Single / double-waveband switchable terahertz absorber

Abstract

The invention provides a single / double-waveband switchable terahertz absorber, which comprises a reflecting layer, a dielectric layer and a pattern layer which are sequentially stacked, and is characterized in that the pattern layer comprises a first metal ring layer, a second metal ring layer and at least one VO2 layer, and the first metal ring layer and the second metal ring layer are flush with the bottom of the VO2 layer and are respectively in contact with the dielectric layer; the first metal circular ring layer is arranged in the second metal circular ring layer, one side of any VO2 layer is in contact with the outer side wall of the first metal circular ring layer, and the other side of any VO2 layer is in contact with the inner side wall of the first metal circular ring layer. According to the invention, free switching between single / double wavebands can be realized, the structure is simple, polarization is not sensitive, wide-angle absorption is realized, and the application requirements in the fields of THz communication, sensing, stealth, modulation and the like are met.

Description

Technical Field

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

[0002] THz waves refer to electromagnetic waves with frequencies in the terahertz range (0.1-10THz). It is between infrared light and microwaves in the electromagnetic spectrum. THz waves are characterized by high frequency and short wavelength. 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 medicine, safety testing, non-destructive testing, communications and other fields. 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 THz devices, THz absorbers have broad application prospects and value in sensing, imaging, stealth and other fields.

[0003] At present, THz absorbers mainly include single-frequency, multi-frequency and broadband absorbers, which have a single absorption function and cannot be flexibly modulated, which greatly limits their scope of application. Therefore, the main difficulty now is to study a THz absorber with diverse functions, simple structure, smaller scale and good performance. Summary of the invention

[0004] The purpose of the present invention is to provide a single / dual-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, comprising 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 VO 2 layer, the first metal ring layer, the second metal ring layer and the VO 2 The bottom of the layer is flush with the dielectric layer, and is in contact with the dielectric layer respectively. The first metal ring layer is arranged in the second metal ring layer. Any of the VO 2 One side of the layer contacts the outer side wall of the first metal ring layer, and any of the VO 2 The other side of the layer contacts the inner side wall of the first metal ring layer.

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

[0008] Furthermore, 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] Furthermore, the dielectric layer is made of polyimide, and the dimensions of the dielectric layer are: 30-54 μm in length, 30-54 μm in width, and 5-6.6 μm in thickness;

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

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

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

[0013]

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

[0015] Further, VO is described according to the following formula 2 Optical properties of layers in the terahertz range:

[0016]

[0017] Among them, the dielectric constant ε at infinite frequency ∞ =12,ω p 2 (σ) is the frequency of 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 (σ 0 )=1.4×10 15 rad / s, σ=2×10 5 S / m is set to VO 2 The metallic phase of the layer, σ = 2 × 10 2 S / m is set to VO 2 The insulating phase of the layer, when the temperature increases, VO 2 The layer exists in a state where metal components and dielectric components coexist;

[0018] The expression of absorption rate is:

[0019] A(ω)=1-R(ω)-T(ω)=1-S 11 (ω) 2 -S 21 (ω) 2

[0020] Where A(ω), T(ω) and R(ω) are the absorptivity, transmittance and reflectivity respectively. 11 (ω) and S 21 (ω) are the reflection and transmission coefficients calculated from the simulation,

[0021] Assumption E 1 and H 2 are the electric field and magnetic field on both sides of the medium, T is the transmission matrix, and

[0022]

[0023] According to the definition of the transfer matrix, we have Where F represents the electromagnetic field before entering the metamaterial, F′ 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 transmission matrix elements have the following relationship with the transmission coefficient and the reflection coefficient:

[0026]

[0027] For uniform dielectric materials, ΔT = 1, and T 11 =T 22 , substituting into

[0028]

[0029] Substituting the transmission matrix T into

[0030]

[0031] It can be solved

[0032]

[0033] Furthermore, the VO 2 The number of layers is four, the VO 2 The layers are arranged between the first metal ring layer and the second metal ring layer at equal intervals, and the VO 2 The layer is fan-shaped, the VO 2The fan angle of the layer ranges from 50° to 70°.

