Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption

Huali Zhu; Yong Zhang; Longfang Ye; Longfang Ye; Yukun Li; Yuehang Xu; Ruimin Xu

doi:10.1364/OE.414039

1. Introduction

Terahertz metamaterial technology, has an operational frequency in the range of 0.1–10 THz, and has broad application prospects in many fields such as super-lens [1–3], perfect absorbers [4–7], and invisible cloaks [8–9]. Terahertz absorber, a device with high absorption rate for incident electromagnetic waves, are in an urgent need in wireless security, radar communication, and selective transceiver applications [10–14]. Due to the difficulty in achieving perfect absorbing in the terahertz band for natural materials, metamaterial absorbers have become a research focus in this field. The metamaterial absorber based on an electromagnetic resonator design was first proposed in 2008 [15], which achieved a peak absorption greater than 88% at 11.5 GHz. Subsequently, various metamaterial absorbers have been studied and designed [16–19], However, the spectral position and corresponding functions of most absorbers are usually fixed, which significantly limits the scope of applications. Therefore, tunable multi-function metamaterials are desirable for terahertz intelligent systems.

Graphene is composed of carbon atoms in the planar hexagonal lattice. It is a two-dimensional monolayer material and a viable candidate for tunable devices owing to its unique features [20–21]. The graphene surface can excite surface plasmons in the mid-infrared and terahertz bands, and its surface conductivity can be manipulated by adjusting the Fermi energy level under a fixed structure [22–24]. Hence, graphene-based tunable terahertz absorbers have been widely reported in recent years. Various graphene-based broadband absorbers with periodically patterned squares, disks, and ribbons have also been investigated [5,16,25]. Furthermore, dual-band or multi-band absorbers [26–28], ultra-bandwidth absorbers with multilayer graphene [29–32], and simple structured absorbers [33–36] have received considerable attention. However, most of these terahertz absorbers are designed for a single functionality, this inherent drawback has been unfavorable to the development of many intelligent systems. In radar communication systems, for example, it is important to selectively transmit and receive terahertz signals. In electromagnetic radiation interference system, flexibly switching the interference frequency band and adjusting the interference amplitude is also necessary. Thus, developing a switchable multi-functional terahertz absorber is urgently needed.

Besides graphene, vanadium dioxide (VO₂) films exhibit excellent and unique reversible insulating-conducting phase transition properties. At room temperature, VO₂ has a monoclinic crystal structure, which transforms into a tetragonal rutile structure when the temperature is increased. When the temperature is decreased to the phase transition temperature, VO₂ transforms into the monoclinic crystal structure again. Notably, when the insulating-conducting phase transition occurs, the electrical [37] and optical properties [38–39] of VO₂ change drastically. In 2006, Jepsen et al. reported that at the ambient temperature of approximately 340 K, the conductivity of VO₂ can undergo a significant and sudden change of five orders of magnitude (40–4×10⁵ S/m); further, a reversible phase transition simultaneously occurs from the insulating state to the conducting state [40]. In addition to the thermal approach [41], electric field [42], light [43], terahertz field [44], and other external excitations can also cause the insulating-conducting phase transition properties. Recently, based on hybrid VO₂ metamaterials, transparent absorbers [45–49], dual broadband absorbers [50], absorption and polarization conversion devices [51,52], and broadband and narrowband absorption conversion absorbers [10,53] have been presented. Furthermore, other devices based on active metamaterials have been developed in the last years [54–58]. Owing to its excellent photoelectric functional performance, VO₂ has tremendous potential applications in THz absorbers.

In this study, we utilized the manipulated property by Fermi energy level of graphene and the phase transition property of VO₂ to demonstrate a switchable and tunable bi-functional terahertz absorber with broadband and multi-band absorption. When VO₂ is in the insulating state, broadband absorption properties are achieved from 1.06 THz to 2.58 THz with optimized geometry. Alternatively, when VO₂ is in the conducting state with the same geometry, the designed device acts as an adjustable multi-band absorber at the absorption frequency of 1 THz, 2.45 THz, and 2.82 THz. The proposed switchable metamaterial absorber exhibits the advantages of bi-functionality, incident angle and polarization independence, and a simple configuration.

