained above 0.15, and the maximum can reach above 0.6, which realizes a more efficient polarization conversion from the incident right-handed circularly polarized wave to the transmitted left-handed circularly polarized wave. At the same time, the left-handed circularly polarized wave in the transmitted wave is The transmission phase covers the range of -π～0; other conditions remain unchanged, when the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillar azimuth angle β becomes 90°, and the long axis length a also gradually increases from 860 nm to 2500 nm, the transmitted wave The transmittance of the mid-left circularly polarized wave is completely consistent with that of the Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillar when the azimuth angle β=0°, but the transmission phase of the left-hand circularly polarized wave in the transmitted wave is shifted upward by a value of π as a whole, and the coverage becomes 0 to π. Therefore, setting the azimuth angle of the rectangular nanopillars to 0° or 90°, combined with the transmission phase (the phase caused by the change of the long axis of the rectangular nanopillars), can ensure that 0-2π phase modulation can be obtained on the basis of high transmittance, providing A specific arrangement structure of a rectangular nano-column array of a metasurface layer enables the metalens to realize the convergence of light waves of any polarization.

The metalens composed of the "Ge ₂ Sb ₂ Se ₄ Te ₁ metasurface-dielectric layer" structure can make the orthogonally polarized transmitted wave phase abruptly relative to the incident wave. By arranging rectangular nanopillars with different long axis lengths and azimuth angles The structural unit can generate a specific phase gradient distribution in the tangential direction of the lens surface, change the wavefront and propagation direction of the transmitted wave, and make it converge. In order to realize the convergence function of the lens, in the x-axis direction of the metasurface, the phase distribution should satisfy the following formula,

来表示，p表示结构单元周期，λ₀表示入射波长。结合图3正交偏振透射波的相位ψ_cross与矩形纳米柱长轴长度a之间的特性关系曲线，根据计算出的透射相位分布

即可确定任意x位置每个矩形纳米柱长轴长度a和方位角(β＝0°或β＝90°)，提供一种具体的超表面层矩形纳米柱阵列的排布结构，使该超构透镜能够实现对任意偏振入射波的高效聚焦。图3“米”字标注的长轴长度对应的单元结构为本发明的一种基于相变材料的任意偏振动态可调谐的双模同时聚焦的超构透镜实施例中选择的所有优化单元结构。Among them, F ₀ represents the preset focal length of the metal lens, and x represents the position coordinates of the geometric center of the rectangular nano-pillars of the metasurface layer. When the number of rectangular nano-pillars in any row is 2n,

to represent, p represents the period of the structural unit, and λ ₀ represents the incident wavelength. Combined with the characteristic relationship curve between the phase ψ _cross of the orthogonally polarized transmitted wave and the length a of the long axis of the rectangular nanopillar, according to the calculated transmission phase distribution

The length a and azimuth angle (β=0° or β=90°) of each rectangular nanopillar at any x position can be determined, and a specific arrangement structure of the rectangular nanopillar array of the metasurface layer is provided, so that the The lens can achieve efficient focusing of incident waves of any polarization. The unit structures corresponding to the long axis length marked with the character "m" in FIG. 3 are all optimized unit structures selected in the embodiment of a phase-change material-based dynamically tunable dual-mode simultaneous focusing metalens embodiment.

The above-designed planar metalens are simulated and tested for a specific wavelength and a specific transmitted wave focusing position (focal length), and the results are shown in Figure 4. Fig. 4-(a) is when Ge ₂ Sb ₂ Se ₄ Te ₁ is in an amorphous state, and the left-handed circularly polarized transmitted wave in the xz plane is incident under the incident right-handed circularly polarized wave with wavelength λ ₀ =4200 nm of the metal lens of the present invention. Focused electric field intensity distribution diagram, Figure 4-(b) is the distribution of the electric field intensity along the z-axis of the left-handed circularly polarized transmitted wave in the xz plane, when x=0, and Figure 4-(c) is the left-handed circularly polarized transmitted wave in the In the xz plane, when z=102 μm (the center of the focal spot), the distribution of the electric field intensity along the x-axis shows that when Ge ₂ Sb ₂ Se ₄ Te ₁ is amorphous, the metal lens of the present invention has a wavelength of λ ₀ =4200 nm. When the right-handed circularly polarized wave is incident, the convergence of the left-handed circularly polarized transmitted wave can be achieved, and the focus position z=102 μm (marked by the gray dotted line), which is almost the same as the preset focal length F ₀ =100 μm, indicating that the metalens can realize the right-handed circularly polarized transmission wave. Efficient focusing of circularly polarized incident waves.

