Electromagnetic wave polarization conversion system and conversion method based on non-hermitian metasurface

Abstract

The present application relates to a kind of electromagnetic wave polarization conversion system and conversion method based on non-hermitian super surface, the conversion system includes the double-port model with x polarization mode and y polarization mode, the double-port model satisfies two polarization modes have same resonance frequency ω0, radiation loss γ and intrinsic loss Γ, there is certain coupling strength κ between two polarization modes, and the size of the resonance frequency ω0, radiation loss γ, intrinsic loss Γ and coupling strength κ satisfy when only x polarized wave / y polarized wave is emitted to the double-side of the double-port model, corresponding exit x polarized wave / y polarized wave is 0, to make the double-port model form equivalent PT symmetric super surface structure with maximum linear polarization conversion efficiency.The present application realizes electromagnetic wave polarization conversion using PT symmetric super surface based on equivalent gain, without material having real gain or requiring the loss of structure oscillator to be different, can simplify material use and structure design.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical systems, in particular to an electromagnetic wave polarization conversion system and method based on a non-Hermite super surface. BACKGROUND

[0002] Polarization is an important degree of freedom of electromagnetic waves, and the regulation of polarization is of great significance for realizing optical communication, enhancing imaging quality, improving wireless energy transmission and sensing efficiency, and realizing advanced technologies such as quantum computing. Traditional polarization regulation devices often have the disadvantages of large size and low efficiency. With the development of technology, people's demand for small, efficient and simple devices is increasing.

[0003] In recent years, the concepts of non-Hermite physics and parity-time (PT) symmetry have attracted great interest. A system that satisfies PT symmetry can have real eigenvalues, and an exceptional point (EP) is a PT symmetry phase transition point. The eigenmodes are self-orthogonal at this point, have inherent chirality, and can be associated with circularly polarized waves, which provides a new method for polarization conversion. Therefore, with the development of super surfaces, many studies have used non-Hermite and PT symmetric super surfaces for polarization conversion.

[0004] However, traditional PT symmetric systems need to balance real gain and loss to achieve polarization conversion, and it is usually difficult to obtain real gain. On the other hand, if PT symmetry is constructed in a passive super surface, it usually requires different losses of oscillators, which makes the structure more complex. SUMMARY

[0005] To solve the above problems, the present application provides an electromagnetic wave polarization conversion system and method based on a non-Hermite super surface, which uses a PT symmetric super surface based on equivalent gain to realize electromagnetic wave polarization conversion, without the need for materials to have real gain or the requirement of different losses of oscillators in the structure, which can simplify material use and structure design.

[0006] The present application is realized by the following scheme, an electromagnetic wave polarization conversion system based on a non-Hermite super surface, comprising a two-port model having x-polarization mode and y-polarization mode, the two-port model satisfies that two polarization modes have the same resonance frequency ω0, radiation loss γ and intrinsic loss Γ, and a certain coupling strength κ between the two polarization modes, and the sizes of the resonance frequency ω0, the radiation loss γ, the intrinsic loss Γ and the coupling strength κ satisfy that when only x-polarized waves / y-polarized waves are emitted to both sides of the two-port model, the corresponding outgoing x-polarized waves / y-polarized waves are 0, so that the two-port model is formed into an equivalent PT symmetric super surface structure with maximum linear polarization conversion efficiency.

[0007] A further improvement of the present invention is that the two-port model satisfies that the magnitude of the coupling strength κ between the two polarization modes is not less than the radiation loss γ.

[0008] A further improvement of the present invention is that the dual-port model includes several unit structures arranged in an array, each unit structure being composed of a first x-direction metal line and a first y-direction metal line intersecting in a cross shape, and at least one of the first x-direction metal line and the first y-direction metal line deviates from the center point of the corresponding unit structure, and the deviation distance satisfies the requirement of the coupling strength κ.

[0009] A further improvement of the present invention is that it further includes an x-axis polarizer and a y-axis polarizer respectively disposed on both sides of the two-port model, wherein the x-axis polarizer and the y-axis polarizer have the same reflection phase and phase change caused by near-field effect, and the x-axis polarizer and the two-port model have the same propagation phase, as do the y-axis polarizer and the two-port model.

