Vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz wave absorber and sensing application

Abstract

The invention relates to the technical field of terahertz metamaterial devices, in particular to an ultra wide band / double narrow band switchable terahertz wave absorber based on vanadium dioxide and sensing application. The wave absorber is formed by periodically arranging and combining a plurality of metamaterial square structure units, and no gap exists between adjacent periodic units. Each metamaterial square structure unit is composed of six layers from top to bottom, namely a VO2 resonance layer, a photonic crystal plate dielectric layer, a VO2 thin film, a metal resonance layer, a polycrystalline silicon dielectric layer and a gold substrate. The reversible phase change process of vanadium dioxide from a metal state to an insulation state is realized through external excitation (light, temperature and voltage). When vanadium dioxide is in a metal state, the terahertz ultra-bandwidth absorption function is achieved, when vanadium dioxide is in an insulated state, the terahertz wave double-narrow-band absorption function is achieved, and conversion between the ultra-wide band and the double narrow bands of the wave absorber is achieved. According to the terahertz wave absorber, the ultra-wideband / dual-narrowband conversion function of terahertz wave absorption can be achieved on one device, the defects that an existing terahertz wave absorber is complex in structure and the absorption characteristic is fixed once the terahertz wave absorber is manufactured are overcome, and the sensing function application of a single narrowband in the terahertz wave absorber is researched.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz waves, and specifically to a terahertz absorber with ultra-wideband / double-narrowband switchable based on vanadium dioxide and its sensing applications. Background Art

[0002] Terahertz (THz) waves refer to electromagnetic waves with frequencies in the range of 0.1 - 10 THz (wavelengths of 30 - 3000 μm), and their spectra are located between microwaves and infrared radiation. Terahertz waves have characteristics such as strong penetration, high safety, excellent signal-to-noise ratio, and high spectral resolution. Due to these unique advantages, terahertz waves have shown broad application prospects in fields such as communication, sensing, and imaging, and have received great attention from researchers. To promote the development of terahertz technology, various functional devices based on metamaterials have been proposed, such as polarization converters, filters, modulators, and absorbers. Among them, terahertz metamaterial absorbers (MMAs) are devices with high absorption rates for incident electromagnetic waves. Through precise engineering design of their periodic surface structures, MMAs can achieve near-perfect absorption of electromagnetic waves.

[0003] In 2008, Landy et al. proposed a polarization-sensitive, single-frequency, perfect-absorbing metamaterial absorber, which triggered a research boom on MMAs. Since then, many researchers have been committed to the study of terahertz metamaterial absorber models and have deeply explored single-frequency, multi-frequency, and broadband absorption characteristics. However, the geometric structure of traditional absorbers is usually fixed after manufacturing, which results in non-adjustable absorption performance and greatly limits their practical applications. With the development of metamaterials, such as vanadium dioxide, graphene, and semiconductors with phase change characteristics, researchers have successfully realized tunable MMAs by combining tunable materials with traditional absorbers.

[0004] In recent years, metamaterial absorbers integrated with vanadium dioxide mainly utilize patterned vanadium dioxide to replace the surface metal pattern of conventional material absorbers, achieving tunable absorption of terahertz absorbers. In 2020, Huang et al. proposed a terahertz absorber based on a VO2 square ring structure, and this device achieved an absorption rate of 90% in the frequency range of 1.85 - 4.3 THz. In 2021, Liu et al. proposed a dual-mode tunable metamaterial absorber based on a vanadium dioxide-graphene hybrid structure. By adjusting the metal-insulating state of vanadium dioxide, this material can be tuned from narrowband absorption to broadband absorption. In 2022, Mou et al. proposed a broadband absorber that achieves an absorption rate of over 90% in the terahertz low-frequency band (0.398 - 1.356 THz), forming broadband absorption and tunable functions through the coupling superposition between annular vanadium dioxide patterns. In 2023, Hu et al. proposed a switchable and tunable terahertz metamaterial absorber. When VO2 is in the insulating state, this design behaves as a dual-band absorber, with an absorption rate exceeding 90% in the ranges of 0.73 - 1.53 THz and 3.03 - 3.64 THz. When VO2 is in the metallic state and the graphene Fermi energy is set to 0.01 eV, it has an absorption rate of over 90% in the range of 1.00 THz - 3.55 THz.

