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Abstract
We propose a dual-controlled switchable broadband terahertz (THz) metamaterial absorber based on a hybrid of vanadium dioxide (VO2) and graphene that demonstrates strong polarization-independent characteristics and works well at a wide range of incidence angles. The peak absorptance of the proposed absorber can be tuned from 26 to 99.2% by changing the Fermi energy of the graphene; the absorptance can be dynamically tuned from 9 to 99.2% by adjusting the conductivity of the vanadium dioxide because of its unique insulator-to-metal transition characteristic. Using these two independent controls in tandem, we found that the state of the proposed absorber can be switched from absorption (>96%) to reflection (>73.5%), and the transmittance can be tuned from 0% to 65% while maintaining broad bandwidth (1.05-1.6 THz), resulting in a better-performing switchable broadband terahertz absorber. Furthermore, we have provided a discussion of the interference theory in which the physical mechanism of the absorption is explained from an optical point of view. The absorber achieves dual-controlled absorptance switching via two independently controllable pathways, offering a new method for switching and modulation of broadband THz radiation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The terahertz (THz) frequency range, which is located between the infrared and microwave bands, is of significant interest to researchers, having potential applications in a wide range of fields, including security systems, biological imaging, and the development of the future sixth-generation wireless network [1–3]. However, most natural materials exhibit weak electric and magnetic responses at THz frequencies [4], a phenomenon known as the “THz gap” [5]. Metamaterials (artificial electromagnetic composites composed of periodically arranged subwavelength units [6–7]) are a potential solution to overcoming the THz gap [8], and their usefulness in manipulating THz radiation based on their electromagnetic properties has been intensively studied. Metamaterials are regarded as ideal platforms for achieving THz-functional devices, such as THz modulators, polarization convertors, and wavefront controllers [9–13]. In particular, THz metamaterial absorbers have attracted a high amount of attention for potential applications in sensing, imaging, and detecting systems [14–17]. Since the first perfect metamaterial-based signal-band absorber was demonstrated by Landy et al. in 2008 [18], a variety of THz metamaterial absorbers, including single-, dual-, multi-, and broadband, have been investigated [19–23]. However, such absorbers are generally based on metallic structures, which suffer from fixed and narrow working bandwidths. This inherent drawback of post-fabrication non-adjustability has been unfavorable to the development of many practical applications.
The non-adjustability problem has been addressed over the past few years by a large amount of research aimed at developing techniques for dynamically controlling the response of metamaterial-based absorbers [24]. Owing to its unique mechanical, electromagnetic, and optical properties, graphene is considered a prime candidate for the development of tunable THz metamaterial [25–28]. Significantly, the sheet conductivity of graphene can be dynamically changed by adjusting its Fermi energy through chemical doping or the application of a bias voltage [29–31], making graphene one of the most promising materials for the construction of tunable THz metamaterial absorbers [32–36]. Several researchers have investigated graphene as a tunable THz metamaterial: Amin et al. investigated the first ultra-broadband graphene THz absorber, which obtained 6.9 THz bandwidth for 90% absorption [34]; Yao et al. proposed a tunable dual-band polarization-insensitive THz-range metamaterial absorber comprising a graphene elliptical nanodisk array and a metal ground plane spaced by a SiO2 dielectric layer [35]; and Ye et al. demonstrated a tunable polarization-insensitive multi-band absorber with six near-unity absorption bands from 0.1 to 10.0 THz [36].
Recently, vanadium dioxide (VO2), a metal oxide that undergoes an insulator-to-metal transition via electrical, thermal, and optical pathways [37–38], has been implemented in metamaterial functional devices [39–45]. The conductivity of VO2 can be changed by five orders of magnitude during transition [46], enabling a large dynamic range and a high modulation depth of absorber following the combination of metamaterials with VO2. Song et al. proposed a broadband tunable THz metamaterial absorber with a sandwiched nanostructure of metal cross/ SiO2 spacer/ VO2 film that achieved a large shift in the resonant absorptance peak from 30 to 100% [47]. Liu et al. demonstrated a broadband tunable metamaterial-VO2 THz absorber that achieved tunable absorption in a range of 5 to 100% [48]. Overall, graphene and VO2 are both promising candidates for achieving dynamically tunable metamaterial absorber in the THz range. However, most studies have reported on the combination of metamaterials with either graphene or VO2 alone, which results in only one available pathway for controlling absorber response.