2. Design and method

Figure 1 shows a three-dimensional (3D) schematic of the proposed switchable metamaterial absorber, which is periodic in x and y directions. The basic unit cell of the proposed absorber consists of five layers: VO₂ hollow ring patch, graphene ring sheet, polysilicon sheet, polyethylene cyclic olefin copolymer (Topas) spacer, and continuous metallic reflector. Topas is a transparent and stiff amorphous thermoplastic copolymer with superior optical properties for advanced terahertz applications, which also has excellent heat resistance, near zero hygroscopicity and high stability. In simulation, the Topas spacer has a relative dielectric permittivity of 2.35 and is assumed to neglect material loss and dispersion. Its thickness (t_topas) and period (p) are set to 27 µm and 44 µm, respectively. Polysilicon, a form of elemental silicon, is an extremely important and excellent semiconductor material, which can act as an electrostatic gating sheet of bias voltage V_g to regulate the Fermi energy level of the graphene sheet in the proposed absorber. In addition, in order not to affect the performance of the absorber, the thickness of polysilicon should be as thin as possible. In simulation, the polysilicon sheet has a relative dielectric permittivity of 3 and is embedded in the Topas spacer 20 nm below the graphene ring sheet, and the thickness of polysilicon is set to t_p=20 nm. Furthermore, based on the Drude model [59], a continuous metallic reflector with a thickness of t_Au=0.5 µm is set at the bottom layer, and its thickness is considerably greater than the skin depth of electromagnetic waves to ensure complete suppression of the terahertz wave transmission.

Fig. 1. Three-dimensional schematic of the proposed switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

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The finite integration technique (FIT) is used to numerically calculate and analyze the properties of the proposed switchable metamaterial absorber. In the simulation, the Kubo formula of ${\sigma _g} = {\sigma _{intra }} + {\sigma _{inter }}$ (unit:S) with intraband and interband [5–7] contributions is used to determine the surface conductivity of graphene.

(1)$${\sigma _{\textrm{intra }}}(\omega ,{\mu _c},\Gamma ,T) = \frac{{j{e^2}}}{{\pi {\hbar ^2}(\omega - j2\Gamma )}}\int\limits_0^\infty {\frac{{\partial {f_d}(\xi ,{\mu _c},T)}}{{\partial \xi }}} - \frac{{\partial {f_d}( - \xi ,{\mu _c},T)}}{{\partial \xi }})\xi \partial \xi, $$

(2)$${\sigma _{\textrm{inter }}}(\omega ,{\mu _c},\Gamma ,T) = \frac{{j{e^2}(\omega - j2\Gamma )}}{{\pi {\hbar ^2}}}\int\limits_0^\infty {\frac{{{f_d}(\xi ,{\mu _c},T) - {f_d}( - \xi ,{\mu _c},T)}}{{{{(\omega - j2\Gamma )}^2} - {{4\xi } / {{\hbar ^2}}}}}} \partial \xi, $$

where ${f_d}(\xi ,{\mu _c},T) = {({e^{{{^{(\xi - {\mu _c})}} / {{k_B}T}}}} + 1)^{ - 1}}$ is the Fermi energy-Dirac distribution, $\Gamma = 2{\tau ^{ - 1}}$ is the phenomenological scattering rate, $\tau$ is the relaxation time, $\hbar$ is the reduced Plank’s constant, and ${k_B}$ is the Boltzmann’s constant. Additionally, $\omega$, $\xi$, ${\mu _c}$, and $T$ denote the radian frequency, energy, Fermi energy level, and absolute temperature, respectively. In this study, the graphene is modeled using the commercial CST Microwave Studio simulation software which possess the created graphene material for optical applications. The Fermi energy level of graphene (${\mu _c}$) is assumed to 0.7 eV, and the relaxation time ($\tau$) of graphene is assumed to be 1.0 ps and 0.1 ps for multi-band and broadband absorption, respectively. Further, the thickness of graphene is set to 0.34 nm, and the graphene ring sheet radius (R_g) is 18 µm, which is connected using g=1-µm-width graphene strips for electric-continuity and gating convenience. It should be noted that to clearly depict the structure diagram of the proposed absorber, some scaling is done in Fig. 1, the actual absorber’s geometry is consistent with that given in this paper.