[0005] However, once the existing terahertz absorber structure is fabricated, its absorption performance is fixed, which severely restricts its practical applications when considering a larger terahertz bandwidth and better flexibility. How to obtain a broadband absorber with tunable and larger working bandwidths and use a simpler tuning method to switch between narrowband and broadband absorption remains a challenge in designing an absorber with switchable absorption functions. Summary of the Invention

[0006] In view of the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a vanadium dioxide-based ultra-wideband / dual-narrowband switchable terahertz absorber and study its application in sensing to solve the problems existing in the prior art.

[0007] To achieve the above purpose and other related purposes, the present invention provides a vanadium dioxide-based ultra-wideband / dual-narrowband switchable terahertz absorber, which is characterized in that: this absorber has a total of six layers, from top to bottom are the vanadium dioxide (VO2) resonant layer, the photonic crystal plate dielectric layer, the VO2 thin film, the metal resonant layer, the polysilicon dielectric layer, and the gold substrate. This absorber is composed of a number of metamaterial square structural units arranged periodically, with no gap between adjacent metamaterial square periodic structural units, and the side length W of its unit structure period is 15 um.

[0008] Among them, the top VO2 resonant layer is composed of two square rings, an open circular ring and a cross. The outer side length of the outer square ring is L1 = 11 µm, the inner side length is L2 = 9 µm, the outer side length of the middle square ring is L3 = 8 µm, the inner side length is L4 = 7 µm, the outer radius of the inner circular ring is R1 = 3.4 µm, the inner radius is R2 = 2.4 µm, the opening is d1 = 1 µm, the length of the cross is b1 = 4 µm, the width is a1 = 1 µm, and the thickness of the VO2 resonant layer is t6 = 0.2 µm.

[0009] Among them, the metal resonant layer is composed of an open square ring and a cross. The outer side length of the square ring is L5 = 11.5 µm, the inner side length is L6 = 10 µm, the opening width is d2 = 0.7 µm, the length of the cross is b2 = 8 µm, and the width is a2 = 0.5 µm. The thickness of the metal resonant layer is t3 = 0.2 µm, the material of the metal resonant layer is gold, and the conductivity of gold is 4.56×10 7 S / m.

[0010] Among them, the thickness of the metal bottom plate is t1 = 0.5 µm, and the material of the metal bottom layer is gold.

[0011] Among them, the thickness of the polysilicon dielectric layer is t2 = 6.5 µm, and its dielectric constant is 3.

[0012] Among them, the thickness of the VO2 thin film is t4 = 0.17 µm.

[0013] Among them, the thickness of the photonic crystal plate dielectric layer is t5 = 7.2 µm, and its dielectric constant is 1.56.

[0014] Among them, the electromagnetic simulation software CST Microwave Studio is used to simulate the designed structure. When the terahertz wave is vertically incident on the surface of the structure, the electric field E is along the y-axis direction, and the magnetic field H is along the x-axis direction. The boundary condition of the absorber structure is set as the periodic boundary condition, and the S parameters obtained by the simulation can be used to calculate the absorption rate of the absorber for the terahertz wave.

[0015] As described above, a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber is proposed. The absorber is composed of a number of metamaterial square structural units arranged periodically, and there is no gap between adjacent periodic units. Each metamaterial square structural unit consists of six layers from top to bottom, namely, a VO2 resonant layer, a photonic crystal plate dielectric layer, a VO2 thin film, a metal resonant layer, a polysilicon dielectric layer, and a gold substrate. The reversible phase change process of vanadium dioxide from the metallic state to the insulating state is achieved through external excitation (light, temperature, and voltage). When vanadium dioxide is in the metallic state, it has an ultra-wideband absorption function for terahertz waves. When vanadium dioxide is in the insulating state, it has a double-narrowband absorption function for terahertz waves, realizing the conversion between the ultra-wideband and double-narrowband of the absorber. The present invention can achieve the ultra-wideband / double-narrowband conversion function of terahertz absorption on a single device, solves the defects of the complex structure of existing terahertz sensors and the fixed functional properties once they are fabricated, and studies the sensing function application of a single narrowband among them. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0017] Figure 1 FIG. is a three-dimensional view and a side view of the structural unit of a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber of the present invention.