[0010] A further improvement of the present invention is that the x-axis polarizer includes a plurality of second y-axis metal lines arranged in an array along the x-axis, and the y-axis polarizer includes a plurality of second x-axis metal lines arranged in an array along the y-axis.

[0011] A further improvement of the present invention is that it further includes two dielectric plates respectively disposed between the x-axis polarizer and the dual-port model, and between the y-axis polarizer and the dual-port model, wherein the two sides of the dual-port model are respectively fixed to the relative inner sides of the two dielectric plates, and the x-axis polarizer and the y-axis polarizer are respectively fixed to the relative outer sides of the two dielectric plates.

[0012] The present invention also provides an electromagnetic wave polarization conversion method based on a non-Hermitian metasurface, which uses the electromagnetic wave polarization conversion system described above to perform polarization conversion.

[0013] A further improvement of the present invention is that the two-port model satisfies that the magnitude of the coupling strength κ between the two polarization modes is equal to the radiation loss γ;

[0014] When it is necessary to convert a linearly polarized wave into a circularly polarized wave, an x-polarized wave / y-polarized wave is emitted to one side of the two-port model, and the polarized wave emitted from the other side of the two-port model is the converted circularly polarized wave.

[0015] A further improvement of the present invention is that the two-port model satisfies that the magnitude of the coupling strength κ between the two polarization modes is not less than the radiation loss γ;

[0016] When it is necessary to convert an x-polarized wave into a y-polarized wave, x-polarized waves are emitted to both sides of the two-port model, and the polarized waves emitted from both sides of the two-port model are the converted y-polarized waves.

[0017] When y-polarized wave needs to be converted into x-polarized wave, y-polarized wave is emitted to the two sides of the dual-port model, and the polarized wave emitted from the two sides of the dual-port model is the converted x-polarized wave.

[0018] Further improvement of the application is that the electromagnetic wave polarization conversion system further comprises an x-polarizer and a y-polarizer respectively arranged on the two sides of the dual-port model, the x-polarizer and the y-polarizer have the same reflection phase and phase change caused by near-field effect, and the x-polarizer and the dual-port model and the y-polarizer and the dual-port model have the same propagation phase.

[0019] When x-polarized wave needs to be converted into y-polarized wave, x-polarized wave is emitted to the side of the x-polarizer, and the polarized wave emitted from the y-polarizer is the converted y-polarized wave.

[0020] When y-polarized wave needs to be converted into x-polarized wave, y-polarized wave is emitted to the side of the y-polarizer, and the polarized wave emitted from the x-polarizer is the converted x-polarized wave.

[0021] The application comprises but is not limited to the following beneficial effects:

[0022] 1. The application adopts a unit structure composed of cross-shaped metal wires to construct an equivalent PT symmetric metasurface structure, the cross-shaped metal wires can break the symmetry in the equivalent PT symmetric metasurface, the equivalent PT symmetric metasurface regards the incident wave as an equivalent gain, can simply design the metasurface structure for polarization conversion, does not need the material to have real gain or the structure to protect different loss oscillators, and can simplify material use and structure design.

[0023] 2. The application sets the dual-port model as an equivalent PT symmetric metasurface structure, and satisfies that the two polarization modes have a certain coupling strength, so that the conversion between linear polarization and circular polarization can be realized when the equivalent PT symmetric metasurface structure is excited on one side.

[0024] 3. The application introduces the x-polarizer and the y-polarizer, so that the conversion system can realize that actual one-side excitation is equivalent to two-side excitation, linear polarization and linear polarization can be more conveniently converted, and the limitation of two-side excitation in actual application is overcome.

[0025] 4. The application introduces the x-polarizer and the y-polarizer, so that the conversion system contains a coherent phase, provides a new degree of freedom except coupling strength and radiation loss for regulating and controlling PT phase change, and is beneficial to change multiple degrees of freedom to control polarization.