这对于高性能成像系统非常重要。By changing the polarization state of the incident wave, it can be tested whether the metalens of this embodiment can realize the focusing performance of the incident wave with any polarization. The incident wavelength is still set to λ ₀ =4200nm, and the simulation test of the focusing effect of any polarization incident wave is carried out on the metalens of this embodiment. The results are shown in Figure 5, and Figure 5-(a) is Ge ₂ Sb ₂ Se ₄ Te ₁ When it is in an amorphous state, the peak light intensity, full width at half maximum FWHM and diffraction limit of the metal lens of the present invention focused under different polarized light incidents, wherein the polarization angle range of the incident linearly polarized light is [0, 180°], and the step size is 5°, the incident circularly polarized light contains left-handed and right-handed circularly polarized light. It can be seen that for the incidence of linearly polarized light at any polarization angle, the peak intensity of the electric field of the transmitted wave passing through the metal lens of the present invention fluctuates between 12 (corresponding to the x-polarized light) and 24 (corresponding to the y-polarized light). , and for the incidence of left-handed circularly polarized light and right-handed circularly polarized light, the peak intensity of the electric field of the transmitted wave passing through the metal lens of the present invention is equal to 36, that is, the electric field intensity under the same conditions under the incidence of x and y linearly polarized light This is because the orthogonally polarized transmission electric field strength under any circularly polarized light incidence can be regarded as the algebraic superposition of the orthogonally polarized electric field strengths under the same conditions of x and y linearly polarized light incidence. In addition, regardless of the incidence of linearly polarized light or circularly polarized light of any polarization angle, the FWHM of the focal spot focused by the transmitted wave through the metalens of the present invention is basically maintained at 0.7λ ₀ , which is lower than that of the metalens. Theoretical diffraction limit

This is very important for high performance imaging systems.

Focusing efficiency is one of the important indicators to measure the performance of flat lens. Fig. 5-(b) shows the focusing efficiency of the metalens of the present invention under different polarized light incidents. The focusing efficiency is defined as the ratio of the transmitted light intensity of the focal spot with a width of three times the FWHM to the entire incident light intensity. It can be seen that when the working wavelength λ ₀ =4200 nm, the focusing efficiency of the transmitted wave through the metal lens of the present invention maintains a relatively constant value of about 70% regardless of whether linearly polarized light or circularly polarized light is incident. The inset of Fig. 5-(b) shows the transmission of the metal lens of the present invention when the polarization angles are 0°, 45°, 90°, 135°, 180°, respectively, when the linearly polarized light and the left- and right-handed circularly polarized light are incident. The focusing effect diagram of the orthogonal polarized wave components in the xz plane further confirms that the metal lens of the present invention can achieve efficient focusing of incident light of any polarization.

进一步证实本发明超构透镜在4000~4500nm的波长范围内可实现较好的聚焦效果。图6-(c)显示了本发明超构透镜实际焦距处的峰值强度、FWHM和聚焦效率对入射波长的依赖关系。可以直观地看出，随着波长的增加，焦斑峰值强度基本呈下降趋势，焦斑的FWHM几乎保持不变，并略低于其对应的衍射极限。模拟还表明，本发明超构透镜在4000～4500nm的波长范围内，聚焦效率始终高于60％，这明显优于或可比拟之前报道的在透射模式下工作的宽带、偏振不敏感超构透镜。Figure 6-(a) shows that when Ge ₂ Sb ₂ Se ₄ Te ₁ is in an amorphous state, the metal lens of the present invention transmits left-handed circularly polarized light under different mid-infrared wavelengths (marked in the figure) under right-handed circularly polarized light incident. The focused electric field intensity distribution diagram of the wave in the xz plane, it can be seen that the metal lens of the present invention has a small focal length shift (relative to the preset focal length F ₀ =100 μm marked by the white dotted line) in the wavelength range of 4000-4500 nm A better focusing effect is achieved. In order to quantitatively characterize the broadband focusing performance of the metalens of the present invention, Fig. 6-(b) shows the actual situation of the left-handed circularly polarized transmitted waves corresponding to different incident wavelengths extracted from the electric field intensity distribution diagram of Fig. 6-(a) in the xz plane. Focusing on the focal length, focal length offset and focal depth, it is found that the actual focal length decreases slightly with the increase of the incident wavelength, which is consistent with the focusing behavior of the diffractive lens, and the focal length offset is defined as the actual focal length minus the preset focal length (F ₀ = 100 μm), it can be seen that in the wavelength range of 4000 to 4500 nm, the focal length offset fluctuates between 0 and 6, but is much smaller than the focal depth of the metal lens of the present invention at the corresponding incident wavelength