In the terahertz range, the dielectric permittivity of VO₂ can be represented using the Drude model [60,61]:

(3)$$\varepsilon (\omega ) = {\varepsilon _\infty } - \frac{{{\omega _p}^2(\sigma )}}{{{\omega ^2} + i\gamma \omega }},$$

where ${\varepsilon _\infty }$ = 12 is the dielectric permittivity at infinite frequency, $\gamma =$ 5.75 × 10¹³ rad/s is the collision frequency, and ${\omega _p}(\sigma )$ is the plasma frequency that depends on conductivity $\sigma$. The plasma frequency can be expressed as ${\omega _p}^2(\sigma ) = {\sigma / {{\sigma _0}}}{\omega _p}^2({\sigma _0})$, where ${\sigma _0}$ = 3 × 10⁵ S/m and ${\omega _p}({\sigma _0})$ = 1.4 × 10¹⁵ rad/s. To mimic the insulating-conducting phase transition properties of VO₂, we used different conductivities for different photoelectric properties of VO₂. In this study, $\sigma$ = 2 × 10⁵ S/m and $\sigma$ = 40 S/m indicate the conducting and insulating states, respectively. The structure of the VO₂ layer is obtained by cutting a central VO₂ ring patch in a continuous VO₂ patch, whose radius is R_v= 22 µm and thickness is t_VO2= 0.5 µm. Notably, the center of the cut VO₂ ring patch coincides with the center of the graphene ring sheet.

3. Results and discussions

For verification, we examined the unit cell of the proposed switchable metamaterial absorber and obtained its electromagnetic response. In FIT numerical simulations, two Floquet ports are assigned to the unit cell of the absorber in the z-direction, one as the source port and the other as the receiving port. Moreover, the periodic boundary conditions in the x and y directions are used to mimic an infinite array. To calculate the absorbance A of the proposed absorber, the transmission coefficient S₂₁ and reflection coefficient S₁₁ are obtained via FIT numerical simulations. The absorbance is calculated as A=1-R-T, where R = |S₁₁|² and T = |S₂₁|². Because the transmittance T can be approximated to 0 using the continuous metallic reflector, maximal absorption efficiency can be achieved by minimizing the reflection coefficient S₁₁.

The absorbance A for TE and TM polarizations under normal incident terahertz waves and Fermi energy level ${\mu _c}$ = 0.7 eV is shown in Fig. 2. The spectra of absorbance A for both TE and TM polarizations completely coincide, indicating an excellent polarization-insensitive property, which is attributed to the axisymmetric structure. When VO₂ is in the insulating state, the absorber exhibits broadband absorption with a 90% absorbance bandwidth of 1.52 THz, from 1.06 to 2.58 THz, for both polarizations. The perfect absorption is achieved at frequencies of 1.28 THz and 2.3 THz. Alternatively, when VO₂ is in the conducting state under the same geometry, the designed device acts as a multi-band absorber at the absorption frequencies of 1 THz, 2.45 THz, and 2.82 THz. As can be seen, the absorbance of the proposed multi-band absorber is relatively high from 2.45 THz to 2.82 THz. This is interesting, in the sensing system, this type of absorption spectrum has potential flexible applications. For example, the perfect absorption points of 1 THz, 2.45 THz and 2.82 THz can be used to accurately distinguish different objects, detect explosives and absorb surveillance radar, while the frequency band between 2.45 THz to 2.82 THz can be used for sensitivity analysis, terahertz filters and attenuators [16,62,63].

Fig. 2. Simulated absorbance of the proposed switchable metamaterial absorber under different VO₂ phase states for both TE and TM polarizations at ${\mu _c} = 0.7\textrm{ }eV$ under normal terahertz incidence.

