CN114114473A - Phase-change-material-based double-mode simultaneous focusing super-structure lens capable of dynamically tuning polarization at will - Google Patents
Phase-change-material-based double-mode simultaneous focusing super-structure lens capable of dynamically tuning polarization at will Download PDFInfo
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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
Technical Field
The invention relates to a phase-change material-based double-mode simultaneous focusing super-structured lens capable of dynamically tuning polarization at will, and belongs to the field of novel artificial electromagnetic materials and optical devices.
Background
The super surface (Metasurface), also called two-dimensional Metamaterials (Metamaterials), is essentially characterized in that the wave front is regulated and controlled by utilizing the phase space change generated by the coupling of a super surface unit structure and incident electromagnetic waves, is a functional film layer device based on a sub-wavelength anisotropic structure, not only retains the singular characteristics of the three-dimensional Metamaterials, but also overcomes the difficulty in the preparation of the three-dimensional Metamaterials. By selecting proper materials and reasonably designing parameters such as the shape, the size, the direction and the like of the super-surface unit structure, the super-surface can flexibly adjust the anisotropy of the nano structure, a nano functional device capable of regulating and controlling various optical parameters such as amplitude, phase, polarization, frequency, spectrum and the like is optimized, and the research and the application of the super-surface in many fields are greatly expanded.
The super-structure lens is an important representative of the practicability of the super-surface, and has attracted great interest of researchers by virtue of the outstanding advantages of super-strong light wave control capability, super-compact structure, multifunctionality, compatibility with semiconductor processes and the like. At present, various functional super-structure lenses are verified theoretically or experimentally, including broadband achromatic super-structure lenses, super-resolution super-structure lenses, multi-focus super-structure lenses, multifunctional super-structure lenses and the like. However, the reported super-structure lenses are limited to phase distribution design, and are difficult to realize arbitrary polarization and broadband focusing functions at the same time; once the structure is determined, the electromagnetic performance of the structure cannot be changed, and the electromagnetic wave is limited in flexible modulation.
Germanium Antimony Selenium Tellurium (Germanium Antimony Selenium Tellurium, Ge)2Sb2Se4Te1GSST) as a new class of phase change materials, has attracted much attention in recent research, especially in the research of dynamically tunable optical devices, mainly thanks to its material specific properties: ge at Normal temperature2Sb2Se4Te1The optical device is in an amorphous state (aGSST), and changes into a crystalline state (cGSST) after reaching a threshold temperature, the difference between the two states is obvious, and the phase change is reversible, so that the GSST can provide more freedom for the reconfigurability and flexibility of the optical device. The dynamic adjustable super-structure lens can be realized by combining the super-structure lens with the phase-change material GSST and by means of external stimulation (thermal, optical, electrical and the like). However, at present, the research on this aspect mainly focuses on the dynamic adjustment and control of the electromagnetic performance by using the phase change of the uniform dielectric layer GSST, and the research on patterning the GSST into the metamaterial unit and realizing the dynamically adjustable metamaterial lens is still in the beginning stage, and has not been paid enough attention.
Disclosure of Invention
In view of the above situation, an object of the present invention is to provide a phase-change material-based metamaterial capable of dynamically tuning polarization and focusing simultaneously in dual modes, so as to solve the technical bottlenecks of incident polarization sensitivity, narrow working bandwidth, difficult dynamic tuning of performance, etc. encountered by the conventional metamaterial.
The scheme provides a design of a double-mode simultaneous focusing super-structure lens capable of being dynamically tunable in polarization at will based on a phase change material, and the phase change material is utilized to realize the designMaterial Ge2Sb2Se4Te1The high refractive index contrast between the amorphous and crystalline states allows dynamic switching of "ON" and "OFF" for a fixed frequency arbitrary polarized wave focusing effect.
The technical scheme adopted by the invention for solving the technical problems is as follows: characterized in that the super-structured lens comprises CaF2Substrate layer and method for implementing transmission phase modulation using phase change material Ge2Sb2Se4Te1The prepared super-surface layer is formed by arranging a plurality of rectangular nano-pillars with the same height on the upper surface of a substrate layer along an x-y plane in a period p to form a rectangular nano-pillar array, wherein the plurality of rectangular nano-pillars with the same height have high aspect ratio structural features, the azimuth angle of the rectangular nano-pillars is 0 DEG or 90 DEG, the period refers to the distance between the geometric centers of two adjacent rectangular nano-pillars on an x axis and a y axis, the number of the rectangular nano-pillars in any row along the x axis direction is 2n, the rectangular nano-pillars in each row are distributed in point symmetry with the row center (x is 0), any side of the row center comprises w row rectangular nano-pillars with long axes along the x axis direction and v column rectangular nano-pillars with long axes along the y axis direction, w + v is n, the long widths of all the rectangular nano-pillars are respectively the long axes and the short axes, the lengths of the short axes are b and the same, and the lengths of the long axes are a and are variable, the transmission phase provided by the lens is used for determining to obtain 0-2 pi phase modulation, and the focusing function of the super-structure lens on the incident wave with fixed frequency and arbitrary polarization is realized.
Preferably, the row of rectangular nano-pillars refers to a rectangular nano-pillar azimuth angle β being 0 °, the column of rectangular nano-pillars refers to a rectangular nano-pillar azimuth angle β being 90 °, and the azimuth angle β refers to a counterclockwise rotation angle of the long axis a of the rectangular nano-pillars with respect to the positive direction of the x-axis.
