A single / double-band switchable terahertz absorber

Abstract

The present invention provides a single / double-band switchable terahertz absorber, which includes a reflective layer, a dielectric layer, and a pattern layer stacked in sequence, wherein: the pattern layer includes a first metal ring layer, a second metal ring layer, and at least one VO<subgt;2< / subgt; layer. The first metal ring layer, the second metal ring layer are flush with the bottom of the VO<subgt;2< / subgt> layer and are respectively in contact with the dielectric layer. The first metal ring layer is disposed inside the second metal ring layer. One side of any VO<subgt;2< / subgt> layer is in contact with the outer sidewall of the first metal ring layer, and the other side of any VO<subgt;2< / subgt> layer is in contact with the inner sidewall of the first metal ring layer. The present invention can be freely switched between single / double bands, and has the advantages of simple structure, polarization insensitivity, and wide-angle absorption, meeting the application requirements in the fields of THz communication, sensing, stealth, modulation, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of resonator unit design, and particularly to a single / double-band switchable terahertz absorber. Background Art

[0002] THz waves refer to electromagnetic waves with frequencies in the terahertz range (0.1 - 10 THz). It lies between infrared light and microwaves in the electromagnetic spectrum. THz waves are characterized by high frequencies and short wavelengths. Since the wavelength is between infrared and microwaves, THz waves have both the imaging and resolution advantages of infrared light and the penetration ability of microwaves. Therefore, THz waves are widely used in fields such as medicine, security detection, non-destructive testing, and communication. Along with the development of THz technology, many THz functional devices (such as modulators, filters, absorbers, etc.) have been studied. As one of the most important terahertz devices, THz absorbers have broad application prospects and values in fields such as sensing, imaging, and stealth.

[0003] Currently, THz absorbers mainly include single-frequency, multi-frequency, and broadband absorbers, whose absorption functions are single and cannot be flexibly modulated, which greatly limits their scope of application. Therefore, studying a THz absorber with diverse functions, simple structure, smaller size, and good performance is the main difficulty faced now. Summary of the Invention

[0004] The purpose of the present invention is to provide a single / double-band switchable terahertz absorber, aiming to solve the problems of single absorption function, large size, and high preparation cost existing in traditional THz resonator units.

[0005] In a first aspect, the present invention provides a resonator unit, including a reflective layer, a dielectric layer, and a pattern layer stacked in sequence, wherein:

[0006] The pattern layer includes a first metal ring layer, a second metal ring layer, and at least one VO2 layer. The first metal ring layer, the second metal ring layer, and the bottom of the VO2 layer are flush and are in contact with the dielectric layer respectively. The first metal ring layer is arranged inside the second metal ring layer. One side of any VO2 layer is in contact with the outer sidewall of the first metal ring layer, and the other side of any VO2 layer is in contact with the inner sidewall of the first metal ring layer.

[0007] Further, the centers of the first metal ring layer, the second metal ring layer, and the center of the upper surface of the dielectric layer are the same point.

[0008] Further, the inner radius of the first metal ring layer is 8 - 10 μm, the inner radius of the second metal ring layer is 14 - 20 μm, and the widths of the first metal ring layer and the second metal ring layer are both 0.5 - 1.5 μm.

[0009] Further, the dielectric layer is made of polyimide, and the dimensions of the dielectric layer are: length 30 - 54 μm, width 30 - 54 μm, and thickness 5 - 6.6 μm;

[0010] The dimensions of the reflective layer are: length 30 - 54 μm, width 30 - 54 μm, and thickness 0.1 - 0.3 μm.

[0011] Further, the first metal ring layer, the second metal ring layer, and the reflective layer are all made of gold material, and the dielectric constants of the first metal ring layer, the second metal ring layer, and the reflective layer are the same.

[0012] Further, the dielectric constant of the reflective layer is set according to the following formula:

[0013]

[0014] where ε Au represents the dielectric constant of the reflective layer, ω p represents the plasma frequency, γ represents the collision frequency, and ω represents the incident wave frequency.

[0015] Further, the optical properties of the VO2 layer in the terahertz range are described according to the following formula:

[0016]

[0017] where the dielectric constant ε at infinite frequency ∞ = 12, ω p 2 (σ) is the frequency of the plasma, γ = 5.75×10 13 rad / s is the collision frequency, σ = 3×10 5 S / m, the initial value of the plasma frequency is ω p (σ0) = 1.4×10 15 rad / s, σ = 2×10 5 S / m is set as the metallic phase of the VO2 layer, σ = 2×10 2 S / m is set as the insulating phase of the VO2 layer. When the temperature rises, the VO2 layer exists in a state where the metallic component and the dielectric component coexist;

[0018] The absorption rate expression is:

[0019] A(ω) = 1 - R(ω) - T(ω) = 1 - |S11 (ω)| 2 -|S 21 (ω)| 2

[0020] where A(ω), T(ω), and R(ω) are the absorption rate, transmittance, and reflectance, respectively. S 11 (ω) and S 21 (ω) are the reflection coefficient and transmission coefficient obtained from simulation calculations,

