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

Abstract

The invention relates to an electromagnetic wave polarization conversion system and conversion method based on a non-Hermite metasurface, the conversion system comprises a dual-port model having an x polarization mode and a y polarization mode, and the dual-port model satisfies that the two polarization modes have the same resonant frequency omega0, radiation loss gamma and intrinsic loss gamma, certain coupling strength kappa exists between the two polarization modes, and the resonant frequency omega0, the radiation loss gamma, the intrinsic loss gamma and the coupling strength kappa meet the condition that when x polarized waves / y polarized waves are only emitted to the two sides of the dual-port model, the corresponding emitted x polarized waves / y polarized waves are 0, and the emitted x polarized waves / y polarized waves are 0. Therefore, the dual-port model forms an equivalent PT symmetrical metasurface structure with the maximum linear polarization conversion efficiency. Electromagnetic wave polarization conversion is achieved through the PT symmetric metasurface based on the equivalent gain, materials do not need to have real gain or different losses of oscillators in a structure are not required, and material use and structural design can be simplified.

Description

Technical Field

[0001] The present invention relates to the technical field of optical systems, and in particular to an electromagnetic wave polarization conversion system and conversion method based on a non-Hermitian metasurface. Background Art

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

[0003] In recent years, the concepts of non-Hermitian physics and parity-time (PT) symmetry have attracted great interest. Systems that satisfy PT symmetry can have real eigenvalues, and singular points (EPs) are PT symmetric phase transition points, where the eigenmodes are self-orthogonal and have inherent chirality, which can be associated with circularly polarized waves, providing a new method for polarization conversion. Therefore, with the development of metasurfaces, there have been many studies on the use of non-Hermitian and PT symmetric metasurfaces for polarization conversion.

[0004] However, in order to achieve polarization conversion, traditional PT symmetric systems need to balance real gain and loss, but it is usually difficult to obtain real gain. On the other hand, if PT symmetry is constructed in a passive metasurface, it usually requires that the losses of the oscillators are different, which makes the structure more complicated. Summary of the invention

[0005] In order to solve the above problems, the present invention provides an electromagnetic wave polarization conversion system and conversion method based on a non-Hermitian metasurface, which realizes electromagnetic wave polarization conversion by using a PT-symmetric metasurface based on equivalent gain. It does not require the material to have real gain or require the loss of the oscillator in the structure to be different, and can simplify material usage and structural design.

[0006] The present invention is implemented through the following scheme: an electromagnetic wave polarization conversion system based on a non-Hermitian metasurface, including a dual-port model with an x ​​polarization mode and a y polarization mode, wherein the dual-port model satisfies that the two polarization modes have the same resonant frequency ω0, radiation loss γ and intrinsic loss Γ, and there is a certain coupling strength κ between the two polarization modes, and the magnitudes of the resonant frequency ω0, radiation loss γ, intrinsic loss Γ and coupling strength κ satisfy that when the x polarization wave / y polarization wave is only emitted to both sides of the dual-port model, the corresponding output x polarization wave / y polarization wave is 0, so that the dual-port model is formed into an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency.

[0007] A further improvement of the present invention is that the dual-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 a plurality of unit structures arranged in an array, the unit structure is composed of a first x-direction metal line and a first y-direction metal line that cross each other, 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 meets the size requirement of the coupling strength κ.

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

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

[0011] A further improvement of the present invention is that it also includes two dielectric plates respectively arranged between the x-direction polarizer and the dual-port model, and between the y-direction 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-direction polarizer and the y-direction 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 performs polarization conversion using the electromagnetic wave polarization conversion system as described above.

[0013] A further improvement of the present invention is that: the dual-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 the linear polarization wave needs to be converted into a circular polarization wave, the x-polarization wave / y-polarization wave is emitted to one side of the dual-port model, and the polarization wave emitted from the other side of the dual-port model is the converted circular polarization wave.