Preferably, along the x-axis direction of the surface, the solving step of the length a (x) of the long axis of each rectangular nano-column is as follows:
1) setting the operating frequency of the super-structured lens to f0(corresponding to the operating wavelength lambda)0) The length a (x) of the long axis of the rectangular nano-column is in the range of [ a ]min，amax]；
2) The azimuth angle beta of each rectangular nano column of the super surface layerAre all set to be 0 DEG, and the frequency f of the incident wave is obtained through simulation0When the left (right) circular polarized light passes through the super-surface unit structure, the transmissivity T of the orthogonal polarized transmitted wavecross Characteristic curve 1 between parameters of length a (x) of long axis of rectangular nano column and phase psi of orthogonal polarization transmission wavecross Characteristic curve 2 between the parameters of length a (x) of the long axis of the rectangular nano-column;
3) the azimuth angle beta of each rectangular nano column of the super surface layer is changed into 90 degrees, and the incident wave frequency f is obtained by simulation0When the left (right) circular polarized light passes through the super-surface unit structure, the transmissivity T of the orthogonal polarized transmitted wavecross Characteristic curve 3 between the parameters of length a (x) of long axis of rectangular nano-column and phase psi of orthogonal polarization transmitted wavecross Characteristic curve 4 with the length a (x) of the long axis of the rectangular nano-column;
4) using the formula of the surface phase distribution of the spherical lens
Calculating to obtain the frequency f of the incident wave0Focal length of F0The transmission phase distribution of the planar super-structure lens along the x-axis direction on the surface
Wherein the content of the first and second substances,
F0the preset focal length of the super-structure lens is represented, and x represents the position coordinate of the geometric center of the rectangular nano-pillar on the super-surface layer, and can be used
To denote λ0Representing the wavelength of the incident electromagnetic wave.
5) Combining the phases ψ of the orthogonally polarized transmitted waves of steps 2) and 3)cross Characteristic curves 2 and 4 between the parameters of the length a (x) of the long axis of the rectangular nano column and the transmission phase distribution at any x position calculated according to the step 4)
And determining the azimuth angle and the long axis length a (x) of the rectangular nano-pillar corresponding to any x position.
Preferably, in order to achieve a better polarization-independent focusing effect, the number n of the rectangular nano-pillars on either side of the center of each row is an integer greater than or equal to 20.
Preferably, the arbitrarily polarized incident electromagnetic wave includes left-handed circularly polarized light, right-handed circularly polarized light, and linearly polarized light with different linear polarization angles, and the linear polarization angle range of the linearly polarized light is [0 ° and 180 ° ], and the step length is 5 °.
Preferably, the wavelength range of the mid-infrared electromagnetic wave is [3950nm,4500nm ], and the step length is 50 nm.
Preferably, the super-surface layer is amorphous at room temperature, changes into a crystalline state after being heated to a threshold value, has a significant difference in dielectric constant between the two states, and is switchable between the two states, and when the super-structure lens changes into the crystalline state, incident wavelengths are set to λ 4000nm, 4200nm, and 4700nm, respectively.
The invention has the beneficial effects that: ge to be characterized by high aspect ratio2Sb2Se4Te1The rectangular nano-columns are distributed on the upper surface of the substrate layer at an azimuth angle of 0 degree or 90 degrees, and the phase and the intensity of transmitted waves are regulated and controlled by adjusting the length of a long shaft, so that the incident light in any polarization state is efficiently converged; in addition, due to the strong robustness of the phase dispersion of the super-surface unit structure, the super-structure lens can realize effective focusing within the bandwidth of 4000-4500 nm of the mid-infrared, the focusing efficiency can reach 70%, and the super-structure lens is more suitable for application of actual scenes; and by regulating and controlling the phase change material Ge2Sb2Se4Te1Phase state, the focusing function of the super-structure lens at fixed frequency can realize the dynamic switching of ON and OFF; in addition, the excitation wavelength is reasonably set, the metamaterial lens can realize the simultaneous high-efficiency focusing of a reflection mode and a transmission mode, and the practicability is higher.
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FIG. 1 is a schematic structural diagram of an embodiment of an arbitrary polarization dynamically tunable dual-mode simultaneous focusing metamaterial lens based on a phase change material according to the present invention;
wherein 1 is CaF2A substrate layer, 2 being Ge2Sb2Se4Te1A super surface layer composed of rectangular nano-pillars;
FIG. 2 (a) shows a phase change material Ge-based phase change material of the present invention2Sb2Se4Te1A schematic of a super surface layer; FIG. 2- (b) is an oblique view of the cell structure of the embodiment of the super-structured lens shown in FIG. 1; FIG. 2- (c) is a top view of the cell structure of the embodiment of the super-structured lens shown in FIG. 1;
wherein a is Ge2Sb2Se4Te1Length of long axis of rectangular nano-column, b is Ge2Sb2Se4Te1The minor axis length of the rectangular nano-pillar is h is Ge2Sb2Se4Te1Height of rectangular nano-pillar, p is Ge2Sb2Se4Te1The period of the rectangular nanorod matrix, beta is an azimuth angle, and refers to Ge2Sb2Se4Te1The long axis of the rectangular nano column rotates anticlockwise relative to the positive direction of the x axis;
along each row of the x-axis Ge2Sb2Se4Te1The minor axis lengths of the rectangular nano-pillars are all equal, the major axis length is determined by the transmission phase provided by the rectangular nano-pillars, and each row of Ge is2Sb2Se4Te1The rectangular nano columns are symmetrically distributed about 0-x, and Ge is arranged on two sides of 0-x2Sb2Se4Te1The number of the rectangular nano columns is n; the rectangular nanopillars along the y-axis direction satisfy column equality.