[0021] Assume and are the electric field and magnetic field on both sides of the medium, respectively, T is the transfer matrix, let

[0022]

[0023] According to the definition of the transfer matrix, there is where represents the electromagnetic field before entering the metamaterial, represents the electromagnetic field after exiting the metamaterial, and T is defined as

[0024]

[0025] where n represents the refractive index, represents the wave vector, Z represents the impedance, d represents the thickness of the metamaterial, and the elements of the transfer matrix have the following relationship with the transmission coefficient and reflection coefficient:

[0026]

[0027] For a homogeneous dielectric material, ΔT = 1, and T 11 = T 22 , substituting gives

[0028]

[0029] Substituting the transfer matrix T gives

[0030]

[0031] It can be solved that

[0032]

[0033] Furthermore, the number of the VO2 layers is four, the VO2 layers are arranged at equal intervals between the first metal ring layer and the second metal ring layer, the VO2 layers are fan-shaped, and the fan angle range of the VO2 layers is 50° - 70°.

[0034] Furthermore, when the conductivity of the VO2 layer is 200 S / m, the resonator unit has two absorption peaks with absorption rates of 95.66% and 99.12% in the dual bands of 1.515 THz and 2.985 THz;

[0035] When the conductivity of the VO2 layer is 200,000 S / m, the resonator unit has an absorption peak with an absorption rate of 99.84% in a single band of 1.815 THz.

[0036] In a second aspect, the present invention provides a single / dual-band switchable terahertz absorber, which comprises a periodic array formed by the above-mentioned resonator units.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] (1) The resonator unit designed in the present invention has a simple structure, is easy to process and manufacture, is polarization-insensitive, and has a good absorption peak for incident angles below 45°, meeting the application requirements in THz communication, security inspection, stealth, imaging, and modulation.

[0039] (2) The resonator unit designed by the present invention can freely switch between two absorption peaks at 1.515THz and 2.985THz with absorption rates of 95.66% and 99.12% respectively, and an absorption peak at 1.815THz with an absorption rate of 99.84%. When the VO2 layer is in an insulating state, the resonator unit exhibits two absorption peaks at 1.515THz and 2.985THz; when the VO2 layer is in a metallic state, the resonator unit has only one absorption peak at 1.815THz, thus being able to freely switch between single and dual bands. It has the advantages of rich absorption functions and flexible modulation, greatly broadening its scope of application. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a schematic diagram of the structure of a resonator unit provided in one embodiment of the present invention;

[0041] Figure 2 is a schematic structural diagram of a pattern layer in one embodiment of the present invention;

[0042] Figure 3 A second structural diagram of a resonator unit provided in one embodiment of the present invention;

[0043] Figure 4 (a) Schematic diagram of the absorption rate of the resonator unit at different conductivity of the VO2 layer; Figure 4 (b) is a schematic diagram of the curve of VO2 layer conductivity changing with temperature; Figure 4 (c) Schematic diagram of the TE mode and TM mode absorption rates when the VO2 layer is in the metallic phase; Figure 4(d) is a schematic diagram of the absorption rates of the TE mode and TM mode when the VO2 layer is in the insulating phase;

[0044] Figure 5 (a) is a schematic diagram of the real part of the dielectric constant of the VO2 layer at different conductivities; Figure 5 (b) is a schematic diagram of the imaginary part of the dielectric constant of VO2 at different conductivities; Figure 5 (c) is a schematic diagram of the relative impedance when the VO2 layer is in the metallic phase; Figure 5 (d) is a schematic diagram of the relative impedance when the VO2 layer is in the insulating phase;

[0045] Figure 6 (a) to Figure 6 (c) are schematic diagrams of the z-axis component (E z ) of the electric field intensity on the upper surface of the pattern layer at 1.815 THz, 1.515 THz, and 2.985 THz respectively, Figure 6 (d) to Figure 6 (f) are schematic diagrams of the E z distribution on the lower surface of the pattern layer at 1.815 THz, 1.515 THz, and 2.985 THz respectively;

[0046] Figure 7 (a) is a schematic diagram of the absorption rate varying with the radius r1 of the first metal ring when the VO2 layer is in the metallic phase; Figure 7 (b) is a schematic diagram of the absorption rate varying with the radius r2 of the second metal ring when the VO2 layer is in the metallic phase; Figure 7 (c) is a schematic diagram of the absorption rate varying with the width w1 of the first metal ring when the VO2 layer is in the metallic phase; Figure 7 (d) is a schematic diagram of the absorption rate varying with the width w2 of the second metal ring when the VO2 layer is in the metallic phase; Figure 7 (e) is a schematic diagram of the absorption rate varying with the width t2 of the dielectric layer when the VO2 layer is in the metallic phase; Figure 7 (f) is a schematic diagram of the absorption rate varying with the width t3 of the first metal ring layer or the second metal ring layer when the VO2 layer is in the metallic phase;