[0015] A further improvement of the present invention is that: the dual-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 the x-polarized wave needs to be converted into the y-polarized wave, the x-polarized wave is emitted to both sides of the dual-port model, and the polarized wave emitted from both sides of the dual-port model is the converted y-polarized wave;

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

[0018] A further improvement of the present invention is that: the electromagnetic wave polarization conversion system further comprises an x-direction polarizer and a y-direction polarizer respectively arranged on both sides of the dual-port model, the x-direction polarizer and the y-direction polarizer have the same reflection phase and phase change caused by near-field effect, and the x-direction polarizer and the dual-port model, as well as the y-direction polarizer and the dual-port model have the same propagation phase;

[0019] When the x-polarized wave needs to be converted into the y-polarized wave, the x-polarized wave is emitted toward the x-polarizer, and the polarized wave emitted through the y-polarizer is the converted y-polarized wave;

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

[0021] The present invention includes but is not limited to the following beneficial effects:

[0022] 1. The present invention 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 internal symmetry of the equivalent PT-symmetric metasurface. The equivalent PT-symmetric metasurface regards the incident wave as an equivalent gain, and can concisely design a metasurface structure for polarization conversion. There is no need for the material to have a real gain or to protect oscillators with different losses in the structure, which can simplify material usage and structural design.

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

[0024] 3. The present invention introduces an x-direction polarizer and a y-direction polarizer, so that the conversion system can achieve the equivalent of actual single-sided excitation to double-sided excitation, which can more conveniently realize the conversion between linear polarization and linear polarization, and overcome the limitations of double-sided excitation in practical applications.

[0025] 4. The present invention introduces an x-direction polarizer and a y-direction polarizer so that the conversion system includes a coherent phase, which provides a new degree of freedom for regulating the PT phase change in addition to coupling strength and radiation loss, and is conducive to changing multiple degrees of freedom to arbitrarily control polarization.

[0026] 5. The present invention provides the system parameter range under the maximum linear polarization conversion efficiency. By changing the size of the system parameters according to actual needs, better conversion efficiency can be ensured, and even expanded to applications in other frequency ranges such as terahertz and near-infrared, providing guidance for the actual design of the conversion system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of polarization conversion of the model of the first embodiment of the present invention when it is excited on one side.

[0028] Figure 2 The present invention Figure 1 Schematic diagram of the unit structure at A in the figure.

[0029] Figure 3 It is a schematic diagram of the principle of verifying that the model of the first embodiment of the present invention is an equivalent PT-symmetric supersurface structure through double-sided excitation.

[0030] Figure 4 yes Figure 3 Schematic diagram of the variation of eigenvalues ​​in a state with parameters.

[0031] Figure 5 yes Figure 1 Schematic diagram of the transmittance and reflectivity of the first y-direction metal line in the state.

[0032] Figure 6 yes Figure 1 Schematic diagram of the antisymmetric and symmetric modes when the coupling strength is large in the state.

[0033] Figure 7 yes Figure 1 In different c The theoretical and simulated transmission coefficient |t yx |Schematic diagram of changes.

[0034] Figure 8 yes Figure 1 Transmission coefficient |t of simulation and experiment when EP condition is met yx |Schematic diagram of changes.

[0035] Fig. 9 yes Figure 1 The resonant frequency ω0 and coupling strength κ in the state are as follows: c Function curve graph of .

[0036] Fig.10 yes Figure 1 Schematic diagram of the theoretical results of the evolution trajectory of the transmitted polarization eigenstate with the coupling strength κ in the state.

[0037] Fig.11 yes Figure 1 In the state at EP |t xx ±ityx |Schematic diagram of the measurement results.

[0038] Fig.12 yes Figure 1 Schematic diagram of the evolution of polarization states with frequency when the incident direction is in the state and the EP condition is met.

[0039] Fig.13 They are Fig.12 Schematic diagram of the electric field distribution corresponding to the eigenfrequency.

