Numerous studies have been made to design switchable terahertz absorber for the application of amplitude modulator. However, it is still a challenge to achieve large modulation range while maintaining broad bandwidth. Here, we propose a switchable broadband absorber/reflector in the low-terahertz regime. By utilizing a hybrid graphene-gold metasurface on SiO2/pSi/PDMS substrate with an aluminum back, an excellent absorption across 0.53–1.05 THz with a wide incident angles for both TE and TM polarizations is achieved. By controlling the voltage across gold electrode and pSi, the chemical potential of graphene can be changed correspondingly. When the chemical potential of graphene varied from 0eV to 0.3eV, the state of the proposed structure can be switched from absorption (>90%) to reflection (>82%) over the whole operation bandwidth. Electric field intensity and surface loss density of the proposed absorber under different chemical potential are given to have a physical insight of the mechanisms. The switchable absorber/reflector can enable a wide application of high performance terahertz devices, such as active camouflage, imaging, modulators and electro-optic switches.
© 2017 Optical Society of America
Metamaterial absorbers (MAs) have been widely explored since the first theoretical and experimental demonstration presented by Landy et al. in 2008 . After that, different sets of MA designs have been demonstrated in almost every technologically relevant spectral range including optical , IR [3,4], terahertz (THz) , and millimeter-wave  bands. Since it is difficult to find strong frequency selective terahertz absorbers , the concept of MAs is especially important at THz frequencies. THz near-perfect MAs have many important technological applications, including sensors , thermal emitters , and imaging devices .
Switchable MAs, a kind of amplitude modulator, are metamaterial structures whose absorption can be tuned by external stimuli. Several tuning mechanisms, such as voltage biased diodes [11–13], micro-electro-mechanical system (MEMS)  and photoconductive semiconductor [15,16] have been reported in previous literatures to make MAs switchable. The switching intensity  (SI) of them are high while the operation bandwidth is narrow. Optically controlled active terahertz devices [17–19] could own high switching intensity and broadband simultaneously. However, externally excited laser irradiation is indispensable for these devices, which limits the scope of application in practice.
Graphene’s complex conductivity depends on the carrier density and can be controlled by gate voltage, electric field, magnetic field, and chemical doping . Hence, graphene can be employed to design switchable absorbers or amplitude modulators [21–23]. For example, Wang et al. fabricated a broadband modulator based on the ternary materials of organic polymer film irradiated with an external excitation laser. Jessop et al. investigated an amplitude modulator based on plasmonic bow-tie antenna arrays with graphene. Recently, an innovative electrically switchable absorber based on graphene is proposed , which provides a significant step in realization of active camouflage systems in microwave frequencies. These reports reveal that graphene is a promising material for switchable absorber, however, it is still a challenge to take high switching intensity and broad bandwidth into account simultaneously.
In this paper, we present a switchable broadband terahertz absorber/reflector through varying the chemical potential of graphene to achieve high switching intensity (72%) while maintaining broad bandwidth (0.53–1.05 THz). By utilizing a bias voltage between gold electrode and p-type silicon, the conductivity of graphene layer can be tuned easily, which is the key to realize the control of absorption. The state of the proposed MA can be switched between reflection (reflection>82%) and absorption (absorption>90%) at low-terahertz spectrum. A physical insight of the absorption principle and switching mechanism will be discussed. The proposed MA also exhibits well performance to large incident angles with a thin thickness (near 1/6 wavelength for mid-frequency).
2. Structure of the switchable absorber
The unit cell of the proposed switchable terahertz broadband absorber/reflector is shown in Figs. 1(a) and 1(b), which consists of six layers: an aluminum layer on the bottom, a lossy PDMS layer, a p-type doped silicon layer, a silicon dioxide layer, a monolayer CVD graphene layer, and finally a gold pattern layer. The gold pattern, with six equilateral triangles and arranged in a regular hexagon, will supply sufficient capacitance effect. The distance between different equilateral triangles and adjacent hexagons are g1 = 2μm and g2 = 90μm, respectively. Graphene patch is designed to be a regular hexagon with a side length of d2 = 60μm, and placed between silicon dioxide layer and gold pattern. The purpose of this design is to improve the use efficiency of graphene, reduce the absorption area in the reflective state, and lower the demand for the variation range of graphene conductivity. The combination of graphene and gold pattern can be regarded as a hybrid graphene-gold metasurface.