[0047] Figure 8 (a) is a schematic diagram of the absorption rate varying with the radius r1 of the first metal ring when the VO2 layer is in the insulating phase; Figure 8 (b) is a schematic diagram of the absorption rate varying with the radius r2 of the second metal ring when the VO2 layer is in the insulating phase; Figure 8 (c) is a schematic diagram of the absorption rate varying with the width w1 of the first metal ring when the VO2 layer is in the insulating phase; Figure 8 (d) is a schematic diagram of the absorption rate varying with the width w2 of the second metal ring when the VO2 layer is in the insulating phase; Figure 8(e) is a schematic diagram of the absorption rate varying with the width t2 of the dielectric layer when the VO2 layer is in the insulating phase; Figure 8 (f) is a schematic diagram of the absorption rate varying with the width t3 of the first metal ring layer or the second metal ring layer when the VO2 layer is in the insulating phase;

[0048] Figure 9 (a) and Figure 9 (b) are respectively the cross-sectional electric field intensity and surface electric field intensity distribution diagrams of the resonator unit at 1.515 THz;

[0049] Figure 10 (a) and Figure 10 (b) are respectively the cross-sectional electric field intensity and surface electric field intensity distribution diagrams of the resonator unit at 2.985 THz;

[0050] Figure 11 (a) and Figure 11 (b) are respectively the cross-sectional electric field intensity and surface electric field intensity distribution diagrams of the resonator unit at 1.815 THz;

[0051] Figure 12 (a) is a schematic diagram of the absorption curve when the pattern layer of the resonator unit has only one second metal ring layer; Figure 12 (b) is a schematic diagram of the absorption curve when the pattern layer of the resonator unit has only one first metal ring layer; Figure 12 (c) is a schematic diagram of the absorption curve when the pattern layer of the resonator unit has only the first metal ring layer and the second metal ring layer; Figure 12 (d) is a schematic diagram of the absorption curve when the pattern layer of the resonator unit has only one VO2 layer;

[0052] Figure 13 (a) is a schematic diagram of the absorption spectrum when the VO2 layer is in the metallic phase at different incident angles; Figure 13 (b) is a schematic diagram of the absorption spectrum when the VO2 layer is in the insulating phase at different incident angles; Figure 13 (c) is a schematic diagram of the absorption spectrum when the VO2 layer is in the metallic phase at different polarization angles; Figure 13 (d) is a schematic diagram of the absorption spectrum when the VO2 layer is in the insulating phase at different polarization angles;

[0053] Figure 14 This is a schematic diagram of the structure of the single / double-band switchable terahertz absorber according to an embodiment of the present invention.

[0054] Symbol description: 10 - dielectric layer; 20 - reflective layer; 30 - VO2 layer; 40 - second metal ring layer; 50 - first metal ring layer.

[0055] The following specific embodiments will further illustrate the present invention in conjunction with the above-mentioned drawings. Specific embodiments

[0056] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meanings understood by those of ordinary skill in the art to which the present invention belongs. The words such as "including" used herein mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects.

[0057] In a first aspect, referring to FIG. 1, an embodiment of the present invention provides a resonator unit, which includes a reflection layer 20, a dielectric layer 10, and a pattern layer stacked in sequence from bottom to top, wherein:

[0058] The pattern layer includes a first metal ring layer 50, a second metal ring layer 40, and at least one VO2 layer 30. The first metal ring layer 50, the second metal ring layer 40, and the bottom of the VO2 layer 30 are flush, and are respectively in contact with the dielectric layer 10. The first metal ring layer 50 is disposed inside the second metal ring layer 40. One side of any VO2 layer 30 is in contact with the outer sidewall of the first metal ring layer 50, and the other side of any VO2 layer 30 is in contact with the inner sidewall of the first metal ring layer 50.

[0059] In some embodiments, the first metal ring layer 50, the second metal ring layer 40, and the reflection layer 20 are all made of gold material. The inner radius of the first metal ring layer 50 is 8 - 10 μm. Exemplarily, the inner radius of the first metal ring layer 50 can be 8 μm, 9 μm, 10 μm, etc.; the inner radius of the second metal ring layer 40 is 14 - 20 μm. Exemplarily, the inner radius of the second metal ring layer 40 can be 14 μm, 16 μm, 18 μm, 20 μm, etc.; the widths of the first metal ring layer 50 and the second metal ring layer 40 are both 0.5 - 1.5 μm. Exemplarily, the widths of the first metal ring layer 50 and the second metal ring layer 40 can be 0.5 μm, 1 μm, 1.5 μm, etc.