[0034] Furthermore, when the VO 2 When the conductivity of the 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 VO 2 When the conductivity of the layer is 200000 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, modulation and other fields.

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

[0040] Figure 1 One of the structural schematic diagrams 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 schematic diagram of a resonator unit provided in one embodiment of the present invention;

[0043] Figure 4 (a) is the resonator unit in VO 2Schematic diagram of the absorption rate of layers with different conductivity; Figure 4 (b) VO 2 Schematic diagram of the curve of layer conductivity changing with temperature; Figure 4 (c) VO 2 The layer is a schematic diagram of the TE mode and TM mode absorptivity in the metallic phase; Figure 4 (d) is VO 2 The layers are schematic diagrams of the TE mode and TM mode absorptivity in the insulating phase;

[0044] Figure 5 (a) VO at different conductivity 2 Schematic diagram of the real part of the dielectric constant of the layer; Figure 5 (b) VO at different conductivity 2 Schematic diagram of the imaginary part of the dielectric constant; Figure 5 (c) VO 2 Schematic diagram of relative impedance when the layer is in the metallic phase; Figure 5 (d) is VO 2 Schematic diagram of relative impedance when the layer is an insulating phase;

[0045] Figure 6 (a) to Figure 6 (c) The components of the electric field strength on the z-axis at 1.815 THz, 1.515 THz, and 2.985 THz on the upper surface of the pattern layer (E z ) distribution diagram, Figure 6 (d) to Figure 6 (f) E of the lower surface of the pattern layer at 1.815THz, 1.515THz, and 2.985THz respectively z Distribution diagram;

[0046] Figure 7 (a) is VO 2 When the layer is a metal phase, the radius of the first metal ring r 1 Schematic diagram of changing absorbance; Figure 7 (b) VO 2 When the layer is a metal phase, the radius of the second metal ring r 2 Schematic diagram of changing absorbance; Figure 7 (c) VO 2 When the layer is a metal phase, the width w of the first metal ring 1 Schematic diagram of changing absorbance; Figure 7 (d) is VO 2 When the layer is a metal phase, the width of the second metal ring w 2 Schematic diagram of changing absorbance; Figure 7 (e) is VO 2 When the layer is a metal phase, the width of the dielectric layer t 2 Schematic diagram of changing absorbance; Figure 7 (f) is VO 2 When the layer is a metal phase, the width t of the first metal ring layer or the second metal ring layer 3 Schematic diagram of changing absorbance;

[0047] Figure 8 (a) is VO 2 When the layer is an insulating phase, the radius of the first metal ring r 1 Schematic diagram of varying absorbance; Figure 8 (b) VO 2 When the layer is an insulating phase, the radius of the second metal ring r 2 Schematic diagram of varying absorbance; Figure 8 (c) VO 2 When the layer is an insulating phase, the width w of the first metal ring 1 Schematic diagram of varying absorbance; Figure 8 (d) is VO 2 When the layer is an insulating phase, the width w of the second metal ring 2 Schematic diagram of varying absorbance; Figure 8 (e) is VO 2 When the layer is an insulating phase, the width of the dielectric layer t 2 Schematic diagram of varying absorbance; Figure 8 (f) is VO 2 When the layer is an insulating phase, the width t of the first metal ring layer or the second metal ring layer 3 Schematic diagram of changing absorbance;

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

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

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

[0051] Fig.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; Fig.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; Fig.12 (c) is a schematic diagram of the absorption curve when the pattern layer of the resonator unit only has the first metal ring layer and the second metal ring layer; Fig.12 (d) The pattern layer of the resonator unit has only one VO 2 Schematic diagram of absorption curve under the layer;

[0052] Fig.13 (a) VO at different incident angles 2 Schematic diagram of the absorption spectrum when the layer is in the metallic phase; Fig.13 (b) VO at different incident angles 2 Schematic diagram of the absorption spectrum when the layer is in the insulating phase; Fig.13 (c) VO at different polarization angles 2 Schematic diagram of the absorption spectrum when the layer is in the metallic phase; Fig.13 (d) VO at different polarization angles 2 Schematic diagram of the absorption spectrum when the layer is in the insulating phase;

[0053] Fig.14 Schematic diagram of the structure of a single / dual-band switchable terahertz absorber in one embodiment of the present invention.