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The electric field distributions are analyzed at different resonance frequencies to reveal the absorption mechanism of the proposed absorber. As shown in Fig. 3, the electric field distributions for TE polarization on the x-y and y-z planes are simulated under the conditions of Fermi energy level ${\mu _c}$ = 0.7 eV and normal incidence. We used a color map to denote the strength of the field. Figure 3(a) shows the top view and side view of the simulated electric field distributions of the proposed broadband absorber when VO₂ is in the insulating state at resonance frequencies of 1.28 THz and 2.3 THz. At these frequencies, the absorber achieves strong field confinement around the graphene ring sheet, which causes high-efficiency terahertz trapping and absorption. The electric fields are forcefully confined in the gap between the two graphene ring sheets at the resonance frequency of 1.28 THz and confined around the edges of the individual graphene ring sheet at 2.3 THz. In other words, the absorption at the 1.28 THz resonance frequency is mainly attributed to the coupling interaction between the graphene ring sheets, whereas that at 2.3 THz resonance frequency is mainly attributed to the fundamental resonance mode (the electric dipole resonance) of the individual graphene ring sheet [10]. These two resonance regions are superimposed to achieve broadband absorption. The electric field distributions of the side view prove the above statement. At 1.28 THz, due to the absorption caused by the coupling interaction between graphene ring sheets, there will be a lot of energy confinement at the edge of the periodic substrate. At 2.3 THz, there is only a small amount of energy bound at the edge of the substrate due to the absorption caused by the resonance of the individual graphene ring sheet. Additionally, this finding indicates that when VO₂ is in an insulating state, it is useless for absorption, and its existence does not affect the broadband absorption characteristics of graphene. Figure 3(b) shows the simulated electric field distributions of the proposed multi-band absorber when VO₂ is in the conducting state at the three resonance frequencies of 1 THz, 2.45 THz, and 2.82 THz. Similarly, the absorption at 1 THz resonance frequency is mainly attributed to the fundamental resonant modes of the graphene ring sheet and VO₂ hollow ring patch, indicating that the VO₂ patch also provides absorption in addition to that provided by the graphene sheet. The field distributions at 1.28 THz and 1.0 THz are similar, but the absorption principle is different. Because the electric field distribution of 1.28 THz is only provided by graphene, while that of 1 THz is provided by both graphene and VO₂. The electric field distributions at 2.45 THz and 2.82 THz are similar, which are confined in the gap between the graphene sheet and VO₂ patch. The two absorption points are mainly attributed to coupling interaction between the graphene ring sheet and VO₂ hollow ring patch. In the same way, the side view of the electric field distributions proves the above statement again. At 1 THz, energy confinement can be observed at the edge of the periodic structure due to the fundamental resonance mode of VO₂. While at 2.45 THz and 2.82 THz, the energy confined by the coupling interaction between VO₂ and graphene is inside the periodic structure and cannot be observed. Notably, because of the axisymmetric structure of the proposed absorber, the TM polarization electric field distributions of the broadband and multi-band absorber is the same as the TE polarization electric field distribution. However, the azimuth angle is rotated by 90°. Furthermore, the type of resonance is the plasmon resonance induced by the designed metamaterial structures, which can effectively enhance the absorption resulted from the Fabry-Perot cavity between the metamaterial structure and the reflector. The effective impedance can be calculated by the transmission coefficient S₂₁ and reflection coefficient S₁₁. In these absorption peaks, the effective impedance perfectly matches the free space impedance with the real part is equal to 1 and the imaginary part is equal to 0.

Fig. 3. Simulated front-view and side-view of the electric field distributions for TE polarization under the conditions of ${\mu _c} = 0.7\textrm{ }eV$ and normal incidence. (a) Electric field distributions of the proposed broadband absorber at 1.28 THz and 2.3 THz. (b) Electric field distributions of the proposed multi-band absorber at 1 THz, 2.45 THz, and 2.82 THz.

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Further, we studied the relationship between terahertz absorption behavior and polarization and incident angles. To simplify the analysis, we fixed Fermi energy level ${\mu _c}$ = 0.7 eV and other parameters as the initial values. Figure 4(a) and Fig. 4(d) show the absorption spectra based on the polarization angle under incident angle θ = 0° of the proposed broadband and multi-band absorber. The proposed absorber exhibits absolute polarization insensitivity with polarization angle φ ranging from 0° to 360°. This polarization-insensitive property is ascribed to the symmetrical structure of the designed metamaterial, which is an important property for terahertz absorbers. Figure 4(b) and Fig. 4(c) show the absorption spectra based on the incident angle θ of the proposed broadband absorber for TE and TM polarizations, respectively. When VO₂ is in the insulating state, the designed broadband absorber exhibits prominent properties with stable absorbance and operating bandwidths in a wide incident angle range of 0° to 60° for both TE and TM polarizations. Moreover, when the incident angle is greater than 60°, the absorption spectrum becomes sensitive to the incident angle. Figure 4(e) and Fig. 4(f) demonstrate the absorption spectrum based on the incident angle of the proposed multi-band absorber for TE and TM polarizations, respectively. The designed multi-band absorber exhibits excellent absorption performance in the first absorption band for both polarizations when VO₂ in the conducting state. For instance, the absorption spectrum with 90% absorbance peak is insensitive to incident angle up to 60°. To this end, the rest of the absorption bands also demonstrate stable absorbance performance in an incident angle range of 0° to 40° for both TE and TM polarizations. When the incident angle is greater than 40°, the absorbance decreases and generates undesirable resonance frequencies. Because as the incident angle θ increases, the tangential electric filed decreases accordingly, and the parasitic resonances of certain parts of the structure increases sharply. This wide-angle behavior is attributed to the absorption properties that are mainly related to the resonances producing the subwavelength structure of the proposed switchable metamaterial absorber, while being less dependent on the incident angle [10]. The subwavelength structure of the proposed absorber can compensate for the momentum mismatch between the metamaterial surface of the graphene-VO₂ and the terahertz wave from free space, thereby exhibiting excellent wide-angle behavior.