In this paper, we propose a polarization-independent broadband THz absorber that combines metamaterials with both graphene and VO2 to achieve dynamically dual-controlled switchable absorptance. The proposed device is a simple sandwich structure comprising a single graphene concentric double square-ring array layer and a VO2 film layer, separated by a SiO2 layer. The double graphene square rings couple with each other to form hybridized plasmonic modes that produce broadband THz wave absorption. Our simulation results demonstrated that the peak absorptance of the proposed absorber can be tuned from 26 to 99.2% by changing the Fermi energy of the graphene under normal incidence from 0.01 to 0.5 eV. By taking advantage of the unique insulator-to-metal transition characteristic of VO2, a maximum tunable range of peak absorptance of 9 to 99.2% can be achieved by increasing the conductivity of the VO2, obtaining a continuous decline in the reflectance and transmittance of the absorber. Moreover, through the use of a bottom layer of VO2, the transmittance of the proposed absorber can be further modulated relative to a conventional tunable absorber with a metal film mirror. Thus, the proposed device can be used as a broadband attenuator or modulator in both absorption or transmission mode. Numerical results show that the state of proposed device can be switched between absorption (>96%) and reflection (>73.5%), and the transmittance can be tuned from 0% to 65%, by simultaneously using these two independent methods of control. The proposed broadband THz absorber also demonstrates a strong polarization-independent characteristic arising from the square-ring symmetry of the unit cell. To quantitatively validate the proposed absorber, we applied interference theory to analyze the numerical simulation results; the simulation absorptance spectra was consistent with the theoretically calculated results. The results of this study could lead to new pathways for the development of tunable devices in the THz and other frequency domains.
2. Design and simulation
Figure 1 shows a schematic of the proposed graphene- and VO2-based THz metamaterial broadband absorber geometry. The top layer of the unit cell comprises double graphene square rings patterned on an SiO2 layer. Unlike most conventional graphene-based metamaterial absorbers, which comprise a three-layer structure with a top graphene structure and a dielectric layer backed by a metal film mirror, the proposed structure uses VO2, a common phase-change material, as the bottom reflective layer. By transforming the VO2 from an insulating dielectric state to a conductive full metallic state, the bottom layer takes on reflective characteristics, allowing the structure to function as a conventional perfect metamaterial absorber. The VO2, graphene, and SiO2 layers have thicknesses of 0.7 µm, 1 nm, and 28 µm, respectively. The substrate can be considered a lossless SiO2 spacer with relative permittivity ${\varepsilon _{si{o_2}}} = 3.9$ [33,35]. The graphene layer comprises an inner and an outer square-ring resonator with widths and side lengths of (win = 0.5 µm, Lin = 3.8 µm) and (wout = 2 µm, Lout = 6.8 µm), respectively. The proposed structure was investigated numerically by applying the finite difference frequency domain method using the commercial CST Microwave Studio simulation software. The unit cell boundary condition was employed along the x-y directions, with Floquet ports used as the two sides along the z-direction. Because of the fourfold symmetry, the proposed broadband absorber exhibits equal responses for the electric polarization along the x-axis and y-axis. Therefore, we defaulted to the y-polarized incident waves, which are vertically illuminate on the device. By applying the S parameters in the simulation, the absorptance was calculated as $\textrm{A} = 1 - R - T = 1 - {|{{S_{11}}} |^2} - {|{{S_{21}}} |^2}$, where $R = {|{{S_{11}}} |^2}$ and $T = {|{{S_{21}}} |^2}$ represent the reflectance and transmittance, respectively.
Fig. 1. Schematic of graphene- and VO2-based metamaterial broadband absorber geometry. P = 15 µm, Lin = 3.8 µm, Lout = 6.8 µm, win = 0.5 µm, wout = 2 µm, d = 28 µm, and h = 0.7 µm.