[0018] Figure 2 FIG. is the resonance pattern of the VO2 resonant layer and the metal resonant layer of the structural unit of a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber of the present invention.

[0019] Figure 3 FIG. is the absorption curve of a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber of the present invention.

[0020] Figure 4 FIG. is a schematic diagram of the equivalent impedance of a simulation example of a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber of the present invention in the metallic state and the insulating state of vanadium dioxide.

[0021] Figure 5 FIG. is the sensing curve of a simulation example of a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber of the present invention at different refractive indices. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can be made. These all belong to the protection scope of the present invention.

[0023] Please refer to Figure 1 and Figure 2 As shown, the present invention proposes a terahertz absorber based on vanadium dioxide that can switch between ultra-wideband and dual-narrowband. The absorber is composed of a number of metamaterial square structural units arranged periodically, and there is no gap between adjacent metamaterial square periodic structural units. Each metamaterial square structural unit consists of six layers from top to bottom, namely, a vanadium dioxide (VO2) resonant layer, a photonic crystal plate dielectric layer, a VO2 thin film, a metal resonant layer, a polysilicon dielectric layer, and a gold substrate. The period side length W of the structural unit is 15um.

[0024] The top VO2 resonant layer consists of two square rings, an open circular ring, and a cross. Among them, the outer side length L1 of the outer square ring is 11um, the inner side length L2 is 9um, the outer side length L3 of the middle square ring is 8um, the inner side length L4 is 7um, the outer radius R1 of the inner circular ring is 3.4um, the inner radius R2 is 2.4um, the opening is d1 = 1um, the length b1 of the cross is 4um, the width a1 is 1um, and the thickness t6 of the VO2 resonant layer is 0.2µm.

[0025] The metal resonant layer consists of an open square ring and a cross. The outer side length L5 of the square ring is 11.5um, the inner side length L6 is 10um, the opening width d2 is 0.7um, the length b2 of the cross is 8um, and the width a2 is 0.5um. The thickness t3 of the metal resonant layer is 0.2um, the material of the metal resonant layer is metal gold, and the conductivity of gold is 4.56×10 7 S / m.

[0026] The thickness t1 of the metal bottom plate is 0.5um, and the material of the metal bottom layer is gold.

[0027] The thickness t2 of the polysilicon dielectric layer is 6.5um, and its dielectric constant is 3.

[0028] The thickness t4 of the VO2 thin film is 0.17um.

[0029] The thickness t5 of the photonic crystal plate dielectric layer is 7.2um, and its dielectric constant is 1.56.

[0030] Furthermore, please refer to Figures 3 to 5, To illustrate the performance characteristics of the vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber described in the present invention, the present invention uses CST Microwave Studio 2022 for performance simulation. During the simulation, a frequency-domain solver is adopted, and the mesh is divided by tetrahedral adaptive meshing. The unit cell boundary condition is adopted in the x and y directions, and the open (add space) boundary condition is adopted in the z direction. The linearly polarized electromagnetic wave is set to transmit along the z axis through the Floquet Port. And the distance between the port position and the top structural layer is set to be greater than two wavelengths to simulate the far-field detection in the actual long-distance view.

[0031] The dielectric constant of vanadium dioxide in the terahertz band is described by the Drude model:

[0032]

[0033] In the formula, ω represents the incident frequency, ε ∞ = 12 is the high-frequency relative dielectric constant, γ = 5.75×10 13 rad / s is the collision frequency. σ is the conductivity of vanadium dioxide, ω p (σ) is the plasma frequency depending on the conductivity, ω p and σ are both proportional to the free carrier density. The relationship between the conductivity of vanadium dioxide and the plasma frequency can be approximately expressed as:

[0034] ω p 2 =(σ / σ0)ω p 2 (σ0).

[0035] In the formula, σ0 = 3×10 5 S / m, ω p (σ0)= 1.4×10 15 rad / s. The conductivity of vanadium dioxide can be regulated by temperature. The conductivity of vanadium dioxide in the metallic state is 3×10 5 S / m, and the conductivity of vanadium dioxide in the insulating state is 300 S / m.