Gold (thickness t1 = 0.5μm) and aluminum (thickness t5 = 1μm) are modeled with a conductivity of 4 × 107 S/m and 3.8 × 107 S/m, respectively. Lossy PDMS layer with thickness t4 = 60μm and permittivity ɛ = 2.35 + i0.047  is used in our design. The dielectric property of the doped silicon (thickness t3 = 0.5μm) in the terahertz range can be described by a Drude model [26,27], typically written by27,28]. Silicon dioxide, with thickness t2 = 0.3 μm and permittivity ɛ = 4, is chosen as insulating layer between graphene and p-type silicon.
The monolayer CVD graphene is modeled as a thin, two-sided film characterized by a surface conductivity σ(ω, μc, Γ, T), where ω is radian frequency, μc is chemical potential, Γ is the phenomenological scattering rate that is assumed to be independent of energy ɛ, and T is temperature. The expression of surface conductivity is determined by Kubo formula [29,30]. We assume Γ = 1meV, a reasonable value according to [29,31].
As shown in Fig. 1(c), a gold feed line, with a width of g3 = 2μm, is employed throughout the structure to provide equivalent bias voltage for each unit cell. A small piece of rectangular graphene is used to connect feed line and gold pattern, and avoid the occurrence of unexpected resonance. By applying a gate voltage (a static electric field) on the electrode, the chemical potential or the conductivity of graphene can be controlled on purpose. A 3D simulation model based on the unit cell shown in Fig. 1(a) is used to investigate the behavior of the switchable absorber/reflector in the low-terahertz regime. Taking advantage of the inherent symmetry of the structure and to reduce simulation time, asymmetric and symmetric boundary conditions are applied in the x and y directions, respectively [32,33].
3. Performance and mechanism of switchable absorber
We assume a terahertz plane wave normally illuminate on the switchable absorber/reflectorwith polarization along the y direction. Based on the finite element method, the simulated absorption spectrum of the unit cell with periodic boundary is plotted in Fig. 2. The absorption is obtained by, where is transmission and equal to zero, due to a 1-μm-thick aluminum layer is used as a ground plane. is the reflectivity of the absorber. For the undoped and ungated case at T = 0K, which means the chemical potential μc = 0eV, absorptivity over 90% is obtained from 0.53THz to 1.05THz, with a wide bandwidth of 65.8%.
We further investigated the origin of the loss to understand the contributions of each part. The absorption spectra under lossless PDMS substrate is similar to the lossy one, the incident electromagnetic energy is not dissipated in the dielectric. Without taking into account the existence of the graphene layer or gold pattern, a maximum absorption of 2% or 19% across the frequency range of interest is obtained, respectively. The slot area, which is formed by two adjacent gold patches and has a perpendicular component to the electric field, will introduce a certain amount of capacitance effect. Graphene patch can be regarded as an impedance element, which will lower the Q factor and expand the impedance matching bandwidth with the existence of gold pattern. The good absorption effect is largely determined by the combined effects of gold pattern and graphene patch.
The thickness of the graphene membrane is small, only about 0.34nm, but it can be found from Fig. 3(a) that as the chemical potential μc (or Fermi energy EF) increases, the influence of the graphene patch on the absorptivity is significant. Varying the graphene chemical potential by using static electric field yields a way to tune the graphene conductivity. When T = 300K, Γ = 1meV, f = 0.8THz, and μc = 0eV, the sheet conductivity of graphene is 0.37-i0.61 mS, and the absorber is in the on state (absorption). When μc = 0.2eV, the graphene sheet conductivity is 2.1-i3.4 mS and the absorptivity is decreased to less than 30%. By using electrode/graphene/SiO2/pSi structure, as several literatures indicate, it is easily to change the graphene sheet conductivity of 5 to 6 times [22,23]. When the chemical potential increased to 0.3eV, the graphene sheet conductivity is 3.1-i5.1 mS, around 8 times bigger than the conductivity of μc = 0eV. The absorber is in the off state and the reflectivity is more than 82%, a high switching intensity (>72%) is obtained.