[0040] Fig.14 It is a schematic diagram of polarization conversion of the model of the second embodiment of the present invention when it is unilaterally excited.

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

[0042] Fig.16 yes Fig.14 The coupling strength κ changes with the deviation distance d c Graph of the changing function.

[0043] Fig.17 yes Fig.14 Radiation loss γ and equivalent coherence phase in the state A function of the propagation distance d.

[0044] Fig.18 yes Fig.14 Different propagation distances d and deviation distances d under different states c Transmission coefficient under |t yx |.

[0045] Fig.19 The second embodiment is a propagation distance d = 2.2 mm. Fig.14 The transmission coefficient of simulation and experiment when the EP condition is met |t yx |.

[0046] Fig. 20 is the simulation and experimental transmission coefficient of the high-order mode of the first embodiment of the present invention and the second embodiment of d = 2.2 mm when the EP condition is met |t yx |Comparison picture. DETAILED DESCRIPTION

[0047] In order to solve the problems of difficulty in implementing polarization conversion and complex structure using traditional PT symmetric systems, the present invention provides an electromagnetic wave polarization conversion system and conversion method based on a non-Hermitian metasurface, which uses a PT symmetric metasurface based on equivalent gain to achieve electromagnetic wave polarization conversion, without requiring the material to have real gain or requiring the oscillator losses in the structure to be different, which can simplify material use and structural design. The electromagnetic wave polarization conversion system and conversion method based on a non-Hermitian metasurface are further described below with specific embodiments and accompanying drawings.

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

[0049] Specifically, see Figure 1 and Figure 2 As shown, the dual-port model includes a plurality of unit structures 1 arranged in an array, the unit structure 1 is composed of a first x-direction metal line 11 and a first y-direction metal line 12 that cross each other, and the plurality of unit structures 1 are arranged and fixed on a first dielectric plate 2. The first x-direction metal line 11 and the first y-direction metal line 12 in each unit structure 1 are of the same material and shape, and at least one of the two deviates from the center point O of the corresponding unit structure 1, so that the two (i.e., the dual-port model satisfies the coupling strength κ between the two polarization modes) that is not less than the radiation loss γ. Through this structural setting, the dual-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 dual-port model to verify that the dual-port model is an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency. Figure 3 As shown, the polarization mode of the two-port model is known Contains x polarization mode a x and y polarization mode a y , and 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 coupled mode equation of the two-port model is:

[0051]

[0052]

[0053] in,

[0054]

[0055]

[0056]

[0057] and They represent the incident wave and the outgoing wave respectively, and the x and y in their subscripts represent the linear polarization directions.

[0058] When only x-polarized waves are incident from both sides, At this time, the maximum efficiency of linear polarization conversion requires the outgoing x polarization wave to be 0, that is, and Substituting this condition into equations (1) and (2) and ignoring the influence of the intrinsic loss Γ, the problem of solving the maximum 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. eff Satisfying PT symmetry, the eigenvalue is like Figure 4 As shown in the figure, when κ>γ, the system is in the PT symmetric phase, and the maximum polarization conversion efficiency is achieved at two real eigenfrequencies. At this time, changing κ does not affect the maximum conversion efficiency. When κ=γ, the system is in the EP, the two eigenfrequencies and eigenstates are degenerate, and the self-orthogonal eigenstates represent the chirality of EP. When κ<γ, the system is in the PT symmetry-breaking phase, the real part of the eigenfrequency is degenerate, and the imaginary part is split, which reduces the polarization conversion efficiency.

[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 maximum linear polarization conversion efficiency.

[0062] The electromagnetic wave polarization conversion system of the first embodiment can be used to convert between linearly polarized waves and circularly polarized waves, and between linearly polarized waves and linearly polarized waves. Specifically,

[0063] Conversion method between linear polarization wave and circular polarization wave:

[0064] To realize the conversion, the dual-port model in the electromagnetic wave polarization conversion system must satisfy κ=γ. At this time, the system is in EP.