[0026] 5、The application gives the system parameter range under the maximum linear polarization conversion efficiency, can ensure better conversion efficiency by changing the size of the system parameter according to actual needs, even extends to the application of terahertz, near infrared and other frequency ranges, provides a guidance direction for the actual design of the conversion system. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is the polarization conversion schematic diagram of the first embodiment model of the application when being excited on one side.

[0028] Figure 2 is the unit structure schematic diagram of A in the application. Figure 1

[0029] Figure 3 is the principle schematic diagram of verifying that the first embodiment model of the application is an equivalent PT symmetric metasurface structure by double side excitation.

[0030] Figure 4 is Figure 3 the schematic diagram of eigenvalue change with parameters in the state.

[0031] Figure 5 is Figure 1 the transmittance and reflectance schematic diagram of the first y direction metal line in the state.

[0032] Figure 6 is Figure 1 the antisymmetric and symmetric mode schematic diagram when the coupling strength is larger in the state.

[0033] Figure 7 is Figure 1 the transmittance |t yx | change schematic diagram of theory and simulation under different d c .

[0034] Figure 8 is Figure 1 the transmittance |t yx | change schematic diagram of simulation and experiment when meeting the EP condition.

[0035] Figure 9 is Figure 1 the function curve diagram of resonance frequency omega0 and coupling strength kappa as the metal line offset distance d c in the state.

[0036] Figure 10 is Figure 1 the theoretical result schematic diagram of the evolution trajectory of the transmittance polarization eigenstate with the coupling strength kappa in the state.

[0037] Figure 11 is Figure 1 the |t xx ±it​yx | A schematic diagram of the measurement results.

[0038] Figure 12 yes Figure 1 A schematic diagram showing the evolution of polarization states with frequency under different incident directions when the EP condition is met.

[0039] Figure 13 They are Figure 12 A schematic diagram of the electric field distribution at the intrinsic frequency.

[0040] Figure 14 This is a schematic diagram of polarization conversion of the model in the second embodiment of the present invention when it is excited on one side.

[0041] Figure 15 This is a schematic diagram illustrating the principle of verifying that the model in the second embodiment of the present invention is an equivalent coherent excitation model through unilateral excitation.

[0042] Figure 16 yes Figure 14 The coupling strength κ varies with the deviation distance d under the state c A graph of a changing function.

[0043] Figure 17 yes Figure 14 Radiation loss γ and equivalent coherent phase under the condition It is a function of the propagation distance d.

[0044] Figure 18 yes Figure 14 Different propagation distances d and deviation distances d under different conditions c The transmission coefficient |t yx |

[0045] Figure 19 The second embodiment has a propagation distance d = 2.2 mm. Figure 14 Transmission coefficient |t in simulation and experiment under the condition of EP and satisfying the EP condition yx |

[0046] Figure 20 The simulation and experimental transmission coefficients |t of higher-order modes of the first embodiment and the second embodiment of the present invention with d=2.2mm are given when the EP condition is met. yx | Comparison chart. Detailed Implementation

[0047] To address the difficulties and structural complexity of polarization conversion using traditional PT-symmetric systems, this invention provides an electromagnetic wave polarization conversion system and method based on a non-Hermitian metasurface. This system utilizes a PT-symmetric metasurface based on equivalent gain to achieve electromagnetic wave polarization conversion, eliminating the need for materials to possess true gain or requiring different losses in the oscillators within the structure, thus simplifying material selection and structural design. The following detailed description, in conjunction with accompanying drawings, illustrates this electromagnetic wave polarization conversion system and method based on a non-Hermitian metasurface.

[0048] First embodiment, see Figures 1-13 As shown, this embodiment provides a single-layer electromagnetic wave polarization conversion system based on a non-Hermitian metasurface, including a two-port model with an x-polarization mode and a y-polarization mode. The two-port model satisfies that the two polarization modes have the same resonant frequency ω0, radiation loss γ, and intrinsic loss Γ, and that there is a certain coupling strength κ between the two polarization modes. The magnitudes of the resonant frequency ω0, radiation loss γ, intrinsic loss Γ, and coupling strength κ satisfy that when x-polarized waves / y-polarized waves are emitted only to both sides of the two-port model, the corresponding emitted x-polarized waves / y-polarized waves are 0, so that the two-port model is formed into an equivalent PT-symmetric metasurface structure with the maximum linear polarization conversion efficiency.