FIG. 3 (a) shows the unit structure of the embodiment of the super-structured lens shown in FIG. 1 at a wavelength λ0At right-handed circularly polarized wave incidence of 4200nm, Ge2Sb2Se4Te1The rectangular nano-pillar satisfies the transmission efficiency of transmitted levorotatory circular polarized wave and Ge when b is 600nm, h is 2800nm, p is 3000nm, beta is 0 DEG and beta is 90 DEG2Sb2Se4Te1A characteristic diagram between the long axis lengths a of the rectangular nano-columns;
FIG. 3- (b) is the super FIG. 1Unit structure of lens-forming embodiment at wavelength lambda0At right-handed circularly polarized wave incidence of 4200nm, Ge2Sb2Se4Te1The rectangular nano-pillar satisfies the conditions that when b is 600nm, h is 2800nm, p is 3000nm, beta is 0 DEG and beta is 90 DEG, the transmitted levorotatory circular polarized wave phase and Ge are formed2Sb2Se4Te1A characteristic diagram between the long axis lengths a of the rectangular nano-columns;
the variation range of the long axis length a is 860nm to 2900nm, R0 represents the case that β is 0 °, R90 represents the case that β is 90 °, and the unit structures corresponding to the long axis length marked by the "meter" on the graph are all optimized unit structures selected by the super lens embodiment of the present invention.
FIG. 4 is Ge2Sb2Se4Te1In the amorphous state, the inventive lens has a working wavelength λ0Right-handed circularly polarized wave from CaF of 4200nm2When incidence is carried out on one side of the substrate layer, a focusing graph of the levorotatory circularly polarized transmitted wave is obtained; (a) the focusing electric field intensity distribution diagram of the left-handed circularly polarized transmitted wave in an x-z plane; (b) the electric field intensity distribution curve of the left-handed circularly polarized transmitted wave in an x-z plane along a z axis when x is 0; (c) the electric field intensity distribution curve of the levorotatory circularly polarized transmitted wave in an x-z plane along an x axis when z is 102 mu m (focal spot center);
FIG. 5 is Ge2Sb2Se4Te1In the amorphous state, the inventive lens has a wavelength λ0A focused image of the transmitted wave at 4200nm, incident light of different polarizations; (a) the peak light intensity, full width at half maximum (FWHM) and diffraction limit of the super-structure lens focused under different polarized incident lights; (b) is the focusing efficiency of the super-structured lens under different polarized incident lights.
The insets are focusing electric field intensity distribution diagrams of transmitted orthogonal polarized waves in an x-z plane when linearly polarized light with polarization angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees and 180 degrees and left-handed and right-handed circularly polarized light are incident on the metamaterial lens;
FIG. 6 is Ge2Sb2Se4Te1In the amorphous state, the inventive lenses have different wavelengths (shown in the figure)Marked) a focusing plot when right-handed circularly polarized light is incident; (a) the distribution diagram of the focusing electric field intensity of the left-handed circularly polarized transmission wave in an x-z plane is formed under the incidence of right-handed circularly polarized light with different wavelengths by the super-structured lens; (b) when right-handed circularly polarized light with different wavelengths enters the super-structured lens, focusing position, focus offset and focus depth of left-handed circularly polarized transmitted wave focusing are adopted; (c) the super-structure lens is focused by left-handed circularly polarized transmitted waves under the incidence of right-handed circularly polarized light with different wavelengths, and has the peak light intensity, full width at half maximum (FWHM), diffraction limit and focusing efficiency.
FIG. 7 is Ge2Sb2Se4Te1When the lens is converted into a crystalline state, the lens is in a focusing picture when right-handed circularly polarized light with different specified wavelengths (marked in the picture) is incident; (a) the distribution diagram of the focusing electric field intensity of the left-handed circularly polarized transmission wave in an x-z plane is formed under the incidence of right-handed circularly polarized light with different specific wavelengths by the super-structured lens; (b) the phase distribution (real) and the achieved phase distribution (Required) curves for the selected optimized 30 cell structures on either side of the center of each row along the x-axis that make up the inventive metamaterial lens for a wavelength of 4000nm when simultaneous focusing of reflection and transmission is achieved with a left-handed circularly polarized light component at | z | ═ 104 μm. (c) Is a wavelength lambda04200nm, distribution of electric field strength along the x-axis in the x-z plane for a left-handed circularly polarized transmitted wave when z 102 μm (focal spot center); (d) for a wavelength of 4700nm, when the reflection focusing is Realized by the left-handed circularly polarized reflected wave at z equal to 90 μm, the Required phase distribution (satisfied) and the Realized phase distribution (satisfied) curves of the selected optimized 30 cell structures on either side of the center of each line along the x axis, which constitute the super-structured lens of the present invention, can be obtained by the formula (1).
Detailed Description
The present invention will be described below by way of examples with reference to the accompanying fig. 1 to 7.