[0065] When converting, see Figure 1 , Figure 4 and Figure 5 As shown, an x-polarized wave is emitted to one side of the dual-port model (y-polarized wave can also be emitted according to actual needs). Since there is a coupling strength κ between the two polarization modes, the polarized wave emitted from the other side of the dual-port model includes the x-polarized wave and the y-polarized wave, which together form a circularly polarized wave.

[0066] Conversion method between linearly polarized waves and linearly polarized waves:

[0067] To achieve this conversion, the two-port model used in the electromagnetic wave polarization conversion system must satisfy κ≥γ, and the system is in the PT symmetric phase or EP.

[0068] The conversion of linear polarization wave into linear polarization wave includes two cases, one is the conversion of x polarization wave into y polarization wave, the other is the conversion of y polarization wave into x polarization wave. The two conversion principles are the same, only the incident and emission directions are different. Now take the conversion of x polarization wave into y polarization wave as an example to illustrate: When performing the conversion, refer to Figure 3 As shown, x-polarized waves are emitted to both sides of the two-port model. Since the two polarization modes (a x and a y ) satisfies that the coupling strength κ between them is not less than the radiation loss γ. Therefore, based on the aforementioned verification principle, it can be known that the polarization conversion efficiency at this time satisfies the maximum. Under the maximum polarization conversion efficiency, the outgoing x-polarization wave is 0, that is to say, the polarization wave emitted from both sides of the dual-port model is only the y-polarization wave, that is, the conversion of the x-polarization wave into the y-polarization wave is realized.

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

[0070] See also Figure 1 and Figure 2 As shown, the first medium 2 in the first embodiment selects a dielectric constant ε r =2.2, thickness d=1mm, the length a of the unit structure 1 in the dual-port model is 12mm, the length l of the first x-direction metal line 11 and the width w=0.6mm, both of which 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 c ≠0, the symmetry of the equivalent PT symmetric metasurface structure is broken by the unit structure 1, and coupling occurs between the first x-direction metal wire 11 and the first y-direction metal wire 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 eigenfrequencies, respectively, as Figure 6 When the x-polarized waves are excited on both sides, the output of the system in EP only contains the converted y-polarized waves, and 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, single-sided excitation is used in simulation and experiment.

[0072] Measured at different d c The transmission coefficient of the x-polarized wave converted to the y-polarized wave |t yx |, such as Figure 7 to Figure 9 As shown. c When it is larger, the system is in the PT symmetric phase, |t yx | shows two peaks close to 0.5, the frequencies of these two peaks correspond to the eigenfrequencies of the Hamiltonian; when d c When κ is reduced to γ, the system is in EP, and the two peaks merge into a peak with a wider bandwidth. c Continue to decrease, the system transitions to the PT symmetry breaking phase, and the peak value decreases. In this experiment, the first x-direction metal line 11 and the first y-direction metal line 12 deviate from the center point O of the unit structure 1 by a distance d c The same, but when the two are offset by a distance d c At the same time, similar phase change results can also be achieved, that is, the unit structure 1 does not need to satisfy the diagonal symmetry.

[0073] Under the condition of unilateral x-polarized wave incidence, the transmitted wave contains x-polarized wave and y-polarized wave with different amplitudes and phases, and therefore has different polarization states. The evolution trajectory of the polarization state of the transmitted wave corresponding to the eigenfrequency with the coupling strength κ is as follows: Fig.10 As shown. The eigenmode of the PT symmetric Hamiltonian is When κ<γ, the Stokes parameters evolve basically along S2=0. At EP, for the case of κ>0, the eigenmode is [i,1] T , indicating that the two orthogonal modes have the same amplitude and a phase difference of π / 2. At this time, the EP chirality is right-handed, and the transmitted wave is a right-handed circularly polarized wave, corresponding to the North Pole on the Poincare sphere, corresponding to Fig.10 The five-pointed star marks the position. xx ±it yx|The measurement results, such as Fig.11 As shown, EP |t xx -it yx |=0 and |t xx +it yx |=1 is satisfied at the same time, indicating that the chirality of EP is degenerate. The spontaneous symmetry breaking at EP causes the evolution trajectory of the intrinsic polarization state on the Poincare sphere to undergo a phase change of π / 2. When κ>γ, the Stokes parameter evolves along S1=0.