In Fig. 3(b), we plot surface loss density of the graphene patch at 0.8THz with two different chemical potentials, μc = 0eV and μc = 0.3eV. The graphene in the slot area, formed by the neighboring gold patches and has a perpendicular component to the electric field, bears the primary responsibility for the absorption of the incident energy.
For the high switching intensity, the possible reasons can be drawn from Fig. 3(b). Since electric field is strongly concentrated in the slot area, the graphene patch in the slot area have a great significant influence on absorptivity. The sheet conductivity of graphene can be varied around 8 times as the chemical potential increases from μc = 0eV to μc = 0.3eV. When μc = 0.3eV, the valuable interaction between graphene patch and gold pattern which dominantly contributes to the absorption is noticeably damped, the excellent impendence matching is destroyed. As a consequence, the absorption is diminished to 16%. Through biasing at different voltages to turn ON and OFF the proposed absorber, we are able to switch the structure between reflection (reflection>82%) and absorption (absorption>90%) at low-terahertz spectrum.
Figures 4(a) and 4(d) give the central side view of the simulated electric field intensity distributions at 1THz, using the coordinate systems shown in Fig. 1(a). Electric field direction of normal incident wave in Fig. 4 is along with Y axis.
As Figs. 4(a) and 4(b) (μc = 0eV) illustrated, the incident electric field is strongly concentrated in the slot area formed by the neighboring gold triangle patches. Figure 4(a) also verifies the existence of capacitance effect in the slot area, which will cooperate harmoniously with the impedance effect introduced by graphene. When the phase of the incident wave is increased from 0° to 90°, the wave peak of the electric field is moving towards the absorber. Obviously, a traveling wave exists above the absorber when μc = 0eV. In Figs. 4(c) and 4(d) (μc = 0.3eV), for the fixed wave peak and the reduced field strength, standing wave is formed and the absorber is switched into a reflector successfully.
Figure 5(a) demonstrate the absorption of the proposed absorber for different polarization angles in the normal incidence case, which reveals that the proposed absorber is polarization insensitive. The C6 symmetric unit cell structure of the MA is the inherent reason for the excellent polarization independent.
We have also investigated the robustness under oblique incidence. The incident wave was modeled as a Floquet port above the unit cell so that both TE and TM polarizations can be easily obtained. For clarity, TE and TM polarization are defined as follows. The wave vector k of the incident light is in the xoz or yoz plane (for TE or TM) and the electric field is in the x direction (TE) or in the yoz plane (TM). The simulated absorption spectra for incident angles up to 70° of the designed structure are plotted in Figs. 5(b) and 5(c) for the TE and theTM polarizations, respectively. The prominent broadband and large-angle working capabilities are evidenced by these two figures. For the TE polarization, the ultra-broadband absorption performance (absorption >80%) is maintained up to 58°. For the TM polarization, the angle limit is 59°. Further investigating Figs. 5(b) and 5(c), we notice that the frequency of the absorption has a slight blue-shifting with the increase of incident angle, for both TE and TM polarizations.
In summary, by depositing the hybrid graphene-gold layout on the SiO2/pSi/ PDMS substrate backed with metallic ground, the switchable properties of the proposed terahertz broadband absorber/reflector have been investigated and discussed. Through altering bias voltage, the chemical potential (Fermi level) of the graphene patch can be controlled. When the chemical potential of graphene varied from 0eV to 0.3eV, we are able to switch the structure between reflector (reflectivity 82%) and absorber (absorptivity 90%) in a wide frequency range (0.53-1.05 THz). Numerical results indicate that our design absorber can fulfill broadband absorption over a wide range of incident angles up to 58° for both TE and TM polarization waves. This switchable broadband reflector/absorber offers a new way for active camouflage systems, photo detector, THz imaging, energy harvesting, beam steering antenna, and dynamic light modulators.
National Key Research and Development Program of China (2016YFB0402505); National Basic Research Program of China (2015CB352100); National Key Laboratory Foundation; National Natural Science Foundation of China (NSFC) (61271017, 61675191, 61178051, 61335010, 11404239); Natural Science Basic Research Plan in Shaanxi Province of China (2013JZ019); Fundamental Research Funds for the Central Universities.
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