[0054] Explanation of symbols: 10 - dielectric layer; 20 - reflective layer; 30 - VO 2 layer; 40-the second metal ring layer; 50-the first metal ring layer.

[0055] The following specific implementation manner will further illustrate the present invention in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0056] In order to make the purpose, 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. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0057] In the first aspect, please refer to 1. An embodiment of the present invention provides a resonator unit, which includes a reflective 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 VO 2 layer 30, the first metal ring layer 50, the second metal ring layer 40 and the VO 2The bottom of the layer 30 is flush and in contact with the dielectric layer 10 respectively. The first metal ring layer 50 is arranged in the second metal ring layer 40. 2 One side of the layer 30 is in contact with the outer wall of the first metal ring layer 50. 2 The other side of the layer 30 contacts the inner side wall 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 reflective layer 20 are all made of gold material, the inner radius of the first metal ring layer 50 is 8-10μm, and exemplarily, the inner radius of the first metal ring layer 50 may be 8μm, 9μm, 10μm, etc.; the inner radius of the second metal ring layer 40 is 14-20μm, and exemplarily, the inner radius of the second metal ring layer 40 may be 14μm, 16μm, 18μm, 20μm, etc.; the width of the first metal ring layer 50 and the second metal ring layer 40 are both 0.5-1.5μm, and exemplarily, the width of the first metal ring layer 50 and the second metal ring layer 40 may be 0.5μm, 1μm, 1.5μm, etc.

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

[0061]

[0062] Among them, ε Au represents the dielectric constant of the reflective 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, in 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 matching layers (PML) are set in the z direction. The Drude model is used to describe the VO 2 Optical properties in the terahertz range:

[0064]

[0065] Among them, the dielectric constant ε at infinite frequency ∞ =12,ω p 2 (σ) is the frequency of plasma, γ = 5.75 × 10 13rad / s is the collision frequency. σ=3×10 5 S / m, the initial value of the plasma frequency is ω p (σ 0 )=1.4×10 15 rad / s. Set σ = 2 × 10 5 S / m is set to VO 2 The metal phase of layer 30, σ = 2 × 10 2 S / m is set to VO 2 The insulating phase of layer 30. When the temperature increases, VO 2 Layer 30 exists in a state where metal components and dielectric components coexist. The absorption rate expression can be expressed by the following formula:

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

[0067] Where A(ω), T(ω) and R(ω) are the absorptivity, transmittance and reflectivity respectively. 11 (ω) and S 21 (ω) is the reflection coefficient and transmission coefficient calculated from the simulation. Since the metal plate grounding at the bottom can prevent electromagnetic waves from transmitting, the transmittance T(ω) is 0. Therefore, the absorption rate is simplified to A(ω) = 1-|S 11 (ω)| 2 The absorption mechanism of the resonator unit can be explained by the equivalent medium theory. When the electromagnetic wave is perpendicular to the absorbing medium, it will be reflected and transmitted in the dielectric layer. From the energy point of view, the energy of the incident electromagnetic wave will be converted into three parts through the resonator unit: the energy reflected by the electromagnetic wave, the energy transmitted through the resonator unit, and the energy lost in the resonator unit. The transmission coefficient S can be extracted by 21 and the reflection coefficient S 11 Thus, the impedance and refractive index of the equivalent dielectric material can be solved. Assume that E 1 and H 2 are the electric field and magnetic field on both sides of the medium, T is the transmission matrix, and

[0068]

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

[0070]

[0071] Where n represents the refractive index, represents the wave vector, Z represents the impedance, and d represents the thickness of the metamaterial. The transmission matrix elements have the following relationship with the transmission coefficient and the reflection coefficient:

[0072]

[0073] For uniform dielectric materials, ΔT = 1, and T 11 =T 22 , substituting into

[0074]

[0075] Substituting the transmission matrix T into

[0076]

[0077] It can be solved

[0078]

[0079] In addition, in some embodiments, the dielectric layer 10 is made of polyimide material, and the dimensions of the dielectric layer 10 are: 30-54 μm in length, 30-54 μm in width, and 5-6.6 μm in thickness. By way of example, 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 all the same point, that is, the two metal rings are aligned with the VO 2 Layer 30 is located in the middle of the structure.