[0049] Specifically, see Figure 1 and Figure 2 As shown, the two-port model includes several unit structures 1 arranged in an array. Each unit structure 1 is composed of a first x-axis metal line 11 and a first y-axis metal line 12 arranged in a cross shape. The several unit structures 1 are arranged and fixed on a first dielectric substrate 2. The first x-axis metal line 11 and the first y-axis metal line 12 in each unit structure 1 are made of the same material and have the same shape, and at least one of them is offset from the center point O of the corresponding unit structure 1, so that the two of them (i.e., the two polarization modes of the two-port model) have a coupling strength κ not less than the radiation loss γ. With this structural configuration, the two-port model can be formed into an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency.

[0050] The electromagnetic wave polarization conversion system also includes an excitation source for emitting polarized waves. The excitation source is used to emit x-polarized waves to both sides of the two-port model to verify that the two-port model is an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency. See details. Figure 3 As shown, the polarization mode of this two-port model is known. Includes x-polarization mode a x and y-polarization mode a y Given that the two modes have the same resonant frequency ω0, radiation loss γ, and intrinsic loss γ, and the coupling strength between the modes is κ, then the coupling mode equations of this two-port model are:

[0051]

[0052]

[0053] in,

[0054]

[0055]

[0056]

[0057] and These represent the incident wave and the emitted wave, respectively, with the x and y subscripts indicating the linear polarization direction.

[0058] When only x-polarized waves are incident from both sides At this point, the most efficient linear polarization conversion requires the output x-polarized wave to be 0, i.e. and Substituting this condition into equations (1) and (2), and neglecting the influence of intrinsic loss Γ, the problem of finding the maximum value of the linear polarization conversion efficiency becomes an eigenvalue problem: The equivalent Hamiltonian is:

[0059]

[0060] At this point, the system becomes a closed system, and the incident wave is treated as a. x The equivalent gain of the mode. H eff Satisfies PT symmetry, and its eigenvalues ​​are like Figure 4 As shown, when κ>γ, the system is in the PT-symmetric phase and reaches the maximum polarization conversion efficiency at the two real eigenfrequency. Changing κ does not affect this maximum conversion efficiency. When κ=γ, the system is in the EP phase, the two eigenfrequency and eigenstate are degenerate, and the self-orthogonal eigenstates represent the chirality of the EP. When κ<γ, the system is in the PT-symmetric broken phase, the real part of the eigenfrequency is degenerate, and the imaginary part is split, which causes the polarization conversion efficiency to decrease.

[0061] In summary, for the two-port model, when the two polarization modes have the same radiation loss γ, by forming a coupling strength κ between the two polarization modes that is not less than the radiation loss γ, the two-port model can be formed into an equivalent PT-symmetric metasurface structure with the maximum linear polarization conversion efficiency.

[0062] The electromagnetic wave polarization conversion system of this first embodiment can perform conversions between linearly polarized waves and circularly polarized waves, as well as conversions between linearly polarized waves. Specifically,

[0063] Method for converting linearly polarized wave and circularly polarized wave:

[0064] In order to realize the conversion, the double-port model in the electromagnetic wave polarization conversion system adopted must satisfy κ = γ, at this time, the system is in EP.

[0065] In the conversion, as shown in Figure 1 , Figure 4 and Figure 5 , x-polarized wave is emitted to one side of the double-port model (y-polarized wave can also be selected according to actual needs), because there is coupling strength κ between the two polarization modes, the polarization wave emitted through the other side of the double-port model contains x-polarized wave and y-polarized wave, which together form circularly polarized wave.

[0066] Method for converting linearly polarized wave and linearly polarized wave:

[0067] In order to realize the conversion, the double-port model in the electromagnetic wave polarization conversion system adopted must satisfy κ ≥ γ, at this time, the system is in PT symmetry phase or EP.