[0036] The absorption rate is an important parameter reflecting the absorption performance of the device. The calculation formula of the absorption rate is:

[0037] A = 1 - |S 11 | 2 - |S 21 | 2

[0038] Among them, S 11 and S 21They represent the reflection coefficient and transmission coefficient generated after the incident wave enters the device, respectively. In the simulation frequency range, since the thickness of the metal bottom layer is much greater than its skin depth, the transmission coefficient S of this device 21 is almost zero. Therefore, the formula for the absorption rate can be simplified to A = 1 - |S 11 | 2 .

[0039] Reference Figure 3 , at a temperature of 350 K, when VO2 is in the metallic state (σ = 3×10 5 S / m), the structure in its metallic state can block the incident THz wave from entering the middle VO2 thin film layer. At this time, the part that plays a broadband absorption role is the first three-layer structure of the device. The absorber has an absorption rate higher than 90% in the range of 3.98 - 10.42 THz. Its absolute bandwidth (f max -f min ) is 6.44 THz, and the relative bandwidth 2(f max -f min ) / (f max +f min ) is 89.44%, where f max = 10.42 THz and f min = 3.98 THz. When VO2 is in the insulating state (σ = 300 S / m), the VO2 resonant layer and the VO2 thin film layer of the device can be regarded as dielectric layers. At the same time, a gold bottom layer with a thickness greater than the skin depth is designed to reduce the transmittance of THz waves. At this time, the resonant structure of the device is the open gold square ring and the gold cross in the middle layer. At this time, the absorption rates of the MMAs at frequencies P1 = 7.16 THz and P2 = 11.44 THz are 98.9% and 99.7% respectively.

[0040] Reference Figure 4 , in the present invention, when vanadium dioxide is in the metallic state and the insulating state respectively, the equivalent impedance of the device obtained by the S-parameter inversion method. The solid line represents the real part Re(z) of the device equivalent impedance, and the dashed line represents the imaginary part Im(z) of the device equivalent impedance. The real part Re(z) of the device equivalent impedance approaches 1, and the imaginary part Im(z) approaches 0. At this time, the equivalent impedance of the device is approximately equal to the free space impedance, realizing approximate impedance matching.

[0041] A typical application of the narrowband absorber is a sensor. To study the sensing performance of the absorber when VO2 is in the insulating phase, we covered the surface of the absorber with test objects of 5 μm thickness having refractive indices of n = 1.0, 1.1, 1.2, 1.3, and 1.4 respectively, and analyzed the absorption characteristic curve using the 11.44 THz absorption peak as the indication peak of the wave-absorbing sensor. The quality factor (Q), sensitivity (S), and figure of merit (FOM) are important parameters for measuring the performance of the sensor, and the calculation formulas are as follows:

[0042] Q = f0 / FWHM

[0043] S = Δf / Δn

[0044] FOM = S * Q where f is the resonance frequency. FWHM is the width between two points corresponding to half of the peak height in a peak curve. Δf is the value of the resonance frequency shift when the refractive index of the test object of the sensor changes, and Δn is the change in the refractive index of the test object, with the unit of GHz / RIU, where RIU is the refractive index unit. The results show that when the absorber is used as a refractive index sensor, the performance is as follows: the quality factor (Q) is 29.6, the sensitivity (S) is 0.2 THz / RIU, and the FOM is 5.93 RIU −1 。

[0045] Reference Figure 5 , as the refractive index n increases, the resonance frequency point of 11.44 THz shows different degrees of frequency shift, but the absorption rate of the absorption peak always remains above 0.9, indicating that the sensor has good absorption stability. As shown in the inset of the figure, the change of the central resonance frequency with the refractive index, and through calculation, the fitting determination coefficient R 2 = 0.99, with a very high fitting degree, indicating that the absorber has high reliability when used as a sensor, can well capture the change law of data, and is suitable for prediction and analysis.