It is further confirmed that the metal lens of the present invention can achieve better focusing effect in the wavelength range of 4000-4500 nm. Figure 6-(c) shows the dependence of the peak intensity, FWHM and focusing efficiency on the incident wavelength at the actual focal length of the metalens of the present invention. It can be seen intuitively that with the increase of wavelength, the peak intensity of the focal spot basically shows a downward trend, and the FWHM of the focal spot remains almost unchanged, and is slightly lower than its corresponding diffraction limit. The simulations also show that in the wavelength range of 4000-4500 nm, the focusing efficiency of the metalens of the present invention is always higher than 60%, which is significantly better than or comparable to the previously reported broadband, polarization-insensitive metalens operating in transmission mode. .

Ge ₂ Sb ₂ Se ₄ Te ₁ exhibits an amorphous state (aGSST) at room temperature, and becomes crystalline (cGSST) after reaching a threshold temperature. In the mid-infrared band, the high refractive index contrast between the two states enables GSST to be an optical device The reconfigurability and flexibility provide more degrees of freedom. Figure 7-(a) shows that when Ge ₂ Sb ₂ Se ₄ Te ₁ is transformed into a crystalline state, when right-handed circularly polarized light is incident at different specific wavelengths (marked in the figure), the left-handed circularly polarized transmitted wave is at A plot of the focused electric field intensity distribution in the xz plane. As expected, when the working wavelength λ ₀ =4200nm and aGSST is converted into cGSST by external stimulation, the metal lens of the present invention can no longer realize focusing, and the focusing function changes from "ON" to "OFF", Fig. 7-(c) shows the wavelength When λ ₀ =4200 nm, the electric field intensity distribution curve of the left-handed circularly polarized transmitted wave in the xz plane and when z = 102 μm (the center of the focal spot) along the x-axis, which further confirms that by controlling the Ge ₂ Sb ₂ Se ₄ Te ₁ phase It can tune the surrounding dielectric environment, so that the metalens of the present invention can realize the dynamic regulation of the focusing function "ON" and "OFF"; when the working wavelength changes to 4700nm, Ge ₂ Sb ₂ Se ₄ Te ₁ is still in a crystalline state (cGSST), the focusing function of the metal lens of the present invention is restored, and the bright focal spot reappears on the focal plane. In order to reveal the underlying physical mechanism, Fig. 7-(d) shows the selected optimized 30 formed on either side of the center of the metalens of the present invention when the wavelength is 4700 nm and the left-handed circularly polarized transmitted wave is focused at z=90 μm. It can be seen that the phase distribution (Required) and the realized phase distribution (Realized) curve of each unit structure are almost completely coincident, which is the fundamental reason for the metal lens of the present invention to achieve perfect focusing at a wavelength of 4700 nm. In particular, Fig. 7-(a) also shows that when Ge ₂ Sb ₂ Se ₄ Te ₁ is in a crystalline state, when right-handed circularly polarized light with a wavelength of λ=4000 nm is incident on the metal lens of the present invention, the left-handed circularly polarized transmitted wave is at xz The in-plane focusing electric field intensity distribution diagram shows that the metal lens of the present invention can achieve dual simultaneous focusing performance of transmission and reflection at a wavelength of λ = 4000 nm, and the focal length is 104 μm. Figure 7-(b) shows that the wavelength is 4000 nm. When the left-handed circularly polarized reflected wave is focused at z=104 μm, the phase distribution (Required) and the realized phase distribution (Realized) of the selected optimized 30 unit structures constituting either side of the metal lens center of the present invention curve, the two phase curves are almost completely coincident, so the metal lens of the present invention can realize reflection focusing at a wavelength of 4000 nm (when λ=4000 nm is not given here, the transmission phase of the 30 unit structures on either side of the center of the metal lens of the present invention is not given. spectrum, which is similar to the case in Fig. 7-(d)). The metal lens of the present invention realizes the simultaneous focusing of reflection and transmission for the first time, which undoubtedly expands the flexibility and diversity of design functions.