[0068] The conversion of linearly polarized wave to linearly polarized wave includes two cases, one is the conversion of x-polarized wave to y-polarized wave, and the other is the conversion of y-polarized wave to x-polarized wave, the conversion principles of the two are the same, only the incident and emission directions are different, and the conversion of x-polarized wave to y-polarized wave is taken as an example for description: in the conversion, as shown in Figure 3 , x-polarized wave is emitted to both sides of the double-port model, because the coupling strength κ between the two polarization modes (a x and a y ) in the double-port model satisfies not less than the radiation loss γ, therefore, based on the verification principle described above, it is known that the polarization conversion efficiency satisfies maximum at this time, and under the maximum polarization conversion efficiency, the emitted x-polarized wave is 0, that is, the polarization wave emitted through both sides of the double-port model only has y-polarized wave, that is, the conversion of x-polarized wave to y-polarized wave is realized.

[0069] The specific parameters of the first embodiment are provided below for simulation and experimental verification, the simulation uses the microwave studio of CST, and the experiment uses microwave experiment.

[0070] As shown in Figure 1 and Figure 2 , the first medium 2 in the first embodiment selects F4BM antenna high-frequency plate with dielectric constant ε r = 2.2 and thickness d = 1 mm, the length a of the unit structure 1 in the double-port model is 12 mm, the length l of the first x-direction metal wire 11 and the first y-direction metal wire 12 is 10.6 mm, the width w is 0.6 mm, both deviate from the center point O of the unit structure 1, and the deviation distance d cSimilarly, the radiation loss γ of the oscillator can be obtained from the transmission and reflection spectrum of the oscillator, γ / 2π=1.8GHz.

[0071] When d c When ≠0, the symmetry within the equivalent PT-symmetric metasurface structure is broken by unit structure 1, and coupling occurs between the first x-axis metal line 11 and the first y-axis metal line 12, changing d. c The coupling strength κ can be changed to control the system in different PT-symmetric phases. When κ is large, antisymmetric and symmetric modes are observed at the two eigenfrequency frequencies, respectively, such as... Figure 6 As shown. When excited by x-polarized waves from both sides, the system outputs only the converted y-polarized wave at EP. The chirality of EP is hidden in the polarization space. Therefore, in order to observe the chirality of EP from the polarization state of the output wave, unilateral excitation is used in simulation and experiment.

[0072] Measurement at different d c Transmission coefficient |t| of x-polarized wave converted to y-polarized wave yx |, such as Figures 7-9 As shown. When d c When |t is large, the system is in the PT-symmetric phase. yx It exhibits two peaks close to 0.5, the frequencies of which correspond to the eigenfrequency of the Hamiltonian; when d c When κ is reduced to γ, the system is in EP, at which point the two peaks merge into a single peak with a wider bandwidth; when d c As the peak value continues to decrease, the system transitions to the PT-symmetric broken phase, and the peak value decreases. In this experiment, the deviation distance d between the first x-direction metal line 11 and the first y-direction metal line 12 from the center point O of the unit structure 1 is... c They are the same, but when the distance d between them is different... c At the same time, similar phase transition results can be achieved, that is, unit structure 1 does not need to satisfy diagonal symmetry.

[0073] Under the condition of single-sided x-polarized wave incidence, the transmitted wave contains x-polarized and y-polarized waves with different amplitudes and phases, and therefore have different polarization states. The evolution trajectory of the transmitted wave polarization state corresponding to the eigenfrequency with the coupling strength κ is as follows: Figure 10 As shown. The eigenmodes of the PT-symmetric Hamiltonian are When κ < γ, the Stokes parameters evolve essentially along S2 = 0. At EP, for the case where κ > 0, the eigenmode is [i, 1]. T This indicates that the two orthogonal modes have the same amplitude and a phase difference of π / 2. At this point, the EP chirality is right-handed, and the transmitted wave is a right-handed circularly polarized wave, corresponding to the north pole on the Poincaré sphere. Figure 10 The position is marked by the central pentagram. Based on |t xx ±it yxThe measurement results, such as Figure 11 As shown, at EP |t xx -it yx |=0 and |t xx +it yx The simultaneous satisfaction of |=1 indicates the chiral degeneracy of EP. The spontaneous symmetry breaking at EP causes a π / 2 phase change in the evolution trajectory of the eigenpolarized state on the Poincaré sphere. When κ>γ, the Stokes parameter evolves along S1=0.