[0060] In addition, in some embodiments, the dielectric constant of the reflection layer 20 is set according to the following formula:

[0061]

[0062] where ε Au represents the dielectric constant of the reflection layer 20, ω p= 1.2×10 16 rad / s represents the plasma frequency, γ = 10.5×10 13 rad / s represents the collision frequency, ω represents the incident wave frequency;

[0063] In addition, during the design process of the resonator unit, the design uses the finite element method for numerical calculation and uses COMSOL Multiphysics software to simulate the electromagnetic characteristics of the resonator unit. In the simulation, periodic boundary conditions are set in the x and y directions, and Floquet ports and perfect matched layers (PML) are set in the z direction. The Drude model is used to describe the optical properties of VO2 in the terahertz range:

[0064]

[0065] Among them, the dielectric constant ε at infinite frequency ∞ = 12, ω p 2 (σ) is the frequency of the plasma, γ = 5.75×10 13 rad / s is the collision frequency. σ = 3×10 5 S / m, the initial value of the plasma frequency is ω p (σ0) = 1.4×10 15 rad / s. Set σ = 2×10 5 S / m as the metallic phase of the VO2 layer 30, and set σ = 2×10 2 S / m as the insulating phase of the VO2 layer 30. When the temperature rises, the VO2 layer 30 exists in a state where the metallic component and the dielectric component coexist. The absorption rate expression can be represented by the following formula:

[0066] A(ω) = 1 - R(ω) - T(ω) = 1 - |S 11 (ω)| 2 - |S 21 (ω)| 2

[0067] Among them, A(ω), T(ω) and R(ω) are the absorption rate, transmittance and reflectance respectively. S 11 (ω) and S 21 (ω) are the reflection coefficient and transmission coefficient obtained from the simulation calculation. Since using a metal plate to ground at the bottom can prevent electromagnetic wave transmission, the transmittance T(ω) is 0. Therefore, the absorption rate simplifies to A(ω) = 1 - |S 11 (ω)| 2. The absorption mechanism of the resonator unit can be explained by the equivalent medium theory. When electromagnetic waves are perpendicular to the absorbing medium, they will be reflected and transmitted in the dielectric layer. From the perspective of energy, the energy of the incident electromagnetic wave will be converted into three parts by the resonator unit: the energy of the reflected electromagnetic wave, the transmitted energy through the resonator unit, and the energy lost in the resonator unit. The equivalent medium material's impedance and refractive index can be solved by extracting the transmission coefficient S 21 and the reflection coefficient S 11 . Assuming and are the electric field strength and magnetic field strength on both sides of the medium respectively, and T is the transfer matrix. Let

[0068]

[0069] According to the definition of the transfer matrix, we have where represents the electromagnetic field before entering the metamaterial, represents the electromagnetic field after exiting the metamaterial, and T is defined as

[0070]

[0071] In the formula, n represents the refractive index, represents the wave vector, Z represents the impedance, and d represents the thickness of the metamaterial. The following relationships exist between the transfer matrix elements and the transmission coefficient and reflection coefficient:

[0072]

[0073] For a homogeneous dielectric material, ΔT = 1, and T 11 = T 22 . Substituting gives

[0074]

[0075] Substituting the transfer matrix T gives

[0076]

[0077] It can be solved that

[0078]

[0079] In addition, in some embodiments, the dielectric layer 10 is made of polyimide material. The dimensions of the dielectric layer 10 are as follows: length 30 - 54 μm, width 30 - 54 μm, and thickness 5 - 6.6 μm. Exemplarily, the length of the dielectric layer 10 can be 30 μm, 36 μm, 42 μm, 48 μm, 54 μm, etc.; the width of the dielectric layer 10 can be 30 μm, 36 μm, 42 μm, 48 μm, 54 μm, etc.; and the thickness of the dielectric layer 10 can be 5 μm, 5.8 μm, 6.6 μm, etc.

[0080] In addition, in some embodiments, the center of the first metal ring layer 50, the center of the second metal ring layer 40, and the center of the upper surface of the dielectric layer 10 are the same point, that is, the two metal rings and the VO2 layer 30 are located in the exact middle of the structure.

[0081] In addition, in some embodiments, the dielectric constants of the two metal rings are the same as that of the reflective layer 20.

[0082] In addition, the number of VO2 layers 30 is four. The VO2 layers 30 are arranged at equal intervals between the first metal ring layer 50 and the second metal ring layer 40. The VO2 layers 30 are fan-shaped, and the fan angle range of the VO2 layers 30 is 50° - 70°. Exemplarily, the fan angle of the VO2 layers 30 can be 50°, 60°, 70°, etc.

[0083] In addition, in some embodiments, the center of any fan-shaped VO2 layer 30 and the center of the first metal ring layer 50 are also at the same point.

[0084] In addition, in some embodiments, the VO2 layer 30 with a conductivity σ = 200 S / m is regarded as an insulating state, and the VO2 layer 30 with a conductivity σ = 200000 S / m is regarded as a metallic state.

