Polarization-insensitive ultra-wideband terahertz metamaterial wave absorber based on vanadium dioxide

Abstract

The invention relates to the technical field of terahertz metamaterial sensors, in particular to a vanadium dioxide-based polarization-insensitive ultra-wideband terahertz metamaterial wave absorber, which consists of a vanadium dioxide resonance layer, a photonic crystal plate dielectric layer and a metal reflecting layer, therefore, the terahertz metamaterial wave absorber formed by the invention is of a metamaterial resonance layer-medium-metal bottom plate structure with a simple configuration. Simulation results show that when VO2 is in a metal state, the wave absorber achieves ultra-wideband absorption within the frequency range of 4.78-13.28 THz, the average absorption rate is as high as 97.5%, the absorption bandwidth is 8.5 THz, and the relative bandwidth is 94.1%. By changing the conductivity of vanadium dioxide, the absorptivity of the wave absorber can be dynamically adjusted between 1.2% and 97.5%. The terahertz ultra-wideband wave absorber has potential application value in the fields of adjustable wideband absorption devices, stealth devices, thermal detection, terahertz switches and the like. In addition, the wave absorber has the characteristics of polarization insensitivity and wide-angle absorption, the practicability of the device in multi-scene application is further improved through the polarization insensitivity characteristic, and the wave absorber has the advantages of being multifunctional and high in reliability.

Description

Technical Field

[0001] This invention relates to the field of terahertz wave technology, specifically to a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide. Background Technology

[0002] Terahertz waves (THz) are a special type of electromagnetic wave with frequencies ranging from 0.1 to 10 THz and corresponding wavelengths from 0.03 mm to 3 mm. Their wavelength range lies between the microwave and infrared bands, and their radiated energy falls between low-frequency electrons and high-frequency photons. In recent decades, the development of ultrafast laser technology has provided reliable and stable excitation sources for terahertz pulses, leading to rapid advancements in the application research of terahertz technology. Terahertz technology has become one of the most important scientific and technological achievements of the 21st century. Due to its unique properties such as broadband bandwidth, low energy, and strong penetrability, terahertz waves have wide applications in biomedicine, sensing, and imaging.

[0003] Metamaterials are artificially fabricated materials with subwavelength dimensions and periodic arrangements. They possess extraordinary physical properties in the terahertz frequency band that natural materials lack (such as perfect absorption, inverse Doppler effect, and negative refractive index). Metamaterials have many practical applications, such as absorbers, filters, antennas, and perfect lenses. 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. Early terahertz metamaterial absorbers generated strong electromagnetic resonance at a certain frequency, resulting in a narrow absorption bandwidth. Furthermore, once the structure of metal absorbers was fabricated, their absorption performance was often fixed, significantly limiting the development and application of absorbers. To address these issues, researchers proposed integrating metamaterials into resonant structures and adjusting their dielectric constant or permeability. This dynamic control method allows for real-time adjustment of absorption characteristics, significantly improving the practical application capabilities of absorbers.

[0004] Currently, metamaterial-based absorbers in the terahertz band mainly suffer from drawbacks such as narrow absorption bandwidth, complex structure, and large size. Therefore, it is necessary to further explore performance improvements such as bandwidth widening, structural simplification, and size reduction.

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide to solve the problems existing in the prior art. Summary of the Invention

[0006] To achieve the above and other related objectives, this invention provides a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide, characterized in that: each structural unit, from top to bottom, consists of a vanadium dioxide resonant layer, a photonic crystal plate (PCS) dielectric layer, and a metal reflective layer, with a periodic side length P of the structural unit. x =P y =33um.

[0007] The absorber includes multiple structural units, which are distributed in an N×N pattern and arranged on a plane perpendicular to the incident direction of the terahertz wave, where N is a natural number.

[0008] The top layer uses patterned VO2 as the resonant layer. The length of the open square resonator is W=25um, the opening width is d=3um, the width of the protruding part of the opening is a1=0.5um, the length is b1=1um, the radius of the inner ring is R=6um, the width of the cross pattern at the center of the resonant layer is a2=2um, the length is b2=9um, and the thickness of vanadium dioxide is t3=0.04um.

[0009] The material of the absorption medium layer is a photonic crystal plate with a dielectric constant of 1.56 and a thickness t2 of 6.5 μm.