In the simulation, the graphene concentric double square rings were modeled as an effective medium with the relative complex permittivity ${{\varepsilon }_\textrm{g}}(\omega )= 1 + \textrm{i}{\sigma _g}(\omega )/{\varepsilon _0}\omega {\textrm{t}_g}$ [49], where ${\sigma _\textrm{g}}$ is the surface conductivity of graphene, ${\varepsilon _0}$ is the permittivity of the vacuum, and ${\textrm{t}_g}$ is the thickness of the graphene. For the THz region, the Fermi energy is much greater than the half of photon energy $\; \hbar {\omega } \ll {E_F}$, and the interband contribution can be neglected when comparing to the intrabancontribution; therefore, the surface conductivity can be simplified to a Drude-like model [49]:
Within the THz range, the relative permittivity of VO2 can be expressed by the Drude model [40]:
The samples of the proposed absorber were fabricated using the following method. First, graphene was transferred to one side of a quartz crystal. Then, a photoresist structure with the same microstructure as the graphene was produced by performing photolithography and development operations on the graphene side. Next, oxygen-plasma technology was used to etch the graphene that was not protected by photoresist. After that, graphene microstructure on the quartz substrate was obtained by dissolving the photoresist in an organic solvent. Finally, molecular beam epitaxy technique was used to grow vanadium dioxide on the other side of the quartz substrate.
3. Results and discussion
To demonstrate the absorption capability of the proposed graphene-VO2 based structure, the absorptance spectra of three different structures were calculated and analyzed (Fig. 2). Owing to the symmetry of the proposed structure, we considered only the case in which y-polarized THz waves illuminated the device. The first structure we examined comprised only an inner square ring, the second had only an outer square ring, and the third had a combined double square-ring structure. To reflect the typical three-layer metamaterial absorber structure comprising a metal ground plane and dielectric and metasurface resonance layers, we assumed that the VO2 was in a fully metallic state with conductivity ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$, a value consistent with the typical metamaterial absorber structure. The calculated absorptance spectra of the respective structures, with the Fermi energy of the graphene fixed at 0.5 eV, are shown in Fig. 2. The blue line in Fig. 2(b), corresponding to the outer ring-only structure, shows a resonance peak at approximately 1.06 THz, which corresponds to the low-frequency absorption peak of the double square-ring structure in Fig. 2(c). Similarly, the inner ring -only structure generates a resonance absorption peak at 1.47 THz, which is close to the high frequency end of the broadband absorption peak. Thus, combining the two single square-ring structures to obtain a double square-ring structure results in a high broadband absorptance above 90% as a result of the coupling effect between the two rings.
Fig. 2. Calculated absorptance spectra and top view of corresponding geometries for structures with (a) inner square ring only, (b) outer square ring only, and (c) double square rings (EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$).
To obtain better insight into the physical mechanism behind the broadband absorptance in the proposed broadband THz absorber, we investigated the electric field distributions of the standalone inner-, standalone outer-, and double-square-ring structures at their corresponding absorption peaks. As shown in Fig. 3, the positive and negative induced charges are accumulated in the direction of the electric field on opposite sides of the separated inner and outer square-ring structures at 1.47 and 1.06 THz, respectively, and the absorption peaks in Figs. 2(a) and (b) are generated at the corresponding frequencies as a result of the electric dipolar responses. In addition, the dipolar modes of the inner- and outer-square-rings can couple to each other due to the double-square-rings being close in the unit cell. Furthermore, the low- and high-frequency absorption peaks of the double-square-ring structure exhibit different coupled plasmonic modes. The electric field distribution on the double-square-ring structure shows two plasmonic modes, namely, + − + − and + + − − (“+” and “−” represent the induced charges at the inner and outer interfaces of the square ring); therefore, it can be deduced that the low-frequency hybridized mode is formed by two anti-parallel dipolar modes while the high-frequency hybridized mode is formed by two parallel dipolar modes. These hybridized plasmonic modes, which are formed by the coupling of the dipole modes of the inner and outer square rings, enable broadband absorptance in the THz range.
Fig. 3. Calculated electric field distributions of standalone inner ring, standalone outer ring, and double-square-ring structures at their corresponding absorption peaks (frequencies are indicated below the respective distributions).