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

[0082] In addition, VO 2 The number of layers 30 is four, VO 2 The layers 30 are arranged between the first metal ring layer 50 and the second metal ring layer 40 at equal intervals. 2 Layer 30 is fan-shaped, VO 2 The fan angle of layer 30 ranges from 50° to 70°. For example, VO 2 The fan angle of the layer 30 may be 50°, 60°, 70°, etc.

[0083] In addition, in some embodiments, any sector VO 2The center of the layer 30 and the center of the first metal ring layer 50 are also at the same point.

[0084] In addition, in some embodiments, VO 2 The conductivity of layer 30 is σ = 200 S / m, which is considered as an insulating state. 2 The electrical conductivity of layer 30 is σ=200000 S / m and is regarded as a metallic state.

[0085] In addition, in some embodiments, the dimensions of the reflective layer 20 are: 30-54 μm in length, 30-54 μm in width, and 0.1-0.3 μm in thickness. By way of example, the length of the reflective layer 20 may be 30 μm, 42 μm, 54 μm, etc., the width of the reflective layer 20 may be 30 μm, 42 μm, 54 μm, etc., and the thickness of the reflective layer 20 may be 0.1 μm, 0.2 μm, 0.3 μm, etc.

[0086] Example 1

[0087] See also Figures 1 to 3 This embodiment provides a resonator unit, including a reflective layer 20, a dielectric layer 10 and a pattern layer stacked in sequence, the pattern layer including a first metal ring layer 50, a second metal ring layer 40 and at least one VO 2 layer 30, the first metal ring layer 50, the second metal ring layer 40 and the VO 2 The bottom of the layer 30 is flush and in contact with the dielectric layer 10 respectively. The first metal ring layer 50 is arranged in the second metal ring layer 40. 2 One side of the layer 30 is in contact with the outer wall of the first metal ring layer 50. 2 The other side of the layer 30 contacts the inner side wall of the first metal ring layer 50, and the unit structure period P = 42 μm;

[0088] 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 all 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 width of the first metal ring layer 50 and the second metal ring layer 40 are both 1 μm, and the first metal ring layer 50, the second metal ring layer 40 and the VO 2The thickness of the layer 30 is 2 μm. The dielectric layer 10 is made of polyimide. The dimensions of the dielectric layer 10 are: 42 μm in length, 42 μm in width, and 5.8 μm in thickness. Its thickness is greater than the skin depth to ensure that the THz wave will not be transmitted. The dielectric constant of the dielectric layer 10 is 3.5 (1 + 0.0027i), where i is an imaginary unit. The dimensions of the reflective layer 20 are: 42 μm in length, 42 μm in width, and 0.2 μm in thickness. The first metal ring layer 50, the second metal ring layer 40, and the reflective 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 reflective layer 20 are the same. 2 The number of layers 30 is four, the VO 2 The layers 30 are arranged between the first metal ring layer 50 and the second metal ring layer 40 at equal intervals. 2 The layer 30 is fan-shaped, and the fan-shaped angle is 60°.

[0089] Test Case

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

[0091] First of all, it should be noted that VO 2 The conductivity of the layer changes with temperature and can switch from an insulating phase to a metallic phase. In this test case, VO 2 When the layer conductivity is 200000S / m, it is a metallic phase, and when it is 200S / m, it is an insulating phase. Figure 4 ,from Figure 4 In (a), it can be seen that the resonator unit is in VO 2 When the layer is in the metal phase, an absorption peak with an absorption rate of 99.84% appears at 1.815 THz; 2 As the conductivity of the layer decreases, the absorption peak of the resonator unit gradually decreases and becomes two absorption peaks. 2 When the layer conductivity is 20000S / m, there is only one absorption peak with a lower absorption rate, while when VO 2 When the layer conductivity is further reduced to 2000S / m, two absorption peaks appear in the resonator unit. The absorption peak at this time is smaller until VO 2 The layer conductivity is reduced to 200S / m, that is, VO 2 When the layer changes to the insulating phase, the resonator unit exhibits two absorption peaks at 1.515THz and 2.985THz, with an absorption rate of 95.66% at 1.515THz and 99.12% at 2.985THz. Based on this, in order to conduct an in-depth study on the resonator unit designed in Example 1, only VO 2 The absorptivity of the layer when it is in the insulating phase and the metallic phase is discussed. Figure 4(b) shows the VO 2 The layer changes with temperature. Figure 4 (c) and Figure 4 It can be seen from (d) that for the incident wave TE mode and TM mode, the absorption rates of these two modes are almost the same regardless of the insulating phase or the metallic phase, which shows that the resonator unit exhibits polarization insensitivity, which is attributed to the symmetrical structure of the resonator unit.