FIG. 1 shows the phase change material Ge-based phase change material of the present invention2Sb2Se4Te1The structural schematic diagram of the arbitrary polarization dynamically tunable dual-mode simultaneous focusing super-structure lens comprises CaF2Substrate layer 1 and Ge for phase modulation2Sb2Se4Te1A super surface layer 2;
FIG. 2- (a) shows Ge for implementing phase modulation according to the present invention2Sb2Se4Te1A super surface layer diagram, the super surface layer is composed of high aspect ratio Ge with different sizes2Sb2Se4Te1The rectangular nano-columns are arranged on the upper surface of the substrate layer in a matrix form at an azimuth angle of 0 degree or 90 degrees and are used for regulating and controlling the phase and the intensity of transmitted waves so as to achieve a focusing effect, and the focusing performance can realize convergence on incident light in any polarization state; fig. 2- (b) and 2- (c) are schematic diagrams of the structural unit of the embodiment shown in fig. 1, wherein the period of the rectangular nanorod matrix is p, the length of the long axis of the rectangular nanorod is a, which is a variable, the length of the short axis is b, the height is h, and the azimuth angle of the rectangular nanorod is β;
along any row of the x-axis Ge2Sb2Se4Te1The number of the rectangular nano columns is 2n, preferably, n is an integer greater than or equal to 20, the rectangular nano columns on the left side of the center of each row and the rectangular nano columns on the right side of the center of each row are distributed in point symmetry about the center of each row, the rectangular nano columns on any side of the center of each row comprise w row rectangular nano columns with long axes along the x-axis direction and v column rectangular nano columns with long axes along the y-axis direction, w + v is equal to n, the lengths of the short axes of all the rectangular nano columns are the same and are set as b, the length a of the long axis is a variable and is determined by a transmission phase provided by the variable, so that 0-2 pi phase modulation is obtained, and the focusing function of the super-structured lens on the incident wave with fixed frequency and arbitrary polarization is realized; the lengths of the long axis and the short axis of the rectangular nano-pillars along the y-axis direction satisfy the column equality.
Preferably, the minor axis length b of the rectangular nano-pillar is 600nm, and the major axis length a is 860-2900 nm;
preferred CaF2The thickness t of the substrate layer is 2 μm;
the period of the rectangular nano-pillar array is a regular quadrilateral array, and the preferable planar period p is 3000 nm.
In the middle infrared band, the phase change material Ge2Sb2Se4Te1Has low optical loss, and can be used in light, electricity, force, etcThe phase change from the amorphous state to the crystalline state can be realized under external stimulation, and the phase change is reversible and nonvolatile. Compared with common phase change material Ge2Sb2Te5，Ge2Sb2Se4Te1The refractive index contrast before and after phase change is larger, so that the method has more remarkable advantages when the electromagnetic property dynamically tunable super-surface is constructed. Based on phase change material Ge2Sb2Se4Te1The super-structure lens can overcome the technical bottlenecks of incident polarization sensitivity, narrow working bandwidth, difficult dynamic tuning of performance and the like of the existing super-structure lens, and has great potential application value and prospect in the technical fields of novel electromagnetic wave devices and electromagnetic waves, such as optical communication, optical encryption and advanced imaging.
The technical solution of the present invention is specifically explained below by examples.
Example a Preset focal Length F of a Superlens0100 μm, radius 90 μm, and numerical aperture NA 0.669. The super-structure lens is composed of Ge with different sizes and high aspect ratios2Sb2Se4Te1The rectangular nano-columns are arranged in a matrix form at an azimuth angle of 0 degree or 90 degrees to form a super surface layer fixed on the CaF2The upper surface of the substrate layer. In the examples, Ge constituting the super surface layer2Sb2Se4Te1The period p of the rectangular nanopillar matrix is 3000 nm. The number of the rectangular nano-pillars in any row along the x-axis direction is 2n, the n rectangular nano-pillars arranged leftwards and rightwards from the center of each row comprise w rectangular nano-pillars in rows with long axes along the x-axis direction and v rectangular nano-pillars in columns with long axes along the y-axis direction, and w + v is equal to n equal to 30, wherein w is equal to 18, and v is equal to 12; the n rectangular nano columns on the left side of the center of each row and the n rectangular nano columns on the right side of the center of each row are symmetrically distributed around the center of each row. All the rectangular nano columns have the same minor axis length, b is 600nm, the major axis length a is variable and has a variation range of 860-2900 nm, and the variation range is determined by the transmission phase provided by the variable, so that 0-2 pi phase modulation can be obtained.
Firstly, the azimuth angle of each rectangular nano-pillar is set to be 0 degree, a relation curve between the orthogonal polarization transmission phase and the length a of the long axis of the rectangular nano-pillar is simulated through simulation,then, rotating the long axis of each rectangular nano-pillar by 90 degrees anticlockwise around the z axis, and simulating a relation curve between the orthogonal polarization transmission phase and the length a of the long axis of the rectangular nano-pillar by the same simulation; according to focal length F0The distribution requirement of the planar lens with the size of 100 mu m on the orthogonal polarization transmission phase is combined with the phase distribution curve to select all optimized unit structures forming the super-structured lens.
The simulation calculations were performed using a wave optics module of finite element electromagnetic field simulation software Comsol Multiphysics (Comsol Inc.). In the mid-infrared band of interest, CaF2The substrate layer is a lossless medium with a refractive index of 1.47 and Ge2Sb2Se4Te1The refractive index varies with wavelength.