[0074] The above results are all from the z of unit structure 1 max The polarization conversion efficiency is the same when the excitation direction is changed, but the polarization state of the transmitted wave is different. Fig.12 As shown. min When the incident wave is incident on the negative end of the z-axis, the output wave at the EP becomes a left-handed circularly polarized wave, indicating that the chirality of the EP is flipped. The flip of the polarization state can be analyzed by the electric field intensity distribution of the structure, such as Fig.13 As shown, when excited from the opposite direction, the electric field intensity of the first y-direction metal line 12 is reversed, that is, the phase difference of the corresponding two orthogonal modes changes from π / 2 to -π / 2. This phase change is related to the sign of the coupling strength κ, and the coupling strength κ changes from positive to negative, causing the EP chirality to flip.

[0075] Although the first embodiment can convert linear polarization waves into linear polarization waves, it requires double-sided excitation. Double-sided excitation has great limitations in practical applications, and the practical application conditions are harsh and not easy to implement. Therefore, the present invention provides a second embodiment, see Figures 14 to 20 As shown, this embodiment provides a three-layer structure of an electromagnetic wave polarization conversion system based on a non-Hermitian metasurface, which, on the basis of the first embodiment, further includes an x-direction polarizer 3 and a y-direction polarizer 4 respectively arranged on both sides of the dual-port model, the x-direction polarizer 3 and the y-direction polarizer 4 having the same reflection phase and phase change caused by the near-field effect, and the x-direction polarizer 3 and the dual-port model, as well as the y-direction polarizer 4 and the dual-port model having the same propagation phase. By limiting the aforementioned phase, it is ensured that the three-layer structure of the electromagnetic wave polarization conversion system satisfies the PT symmetry, and by introducing the x-direction polarizer 3 and the y-direction polarizer 4, the electromagnetic wave conversion system forms an equivalent coherent excitation model, which can realize single-side excitation equivalent to double-side excitation.

[0076] Specifically, see Fig.14As shown, the x-direction polarizer 3 includes a plurality of second y-direction metal wires arranged in an x-direction array, and the array allows the polarized waves in the x-direction to pass through, while reflecting the polarized waves in the y-direction. The y-direction polarizer 4 includes a plurality of second x-direction metal wires arranged in a y-direction array, and the array allows the polarized waves in the y-direction to pass through, while reflecting the polarized waves in the x-direction. Dielectric plates 5 are also provided between the x-direction polarizer 3 and the dual-port model, and between the y-direction polarizer 4 and the dual-port model, respectively. The two sides of the dual-port model are respectively fixed to the relative inner sides of the two dielectric plates 5, and the x-direction polarizer and the y-direction polarizer are respectively fixed to the relative outer sides of the two dielectric plates 5. In order to make the system satisfy the equivalent coherent phase (i.e. is the total phase of the x-direction polarizer, is the total phase of the polarizer in the y direction, which includes the reflection phase π of the polarized wave perpendicular to the polarizer, the propagation phase The two dielectric plates 5 have the same thickness, 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 of the second embodiment can also be used to convert linear polarization waves into linear polarization waves. Although the electromagnetic wave polarization conversion system of the first embodiment and the second embodiment is a single-layer structure and a three-layer structure, both satisfy PT symmetry. Specifically, the method for converting linear polarization waves into linear polarization waves using the second embodiment is as follows:

[0078] The conversion of linear polarization wave into linear polarization wave also includes two cases, one is the conversion of x polarization wave into y polarization wave, the other is the conversion of y polarization wave into x polarization wave. The two conversion principles are the same, only the incident and emission directions are different. Now take the conversion of x polarization wave into y polarization wave as an example to illustrate: When performing the conversion, refer to Fig.15 As shown, the x-polarized wave is emitted to the side of the x-polarizer 4, and the converted y-polarized wave is directly transmitted, while the x-polarized wave is reflected by the y-polarizer, thereby forming an incident wave on the right side, so that the system can achieve equivalent double-sided excitation under actual single-sided excitation. Its coupled mode equation is:

[0079]

[0080]

[0081] in,

[0082]

[0083]

[0084]

[0085]

[0086]

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

[0088] Substitute the unilateral incentive condition And the condition that the outgoing x polarization wave is 0, that is Substituting into formulas (5) and (6), we can obtain the system equivalent Hamiltonian:

[0089]

[0090] make H eff is PT symmetric, and the phase transition condition is Compared with the traditional two-port model, it can be seen that on the basis of adjusting the coupling strength κ and radiation loss γ, the equivalent coherent excitation model also provides the phase degrees of freedom, change The system resonant frequency, equivalent gain and polarization conversion efficiency can be changed.

[0091] The following example shows the propagation distance d and the deviation distance d. c The specific structure of the second embodiment is simulated.

[0092] See also Fig.14 , Figure 16 to Figure 18 As shown in Figure 2, under the condition of different propagation distances d, the system realizes PT phase transition at different coupling strengths κ, and the transmission coefficient |t yx |The spectrum shows different resonant frequencies and line widths. When the propagation distance d is large, although the polarization conversion frequency range becomes wider, the excessive thickness of the medium (i.e., the propagation distance d is too large) is often accompanied by large losses and is not conducive to practical applications. Therefore, it is necessary to select a suitable medium thickness to achieve perfect polarization conversion. After comprehensive consideration, the thickness of the second medium 5 is finally selected to be 2.2 mm.

[0093] See also Fig.19 As shown in Figure 2, at EP, broadband polarization conversion is achieved due to the merging of two peaks. Fig.18 It shows that when d c When is small, there is a peak at 14 GHz that does not conform to the equivalent coherent excitation model. This peak corresponds to the high-order mode. Fig. 20As shown, in the three-layer structure, the influence of this mode is amplified relative to the single-layer structure. Although the corresponding polarization conversion efficiency is lower than that of the low-frequency mode, it can still be used as a frequency for polarization conversion, which is beneficial to achieve multi-band conversion.

[0094] The present invention first regards the incident wave as an equivalent gain, and constructs an equivalent PT symmetric metasurface using a unit structure composed of cross-shaped metal wires that break the in-plane symmetry. Under the condition of unilateral excitation, the conversion of linear polarization to circular polarization at EP can be realized, and the flipping of EP chirality is realized by changing the sign of the coupling coefficient. In addition, the present invention can realize equivalent coherent excitation under unilateral excitation by adding a metal wire array with a perpendicular direction before and after the above-mentioned metasurface as a polarizer, and by changing the thickness of the medium in the structure and the coherent phase, a new degree of freedom is introduced for regulating the PT phase change, and the conversion of linear polarization to linear polarization can be realized at the PT symmetric phase and EP. In addition, by changing the size of the structural geometric parameters, the present invention can be easily expanded to other bands such as terahertz and near-infrared. The present invention provides a new platform for studying non-Hermitian physics and is conducive to the development of advanced devices for regulating electromagnetic waves.

[0095] The present invention is described in detail above in conjunction with the embodiments of the accompanying drawings. A person skilled in the art can make various variations of the present invention according to the above description. Therefore, certain details in the embodiments should not constitute a limitation of the present invention, and the scope of protection of the present invention shall be defined by the scope of the attached claims.