[0010] The metal reflective layer is made of gold, which has an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal reflective layer t1=1um.

[0011] To verify the performance of the designed terahertz absorber, finite element analysis was performed using the electromagnetic simulation software CST. Periodic boundary conditions were set in the x and y axes, and open boundary conditions were set in the z-axis to simulate an infinite periodic array. In the simulation, the incident electromagnetic wave was divided into transverse electric (TE) waves and transverse magnetic (TM) waves. The electric field of the TE wave was parallel to the y-axis, and the magnetic field was parallel to the x-axis; the electric field of the TM wave was parallel to the x-axis, and the magnetic field was parallel to the y-axis.

[0012] As described above, this invention provides a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide. This absorber consists of a vanadium dioxide resonant layer, a photonic crystal substrate dielectric layer, and a metal reflective layer, resulting in a simple metamaterial resonant layer-dielectric-metal substrate structure for the terahertz metamaterial sensor. Simulation results show that when VO2 is in a metallic state, the absorber achieves ultrawideband absorption in the frequency range of 4.78-13.28 THz, with an average absorptivity of 97.5%, an absorption bandwidth of 8.5 THz, and a relative bandwidth of 94.1%. By changing the conductivity of vanadium dioxide, the absorber's absorptivity can be dynamically adjusted between 1.2% and 97.5%. This ultrawideband terahertz absorber has potential applications in tunable broadband absorption devices, stealth devices, thermal detection, and terahertz switches. Furthermore, the absorber exhibits polarization insensitivity and wide-angle absorption characteristics. Its polarization insensitivity further enhances the device's practicality in various applications, giving it advantages of multifunctionality and high reliability. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0014] Figure 1 This is a three-dimensional diagram of a polarization-insensitive ultrawideband terahertz metamaterial absorber structure unit based on vanadium dioxide according to the present invention.

[0015] Figure 2 This is a top view of a polarization-insensitive ultrawideband terahertz metamaterial absorber structural unit based on vanadium dioxide according to the present invention.

[0016] Figure 3 This is a side view of a polarization-insensitive ultrawideband terahertz metamaterial absorber structural unit based on vanadium dioxide according to the present invention.

[0017] Figure 4 This is the absorption spectrum of a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide according to the present invention.

[0018] Figure 5 This is a schematic diagram of the equivalent impedance of a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide in the metallic state, according to a simulation example of the present invention.

[0019] Figure 6 This is an absorption spectrum of a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide at different polarization angles according to the present invention.

[0020] Figure 7This is a simulation example of a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide, showing the absorption spectra at different vanadium dioxide conductivity levels. Detailed Implementation

[0021] To more clearly illustrate the purpose and advantages of this invention, a detailed description of its specific implementation will be provided below with reference to the accompanying drawings. Through illustrations and related descriptions, various aspects of the embodiments of this invention will be shown, including structural features and absorption curves, to enable readers to gain a deeper understanding and mastery of the essence of this invention. This detailed description will help readers to more comprehensively understand this invention at the technical level and provide useful reference for further research, application, and improvement. By combining text and illustrations, this description will present the specific details of the embodiments of this invention, enabling readers to more intuitively grasp the technical features and innovations of this invention.

[0022] Please see Figures 1 to 3 As shown, this invention proposes a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide, comprising multiple structural units, which are distributed in an N×N pattern and arranged on a plane perpendicular to the incident direction of the terahertz wave, where N is a natural number.

[0023] Each of the structural units consists of a vanadium dioxide resonant layer, a photonic crystal plate dielectric layer, and a metal reflective layer from top to bottom, making the terahertz metamaterial absorber formed by the present invention a simple "sandwich" structure, thereby solving the problems of complex structure and difficult fabrication of traditional terahertz absorbers.

[0024] The top layer uses patterned VO2 as the resonant layer. The length of the open square resonator is W=25um, the opening width is d=3um, the width of the protruding part of the opening is a1=0.5um, the length is b1=1um, the radius of the inner ring is R=6um, the width of the cross pattern at the center of the resonant layer is a2=2um, the length is b2=9um, and the thickness of vanadium dioxide is t3=0.04um.

[0025] The material of the absorption medium layer is a photonic crystal plate with a dielectric constant of 1.56 and a thickness t2 = 6.5 μm.