FIG. 3 shows a cell structure at wavelength λ according to an embodiment of the present invention0Transmission efficiency and phase of transmitted levorotatory circularly polarized wave and Ge under incidence of right-handed circularly polarized wave of 4200nm2Sb2Se4Te1Characteristic diagram between long axis length a of rectangular nano-pillar, showing that right-handed circularly polarized wave is incident to' Ge2Sb2Se4Te1Transmission spectrum in a super surface-dielectric layer' cell structure, here Ge of the super structure lens cell structure2Sb2Se4Te1The rectangular nano-pillars satisfy b ═ 600nm, h ═ 2800nm, and p ═ 3000 nm. At a wavelength λ0At right-handed circularly polarized wave incidence of 4200nm, when Ge is present2Sb2Se4Te1The azimuth angle beta of the rectangular nano-column is equal to 0 DEG, and in the process that the length a of the long axis is gradually increased from 860nm to 2500nm, the left-handed circularly polarized wave transmittance in the transmitted wave is higher and is maintained to be more than 0.15 and can be up to more than 0.6, so that the relatively high-efficiency polarization conversion from the incident right-handed circularly polarized wave to the transmitted left-handed circularly polarized wave is realized, and meanwhile, the transmission phase of the left-handed circularly polarized wave in the transmitted wave covers the range of-pi-0; other conditions are not changed when Ge2Sb2Se4Te1The azimuth angle beta of the rectangular nano-pillar is changed into 90 degrees, the length a of the long axis is gradually increased from 860nm to 2500nm, and the left-handed circularly polarized wave transmittance and Ge in the transmitted wave2Sb2Se4Te1The azimuth angle beta of the rectangular nano-column is completely consistent when being equal to 0 degrees, but the transmission phase of the left-handed circularly polarized wave in the transmitted wave is wholly shifted by a value pi, and the coverage range is changed into 0-pi. Therefore, the azimuth angle of the rectangular nano-columns is set to be 0 degree or 90 degrees, and the transmission phase (the phase caused by the change of the long axis of the rectangular nano-columns) is combined, so that the 0-2 pi phase modulation can be obtained on the basis of high transmittance, a specific arrangement structure of the rectangular nano-column array with the ultra-surface layer is provided, and the ultra-structure lens can realize the convergence of randomly polarized light waves.
From "Ge2Sb2Se4Te1The super-structure lens formed by the super-surface-dielectric layer structure can enable orthogonal polarization transmitted waves to have phase mutation relative to incident waves, and can generate specific phase gradient distribution in the tangential direction of the surface of the lens by arranging structural units of rectangular nano-columns with different long axis lengths and azimuth angles, so that the wave fronts and the transmission directions of the transmitted waves are changed and converged. In order to realize the converging function of the lens, in the x-axis direction of the super surface, the phase distribution should satisfy the following formula,
wherein, F0The preset focal length of the super-structure lens is represented, x represents the position coordinate of the geometric center of the rectangular nano-pillars on the super-surface layer, when the number of the rectangular nano-pillars on any row is 2n,
p represents a structural unit period, lambda0Indicating the incident wavelength. Phase psi of orthogonally polarized transmitted waves in conjunction with FIG. 3crossThe characteristic relation curve between the length a of the long axis of the rectangular nano column is calculated according to the calculated transmission phase distribution
Namely, the length a and the azimuth angle (beta is 0 degree or beta is 90 degrees) of the long axis of each rectangular nano column at any x position can be determined, and a specific arrangement structure of the rectangular nano column array of the ultra-surface layer is provided, so that the ultra-structure lens can be used for determining the length a and the azimuth angle of each rectangular nano column at any x positionThe high-efficiency focusing of incident waves with any polarization can be realized. The unit structures corresponding to the long axis length marked by the Chinese character 'mi' in fig. 3 are all optimized unit structures selected in the random polarization dynamically tunable dual-mode simultaneous focusing metamaterial lens embodiment based on the phase change material.
The results of analog simulation detection of the specific wavelength and the specific transmitted wave focal position (focal length) of the planar super-structured lens designed as described above are shown in fig. 4. FIG. 4- (a) is Ge2Sb2Se4Te1In the amorphous state, the inventive lens has a wavelength λ0When a right-handed circularly polarized wave is incident at 4200nm, the distribution of the electric field intensity of the left-handed circularly polarized transmitted wave in the x-z plane is shown in fig. 4- (b) as the distribution of the electric field intensity along the z axis when x is 0, and the distribution of the electric field intensity along the x axis when z is 102 μm (focal spot center) in the x-z plane is shown in fig. 4- (c), which shows Ge is the distribution of the electric field intensity along the x axis when z is 102 μm (focal spot center)2Sb2Se4Te1In the amorphous state, the inventive lens has a wavelength λ0Under the incidence of a right-handed circularly polarized wave of 4200nm, the convergence of a left-handed circularly polarized transmitted wave can be realized, and the focusing position z is 102 mu m (marked by a grey dotted line) and the preset focal length F0Nearly identical at 100 μm, indicating that the metamaterial lens is capable of achieving efficient focusing of right-handed circularly polarized incident waves.
