A polarization-selective perfect absorber based on open-hole photonic crystal plate and graphene

Abstract

The present invention discloses a polarization-selective perfect absorber based on an apertured photonic crystal plate and graphene, comprising a reflective metal plate, an air layer, an apertured photonic crystal plate, and a graphene film arranged in sequence from bottom to top; the intrinsic loss of the graphene material is changed by changing the Fermi level of the graphene to match the radiation loss of the apertured photonic crystal plate, and the critical perfect absorption conditions of two different bands are matched at different Fermi levels. The present invention overcomes the problem that the electromagnetic wave absorption rate of a single all-dielectric metasurface or a single graphene is nearly equal to 0, and combines the all-dielectric metasurface with graphene to achieve polarization-selective narrow-band perfect absorption at specific frequencies in different bands.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic supersurfaces, and in particular to a polarization selective perfect absorber based on an open-hole photonic crystal plate and graphene. Background Art

[0002] At present, metasurfaces are developing rapidly, and absorbers have a wide range of potential applications in couplers, filters and electromagnetic stealth. Narrowband perfect absorbers have broad application prospects in detection, imaging and modulation, and therefore have attracted more and more attention from researchers. When the resonance peak is made extremely narrow, the metasurface also has broad application prospects in encrypted communications. Narrowband fano resonance based on continuous domain bound states has great application potential in the fields of high-sensitivity refractive index sensing and narrowband absorption. However, efficient absorption of electromagnetic waves is still a difficult problem to be solved in the field, and most of the current absorption can only target a single polarization and cannot achieve polarization-selective absorption, which limits the practical application of metasurface absorbers.

[0003] At present, most metasurface absorption devices have limited absorptivity for electromagnetic waves, or can only absorb electromagnetic waves with a single polarization. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a polarization-selective perfect absorber based on an open-hole photonic crystal plate and graphene, which can perfectly absorb electromagnetic waves of a specific polarization in a specific band at different Fermi levels.

[0005] Technical solution: A polarization-selective perfect absorber based on an open-hole photonic crystal plate and graphene, comprising a reflective metal plate, an air layer, an open-hole photonic crystal plate, and a graphene film arranged from bottom to top; by changing the Fermi level of graphene to change the intrinsic loss of the graphene material, the radiation loss of the open-hole photonic crystal plate is matched, and the critical perfect absorption conditions of two different bands are matched at different Fermi levels.

[0006] Further, the perforated photonic crystal plate is an asymmetric structure, the photonic crystal plate is a rectangular parallelepiped, the thickness is t, the length and width are 2a respectively, the value range of t is 150nm~300nm, the value range of a is 400nm~600nm; the perforation method is as follows: when viewed from above, it is divided into four identical square areas according to the upper left, lower left, upper right and lower right, and air holes are provided with the center of each area as the center of the circle, the horizontal axis where the center of the air hole in the upper left part is located coincides with the upper side of the rectangular hole, the center of the circle coincides with the midpoint of the side length, and the length and width of the rectangle are A and B, where A is 0~2r, B is 0~a / 2, r is the radius of the air hole, and its value range is 50nm~150nm; the horizontal axis where the center of the air hole in the upper right part is located coincides with the lower side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the horizontal axis where the center of the air hole in the lower left part is located coincides with the lower side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the horizontal axis where the center of the air hole in the lower right part is located coincides with the upper side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the depths of the circular hole and the rectangular hole are consistent with the thickness of the photonic crystal plate.

[0007] Furthermore, the material of the perforated photonic crystal plate is silicon or silicon nitride.

[0008] Further, ways to change the Fermi level include applying voltage, doping, chemical functionalization, strain regulation, interface effects, light irradiation and temperature changes.

[0009] Furthermore, the critical absorption condition is that the radiation loss of the structure is equal to the material loss of the structure.