[0026] The metal substrate is made of metallic gold, which has an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal substrate t1=1um.

[0027] For further details, please refer to Figures 4 to 7To illustrate the performance characteristics of the polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide described in this invention, performance simulations were performed using CST Microwave Studio 2022. A frequency domain solver was used in the simulation, and a tetrahedral adaptive mesh was employed. Unit cell boundary conditions were used in the x and y directions, and open (add space) boundary conditions were used in the z direction. Linearly polarized electromagnetic waves were propagated along the z-axis via a Floquet Port. Furthermore, the distance between the port location and the top structural layer was set to be greater than two wavelengths to simulate far-field detection in a real-world scenario.

[0028] The relative permittivity ε of vanadium dioxide in the terahertz band is defined using the Drude model, and ε is expressed as:

[0029]

[0030] In the formula, the high-frequency dielectric constant ε ∞ =12, collision frequency γ=5.75×10 13 rad / s, plasma frequency ω p (σ) is related to the conductivity σ0, and the formula is:

[0031]

[0032] Where σ0 = 3 × 10 5 S / m, ω p (σ0) = 1.4 × 10 15 rad / s. In the simulation, the phase transition characteristics of vanadium dioxide were simulated by setting different values ​​of conductivity σ. When VO2 is in the metallic state, the conductivity is 3 × 10 rad / s. 5 S / m; when VO2 is in an insulating state, the conductivity is 300 S / m.

[0033] Absorption rate A is an important parameter reflecting the absorption performance of a device. Assuming that the electromagnetic wave is incident perpendicularly to the surface of the absorber structure along the z-axis, according to electromagnetic theory, the formula for calculating the absorption rate is:

[0034]

[0035] Among them, S 11 and S 21 Let S represent the reflection coefficient and transmission coefficient of the incident wave after it strikes the device, respectively. Within the simulation frequency range, because the thickness of the metal substrate is much greater than its skin depth, the transmission coefficient S of this device is... 21 Since it is almost zero, the formula for the absorption rate can be simplified to A(ω) = 1 - R(ω) = 1 - |S 11 | 2 .

[0036] Reference Figure 4 Vanadium dioxide is in the metallic state (σ=3×10). 5 At a frequency of f = 8.78 THz (S / m), the absorber exhibits an absorption rate greater than 90% in the frequency range of 4.78-13.28 THz, and achieves perfect absorption at f = 8.78 THz. Furthermore, according to the formula RBW = 2(f... h -f l ) / (f h +f l The relative bandwidth of the absorber can be calculated to be approximately 94.1%, where f h =13.28THz, f l =4.78THz, the designed broadband absorber has great potential for application in broadband absorption.

[0037] The equivalent impedance of a metamaterial absorber is closely related to its dielectric constant and permeability, and this characteristic can be adjusted by changing its structural dimensions. At different incident wavelengths, metamaterial absorbers can achieve better impedance matching over a wider frequency band, thus maintaining good absorption performance over a large range. For metamaterials, the geometric dimensions of their periodic units are much smaller than the wavelength corresponding to the operating frequency. Therefore, when analyzing their electromagnetic properties, they can be considered as a whole, ignoring the interactions between different parts within the structure. Furthermore, metamaterials typically employ symmetrical designs, thus they can be equivalently represented by a homogeneous dielectric plate. The normalized impedance of a metamaterial absorber can be expressed as:

[0038]

[0039] Reference Figure 5 Within the absorption bandwidth, the real part of the relative impedance is close to 1 and the imaginary part is close to zero. At this time, the characteristic impedance of the absorber is basically the same as the impedance of free space, achieving impedance matching. This significantly reduces the reflection of incident electromagnetic waves on the absorber surface and maximizes the absorption effect of electromagnetic waves.

[0040] In practical applications, electromagnetic waves are often incident obliquely onto the metasurface in any direction. Therefore, it is necessary to ensure that electromagnetic waves incident from different directions can be effectively absorbed by the absorber. (Refer to...) Figure 6 This paper presents the absorption spectrum of terahertz waves under normal incidence conditions, with polarization angles varying in the range of 0–90°. Due to the highly symmetrical surface resonant layer structure of this absorber, the absorbance and broadband frequency range remain unchanged as the polarization angle increases, exhibiting polarization insensitivity.

[0041] The modulation depth (MD) of a tunable metamaterial absorber can be calculated using the following formula.