By changing the polarization state of the incident wave, whether the metamaterial lens can realize the focusing performance on the incident wave with any polarization can be checked. The incident wavelength is still set to λ0The results of the simulation test of the focusing effect of the arbitrarily polarized incident wave on the metamaterial lens of this example at 4200nm are shown in fig. 5, where (a) of fig. 5 is Ge2Sb2Se4Te1When the amorphous state is adopted, the lens has the focused peak light intensity, full width at half maximum (FWHM) and diffraction limit under the incidence of different polarized light, wherein the linear polarization angle range of the incident linearly polarized light is [0,180 DEG ]]Step size is 5 °, and the incident circularly polarized light includes left-handed and right-handed circularly polarized light. It can be seen that the peak intensity of the electric field of the transmitted wave transmitted through the inventive super-structured lens is 12 (corresponding to the peak intensity of the electric field) for the incident linearly polarized light of an arbitrary polarization anglex-linearly polarized light) and 24 (corresponding to y-linearly polarized light), and for incidence of left-handed circularly polarized light and right-handed circularly polarized light, the peak intensities of the electric fields of the transmitted waves transmitted through the super-structured lens of the invention are equal to 36, i.e. the sum of the electric field intensities under incidence of the x-linearly polarized light and the y-linearly polarized light under the same condition, which is because the electric field intensity of the orthogonal polarization transmitted under incidence of any circularly polarized light can be regarded as algebraic superposition of the electric field intensities of the orthogonal polarization under incidence of the x-linearly polarized light and the y-linearly polarized light under the same condition. In addition, the focal spot full width at half maximum FWHM of the transmitted wave focus transmitted through the super-structured lens of the present invention is substantially maintained at 0.7 lambda for either linearly polarized light or circularly polarized light incident at any polarization angle0All below the theoretical diffraction limit of the super-structured lens
This is very important for high performance imaging systems.
The focusing efficiency is one of the important indicators for measuring the performance of a planar lens. Fig. 5- (b) is the focusing efficiency of the super-structured lens of the present invention under different polarized light incidence, which is defined as the ratio of the transmitted light intensity of the focal spot at three times the FWHM width to the whole incident light intensity. It can be seen that at the operating wavelength λ0When the wavelength is 4200nm, the focusing efficiency of the transmitted wave transmitted through the inventive lens is maintained at a relatively constant value, about 70%, regardless of whether the light is linearly polarized or circularly polarized. Fig. 5- (b) is an illustration showing the focusing effect of the transmitted orthogonal polarized wave component in the x-z plane when the polarized light with the polarization angles of 0 °,45 °, 90 °, 135 °,180 ° and the left-handed and right-handed circularly polarized light are incident on the metamaterial lens according to the present invention, which further proves that the metamaterial lens according to the present invention can achieve the efficient focusing of the incident light with any polarization.
FIG. 6- (a) shows Ge2Sb2Se4Te1In the amorphous state, the distribution diagram of the focusing electric field intensity of the levorotatory circularly polarized transmission wave in the x-z plane is obtained when the right-handed circularly polarized light with different mid-infrared wavelengths (marked in the figure) is incident on the super-structure lens, and the super-structure lens is deviated by a smaller focal length (relative to the preset focal length F marked by the white dotted line)0100 μm) achieves a good focusing effect in the wavelength range of 4000 to 4500 nm. To quantitatively characterize the broadband focusing performance of the inventive super-structured lens, fig. 6- (b) shows the actual focal length, the focal offset and the focal depth of the levorotatory circularly polarized transmitted wave corresponding to different incident wavelengths in the x-z plane extracted from the electric field intensity distribution diagram of fig. 6- (a), and it is found that the actual focal length slightly decreases with the increase of the incident wavelength, which is consistent with the focusing behavior of the diffractive lens, and the focal offset is defined as the actual focal length minus a preset focal length (F) minus the focal length (F)0100 μm), the focus offset fluctuates between 0 and 6 within the wavelength range of 4000 to 4500nm, but is much smaller than the depth of focus of the inventive metamaterial lens at the corresponding incident wavelength
Further proves that the super-structure lens can realize better focusing effect within the wavelength range of 4000-4500 nm. Fig. 6- (c) shows the dependence of the peak intensity, FWHM and focusing efficiency at the actual focal length of the inventive super-structured lens on the incident wavelength. It can be seen intuitively that as the wavelength increases, the peak intensity of the focal spot tends to decrease substantially, and the FWHM of the focal spot remains almost constant and slightly below its corresponding diffraction limit. The simulation also shows that the focusing efficiency of the metamaterial lens is always higher than 60% in the wavelength range of 4000-4500 nm, which is obviously superior to or comparable to the broadband and polarization-insensitive metamaterial lens which works in a transmission mode and is reported before.