[0010] Compared with the prior art, the present invention has the following significant effects:

[0011] 1. The present invention breaks the symmetry by combining a circle and a rectangle, and combines with graphene, and through the reflection of the lower bottom plate, can reach the critical coupling condition of perfect absorption when the material loss of graphene is equal to the radiation loss of the photonic crystal plate, thereby achieving perfect absorption; it overcomes the disadvantage of low absorption rate of a single material, and can achieve selective narrow-band perfect absorption of electromagnetic waves with different polarizations in different bands by adjusting the Fermi level of graphene;

[0012] 2. The metasurface proposed in the present invention overcomes the low absorption rate problem of the all-dielectric metasurface. In the frequency range of 250THz to 300THz, when the graphene Fermi level adjustment range is 0eV to 1eV, it can achieve selective perfect narrowband absorption of x-polarized and y-polarized electromagnetic waves at different graphene Fermi levels;

[0013] 3. The metasurface proposed in the present invention overcomes the near-zero absorption rate of electromagnetic waves by a single metasurface or graphene, and achieves perfect absorption in a narrow band. It can also achieve selective perfect absorption of different polarizations by regulating the Fermi level of graphene, so it has broad application prospects in encrypted communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a stereogram of the present invention;

[0015] Figure 2 yes Figure 1 A top view of

[0016] Figure 3 yes Figure 1 Side view of

[0017] Figure 4 is a graph showing the change of x-polarization absorptivity with graphene Fermi level obtained by simulation in the embodiment;

[0018] Figure 5 3 is a graph showing the change of the y-polarization absorptivity with the graphene Fermi level obtained by simulation in the embodiment. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0020] like Figure 1 As shown, it is a stereoscopic diagram of an open-hole photonic crystal plate and a graphene polarization-selective perfect absorber proposed by the present invention, which consists of a metal reflector 1, an air layer 2, an air hole 3, a photonic crystal plate 4, and a graphene film 5 from bottom to top. By changing the graphene Fermi level, perfect absorption of x-polarization is first achieved, and at this time, perfect absorption of y-polarized electromagnetic waves cannot be achieved; by properly adjusting the graphene Fermi level, perfect absorption of y-polarization is achieved.

[0021] By changing the Fermi level of graphene, the intrinsic loss of graphene material is changed to match the radiation loss of the open-hole photonic crystal plate. Ways to change the Fermi level include applying voltage, doping, chemical functionalization, stress (strain regulation), interface effects, light and temperature changes.

[0022] The photonic crystal plate 4 is made of silicon nitride. Figure 2When the symmetry of the structure is broken, the structure can couple with x-polarized electromagnetic waves in the resonant frequency range of 250THz to 350THz. When the x-polarized wave is incident, the reflection or transmission spectrum will show a typical Fano resonance curve, but there is almost no absorption effect. At the same time, it can couple with y-polarized electromagnetic waves. When the y-polarized wave is incident, the reflection or transmission spectrum will show a typical Fano resonance curve, but there is almost no absorption effect. The resonant frequency in the y-polarization is different from that in the x-polarization.

[0023] The intrinsic loss is adjusted by adjusting the Fermi level of graphene to match the radiation loss caused by the symmetry breaking of the bound state of the symmetry-protected continuous domain. There are both radiation loss and material loss, and the calculation formula for the absorption rate A1 of the structure is:

[0024]

[0025] Among them, ω 0 is the resonant frequency of the structure, γ is the radiation loss of the structure, and γ 0 is the material loss of the structure.

[0026] When the critical coupling condition is reached, γ 0 =γ, it can be seen that the theoretical maximum absorptivity of the structure at the resonant frequency is 0.5. When there is a reflector in the lower layer, the absorptivity A2 of the structure (i.e., the absorptivity of the absorber of the present invention) is calculated as follows:

[0027]

[0028] At this time, under critical coupling conditions, the absorption rate is a maximum of 1.