Based on the properties of the VO2 transition from insulating to metallic states, the absorptance can be dynamically controlled by changing the conductivity of the bottom VO2 layer. It is seen from Figs. 4(a) and (b) that as the conductivity of the VO2 increases while at a fixed graphene Fermi energy of 0.5 eV, the reflectance and transmittance of the proposed absorber both decrease. In addition, when the VO2 is in the fully metallic state, with a conductivity of ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$, the transmission spectrum of the broadband absorber remains constant with a frequency that is close to zero (Fig. 4(b)), while the reflectance value reaches minimum (Fig. 4(a)). In this state, the structure is similar to the three-layer structure of a classical perfect metamaterial absorber. As the VO2 transitions to an insulator state, the reflectance and transmittance spectra undergo noticeable shifts. As shown in Fig. 4(c), the absorptance of the broadband absorber at peak frequencies can be continuously tuned from 9 to 99.2% by increasing the conductivity from 10 to 200000 S/m. In the VO2 insulator state, the absorptance of the broadband absorber is very close to zero. As the VO2 metallic properties increase, the rising conductivity leads to a continuous increase in absorptance. Once the VO2 assumes a fully metallic state, the broadband absorber can be considered a perfect absorber with high broadband absorptance in the THz range. To reveal the mechanism of how changing the conductivity of VO2 achieves this phenomenon, Fig. 5 shows the real and imaginary parts of VO2 permittivity as a function of frequency under varying conductivity. The real and imaginary parts of permittivity change with the conductivity of VO2 (from 10 to 200000 S/m) and the imaginary part changes more than the real part. Therefore, the center frequency of the absorption peak is essentially consistent, but the intensity of the absorption spectrum differs significantly due to the shift of the imaginary part of permittivity. As the conductivity of the VO2 increases, the metallic behavior increases, and with it, the absorption. This ability to dynamically control the reflectance, transmittance, and absorptance of the proposed broadband absorber by changing the conductivity of the VO2 represents an improvement over previously developed tunable absorbers, which can only tune for reflectance and absorptance. As a result of this flexibility, the proposed device can be used as a broadband attenuator and modulator at THz frequencies in either absorption or transmission mode.
Fig. 4. Calculated (a) reflectance, (b) transmittance, and (c) absorptance spectra of the proposed absorber at various VO2 conductivities and a fixed graphene Fermi energy of 0.5 eV.
To illustrate the effect of VO2 thickness on absorptance, the absorptance at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$ under different VO2 thicknesses was investigated (Fig. 6). It is obvious from the figure that the absorptance of the proposed absorber gradually increases as the thickness of the VO2 increases, although at a gradually diminishing rate. The absorptance reaches a maximum at h = 0.7 µm, after which it remains essentially unchanged with further increases in thickness.
Fig. 6. Calculated absorptance spectra at varying VO2 thickness (h) for EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$.
Graphene is also a tunable material whose surface conductivity can be altered by varying the Fermi energy. To demonstrate the broadband reflectance and absorptance adjustability of the proposed broadband absorber, we altered the graphene Fermi energy at a fixed VO2 conductivity of ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$. Figure 7 shows that the peak absorptance of the absorber can be tuned from 26 to 99.2% by adjusting the Fermi energy under normal incidence from 0.01 to 0.5 eV.
Fig. 7. Calculated (a) reflectance and (b) absorptance spectra of the proposed absorber at graphene Fermi energies ranging from 0.01 to 0.5 eV under a fixed VO2 conductivity of ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$.
Next, we analyzed the reflectance, transmission, and absorption spectra of the proposed absorber at various VO2 conductivities when the graphene Fermi energy was fixed at 0.01 eV, as shown in Fig. 8. Figure 8(a) and (b) illustrate the VO2 transitioning from an insulator state to a metallic state with increasing conductivity, while the transmittance gradually decreases and the corresponding reflectance first decreases and then increases. Therefore, the calculated absorptance spectra increases first and then decreases, staying between 0% and 79.5% with an increase in VO2 conductivity. When the VO2 conductivity is 10 S/m, the transmittance is at its highest value; the reflectance is also high, so that the absorption is nearly zero. Thus, by applying these independent controls in tandem, we can conclude that the proposed device is able to be switched from absorption (>96%) to reflection (>73.5%) and the transmittance can be tuned from 0% to 65% in a wide frequency range (1.05-1.6 THz).