[0042]

[0043] Amax and Amin represent the absorption rates of VO2 in its metallic and insulating states, respectively.

[0044] Typically, the conductivity of VO2 can be controlled by adjusting the temperature. Since conductivity affects the dielectric constant of VO2, the absorber's absorption rate can be continuously adjusted by changing the temperature. (Refer to...) Figure 7 When the conductivity of VO2 increases from 300 S / m to 3 × 10 5 At a current S / m, both the absorption rate and absorption bandwidth of the absorber gradually increase, and the absorption rate can be dynamically adjusted from 1.2% to 97.5%. When the conductivity of VO2 increases to 3×10... 5 At a wavelength of S / m, a perfect absorption peak appears at f=8.78THz, exhibiting the best bandwidth absorption effect. Based on this design, a metamaterial absorber with an excellent MD value (98.9%) was realized. By adjusting the conductivity of VO2, the absorptivity of the device can be controlled, thereby achieving absorption or reflection effects. Since VO2 is a phase change material, its conductivity can be modulated by electrical or thermal stimulation, thus enabling the device to switch between absorber and reflector functions.

[0045] In summary, a polarization-insensitive ultrawideband terahertz metamaterial absorber based on vanadium dioxide was designed. This device consists of a VO2 resonant layer, a photonic crystal substrate dielectric layer, and a metallic reflective layer. Simulation results show that when the conductivity of VO2 is between 300 S / m and 3 × 10⁻⁶ S / m… 5 When the S / m ratio is adjusted, the absorber's absorptivity can be dynamically adjusted from 1.2% to 97.5%. When VO2 is in an insulating state, the absorber's average absorptivity is 1.2%; when VO2 is in a metallic state, the absorber achieves an absorptivity exceeding 90% in the 4.78-13.28 THz frequency range, with an absorption bandwidth of 8.5 THz. Furthermore, the symmetry of the device's surface pattern makes the terahertz perfect absorber polarization-insensitive, and it also possesses wide-angle absorption characteristics. This design offers advantages such as simple structure, large adjustment range, high absorptivity, and ultra-wide absorption bandwidth, making it potentially valuable for applications in adjustable bandwidth absorption devices, stealth devices, thermal detection, and terahertz switches.

Claims

1. A polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide, which is composed of N×N periodic structural units arranged periodically in the same horizontal plane, where N is a natural number, and arranged on a plane perpendicular to the incident direction of the terahertz wave. Each of the structural units is composed of a vanadium dioxide (VO2) resonant layer, a photonic crystal slab (PCS) dielectric layer, and a metal reflective layer from top to bottom, and the periodic side length of the structural unit is P x =P y =33um. The metamaterial absorber has the characteristics of polarization insensitivity and wide-angle absorption, and has potential application value in terahertz communication, stealth technology, imaging and terahertz switching.

2. The polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide according to claim 1, characterized in that: Based on the terahertz vanadium dioxide metamaterial absorber, the average absorption rate is as high as 97.5%, the absorption bandwidth is 8.5THz, and the relative bandwidth is 94.1%.

3. The polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide according to claim 2, characterized in that: The top layer uses patterned VO2 as the resonance layer. The length of the open square resonator is W=25um, the opening width is d=3um, the width of the protruding part of the opening is a1=0.5um, the length is b1=1um, the inner ring radius is R=6um, the cross pattern located in the center of the resonance layer is a2=2um wide and b2=9um long, and the thickness of vanadium dioxide is t3=0.04um.

4. The polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide according to claim 3, characterized in that: The material of the dielectric layer is a photonic crystal plate, the dielectric constant of the photonic crystal plate is 1.56, and the thickness t2 of the dielectric layer is 6.5 um.

5. The polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide according to claim 4, characterized in that: The material of the bottom metal reflective layer is metal gold, and the electrical conductivity of gold is 4.56×10 7 S / m, the thickness of the metal reflective layer is t1=1um.

6. The polarization-insensitive ultra-wideband terahertz metamaterial absorber based on vanadium dioxide according to claim 5, characterized in that: The conductivity of VO2 can be controlled by adjusting the temperature. When the conductivity of VO2 is 300 S / m, VO2 is in an insulating state; when the conductivity of VO2 is 3×10 5 S / m, VO2 is in a metallic state.

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