The manufacture of the metal lens of the present invention can be prepared by the following method:

(1) Using the thermal evaporation method, by controlling the evaporation rate ratio of Ge ₂ Sb ₂ Te ₅ and Ge ₂ Sb ₂ Se ₅ two isolated targets, a 2800 nm thick Ge ₂ Sb was deposited on a double-sided polished CaF ₂ substrate _{2Se 4} Te ₁ film.

(2) The Ge ₂ Sb ₂ Se ₄ Te ₁ thin film was patterned into rectangular nanopillar arrays by electron beam lithography and reactive ion etching techniques to form a metasurface layer.

(3) The transformation and regulation of Ge ₂ Sb ₂ Se ₄ Te ₁ from amorphous state to crystalline state are realized by thermal annealing process.

In the embodiment of the present invention, the rectangular nano-columns of the Ge ₂ Sb ₂ Se ₄ Te ₁ metasurface layer along the y-axis direction satisfy the column equality in order to finally realize the line convergence of the metalens, and of course, in order to realize the point convergence or other convergence of the metalens It can also be flexibly designed according to actual needs, and the above solutions should also fall within the scope of protection of the present invention.

Figure 2-(a) is only to illustrate the arrangement of the rectangular nanocolumns of the present invention, so the lengths of the nanopillars are not set according to the content of the scheme, which does not affect the protection of the present invention to the above scheme.

The incident light in Figures 6-7 adopts right-handed circularly polarized wave, which is only to illustrate the wide-band focusing of metalens of the present invention, the dynamic tuning of focusing functions "ON" and "OFF" and the dual-mode simultaneous focusing performance, of course, other can also be used. The above solutions should also fall within the scope of protection of the present invention.

To sum up, the present invention proposes a method to realize the convergence of arbitrary polarized waves of specific wavelengths by fixing different sizes of high aspect ratio Ge ₂ Sb ₂ Se ₄ Te ₁ rectangular nanopillars on the upper surface of the substrate layer at an azimuth angle of 0° or 90°. of transmissive metalens. By regulating the phase state of Ge ₂ Sb ₂ Se ₄ Te ₁ , the present invention can realize the dynamic regulation of “ON” and “OFF” focusing functions; by reasonably setting the excitation wavelength, the present invention can realize the simultaneous dual modes of “reflection” and “transmission” Focus efficiently. In addition, the present invention also realizes high-efficiency focusing performance with a broadband of 4000-4500 nm and below the diffraction limit.

The examples given are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or improvements within the technical scope disclosed by the present invention. , such as scaling the structure size to make it work in different bands such as terahertz and visible, all should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (7)