Fig. 8. Calculated (a) reflectance, (b) transmittance, and (c) absorptance spectra of the proposed absorber at various VO2 conductivities and a fixed graphene Fermi energy of 0.01 eV.
We next investigated the absorptance tunability of the proposed broadband absorber by altering the two independently controllable parameters, VO2 conductivity and Fermi energy, to achieve a wider range of absorptance. As shown in Fig. 9(a), by setting the Fermi energy and VO2 conductivity at EF = 0.01 eV and ${\sigma _{v{o_2}}} = 10\; \textrm{S}/\textrm{m}$, respectively, the absorptance spectra of the proposed absorber reaches nearly 0%. In addition, when the VO2 is in the fully metallic state with a conductivity of ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$ and a Fermi energy of EF = 0.5 eV, the absorptance spectrum exhibits a high absorption at broadband frequency from 1.05 Thz to 1.6 Thz. Thus, we can achieve a tunable wide absorptance range from 0 to 99.2% by simultaneously controlling these two independent parameters. Furthermore, Fig. 9(b) shows the side view of the electric field distributions with Fermi energy and VO2 conductivity fixed at EF = 0.5 eV, ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$ and EF = 0.01 eV, ${\sigma _{v{o_2}}} = 10\; \textrm{S}/\textrm{m}$, respectively. A strong electric field is concentrated in the interface between the substrate and the graphene, which indicates that graphene plasmon resonances can enhance the absorption of the proposed absorber, while the electric fields are practically diminished to zero when the VO2 is in the insulator state (and EF = 0.01 eV) at 1.05 THz. This wide range of adjustable absorptance results in a better-performing switchable broadband terahertz absorber. Here, it can be seen from the blue line and green line in Fig. 9(a) that the proposed absorber still has a low absorptance with Fermi energy and VO2 conductivity fixed at EF = 0.01 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$, respectively, and at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 10\; \textrm{S}/\textrm{m}$.
Fig. 9. (a) Calculated absorptance spectra of the proposed absorber system for different fixed values of the Fermi energy, EF, and VO2 conductivity, ${\sigma _{v{o_2}}}$, as designated in the legend of the figure. (b) Side view of electric field distributions at 1.05 THz.
To get further insight into the physics of the above phenomenon, we introduced the impedance matching theory [54], which discusses the S-parameter retrieval method. The relative impedance can be obtained using
Fig. 10. Real part and imaginary part of the relative impedance with Fermi energy and VO2 conductivity fixed at (a) EF = 0.01 eV, ${\sigma _{v{o_2}}} = 10\; \textrm{S}/\textrm{m}$; (b) EF = 0.01 eV, ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$; (c) EF = 0.5 eV, ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$, and (d) EF = 0.5 eV, ${\sigma _{v{o_2}}} = 10\; \textrm{S}/\textrm{m}$.
The proposed structure was also found to exhibit polarization-independent behavior as a result of the symmetry of its structure. Figure 11(a) shows the influence of the polarization angle on absorptance spectra under the normal incident case for EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$. The absorptance remains highly consistent as the polarization angle, θ, is varied from 0 to 90°. For further explanation of this effect, we calculated the electric field distributions when the polarization angle θ = 0°, 15°, 30° and 45°, as shown in Fig. 11(b). The hybridized plasmonic mode is stably excited at different polarization angles, which demonstrates the polarization-independent quality of the absorber. Because the polarization angle typically is randomly oblique under practical conditions, this polarization-independent property is particularly important in absorbers and enhances the usability of the proposed broadband absorber in numerous devices.
Fig. 11. (a) Calculated absorptance color map of the broadband absorber with respect to θ, the angle between the polarized electric field and the x-axis of the structure, at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$. (b) Calculated the electric field distributions at 1.05THz for θ = 0°, 15°, 30°, and 45°.
We also studied the properties of absorptance under oblique incidence for both transverse electric (TE) and transverse magnetic (TM) polarized illumination. Figure 12 shows the absorptance of the proposed device as a function of incidence angle and frequency as the incidence angle is varied from 0 to 80° at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$. Desirable values of absorptance are maintained up to incidence angles of 50 and 60° under TE and TM mode oblique incidence, respectively.