[0092] Also, see Figure 5 In this test case, VO is also controlled by thermal induction. 2 The layer undergoes a phase change from an insulating phase to a metallic phase, which results in a change in the absorption rate. The main physical mechanism is that the dielectric constant changes, as shown in Figures 5(a) and Figure 5 (b) shows that VO 2 The relationship between the real and imaginary parts of the dielectric constant of the layer and the conductivity. Obviously, the real part under different conductivity is much smaller than the imaginary part. The real part of the dielectric constant mainly affects the resonant frequency, and the imaginary part mainly affects the loss. The physical mechanism of perfect absorption can be explained by citing impedance matching. When the THz wave is incident vertically, the relative impedance and absorptivity can be expressed as:

[0093]

[0094] Where Z is the effective impedance of the resonator unit, Z 0 is the free space impedance, Z r Represents the relative impedance of the resonator unit. Figure 5 (c) and Figure 5 (d) shows the VO 2 When the layer conductivity is 200S / m and 200000S / m respectively, the relative impedance of the resonator unit can be obtained. 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 is close to 1, and the imaginary part is close to 0, that is, the resonator unit matches the free space impedance, indicating that the definition of relative impedance is met, that is, the resonator unit designed in 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 recorded as Peak 1, Peak 2, and Peak 3. Figure 6 , showing the VO 2 The three absorption peaks of the metal pattern layer (f 1 ,f 2 ,f 3 ) z and H z Distribution, from Figure 6 It can be seen from (a) that when VO 2When the layer is in the metallic phase, the positive charge at 1.815 THz is mainly distributed in the upper part of the inner and outer rings, while the negative charge is mainly distributed in the lower part of the inner and outer rings. Figure 6 (b) It can be seen that when VO 2 When the 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 charge distributed in the upper half of the ring and the negative charge distributed in the lower half; for 2.985 THz, Figure 6 (c) shows that the charge distribution of peak 3 is mainly concentrated in the inner ring (the first metal ring layer), with positive charges distributed in the upper half of the inner ring and negative charges distributed in the lower half. This accumulation of opposite charges indicates electric dipole resonance. Figure 6 (d) shows the surface H at 1.815 THz z Distribution, concentrated on the inner and outer rings; Figure 6 (e) shows the surface H at 1.515 THz z Distribution, concentrated on the outer ring (the second metal ring layer); Figure 6 (f) shows the surface H at 2.985 THz z Distribution, concentrated on the inner ring.