[0074] All the above results are from the z-axis of unit structure 1. max When excited at the z-end (i.e., the positive z-axis end), changing the excitation direction results in the same polarization conversion efficiency, but the polarization state of the transmitted wave differs, such as... Figure 12 As shown. From z min When incident at the z-axis negative end, the outgoing wave at EP becomes a left-handed circularly polarized wave, indicating that the chirality of EP has reversed. This polarization reversal can be analyzed by the electric field distribution of the structure, such as... Figure 13 As shown, when excited from the opposite direction, the electric field intensity of the first y-direction metal line 12 is out of phase, that is, the phase difference between the two orthogonal modes changes from π / 2 to -π / 2. This phase change is related to the sign of the coupling strength κ; when the coupling strength κ changes from positive to negative, the chirality of EP is flipped.

[0075] While the first embodiment can convert between linearly polarized waves, it requires bi-sided excitation. Bi-sided excitation has significant limitations in practical applications due to stringent conditions and is difficult to implement. Therefore, this invention provides a second embodiment, see below. Figures 14-20 As shown, this embodiment provides a three-layer electromagnetic wave polarization conversion system based on a non-Hermitian metasurface. Building upon the first embodiment, it further includes an x-axis polarizer 3 and a y-axis polarizer 4 respectively disposed on both sides of a two-port model. The x-axis polarizer 3 and the y-axis polarizer 4 have the same reflection phase and phase change caused by near-field effects. Furthermore, the x-axis polarizer 3 and the two-port model, as well as the y-axis polarizer 4 and the two-port model, have the same propagation phase. Through the aforementioned phase constraints, the three-layer electromagnetic wave polarization conversion system satisfies PT symmetry. The introduction of the x-axis polarizer 3 and the y-axis polarizer 4 enables the electromagnetic wave conversion system to form an equivalent coherent excitation model, achieving single-sided excitation equivalent to double-sided excitation.

[0076] Specifically, see Figure 14As shown, the x-direction polarizer 3 includes a plurality of second y-direction metal wires arranged in an x-direction array, which allows the polarization wave in the x-direction to pass through while reflecting the polarization wave in the y-direction. The y-direction polarizer 4 includes a plurality of second x-direction metal wires arranged in a y-direction array, which allows the polarization wave in the y-direction to pass through while reflecting the polarization wave in the x-direction. The x-direction polarizer 3 and the dual-port model are further provided with a dielectric plate 5 therebetween, and the two sides of the dual-port model are fixed to the opposite inner sides of the two dielectric plates 5, and the x-direction polarizer and the y-direction polarizer are fixed to the opposite outer sides of the two dielectric plates 5. In order to make the system satisfy the equivalent coherent phase (i.e. the total phase of the x-direction polarizer, the total phase of the y-direction polarizer, which includes the reflection phase π of the polarizer to the polarization wave perpendicular to the direction, the propagation phase and the phase change caused by the near-field effect), the thickness of the two dielectric plates 5 is the same, which corresponds to the propagation distance d between the x-direction polarizer 3 and the dual-port model, and between the y-direction polarizer 4 and the dual-port model.