Ge at Normal temperature2Sb2Se4Te1Exhibiting an amorphous state (aGSST) and changing to a crystalline state (cGSST) upon reaching a threshold temperature, the high refractive index contrast between the two states enables GSST to provide more freedom for the reconfigurability and flexibility of the optical device in the mid-infrared band. FIG. 7- (a) is Ge2Sb2Se4Te1When the metamaterial lens is converted into a crystalline state, when right-handed circularly polarized light with different specific wavelengths (marked in the figure) enters, the distribution diagram of the focusing electric field intensity of a left-handed circularly polarized transmission wave in an x-z plane is obtained. As expected, when operating at wavelength λ04200nm, aGSST is converted into cGST by external stimulationIn time, the super-structure lens of the invention can not realize focusing any more, the focusing function is changed from 'ON' to 'OFF', and the wavelength lambda is shown in figure 7- (c)0The electric field strength profile of a left-handed circularly polarized transmitted wave in the x-z plane along the x-axis when z is 102 μm (focal spot center) at 4200nm, further confirming the regulation of Ge2Sb2Se4Te1The phase state can be tuned to the surrounding dielectric environment, so that the metamaterial lens can realize dynamic regulation and control ON focusing functions of ON and OFF; when the working wavelength is changed to 4700nm, Ge2Sb2Se4Te1Still in the crystalline state (cGSST), the focusing function of the inventive super-structured lens is restored again, and a bright focal spot reappears at the focal plane. To reveal the underlying physical mechanism, fig. 7- (d) shows the phase distribution (Required) and the achieved phase distribution (real) curves Required for the selected optimized 30 cell structures on either side of the center of the inventive metamaterial lens when focusing is achieved at z 90 μm for a wavelength of 4700nm, which is the fundamental reason why the inventive metamaterial lens achieves perfect focusing at a wavelength of 4700 nm. In particular, FIG. 7- (a) also shows Ge2Sb2Se4Te1When the optical lens is in a crystalline state, when right-handed circularly polarized light with the wavelength of lambda being 4000nm is incident, the distribution diagram of the focusing electric field intensity of a left-handed circularly polarized transmission wave in an x-z plane is shown, the optical lens can realize the double simultaneous focusing performance of transmission and reflection when the wavelength of lambda is 4000nm, the focal length is 104 μm, and fig. 7- (b) shows that when the wavelength of 4000nm is reached, and the left-handed circularly polarized reflection wave is focused when the wavelength of z is 104 μm, the phase distribution (Required) and the Realized phase distribution (real) curve which are Required by the selected optimized 30 unit structures on any side of the center of the optical lens can be almost completely superposed, so that the optical lens can realize the reflection focusing when the wavelength of 4000nm (when the wavelength of lambda is not given, the transmission phase spectrum of 30 unit structures on any side of the center of the optical lens can be formed, which is similar to the case in fig. 7- (d). The inventive super-structured lens realizes simultaneous focusing of reflection and transmission for the first time, which undoubtedly expands the designFlexibility and diversity of functions.
The super-structure lens can be prepared by the following method:
(1) by controlling Ge by thermal evaporation2Sb2Te5And Ge2Sb2Se5Evaporation rate ratio of two isolated targets, CaF polished on both sides2Deposition of 2800nm thick Ge on a substrate2Sb2Se4Te1A film.
(2) Ge is prepared by electron beam lithography and reactive ion etching2Sb2Se4Te1The thin film is patterned into a rectangular nano-pillar array to form a super-surface layer.
(3) By thermal annealing process, Ge is realized2Sb2Se4Te1And (3) transformation and regulation from an amorphous state to a crystalline state.
In embodiments of the invention, Ge2Sb2Se4Te1The reason why the rectangular nano-pillars of the super surface layer along the y-axis direction meet the column equality is to finally realize the line convergence of the super-structured lens, and certainly, in order to realize the point convergence or other convergence types of the super lens, the design can be correspondingly flexibly designed according to the actual needs, and the scheme also falls into the protection scope of the invention.
The figure 2- (a) is only used for illustrating the arrangement of the rectangular nano-pillars of the invention, and therefore, the length of the rectangular nano-pillars is not set according to the content of the scheme, which cannot influence the protection of the scheme of the invention.
The right-handed circularly polarized waves are used for the incident lights in fig. 6-7, which are only for illustrating the broadband focusing, dynamically tunable dual-mode simultaneous focusing performance of the metamaterial lens according to the present invention, and the ON and OFF functions of the metamaterial lens according to the present invention, and of course, other polarized light sources may be used for illustration, and the above-mentioned schemes also fall within the scope of the present invention.
In summary, the present invention proposes a method for forming a high aspect ratio Ge film by combining different sizes of high aspect ratio Ge2Sb2Se4Te1The rectangular nano-column is fixed on the upper surface of the substrate layer at an azimuth angle of 0 degree or 90 degrees to realize the transmission type super-structure lens with arbitrary polarized wave convergence of specific wavelength. Tong (Chinese character of 'tong')Over-regulation of Ge2Sb2Se4Te1The phase state, the invention can realize the dynamic regulation and control of the focusing functions of ON and OFF; by reasonably setting the excitation wavelength, the invention can realize the double modes of reflection and transmission and simultaneously realize high-efficiency focusing. In addition, the invention also realizes the high-efficiency focusing performance of 4000-4500 nm broadband and lower than the diffraction limit.
The examples given are only preferred embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or modifications, such as scaling the structure size to work in different bands such as terahertz, visible, etc., that can be easily conceived by those skilled in the art within the technical scope of the present invention shall be covered by the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (7)
1. A random polarization dynamically tunable dual-mode simultaneous focusing super-structure lens based on a phase-change material is characterized in that the super-structure lens comprises CaF2Substrate layer and method for implementing transmission phase modulation using phase change material Ge2Sb2Se4Te1The prepared super-surface layer is formed by arranging a plurality of rectangular nano-pillars with the same height on the upper surface of a substrate layer along an x-y plane in a period p to form a rectangular nano-pillar array, wherein the plurality of rectangular nano-pillars with the same height have high aspect ratio structural features, the azimuth angle of the rectangular nano-pillars is 0 DEG or 90 DEG, the period refers to the distance between the geometric centers of two adjacent rectangular nano-pillars on an x axis and a y axis, the number of the rectangular nano-pillars in any row along the x axis direction is 2n, the rectangular nano-pillars in each row are distributed in point symmetry with the row center (x is 0), any side of the row center comprises w row rectangular nano-pillars with long axes along the x axis direction and v column rectangular nano-pillars with long axes along the y axis direction, w + v is n, the long widths of all the rectangular nano-pillars are respectively the long axes and the short axes, the lengths of the short axes are b and the same, and the lengths of the long axes are a and are variable, the transmission phase provided by the lens is used for determining to obtain 0-2 pi phase modulation, and the focusing function of the super-structure lens on the incident wave with fixed frequency and arbitrary polarization is realized.