Fig. 4. Simulated absorption spectrum of the broadband (a-c) and multi-band (d-f) absorber as functions of frequency and the polarization angle φ and incident angle θ under the condition of ${\mu _c} = 0.7\textrm{ }eV$.

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Moreover, the impact of the geometry on the absorption performance of the broadband and multi-band absorber is discussed. To simplify the analysis, we retain Fermi energy level ${\mu _c}$ = 0.7 eV and the thickness of Topas t_topas= 27 µm, where initial t_topas is selected as a quarter wavelength and then slightly optimized to achieve the final value. The analysis is performed under the normal incidence of TE polarization. The absorption performance of the broadband absorber as functions of frequency and graphene radius (R_g), VO₂ radius (R_v), and VO₂ thickness (t_VO2) are shown in Fig. 5(a)-Fig. 5(c), respectively. When VO₂ is in the insulating state, the absorption performance of the broadband absorber is unaffected by the geometrical dimensions of the VO₂ hollow ring patch. In other words, VO₂ does not contribute to the broadband absorption performance when it is in the insulating state. The absorption performance of the absorber when R_g is increased is shown in Fig. 5(a). As R_g increases, the graphene area and absorption bandwidth also increase. Figure 5(d) –Fig. 5(f) show that when VO₂ is in the conducting state, the geometrical dimensions of the graphene ring sheet and VO₂ hollow ring patch have a significant influence on the multi-band absorbance performance. As R_g increases, the graphene area increases, the first resonance frequency moves to a lower frequency, and the second resonance frequency is relatively fixed. When R_g = 18 µm, the resonance frequency caused by graphene is adjacent with the fundamental resonance frequency of VO₂, the interaction between them forms the third perfect absorption frequency point. As shown in Fig. 5(e), when R_v is less than the initial R_g= 18 µm, the graphene is covered by VO₂ and only one resonance frequency is generated. It is obvious from Fig. 3(b) that, the electric field energy is mainly confined to the sub-wavelength structure between the edge of graphene and VO₂. When VO₂ covers the edge of graphene, it will hinder the absorption of the sub-wavelength structure, and most of the terahertz wave will be reflected. When R_v= 22 µm, optimal performance with three-band absorption is achieved. As the R_v increases, the VO₂ area decreases and cannot provide sufficient absorption intensity, so that the absorption value gradually decreases. Figure 5(f) shows that the absorption performance of multi-band absorber is generally unaffected by t_VO2 when it is greater than 0.5 µm, because this thickness is much greater than the skin depth of electromagnetic waves.

Fig. 5. Simulated absorption performance of the broadband (a-c) and multi-band (d-f) absorber as functions of frequency and graphene radius R_g, VO₂ radius R_v, and VO₂ thickness t_VO2.

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Next, to verify the adjustable absorption performance of the absorber using the bias voltage, we simulated the absorption spectrum as a function of frequency and Fermi energy level ${\mu _c}$ under normal incidence for TE polarization. As shown in Fig. 6(a), when VO₂ is in the insulating state, the broadband absorbance peak can be adjusted while maintaining the same center frequency from 20% to nearly 100% by changing ${\mu _c}$ from 0 eV to 0.7 eV, thus demonstrating excellent broadband absorption modulation above 80%. While when VO₂ is in conducting state, as shown in Fig. 6(b), due to the interaction between the fundamental resonance frequency of VO₂ and the gradually increasing absorption value of graphene, resonance frequencies for multi-band absorber are reconfigurable by changing the Fermi energy level. As ${\mu _c}$ increases from 0.5 eV to 1 eV, the first resonance frequency is reconfigurable from 1 THz to 1.1 THz with over 90% absorbance peak. For the second and third resonance frequencies, the reconfigurable resonance frequency with over 90% absorbance peak switchs from 2.2 THz to 2.65 THz and 2.55 THz to 3.16 THz, respectively. In summary, the broadband and multi-band absorber can be tuned by changing the Fermi energy levels. This absorber can be widely used in terahertz intelligent systems.