[0077] The electromagnetic wave polarization conversion system using the second embodiment can also convert linearly polarized waves into linearly polarized waves. Although the electromagnetic wave polarization conversion systems of the first embodiment and the second embodiment are single-layer structure and three-layer structure respectively, both of them satisfy PT symmetry. Specifically, the method for converting linearly polarized waves into linearly polarized waves using the second embodiment is as follows:

[0078] The conversion of linearly polarized waves into linearly polarized waves also includes two cases, one is the conversion of x-polarized waves into y-polarized waves, and the other is the conversion of y-polarized waves into x-polarized waves, and the conversion principles of the two cases are the same, only the incident and outgoing directions are different. Taking the conversion of x-polarized waves into y-polarized waves as an example for illustration: Figure 15 As shown, x-polarized waves are emitted to the side of the x-direction polarizer 4, and the converted y-polarized waves are directly transmitted, while the x-polarized waves are reflected by the y-direction polarizer, thereby forming the incident wave on the right side, so that the system realizes equivalent two-side excitation under actual single-side excitation. The coupled mode equation is:

[0079]

[0080]

[0081] wherein,

[0082]

[0083]

[0084]

[0085]

[0086]

[0087] The incident wave becomes The emitted wave becomes and It is the equivalent coherent incident part. It is the propagation phase of the electromagnetic wave propagating between the x-axis polarizer (y-axis polarizer) and the equivalent PT-symmetric metasurface. k is the wave vector, and d is the propagation distance.

[0088] Substitute the one-sided excitation conditions The condition for the emitted x-polarized wave to be 0, i.e. Substituting into formulas (5) and (6), we obtain the equivalent Hamiltonian of the system:

[0089]

[0090] make H eff It is PT symmetric, and the phase transition condition is: Compared with the traditional two-port model, this equivalent coherent excitation model provides phase information by adjusting the coupling strength κ and radiation loss γ. Degree of freedom, change It can change the system's resonant frequency, equivalent gain, and polarization conversion efficiency.

[0091] The following examples illustrate the differences between propagation distances d and deviation distances d. c The specific structure of the second embodiment is simulated.

[0092] See Figure 14 , Figures 16-18 As shown, under different propagation distances d, the system achieves the PT phase transition at different coupling strengths κ, and the transmission coefficient |t yx The spectral lines exhibit different resonant frequencies and linewidths. While a larger propagation distance *d* widens the polarization conversion frequency range, excessively thick media (i.e., excessively large propagation distance *d*) often results in significant losses and is detrimental to practical applications. Therefore, achieving perfect polarization conversion requires selecting an appropriate medium thickness. Taking all factors into consideration, a thickness of 2.2 mm was ultimately chosen for the second medium 5.

[0093] See Figure 19 As shown, at EP, broadband polarization conversion is achieved due to the merging of the two peaks. In addition, Figure 18 This indicates that when d c At lower frequencies, a peak at 14 GHz occurs that does not conform to the equivalent coherent excitation model; this peak corresponds to a higher-order mode. (See also...) Figure 20As shown, in the three-layer structure, the influence of the mode is amplified relative to the single-layer structure, and the corresponding polarization conversion efficiency, although smaller than the low-frequency mode, can still serve as a frequency for polarization conversion, facilitating the realization of multi-band conversion.

[0094] The application firstly takes the incident wave as equivalent gain, constructs an equivalent PT symmetric metasurface by using a unit structure of cross-shaped metal lines that breaks the in-plane symmetry, realizes the conversion from linear polarization to circular polarization at the EP under the condition of single-sided excitation, and realizes the inversion of the EP chirality by changing the sign of the coupling coefficient. In addition, the application can realize equivalent coherent excitation under single-sided excitation by adding a metal line array perpendicular to the direction as a polarizer in front of and behind the above-mentioned metasurface, introduces a new degree of freedom for the regulation of PT phase transition by changing the coherent phase through the change of the thickness of the medium in the structure, and realizes the conversion from linear polarization to linear polarization at the PT symmetric phase and the EP. In addition, by changing the size of the geometric parameters of the structure, the application can be easily extended to other wave bands such as terahertz and near-infrared. The application provides a new platform for the study of non-Hermite physics, and is conducive to the development of advanced devices for regulating electromagnetic waves.

[0095] The above embodiments of the application are described in detail with reference to the accompanying drawings, and those skilled in the art can make various changes to the application according to the above description. Therefore, some details in the embodiments should not constitute a limitation on the application, and the scope of protection of the application will be defined by the appended claims.