[0096] In this test case, in order to further understand the absorption mechanism, the influence of some structural parameters on the resonator unit is discussed. Figure 7 , it can be seen that when VO 2 When the layer is in the metallic phase, Figure 7 (a) and Figure 7 As can be seen from (b), whether r 1 Or r 2 As the absorption intensity of peak 1 increases, it increases to a maximum value and then decreases, and both of them show red shift, but the difference is that r 2 The change amplitude of peak 1 is greater than r 1 The change amplitude of peak 1 is greater when it changes. Figure 7 (c) and Figure 7 (d) shows that changing the width of the corresponding metal ring will result in similar changes. This is consistent with the previous electric field E z The distribution of is consistent with that of peak 1, the electric field E z The charge on the outer ring of the distribution is greater than the charge on the inner ring. Therefore, changing r 2 Natural ratio change 1 The change is larger. 2 When it increases from 5.4μm to 6.2μm, Figure 7 As shown in (e), the absorption intensity of peak 1 increases first, t 2 When =5.8μm, the absorption peak gradually becomes smaller after reaching the extreme value, and there is a slight red shift. Figure 7(f) shows the absorption rate as a function of the thickness t of the first metal ring layer or the second metal ring layer. 3 The relationship between 3 = 0.2μm when the absorption peak is the highest. Continue to discuss VO 2 When the layer is in the insulating phase, the resonator unit changes with the parameters. Figure 8 (a), when r 1 When r changes, 1 As r increases, the resonance frequency of peak 2 remains unchanged, and the absorption intensity decreases slightly; while the absorption intensity and resonance frequency of peak 3 change greatly. 1 The absorption peak gradually redshifts and the absorption intensity decreases. In the previous electric field distribution, it can be seen that peak 3 is mainly determined by the inner ring, which changes r 1 The natural peak 3 has a large change. Figure 8 As can be seen from (b), VO 2 When the layer is in the insulating phase, change r 2 When r 2 As the absorption intensity of peak 3 increases slightly, the resonance frequency remains unchanged, while peak 2 redshifts and the absorption intensity decreases. Figure 8 (c) and Figure 8 (d) It can be seen that by changing the width of the metal ring, the corresponding resonant frequency will be red-shifted. Figure 8 (e) and Figure 8 (f) shows that when the intermediate dielectric layer t 2 As t increases, the absorption intensities of peaks 2 and 3 gradually increase and are accompanied by a slight red shift. 3 When the absorption peak increases, it first becomes larger and then becomes smaller. By changing the parameters and analyzing its absorption, the structural parameters of the resonator unit can be better optimized to obtain a resonator unit with suitable performance. For a ring structure with a frequency selective surface, the resonant frequency can be given by the following formula according to the resonant frequency:

[0097]

[0098] Where r = (r 1 +r 2 ) / 2 is the average radius of the ring, λ and f are the resonant wavelength and resonant frequency, c is the speed of light in free space, ε eff is the effective dielectric constant of the dielectric layer. 1 and r 2 The increase of r causes the absorption frequency to shift to a low frequency. 2 As the effective dielectric constant of the dielectric layer increases, the resonant frequency decreases, r 1 is the inner radius of the second metal ring layer, r 2 is the inner radius of the first metal ring layer, w1 is the width of the first metal ring layer, w 2 is the width of the second metal ring layer, t 2 is the width of the dielectric layer, t 3 is the thickness of the first metal ring layer or the second metal ring layer.

[0099] Fig. 9 The electric field distribution of the resonator unit at 1.515 THz is shown, where Fig. 9 (a) shows a side view. Fig. 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; Fig.10 The electric field intensity distribution of the resonator unit at 2.985THz is shown. Fig.10 (a) shows a side view. Fig.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 resonance of this ring; Fig.11 The electric field intensity distribution of the resonator unit at 1.815THz is shown. Fig.11 (a) shows a side view. Fig.11 (b) The surface pattern layer shows that the electric field strength at 1.815 THz is in the inner and outer rings and VO 2 Layers are distributed, but mainly concentrated in the outer ring, inner ring (first metal ring layer) and VO 2 The layer has only a slight electric field distribution, so the 1.815THz absorption peak is generated by the inner and outer rings and the VO 2 The common resonance of the layers.

[0100] In addition, this test case also decomposes and explores the structure of the resonator unit, such as Fig.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, Fig.12 (b) shows that the absorption rate at 1.560 THz is 97.16%. Fig.12 As shown in (c), when there are two metal rings, two absorption peaks appear, and the absorption rates at 1.565THz and 3.115THz are 97.31% and 99.74% respectively. 2 The structure of the layer, its absorption rate is as follows Fig.12 As shown in (d), when VO 2 When the layer is in the insulating phase, its absorption rate is almost 0. As the conductivity increases and gradually transitions to the metallic phase, its absorption rate slowly increases. 2When the layer is in the insulating phase, the originally designed resonator unit is similar to a structure with only two metal rings, VO 2 The layer has a slight effect, so its absorption peak and resonance frequency have a slight change. 2 When the layer is in the metal phase, VO 2 The layers have a great 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 Fig.13 As shown. It can be seen that for VO 2 When the layer is in the metallic phase, Fig.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%. 2 When the layer is in the insulating phase, Fig.13 (b), when the incident angle changes from 0° to 45°, the peak 2 decreases rapidly but still has a good absorption rate, and the peak 3 shows insensitivity. Next, the polarization angle is discussed, such as Fig.13 (c) and Fig.13 (d) It can be seen that whether VO 2 Whether the layer is in the metallic or insulating phase ensures polarization insensitivity.