Fig. 6. Simulated absorption performances of the broadband (a) and multi-band (b) absorber as functions of frequency and the Fermi energy levels.

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Finally, the potential application of the metamaterial absorber in sensing is studied. According to the above analysis, the metamaterial absorber has a strong local field at the resonance frequency and is very sensitive to the changes of the surrounding medium. Therefore, in the implementation, placing objects above the absorber. Terahertz wave will pass through the objects and then incident inside the absorber. When the parameters of objects such as the permittivity change, the resonance frequency of the absorber will be shifted, and the offset of the resonance frequency can be obtained by theoretical calculation according to the electromagnetic perturbation theory. Therefore, based on the resonance frequency offset, the permittivity of the objects can be measured. However, the bottleneck restricting the industrialization of terahertz metamaterial absorbers is mainly related to the processing of devices, which is also a key point in the future research of terahertz metamaterials devices.

4. Conclusion

In conclusion, we proposed and demonstrated a switchable metamaterial absorber with broadband and multi-band absorption operating in the terahertz regime. Our absorber design is well structured and consists of the VO₂ hollow ring patch, graphene ring sheet, polysilicon sheet, Topas spacer, and continuous metallic reflector. By changing the phase transition property of VO₂, the switchable functional properties of the absorber can be achieved. When VO₂ is in the insulating state, a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz is achieved. The perfect absorption is achieved at frequencies of 1.28 THz and 2.3 THz. The broadband absorber demonstrates excellent absorption performance with 90% terahertz absorbance in a wide incident angle range of 0°−60° for TE and TM polarizations. The absorption bandwidth and intensity of the broadband absorber can be adjusted by changing the area or Fermi energy level of graphene. When VO₂ is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies of 1 THz, 2.45 THz, and 2.82 THz. The simulated electric field distributions show that the multi-band absorption is attributed to the fundamental resonance modes of the graphene ring sheet and VO₂ hollow ring patch, as well as the coupling interaction between them. Moreover, the multi-band absorber exhibits more than 90% peak absorbance in an incident angle range of 0°−40° for both TE and TM polarizations. By carefully optimizing the geometric parameters, optimal multi-band absorption characteristics of the absorber can be obtained. Additionally, the resonance frequencies of the multi-band absorber can be reconfigured by changing the Fermi energy level of graphene.

Feasibility of the proposed design and a possible fabrication procedure are discussed here. (1) Use chemical vapor deposition (CVD) to generate graphene on the copper film and use magnetron sputtering technique to fabricate VO₂, meanwhile, the metallic reflector can be deposited on the Si wafer by gilding. (2) Topas gum should be spin-coated over the metallic reflector to form an Au-Topas wafer, and use the same process to spin-coat Topas on the polysilicon. (3) Transfer the polysilicon-Topas wafer onto the Au-Topas wafer to obtain an Au-Topas-polysilicon-Topas substrate. (4) Coat polymethyl methacrylate (PMMA) gum on the graphene-copper sample, and use FeCl₃ solution to dissolve the copper from the copper-graphene-PMMA sample. (5) Transfer the graphene-PMMA sample onto the Au-Topas-polysilicon-Topas substrate, then use acetone solution to remove the PMMA gum to obtain an Au-Topas-polysilicon-Topas-graphene wafer. (6) Etch patterned graphene using reactive ion etching system, and finally, etch VO₂ film onto the patterned graphene to form the proposed absorber.

The switchable metamaterial absorber presented in this study shows the merits of bi-functionality, incident angle and polarization independence, a simple configuration, and is adjustable by changing the Fermi energy level of graphene. For the terahertz intelligent system, this bi-functional device is important, which can be used for objects imaging under the broadband absorption and objects distinguishing under the multi-band absorption. Using one device to achieve two different functions can greatly reduce the size of terahertz system. And this absorber is also very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Funding

National Natural Science Foundation of China (61871072).

Disclosures

The authors declare no conflicts of interest.

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Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption

Abstract

1. Introduction

2. Design and method

3. Results and discussions

4. Conclusion

Funding

Disclosures

References

Cited By

Figures (6)

Equations (3)

Optics Express