Claims

1. An electromagnetic wave polarization conversion system based on a non-Hermitian metasurface, characterized in that, This includes a two-port model with x-polarization and y-polarization modes, wherein the two polarization modes have the same resonant frequency. Radiation loss and intrinsic loss There is a certain coupling strength between the two polarization modes. And the resonance frequency Radiation loss Intrinsic losses and coupling strength The magnitude of the coupling strength satisfies the condition that when x-polarized or y-polarized waves are emitted only to both sides of the two-port model, the corresponding emitted x-polarized or y-polarized wave is 0, so that the two-port model forms an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency; wherein, the two-port model includes several unit structures arranged in an array, each unit structure is composed of a first x-axis metal line and a first y-axis metal line intersecting in a cross shape, and at least one of the first x-axis metal line and the first y-axis metal line deviates from the center point of the corresponding unit structure, and the deviation distance satisfies the coupling strength. Size requirements.

2. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface as described in claim 1, characterized in that, The two-port model satisfies the coupling strength between the two polarization modes. The size is not less than the radiation loss. .

3. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface as described in claim 1, characterized in that, It also includes an x-axis polarizer and a y-axis polarizer respectively disposed on both sides of the two-port model. The x-axis polarizer and the y-axis polarizer have the same reflection phase and phase change caused by near-field effect, and the x-axis polarizer and the two-port model have the same propagation phase, as do the y-axis polarizer and the two-port model.

4. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface as described in claim 3, characterized in that, The x-axis polarizer includes a plurality of second y-axis metal lines arranged in an array along the x-axis, and the y-axis polarizer includes a plurality of second x-axis metal lines arranged in an array along the y-axis.

5. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface as described in claim 4, characterized in that, It also includes two dielectric plates respectively disposed between the x-axis polarizer and the dual-port model, and between the y-axis polarizer and the dual-port model. The two sides of the dual-port model are respectively fixed to the relative inner sides of the two dielectric plates, and the x-axis polarizer and the y-axis polarizer are respectively fixed to the relative outer sides of the two dielectric plates.

6. A method for electromagnetic wave polarization conversion based on a non-Hermitian metasurface, characterized in that, Polarization conversion is performed using the electromagnetic wave polarization conversion system as described in claim 1.

7. The electromagnetic wave polarization conversion method based on a non-Hermitian metasurface as described in claim 6, characterized in that: The two-port model satisfies the coupling strength between the two polarization modes. The size is equal to the radiation loss. ; When it is necessary to convert a linearly polarized wave into a circularly polarized wave, an x-polarized wave or a y-polarized wave is emitted to one side of the two-port model, and the polarized wave emitted from the other side of the two-port model is the converted circularly polarized wave.

8. The electromagnetic wave polarization conversion method based on a non-Hermitian metasurface as described in claim 6, characterized in that: The two-port model satisfies the coupling strength between the two polarization modes. The size is not less than the radiation loss. ; When it is necessary to convert an x-polarized wave into a y-polarized wave, x-polarized waves are emitted to both sides of the two-port model, and the polarized waves emitted from both sides of the two-port model are the converted y-polarized waves. When it is necessary to convert a y-polarized wave into an x-polarized wave, a y-polarized wave is emitted to both sides of the two-port model. The polarized wave emitted from both sides of the two-port model is the converted x-polarized wave.

9. The electromagnetic wave polarization conversion method based on a non-Hermitian metasurface as described in claim 6, characterized in that: The electromagnetic wave polarization conversion system further includes an x-axis polarizer and a y-axis polarizer respectively disposed on both sides of the two-port model. The x-axis polarizer and the y-axis polarizer have the same reflection phase and phase change caused by near-field effect, and the x-axis polarizer and the two-port model have the same propagation phase, as do the y-axis polarizer and the two-port model. When it is necessary to convert an x-polarized wave into a y-polarized wave, an x-polarized wave is emitted toward the x-polarizer, and the polarized wave emitted through the y-polarizer is the converted y-polarized wave. When it is necessary to convert a y-polarized wave into an x-polarized wave, a y-polarized wave is emitted towards the y-polarizer, and the polarized wave emitted through the x-polarizer is the converted x-polarized wave.

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