[0102] In summary, the resonator unit designed in the present invention realizes free switching between two absorption peaks with absorption rates of 95.66% and 99.12% at 1.515THz and 2.985THz respectively and an absorption peak with an absorption rate of 99.84% at 1.815THz. 2 When the VO layer is in an insulating state, the resonator unit exhibits two absorption peaks at 1.515 THz and 2.985 THz; 2 When the layer is in the metallic state, the resonator unit has only one absorption peak at 1.815 THz, that is, by thermally controlling the change of VO 2 When the conductivity of the layer changes to an insulating phase, the resonator unit can change from single-band absorption to dual-band absorption, and vice versa. In addition, the resonator unit designed by the present invention has a simple structure and is easy to process and manufacture; in addition, the resonator unit designed by the present invention is polarization-insensitive and has a good absorption peak for incident angles below 45°. Based on these characteristics, it has a wide range of applications in communications, security inspection, stealth, imaging, modulation and other technologies.

[0103] Second, as Fig.14As shown, 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 M×N bins, wherein M and N are both 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 units and will not be described in detail in this embodiment.

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

Claims

1. A resonator unit, characterized in that: It 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 VO2 layer. The first metal ring layer and the second metal ring layer are flush with the bottom of the VO2 layer and are in contact with the dielectric layer respectively. The first metal ring layer is arranged in the second metal ring layer. One side of any VO2 layer is in contact with the outer side wall of the first metal ring layer, and the other side of any VO2 layer is in contact with the inner side wall of the first metal ring layer.

2. The resonator unit according to claim 1, characterized in that The center of the first metal ring layer, the center of the second metal ring layer, and the center of the upper surface of the dielectric layer are all the same point.

3. The resonator unit according to claim 1, characterized in that 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.

4. 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: 30-54 μm in length, 30-54 μm in width, and 5-6.6 μm in thickness; The dimensions of the reflective layer are: 30-54 μm in length, 30-54 μm in width, and 0.1-0.3 μm in thickness.

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

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

7. The resonator unit according to claim 5, 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 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 to the metal phase of the VO2 layer, and σ = 2 × 10 2 When S / m is set as the insulating phase of the VO2 layer, the VO2 layer exists in a state where the metal component and the dielectric component coexist; The expression of absorption rate is: A(ω)=1-R(ω)-T(ω)=1-S 11 (oh) 2 -S 21 (oh) 2 Where A(ω), T(ω) and R(ω) are the absorptivity, transmittance and reflectivity respectively. 11 (ω) and S 21 (ω) are the reflection and transmission coefficients calculated from the simulation, Assume that E1 and H2 are the electric field strength and magnetic field strength on both sides of the medium, respectively, and T is the transmission matrix. According to the definition of the transfer matrix, we have Where F represents the electromagnetic field before entering the metamaterial, F′ 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 transmission matrix elements have the following relationship with the transmission coefficient and the reflection coefficient: For uniform dielectric materials, ΔT = 1, and T 11 =T 22 , substituting into Substituting the transmission matrix T into It can be solved 8. The resonator unit according to claim 1, characterized in that The number of the VO2 layers is four, and the VO2 layers are arranged between the first metal ring layer and the second metal ring layer at equal intervals. The VO2 layers are fan-shaped, and the fan-shaped angle range of the VO2 layers is 50°-70°.

9. The resonator unit according to claim 1, characterized in that 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; When the conductivity of the VO2 layer is 200000 S / m, the resonator unit has an absorption peak with an absorption rate of 99.84% in a single band of 1.815 THz.

10. A single / dual-band switchable terahertz absorber, characterized in that: A periodic array comprising resonator units as claimed in any one of claims 1 to 9.

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