[0085] In addition, in some embodiments, the dimensions of the reflective layer 20 are as follows: length 30 - 54 μm, width 30 - 54 μm, and thickness 0.1 - 0.3 μm. Exemplarily, the length of the reflective layer 20 can be 30 μm, 42 μm, 54 μm, etc.; the width of the reflective layer 20 can be 30 μm, 42 μm, 54 μm, etc.; and the thickness of the reflective layer 20 can be 0.1 μm, 0.2 μm, 0.3 μm, etc.

[0086] Embodiment 1

[0087] Please refer to Figures 1 to 3, this embodiment proposes a resonator unit, which includes a reflection layer 20, a dielectric layer 10, and a pattern layer stacked in sequence. The pattern layer includes a first metal ring layer 50, a second metal ring layer 40, and at least one VO2 layer 30. The first metal ring layer 50, the second metal ring layer 40, and the bottom of the VO2 layer 30 are flush and are in contact with the dielectric layer 10 respectively. The first metal ring layer 50 is disposed inside the second metal ring layer 40. One side of any VO2 layer 30 is in contact with the outer sidewall of the first metal ring layer 50, and the other side of any VO2 layer 30 is in contact with the inner sidewall of the first metal ring layer 50. The unit structure period P = 42 μm;

[0088] The centers of the first metal ring layer 50, the second metal ring layer 40, and the center of the upper surface of the dielectric layer 10 are the same point. The inner radius of the first metal ring layer 50 is 9 μm, the inner radius of the second metal ring layer 40 is 17 μm, the widths of the first metal ring layer 50 and the second metal ring layer 40 are both 1 μm, and the thicknesses of the first metal ring layer 50, the second metal ring layer 40, and the VO2 layer 30 are all 2 μm. The dielectric layer 10 is made of polyimide. The size of the dielectric layer 10 is: length 42 μm, width 42 μm, and thickness 5.8 μm. Its thickness is greater than the skin depth, which can ensure that the THz wave will not transmit. The dielectric constant of the dielectric layer 10 is 3.5(1 + 0.0027i), where i is the imaginary unit. The size of the reflection layer 20 is: length 42 μm, width 42 μm, and thickness 0.2 μm. The first metal ring layer 50, the second metal ring layer 40, and the reflection layer 20 are all made of gold material. The dielectric constants of the first metal ring layer 50, the second metal ring layer 40, and the reflection layer 20 are the same. The number of VO2 layers is four. The VO2 layers 30 are equally spaced between the first metal ring layer 50 and the second metal ring layer 40. The VO2 layer 30 is fan-shaped, and the fan angle is 60°.

[0089] Test example

[0090] It should be noted that the product used in this test example is the resonator unit designed in Embodiment 1.

[0091] First of all, it should be noted that the conductivity of the VO2 layer changes with temperature and can turn from the insulating phase to the metallic phase. In this test example, the conductivity of the VO2 layer is set to 200000 S / m for the metallic phase and 200 S / m for the insulating phase. Please refer to Figure 4 , from Figure 4It can be seen from (a) that when the resonator unit is in the metallic phase of the VO2 layer, an absorption peak with an absorption rate reaching 99.84% appears at 1.815 THz. As the conductivity of the VO2 layer decreases, the absorption peak of the resonator unit gradually decreases and becomes two absorption peaks. When the conductivity of the VO2 layer is 20000 S / m, there is only one absorption peak with a relatively low absorption rate. When the conductivity of the VO2 layer is further reduced to 2000 S / m, two absorption peaks appear in the resonator unit, and the absorption peaks are relatively small at this time. Until the conductivity of the VO2 layer is reduced to 200 S / m, that is, when the VO2 layer becomes an insulating phase, the resonator unit exhibits two absorption peaks at 1.515 THz and 2.985 THz, with an absorption rate of 95.66% at 1.515 THz and 99.12% at 2.985 THz. Based on this, in order to conduct an in-depth study on the resonator unit designed in Example 1, only the absorption rates when the VO2 layer is in the insulating phase and the metallic phase will be discussed next. Figure 4 (b) shows the variation of the VO2 layer with temperature. From Figure 4 (c) and Figure 4 (d), it can be seen that for the TE mode and TM mode of the incident wave, the absorption rates are almost the same whether in the insulating phase or the metallic phase, which indicates that the resonator unit exhibits polarization insensitivity, which is attributed to the symmetric structure of the resonator unit.