Claims

1. An electromagnetic wave polarization conversion system based on a non-Hermitian metasurface, characterized in that: The invention comprises a dual-port model having an x-polarization mode and a y-polarization mode, wherein the dual-port model satisfies that the two polarization modes have the same resonant frequency ω0, radiation loss γ and intrinsic loss Γ, there is a certain coupling strength κ between the two polarization modes, and the magnitudes of the resonant frequency ω0, radiation loss γ, intrinsic loss Γ and coupling strength κ satisfy that when the x-polarization wave / y-polarization wave is only emitted to both sides of the dual-port model, the corresponding output x-polarization wave / y-polarization wave is 0, so that the dual-port model is formed into an equivalent PT-symmetric metasurface structure with maximum linear polarization conversion efficiency.

2. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface according to claim 1, characterized in 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.

3. The electromagnetic wave polarization conversion system based on a non-Hermitian metasurface according to claim 1 or 2, characterized in that: The dual-port model includes a plurality of unit structures arranged in an array, wherein the unit structure is composed of a first x-direction metal line and a first y-direction metal line that cross each other, and at least one of the first x-direction metal line and the first y-direction metal line deviates from a center point corresponding to the unit structure, and the deviation distance meets the size requirement of the coupling strength κ.

4. The electromagnetic wave polarization conversion system based on non-Hermitian metasurface according to claim 1, characterized in that: It also includes an x-direction polarizer and a y-direction polarizer respectively arranged on both sides of the dual-port model, wherein the x-direction polarizer and the y-direction polarizer have the same reflection phase and phase change caused by near-field effect, and the x-direction polarizer and the dual-port model, as well as the y-direction polarizer and the dual-port model have the same propagation phase.

5. The electromagnetic wave polarization conversion system based on non-Hermitian metasurface according to claim 4, characterized in that: The x-direction polarizer includes a plurality of second y-direction metal wires arranged in an x-direction array, and the y-direction polarizer includes a plurality of second x-direction metal wires arranged in a y-direction array.

6. The electromagnetic wave polarization conversion system based on non-Hermitian metasurface according to claim 5, characterized in that: It also includes two dielectric plates respectively arranged between the x-direction polarizer and the dual-port model, and between the y-direction 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-direction polarizer and the y-direction polarizer are respectively fixed to the relative outer sides of the two dielectric plates.

7. 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 claimed in claim 1.

8. The electromagnetic wave polarization conversion method based on a non-Hermitian metasurface according to claim 7, characterized in 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; When the linear polarization wave needs to be converted into a circular polarization wave, the x-polarization wave / y-polarization wave is emitted to one side of the dual-port model, and the polarization wave emitted from the other side of the dual-port model is the converted circular polarization wave.

9. The electromagnetic wave polarization conversion method based on non-Hermitian metasurface according to claim 7, characterized in 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; When the x-polarized wave needs to be converted into the y-polarized wave, the x-polarized wave is emitted to both sides of the dual-port model, and the polarized wave emitted from both sides of the dual-port model is the converted y-polarized wave; When the y-polarized wave needs to be converted into the x-polarized wave, the y-polarized wave is emitted to both sides of the dual-port model, and the polarized waves emitted from both sides of the dual-port model are the converted x-polarized waves.

10. The electromagnetic wave polarization conversion method based on non-Hermitian metasurface according to claim 7, characterized in that: The electromagnetic wave polarization conversion system further comprises an x-direction polarizer and a y-direction polarizer respectively arranged on both sides of the dual-port model, wherein the x-direction polarizer and the y-direction polarizer have the same reflection phase and phase change caused by near-field effect, and the x-direction polarizer and the dual-port model, as well as the y-direction polarizer and the dual-port model have the same propagation phase; When the x-polarized wave needs to be converted into the y-polarized wave, the 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 the y-polarized wave needs to be converted into the x-polarized wave, the y-polarized wave is emitted to one side of the y-direction polarizer, and the polarized wave emitted through the x-direction polarizer is the converted x-polarized wave.

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