Fig. 12. Calculated color maps of absorptance as a function of incidence angle and frequency under incidence angles ranging from 0 to 80° for (a) TE and (b) TM polarized illumination at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$.
Our results are best discussed in the context of two recent works involving dual or triple control of absorptance. The Ref. [55] was the first study on the polarization-dependent hBN∕graphene∕hBN∕VO2 multilayer system with bi-tunable and bi-functional characteristics at the mid-infrared wavelengths. When it is in the perfect resonant absorption functionality, the frequency of the resonant absorption response can be tuned by changing the chemical potential of graphene, and the absorptance amplitude can only be adjusted in a small range by varying the metallic fraction coefficient of VO2. In addition, the researchers in Ref. [56] proposed a tricontrollable metasurface with graphene and InSb pixel patches which can achieve the maximum-absorptance frequency controllable by thermal, magnetic and electrical approaches. Although these works presented dual and triple control of the absorptance, they all focused on the tuning of the absorptance frequency or absorptance amplitude in a small range. However, our designed absorber achieves a widened range of dynamically tunable absorptance amplitude and has switching functionality by utilizing dual control in the THz range. Meanwhile, the structure of our present work shows the strong polarization-independent property and function over a wide range of incidence angles.
4. Comparison with the interference theory
To better understand the physical mechanism of absorption occurring in the proposed broadband absorber, we applied interference theory [20,57–59] to emulate the structure as the asymmetric Fabry-Perot cavity, as shown in Fig. 13(d). We limited our analysis to the case in which VO2 was in a fully metallic state with a conductivity of ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$, which can be assumed to correspond to a perfect absorber. As a result, r23 = −1, and there was no THz wave transmission through the structure. In this model, the overall reflection can be regarded as a superposition of multiple reflections between the top and bottom layers as follows:
Fig. 13. (a) Amplitude and (b) phase of reflection and transmission coefficients at the broadband metamaterial absorber interface (the t21 nearly coincides with t12, and the θ21 is nearly coincides with θ12). (c) Comparison of absorptance spectra produced by theoretical calculation and numerical simulation with the parameters of the structure in Fig. 1 applied at EF = 0.5 eV and ${\sigma _{v{o_2}}} = 200000\; \textrm{S}/\textrm{m}$. (d) Schematic of the multiple reflection and interference model.
We can use this multiple reflection and interference model to analyze the broadband metamaterial absorber from an optical approach. The amplitude and phase of the reflection and transmission coefficients calculated by the model under normal incidence are shown in Figs. 13(a) and (b), respectively, and a comparison of the absorptance spectra produced by theoretical calculation and numerical simulation is provided in Fig. 13(c), from which it is apparent that the simulated absorptance spectra are essentially consistent with the theoretical calculation results.
5. Conclusions
In this study, we numerically investigated a polarization-independent broadband THz absorber in which metamaterials were combined with both graphene and VO2. The absorptance of the proposed device can be tuned from 26 to 99.2% by varying the graphene Fermi energy and from 9 to 99.2% by increasing the conductivity of the VO2. More importantly, by utilizing dual control in the THz range, the state of the proposed device can be switched from absorption (>96%) to reflection (>73.5%) and the transmittance can be tuned from 0% to 65% in a widened range (1.05-1.6 THz). The broadband absorber also has excellent absorption properties over a polarization angle ranging from 0 to 90° and functions well over a wide range of incidence angles. Comparison of the obtained numerical results with the interference theory was carried out and they agreed well. The dynamic tunability properties and large-angle functionality in terms of the polarization and incidence angle suggest that the proposed absorber can be effectively used in THz devices such as tunable sensors and modulators.
Funding
National Natural Science Foundation of China (61875106, 61775123); Key Technology Research and Development Program of Shandong (2019GGX104039, 2019GGX104053); Shandong University of Science and Technology Top-Notch Talent Project for Young Teachers (BJRC20160505) Shandong Graduate Student Tutor Guidance Ability Promotion Program Project (SDYY17030); National Key Research and Development Program of China (2017YFA0701000).
Acknowledgments
Thanks are due to John F. O’Hara for valuable discussion.
Disclosures
The authors declare no conflicts of interest.
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