[0092] In addition, please refer to Figure 5 , this test example also controls the phase change of the VO2 layer from the insulating phase to the metallic phase through the thermal induction method, thereby causing a change in the absorption rate. The main physical mechanism is that the dielectric constant has changed, as shown in Figs. 5(a) and Figure 5 (b). It shows the relationship between the real part and the imaginary part of the dielectric constant of the VO2 layer with the change of conductivity. Obviously, the real part is much smaller than the imaginary part at different conductivities. The real part of the dielectric constant mainly affects the resonance frequency, and the imaginary part mainly affects the loss. The physical mechanism of perfect absorption can be explained by impedance matching. When the THz wave is incident perpendicularly, the relative impedance and the absorption rate can be expressed as:

[0093]

[0094] where Z is the effective impedance of the resonator unit, Z0 is the free space impedance, and Z r represents the relative impedance of the resonator unit. Figure 5 (c) and Figure 5 (d) show that when the conductivity of the VO2 layer is 200 S / m and 200000 S / m respectively, the relative impedance of the resonator unit can be obtained. From Figure 5 (c) and Figure 5(d) It can be clearly seen that at the peak absorption frequency, the real part of the relative impedance approaches 1 and the imaginary part approaches 0, that is, the resonator unit is impedance-matched with free space, indicating that the definition of relative impedance is satisfied. In other words, the resonator unit designed by the present invention is reasonable.

[0095] In addition, in order to understand the absorption mechanism of the resonator unit, in this test example, the three peaks are denoted as peak 1, peak 2, and peak 3. Please refer to Figure 6 , which shows the E z and H z distributions of the three absorption peaks (f1, f2, f3) of the VO2 layer and the metal pattern layer. From Figure 6 (a), it can be seen that when the VO2 layer is in the metallic phase, the positive charges at 1.815 THz are mainly distributed in the upper half of the inner and outer rings, while the negative charges are mainly distributed in the lower half of the inner and outer rings. From Figure 6 (b), it can be seen that when the VO2 layer is in the insulating phase, at 1.515 THz, the positive and negative charges are mainly concentrated on the outer ring, with the positive charges distributed in the upper half of the ring and the negative charges distributed in the lower half; for 2.985 THz, from Figure 6 (c), it can be seen that the charge distribution of peak 3 is mainly concentrated on the inner ring (the first metal ring layer), with the positive charges distributed in the upper half of the inner ring and the negative charges distributed in the lower half. This accumulation of opposite charges indicates an electric dipole resonance. Figure 6 (d) shows the surface H z distribution at 1.815 THz, which is concentrated on the inner and outer rings; Figure 6 (e) shows the surface H z distribution at 1.515 THz, which is concentrated on the outer ring (the second metal ring layer); Figure 6 (f) shows the surface H z distribution at 2.985 THz, which is concentrated on the inner ring.

[0096] In this test example, in order to further understand the absorption mechanism, the influence of some structural parameters on the resonator unit was continued to be discussed. For example, Figure 7 , it can be seen that when the VO2 layer is in the metallic phase, from Figure 7 (a) and Figure 7 (b), it can be seen that whether r1 or r2 increases, the absorption intensity of peak 1 first increases to reach an extreme value and then decreases, and there is a red shift in both cases. However, the difference is that the change amplitude of peak 1 when r2 changes is larger than that when r1 changes. Figure 7 (c) and Figure 7 (d) show that similar changes occur when changing the corresponding metal ring width. This is consistent with the previous distribution of the electric field E z . For peak 1, the electric field E zThe charge of the outer ring is greater than that of the inner ring. Therefore, changing r2 naturally results in a greater change than changing r1. When the dielectric layer t2 increases from 5.4 μm to 6.2 μm, as Figure 7 shown in (e), the absorption intensity of peak 1 first increases. When t2 = 5.8 μm, the absorption peak reaches its maximum and then gradually decreases, with a slight red shift. Figure 7 (f) shows the relationship between the absorption rate and the thickness t3 of the first metal ring layer or the second metal ring layer. The absorption peak is highest when t3 = 0.2 μm. Continuing the discussion on the resonator unit when the VO2 layer is in the insulating phase and how it changes with parameters. As Figure 8 (a) shows, when r1 changes, as r1 increases, the resonance frequency of peak 2 remains unchanged, and the absorption intensity slightly decreases; while for peak 3, both the absorption intensity and the resonance frequency change significantly. As r1 gradually increases, the absorption peak slowly redshifts and the absorption intensity decreases. From the previous electric field distribution, it can be seen that peak 3 is mainly determined by the inner ring, so changing r1 naturally causes a large change in peak 3. Similarly, from Figure 8 (b), it can be seen that when the VO2 layer is in the insulating phase and r2 is changed, peak 3 remains almost unchanged. As r2 increases, the absorption intensity of peak 3 slightly increases and the resonance frequency remains unchanged, while peak 2 redshifts and the absorption intensity decreases. From Figure 8 (c) and Figure 8 (d), it can be seen that when the width of the metal ring is changed, the corresponding resonance frequencies all show a red shift. Figure 8 (e) and Figure 8 (f) show that when the intermediate dielectric layer t2 increases, the absorption intensities of peak 2 and peak 3 gradually increase with a slight red shift; when t3 increases, the absorption peak first increases and then decreases. Analyzing the absorption by changing the parameters can better optimize the structural parameters of the resonator unit to obtain a resonator unit with suitable performance. For the annular structure with a frequency selective surface, the resonance frequency can be given by the following formula:

[0097]

[0098] where r = (r1 + r2) / 2 is the average radius of the ring, λ and f are the resonance wavelength and resonance frequency respectively, c is the speed of light in free space, and ε eff is the effective dielectric constant of the dielectric layer. As r1 and r2 increase, the increase in r causes the absorption frequency to shift towards the low frequency. As t2 increases, the effective dielectric constant of the dielectric layer becomes larger, resulting in a decrease in the resonance frequency. r1 is the inner radius of the second metal ring layer, r2 is the inner radius of the first metal ring layer, w1 is the width of the first metal ring layer, w2 is the width of the second metal ring layer, t2 is the width of the dielectric layer, and t3 is the thickness of the first metal ring layer or the second metal ring layer.

[0099] Figure 9The electric field distribution of the resonator unit at 1.515THz is shown, where Figure 9 (a) shows a side view. Figure 9 (b) The surface pattern layer is shown. It can be seen that the dielectric layer is near the pattern layer and is distributed on the outer ring (the second metal ring layer), indicating that the absorption peak is mainly caused by the resonance of the outer ring; Figure 10 The electric field intensity distribution of the resonator unit at 2.985THz is shown. Figure 10 (a) shows a side view. Figure 10 (b) The surface pattern layer is shown. It can be seen that the electric field is mainly concentrated on the inner ring, indicating that the 2.985 THz absorption peak is caused by the ring resonance; Figure 11 The electric field intensity distribution of the resonator unit at 1.815THz is shown. Figure 11 (a) shows a side view. Figure 11 (b) The surface pattern layer is shown. It can be seen that the electric field intensity at 1.815 THz is distributed in the inner and outer rings and the VO2 layer, but is mainly concentrated in the outer ring. The inner ring (the first metal ring layer) and the VO2 layer have only a slight electric field distribution. Therefore, the 1.815 THz absorption peak is caused by the joint resonance of the inner and outer rings and the VO2 layer.

[0100] In addition, this test case also decomposes and explores the structure of the resonator unit, such as Figure 12 As shown in (a), a single metal inner ring structure can see an absorption peak at 3.075THz, and its absorption rate reaches 94.35%. Similarly, for a single metal outer ring structure, Figure 12 (b) shows that the absorption rate at 1.560 THz is 97.16%. Figure 12 As shown in (c), when there are two metal rings, two absorption peaks appear, with absorption rates of 97.31% and 99.74% at 1.565THz and 3.115THz respectively. For the structure with only VO2 layer, the absorption rate is as follows: Figure 12 As shown in (d), when the VO2 layer is in the insulating phase, its absorption rate is almost zero. As the conductivity increases and the transition to the metallic phase occurs, the absorption rate slowly increases. When the VO2 layer is in the insulating phase, the originally designed resonator unit resembles a structure consisting of only two metal rings, with the VO2 layer having a slight influence, resulting in a slight shift in the absorption peak and resonant frequency. When the VO2 layer is in the metallic phase, the VO2 layer has a greater influence on the resonator unit, and the combined design forms a new resonant structure that becomes an absorption peak.

[0101] Finally, considering that in practical applications, the spatial incident wave cannot be guaranteed to be vertically incident, the angle sensitivity is discussed, such as Figure 13 As shown. It can be seen that when the VO2 layer is in the metal phase, Figure 13(a) It can be seen that peak 1 has always maintained a high absorption rate, and the absorption rate has always remained above 90%; when the VO2 layer is in the insulating phase, such as Figure 13 (b), when the incident angle ranges from 0° to 45°, peak 2 decreases rapidly but still has a good absorption rate, and peak 3 shows insensitivity. Next, the polarization angle is discussed. As shown in Figure 13 (c) and Figure 13 (d), it can be seen that regardless of whether the VO2 layer is in the metallic phase or the insulating phase, polarization insensitivity is ensured.

[0102] In summary, the resonator unit designed in the present invention realizes the free switching between two absorption peaks with absorption rates of 95.66% and 99.12% at 1.515 THz and 2.985 THz respectively, and an absorption peak with an absorption rate of 99.84% at 1.815 THz. When the VO2 layer is in the insulating state, the resonator unit exhibits two absorption peaks at 1.515 THz and 2.985 THz; while when the VO2 layer is in the metallic state, the resonator unit has only one absorption peak at . That is to say, by thermally controlling the conductivity of the VO2 layer to become the insulating phase, the resonator unit can switch from single-band absorption to dual-band absorption, and vice versa. In addition, the resonator unit designed in the present invention has a simple structure and is easy to process and manufacture; in addition, the resonator unit designed in the present invention has polarization insensitivity and has good absorption peaks for incident angles below 45°. Based on these characteristics, it has a wide range of applications in technologies such as communication, security inspection, stealth, imaging, and modulation.