1. A metalens that can be arbitrarily polarized dynamically tunable dual-mode simultaneous focusing based on phase change material, it is characterized in that, this metalens comprises CaF ₂ substrate layer and is used for realizing transmission phase modulation and using phase change material A metasurface layer made of Ge ₂ Sb ₂ Se ₄ Te ₁ , the metasurface layer is formed by a plurality of rectangular nano-columns with the same height arranged on the upper surface of the substrate layer as a rectangular nano-column array with a period p along the xy plane, so The plurality of rectangular nanopillars with the same height all have high aspect ratio structural features and their azimuth angles are 0° or 90°, and the period refers to the distance between the geometric centers of two adjacent rectangular nanopillars on the x and y axes, The number of rectangular nanopillars in any row along the x-axis direction is 2n, the rectangular nanopillars in each row are point-symmetrically distributed about the row center (x=0), and either side of the center of each row contains w long axes along the x-axis direction Row of rectangular nanopillars and v columns of rectangular nanopillars with the long axis along the y-axis direction, and w+v=n, the length and width of all rectangular nanopillars are the long axis and the short axis, respectively, and the short axis length is b and the same, The length of the major axis is a and is a variable, which is determined by the transmission phase provided by it, so as to obtain 0-2π phase modulation, and realize the focusing function of the metalens on the incident wave of fixed frequency and arbitrary polarization. 2. a kind of metalens based on phase-change material that can be arbitrarily polarized dynamically tunable dual-mode focusing simultaneously, it is characterized in that, described row rectangular nano-column refers to rectangular nano-column azimuth angle β=0°, the column of rectangular nanopillars refers to the rectangular nanopillar azimuth angle β=90°, and the azimuth angle β refers to the counterclockwise rotation angle of the long axis of the rectangular nanopillars with respect to the positive direction of the x-axis. 3. a kind of metalens based on phase-change material that can be arbitrarily polarized dynamically tunable dual-mode focusing simultaneously, it is characterized in that, along the surface x-axis direction, each rectangular nano-column long-axis length a The steps to solve (x) are: 1) Set the working frequency of the metalens to f ₀ (corresponding to the working wavelength λ ₀ ), and the variation range of the long-axis length a(x) of the rectangular nanopillars is [a _min , a _max ]; 2) The azimuth angle β of each rectangular nanocolumn in the metasurface layer is set to 0°, and it is obtained through simulation that when the incident wave frequency is f ₀ , the left (right) circularly polarized light passes through the metasurface unit structure and is orthogonally polarized. The characteristic curve 1 between the transmittance T _cross of the transmitted wave and the long-axis length a(x) parameter of the rectangular nanocolumn and the relationship between the phase ψ _cross of the orthogonally polarized transmitted wave and the long-axis length a(x) parameter of the rectangular nanocolumn characteristic curve 2; 3) Change the azimuth angle β of each rectangular nanocolumn in the metasurface layer to 90°, and also obtain through the simulation simulation that when the incident wave frequency is f ₀ , the left (right) circularly polarized light passes through the metasurface unit structure and is orthogonally polarized. The characteristic curve 3 between the transmittance T _cross of the transmitted wave and the long-axis length a(x) parameter of the rectangular nanopillar and the phase _ψcross of the orthogonally polarized transmitted wave and the long-axis length a(x) parameter of the rectangular nanopillar characteristic curve 4; 4) Using the surface phase distribution formula of spherical lens

The transmission phase distribution along the x-axis direction of the plane metalens with the incident wave frequency f ₀ and the focal length F ₀ is obtained by calculation

in,

F ₀ represents the preset focal length of the metalens, and x represents the position coordinates of the geometric center of the rectangular nanopillars of the metasurface layer.

N=±1,±2,...±n, and λ ₀ represents the wavelength of the incident electromagnetic wave. 5) Combine the characteristic curves 2 and 4 between the phase ψ _cross of the orthogonally polarized transmitted wave in steps 2) and 3) and the long-axis length a(x) parameter of the rectangular nanopillar, according to the arbitrary x position calculated in step 4). Transmission phase distribution at

Determine the azimuth angle and major axis length a(x) of the rectangular nanopillars corresponding to any x position. 4. a kind of metalens based on a phase change material that can be arbitrarily polarized dynamically tunable dual-mode focusing at the same time, it is characterized in that, in order to realize better polarization independent focusing effect, described each The number n of rectangular nanopillars on either side of the row center is an integer greater than or equal to 20. 5. according to a kind of metalens that can be arbitrarily polarized dynamically tunable dual-mode focusing simultaneously based on phase-change material according to claim 4, it is characterized in that, described arbitrary polarization incident electromagnetic wave comprises left-handed circularly polarized light, right-handed circularly polarized light Polarized light and linearly polarized light with different linear polarization angles, the linear polarization angle range of the linearly polarized light is [0°, 180°], and the step size is 5°. 6. A kind of metalens that can be arbitrarily polarized dynamically tunable dual-mode simultaneous focusing based on phase change material according to claim 4, it is characterized in that, the wavelength value range of described mid-infrared electromagnetic wave is [3950nm, 4500nm ] with a step size of 50 nm. 7. A kind of metalens that can be arbitrarily polarized dynamically tunable dual-mode simultaneous focusing based on phase change material according to claim 4, it is characterized in that, described phase change material that forms metasurface layer is Ge ₂ Sb ₂ Se ₄ Te ₁ , which is amorphous at room temperature, and changes to a crystalline state after being heated to a threshold temperature. There is a significant difference in the permittivity of the two states, and the two states can be converted to each other. The metalens are in Ge ₂ Sb ₂ When Se ₄ Te ₁ is in a crystalline state, the specific incident wavelengths are set to λ=4000 nm, 4200 nm, and 4700 nm, respectively.

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