2. The phase change material based bimodal simultaneous focusing metamaterial capable of arbitrary polarization dynamics tuning as claimed in claim 1, wherein the row of rectangular nano-pillars refers to a rectangular nano-pillar azimuth angle β ═ 0 °, the column of rectangular nano-pillars refers to a rectangular nano-pillar azimuth angle β ═ 90 °, and the azimuth angle β refers to a counterclockwise rotation angle of a long axis of the rectangular nano-pillars with respect to a positive direction of an x axis.
3. The dual-mode simultaneous focusing metamaterial capable of arbitrary polarization dynamics tuning based on phase change material as claimed in claim 2, wherein along the surface x-axis direction, the solving step of the length a (x) of each rectangular nanorod in the long axis direction is:
1) setting the operating frequency of the super-structured lens to f0(corresponding to the operating wavelength lambda)0) The length a (x) of the long axis of the rectangular nano-column is in the range of [ a ]min，amax]；
2) Setting the azimuth angle beta of each rectangular nano column of the super surface layer to be 0 DEG, and acquiring the frequency f of an incident wave through simulation0When the left (right) circular polarized light passes through the super-surface unit structure, the transmissivity T of the orthogonal polarized transmitted wavecrossCharacteristic curve 1 between parameters of length a (x) of long axis of rectangular nano column and phase psi of orthogonal polarization transmission wavecrossCharacteristic curve 2 between the parameters of length a (x) of the long axis of the rectangular nano-column;
3) the azimuth angle beta of each rectangular nano column of the super surface layer is changed into 90 degrees, and the incident wave frequency f is obtained through simulation0When the left (right) circular polarized light passes through the super-surface unit structure, the transmissivity T of the orthogonal polarized transmitted wavecrossCharacteristic curve 3 between the parameters of length a (x) of long axis of rectangular nano-column and phase psi of orthogonal polarization transmitted wavecrossCharacteristic curve 4 with the length a (x) of the long axis of the rectangular nano-column;
4) using the formula of the surface phase distribution of the spherical lens
Calculating to obtain the frequency f of the incident wave0Focal length of F0The transmission phase distribution of the planar super-structure lens along the x-axis direction on the surface
Wherein the content of the first and second substances,
F0the preset focal length of the super-structure lens is represented, and x represents the position coordinate of the geometric center of the rectangular nano-pillar on the super-surface layer, and can be used
N ± 1, ± 2, … ± N, λ0Representing the wavelength of the incident electromagnetic wave.
5) Combining the phases ψ of the orthogonally polarized transmitted waves of steps 2) and 3)crossCharacteristic curves 2 and 4 between the parameters of the length a (x) of the long axis of the rectangular nano column and the transmission phase distribution at any x position calculated according to the step 4)
And determining the azimuth angle and the long axis length a (x) of the rectangular nano-pillar corresponding to any x position.
4. The phase change material based arbitrary polarization dynamically tunable two-mode simultaneous focusing super-structured lens according to claim 3, wherein 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 for achieving better polarization independent focusing effect.
5. The phase-change-material-based bimodulus simultaneous-focusing metamaterial with dynamically tunable arbitrary polarization as claimed in claim 4, wherein the incident electromagnetic waves with arbitrary polarization include left-handed circularly polarized light, right-handed circularly polarized light and linearly polarized light with different linear polarization angles, the linearly polarized light has a linear polarization angle range of [0 °,180 ° ], and the step length is 5 °.
6. The phase-change-material-based bimodal simultaneous focusing metamaterial capable of arbitrary polarization dynamics tuning as claimed in claim 4, wherein the wavelength range of the mid-infrared electromagnetic wave is [3950nm,4500nm ], and the step length is 50 nm.
7. A dynamically tunable bimodal simultaneous focusing metamaterial of arbitrary polarization based on phase change material as claimed in claim 4, characterized in that the phase change material constituting the super surface layer is Ge2Sb2Se4Te1The amorphous state at normal temperature is changed into the crystalline state after being heated to reach the threshold temperature, the dielectric constants of the two states are obviously different and can be mutually converted, and the super-structure lens is in the Ge state2Sb2Se4Te1When the crystal state is obtained, the specific incident wavelengths are set to λ 4000nm, 4200nm and 4700nm, respectively.
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- 2022-01-10 ZA ZA2022/00430A patent/ZA202200430B/en unknown
- 2022-01-17 CN CN202210047842.7A patent/CN114114473A/en not_active Withdrawn
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| CN114527521A (en) * | 2022-03-04 | 2022-05-24 | 郑州航空工业管理学院 | Polarization-insensitive active super-surface cloak |
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| CN114994813B (en) * | 2022-07-15 | 2024-01-30 | 南京大学 | On-chip transflective superlens, design method and 4f optical system with transflective dual channels |
| CN117369028A (en) * | 2023-11-20 | 2024-01-09 | 重庆邮电大学空间通信研究院 | Sb based on geometric phase 2 Se 3 -SiO 2 Reconfigurable beam switching supersurface |
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