[0103] In a second aspect, as shown in Figure 14 , an embodiment of the present invention further provides a single / dual-band switchable terahertz absorber, which includes a periodic array formed by the above-mentioned resonator units. Specifically, the periodic structure of the absorber is arranged in an M×N two-dimensional layout, where both M and N are positive integers. Since the absorber includes several of the above-mentioned resonator units, the absorber in this embodiment has all the advantages of the resonator unit and will not be described in detail in this embodiment.

[0104] Although the embodiments of the present invention have been described in detail above, it is obvious to those skilled in the art that various modifications and changes can be made to these embodiments. However, it should be understood that such modifications and changes are all within the scope and spirit of the present invention described in the claims. Moreover, the present invention described herein may have other embodiments and can be implemented or realized in various ways.

Claims

1. A resonator unit, characterized in that, It includes a reflective layer, a dielectric layer, and a pattern layer stacked in sequence, where: The pattern layer includes a first metal ring layer, a second metal ring layer, and at least one VO2 layer. The bottom surfaces of the first metal ring layer, the second metal ring layer, and the VO2 layer are flush, and are respectively in contact with the dielectric layer. The first metal ring layer is disposed inside the second metal ring layer. One side of any VO2 layer is in contact with the outer sidewall of the first metal ring layer, and the other side of any VO2 layer is in contact with the inner sidewall of the first metal ring layer. The centers of the first metal ring layer, the second metal ring layer, and the upper surface of the dielectric layer are the same point. The number of VO2 layers is four, and the VO2 layers are equally spaced between the first metal ring layer and the second metal ring layer. The VO2 layer is fan-shaped, and the fan angle range of the VO2 layer is 50°-70°.

2. The resonator unit according to claim 1, wherein The inner radius of the first metal ring layer is 8-10 μm, the inner radius of the second metal ring layer is 14-20 μm, and the widths of the first metal ring layer and the second metal ring layer are both 0.5-1.5 μm.

3. The resonator unit according to claim 1, characterized in that, The dielectric layer is made of polyimide, and the dimensions of the dielectric layer are: length 30-54 μm, width 30-54 μm, and thickness 5-6.6 μm; The dimensions of the reflective layer are: length 30-54 μm, width 30-54 μm, and thickness 0.1-0.3 μm.

4. The resonator unit according to any one of claims 1-3, characterized in that The first metal ring layer, the second metal ring layer, and the reflective layer are all made of gold material (Au), and the dielectric constants of the first metal ring layer, the second metal ring layer, and the reflective layer are the same.

5. The resonator unit according to claim 4, characterized in that, The dielectric constant of the reflective layer is set according to the following formula: ； where ε Au represents the permittivity of the reflective layer, ω p represents the plasma frequency, γ represents the collision frequency, and ω represents the incident wave frequency.

6. The resonator unit according to claim 4, characterized in that, The optical properties of the VO2 layer in the terahertz range are described according to the following formula: ； ； Among them, the dielectric constant ε at infinite frequency ∞ = 12, ω p 2 (σ) is the frequency of the plasma, γ = 5.75×10 13 rad / s is the collision frequency, σ = 3×10 5 S / m, the initial value of the plasma frequency is ω p (σ0) = 1.4 × 10 15 rad / s, set σ = 2×10 5 S / m as the metallic phase of the VO2 layer, set σ = 2×10 2 S / m as the insulating phase of the VO2 layer. When the temperature rises, the VO2 layer exists in a state where the metallic component and the dielectric component coexist; The absorption rate expression is: ； where A(ω), T(ω), and R(ω) are the absorptance, transmittance, and reflectance, respectively, and S 11 (ω) and S 21 (ω) are the reflection and transmission coefficients obtained from the simulation calculation, Assume that E1 and H2 are the electric field strength and magnetic field strength on both sides of the medium, and T is the transfer matrix. Let ； According to the definition of the transfer matrix, we have , where represents the electromagnetic field before entering the metamaterial, represents the electromagnetic field after exiting the metamaterial, and T is defined as ； where n represents the refractive index, represents the wave vector, Z represents the impedance, d represents the thickness of the metamaterial, and the following relationships exist between the elements of the transfer matrix and the transmission coefficient and reflection coefficient: ； For a homogeneous dielectric material, there is , and , substituting gives ； Substituting the transfer matrix T, we get ； It can be solved that ； 。 7. The resonator unit according to claim 1, wherein, When the conductivity of the VO2 layer is 200 S / m, the resonator unit has two absorption peaks with absorption rates of 95.66% and 99.12% at 1.515 THz and 2.985 THz in the dual-band; When the conductivity of the VO2 layer is 200000 S / m, the resonator unit has one absorption peak with an absorption rate of 99.84% at 1.815 THz in the single-band.

8. A single / double-band switchable terahertz absorber, characterized in that, It includes a periodic array formed by the resonator unit according to any one of claims 1-7.

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