[0046] In summary, this paper presents a vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber and its sensing applications. The absorber is composed of a number of metamaterial square structural units arranged periodically, and there is no gap between adjacent periodic units. Each metamaterial square structural unit consists of six layers from top to bottom, namely the VO2 resonant layer, the photonic crystal plate dielectric layer, the VO2 thin film, the metal resonant layer, the polysilicon dielectric layer, and the gold substrate. The reversible phase change process of vanadium dioxide from the metallic state to the insulating state is achieved through external excitation (light, temperature, and voltage). When vanadium dioxide is in the metallic state, it functions as an ultra-wideband absorber for terahertz waves. When vanadium dioxide is in the insulating state, it functions as a double-narrowband absorber for terahertz waves, realizing the conversion between the ultra-wideband and double-narrowband of the absorber. The present invention can achieve the ultra-wideband / double-narrowband conversion function of terahertz wave absorption on a single device, solving the defects of the complex structure of existing terahertz sensors and the fixed functional properties once they are fabricated, and studying the sensing functional applications of the single narrowband among them.

[0047] The above embodiments should be understood as being only for illustrative purposes of the present invention and not for limiting the protection scope of the present invention. After reading the content described in the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber, which is composed of a number of metamaterial square structural units arranged periodically, with no gap between adjacent metamaterial square periodic structural units. Each metamaterial square structural unit consists of six layers from top to bottom, namely, a vanadium dioxide (VO2) resonant layer, a photonic crystal plate dielectric layer, a VO2 thin film, a metal resonant layer, a polysilicon dielectric layer, and a gold substrate. The period side length W of the structural unit is 15 µm.

2. The terahertz absorber based on vanadium dioxide with ultra-wideband / double narrowband switchability as claimed in claim 1, wherein: The VO2 resonant layer consists of two square rings, an open circular ring, and a cross. The outer side length L1 of the outer square ring is 11 µm, the inner side length L2 is 9 µm, the outer side length L3 of the middle square ring is 8 µm, the inner side length L4 is 7 µm, the outer diameter R1 of the inner circular ring is 3.4 µm, the inner diameter R2 is 2.4 µm, the opening d1 is 1 µm, the length b1 of the cross is 4 µm, the width a1 is 1 µm, and the thickness t6 of the VO2 resonant layer is 0.2 µm.

3. The terahertz absorber based on vanadium dioxide and capable of switching between ultra-wideband and dual-narrowband as claimed in claim 2, wherein: The metal resonant layer is composed of an open square loop and a cross. The outer side length of the square loop is L5 = 11.5 um, the inner side length is L6 = 10 um, the opening width is d2 = 0.7 um, the length of the cross is b2 = 8 um, and the width is a2 = 0.5 um. The thickness of the metal resonant layer is t3 = 0.2 um. The material of the metal resonant layer is gold, and the conductivity of gold is 4.56×10 7 S / m.

4. The terahertz absorber based on vanadium dioxide and capable of switching between ultra-wideband and dual-narrowband as claimed in claim 3, wherein: The thickness t1 of the metal bottom plate is 0.5 µm, and the material of the metal bottom layer is gold.

5. A vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber according to claim 4, characterized in that: The thickness t2 of the polysilicon dielectric layer is 6.5 µm, and its dielectric constant is 3.

6. The terahertz absorber based on vanadium dioxide and capable of switching between ultra-wideband and dual-narrowband as described in claim 5, wherein: The thickness t4 of the VO2 thin film is 0.17 µm.

7. The terahertz absorber based on vanadium dioxide and capable of switching between ultra-wideband and dual-narrowband as claimed in claim 6, wherein: The thickness t5 of the photonic crystal plate dielectric layer is 7.2 µm, and its dielectric constant is 1.

56.

8. A vanadium dioxide-based ultra-wideband / double-narrowband switchable terahertz absorber according to claim 7, characterized in that: By changing the conductivity of vanadium dioxide through external excitation, a reversible phase change process of vanadium dioxide from the metallic state to the insulating state can be achieved. The external excitation can be light, temperature, and voltage. When vanadium dioxide is in the metallic state, it has an ultra-wideband absorption function for terahertz waves. When vanadium dioxide is in the insulating state, it has a double-narrowband absorption function for terahertz waves, realizing the narrow / ultra-wideband absorption conversion of the absorber.

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