[0029] Example

[0030] A perfect absorber based on an open-hole photonic crystal plate and graphene polarization selection, firstly, metallic silver is used as the bottom reflective layer, and then there is a symmetry-breaking silicon nitride photonic crystal plate on the upper layer, whose real part of refractive index is 2.02 and imaginary part is 0. The thickness of the photonic crystal plate 4 is t=200nm, and a graphene film 5 with a thickness of h=0.34nm (single-layer graphene thickness, if multi-layer graphene is used, the thickness is a corresponding multiple) is covered on the upper layer. Among them, the punching process of the middle photonic crystal plate is as follows: a square dielectric plate with a certain thickness is divided into four identical square areas according to the upper left, lower left, upper right and lower right. First, the air holes are opened with the center of the four identical square areas as the center of the circle, and then the horizontal axis where the center of the air hole in the upper right part is located is used as the lower side of the rectangular hole to coincide with the center of the circle and the midpoint of the side length; the horizontal axis where the center of the air hole in the lower left part is located is used as the lower side of the rectangular hole to coincide with the center of the circle and the midpoint of the side length; the horizontal axis where the center of the air hole in the lower right part is located is used as the upper side of the rectangular hole to coincide with the center of the circle and the midpoint of the side length. The depth of the circular hole and the rectangular hole are consistent with the thickness of the photonic crystal plate. The length of the quarter unit is a = 500nm, the radius of the circular hole is r = 100nm, and the length and width of the rectangle are A = 2r = 200nm and B = 1.45r = 145nm respectively. After such parameterized modeling, it is extremely convenient for model optimization.

[0031] The present invention uses the simulation software COMSOL to model, sets periodic boundary conditions in the x and y directions, and sets a perfect matching layer in the z direction. The structure is incident with x polarization and y polarization, and the absorption curves are as follows: Figure 3 and Figure 4 As shown: You can see Figure 3 When the graphene Fermi level is 0.73 eV, the absorption rate for x-polarization reaches 0.99, while the absorption rate decreases when the Fermi level deviates slightly; Figure 4 It is shown in the figure that when the Fermi level of graphene is 0.7eV, the absorption of the structure for y polarization reaches 0.99, while when there is a slight deviation from the Fermi level, the absorption rate drops significantly.

[0032] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A polarization-selective perfect absorber based on an open-hole photonic crystal plate and graphene, characterized in that: It includes a reflective metal plate, an air layer, an open-hole photonic crystal plate, and a graphene film arranged in sequence from bottom to top; the intrinsic loss of the graphene material is changed by changing the Fermi level of graphene to match the radiation loss of the open-hole photonic crystal plate, and the critical perfect absorption conditions of two different bands are matched at different Fermi levels.

2. The polarization selective perfect absorber based on an open-hole photonic crystal plate and graphene according to claim 1, characterized in that: The perforated photonic crystal plate is an asymmetric structure. The photonic crystal plate is a rectangular parallelepiped with a thickness of t, a length and a width of 2a, a value range of t is 150nm-300nm, and a value range of a is 400nm-600nm. The perforation method is as follows: when viewed from above, it is divided into four identical square areas according to the upper left, lower left, upper right, and lower right, and air holes are provided with the center of each area as the center of the circle, and the horizontal axis where the center of the air hole in the upper left part is located coincides with the upper side of the rectangular hole, the center of the circle coincides with the midpoint of the side length, and the length and width of the rectangle are A and B respectively. Among them, the value of A is 0~2r, the value of B is 0~a / 2, r is the radius of the air hole, and the value range is 50nm~150nm; the horizontal axis where the center of the air hole in the upper right part is located coincides with the lower side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the horizontal axis where the center of the air hole in the lower left part is located coincides with the lower side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the horizontal axis where the center of the air hole in the lower right part is located coincides with the upper side of the rectangular hole, and the center of the circle coincides with the midpoint of the side length; the depths of the circular hole and the rectangular hole are consistent with the thickness of the photonic crystal plate.

3. The polarization selective perfect absorber based on an open-hole photonic crystal plate and graphene according to claim 1, characterized in that: The material of the perforated photonic crystal plate is silicon or silicon nitride.

4. The polarization selective absorber based on an open-hole photonic crystal plate and graphene according to claim 1, characterized in that: Ways to change the Fermi level include applying voltage, doping, chemical functionalization, strain regulation, interface effects, light and temperature changes.

5. The polarization selective absorber based on an open-hole photonic crystal plate and graphene according to any one of claims 1 to 4, characterized in that: The critical absorption condition is that the radiation loss of the structure is equal to the material loss of the structure.

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