We demonstrate that a broadband terahertz absorber with near-unity absorption can be realized using a net-shaped periodically sinusoidally-patterned graphene sheet, placed on a dielectric spacer supported on a metallic reflecting plate. Because of the gradient width modulation of the unit graphene sheet, continuous plasmon resonances can be excited, and therefore broadband terahertz absorption can be achieved. The results show that the absorber’s normalized bandwidth of 90% terahertz absorbance is over 65% under normal incidence for both TE and TM polarizations when the graphene chemical potential is set as 0.7 eV. And the broadband absorption is insensitive to the incident angles and the polarizations. The peak absorbance remains more than 70% over a wide range of the incident angles up to 60° for both polarizations. Furthermore, this absorber also has the advantage of flexible tunability via electrostatic doping of graphene sheet, which peak absorbance can be continuously tuned from 14% to 100% by controlling the chemical potential from 0 eV to 0.8 eV. The design scheme is scalable to develop various graphene-based tunable broadband absorbers at other terahertz, infrared, and visible frequencies, which may have promising applications in sensing, detecting, and optoelectronic devices.
© 2017 Optical Society of America
Broadband terahertz absorbers have recently attracted increasing interests for their promising applications in terahertz trapping [1,2], sensing [3,4], imaging [5,6], and detecting . The absorbers can be realized by using various structures like layered Salisbury screen and periodic structures . The physical mechanism for achieving high absorption is that the electromagnetic fields need to be strongly confined inside the lossy materials. Graphene, as a material of single-layered carbon atoms arranged in a plane with honeycomb lattice, has excellent mechanical, chemical, and electrically tunable properties, which offer many interesting possibilities for terahertz and optical technologies. Especially, with the ability to support surface plasmons in the terahertz and infrared ranges, graphene has been applied in a wide range of terahertz and optical devices, such as polarizers , photodetectors [10,11], hyperlenses , modulators [13,14] and absorbers [15–17]. It is found that a single sheet of undoped graphene is highly transparent with the light absorbance of 2.3% [8,18]. To enhance the light confinement and transform this poorly absorbing graphene into a perfect absorber, various graphene absorbers with periodic structures including arrays of disks [8,11], ribbons [19–21], cross-shaped structures , as well as graphene and metal combined structures  have been proposed. However, these absorbers relying on the resonant structures of unit cells are usually very narrowband, which greatly limits their further applications in optoelectronic devices.
To achieve broadband absorption, various structures have been investigated. As one class of the typical structures, multi-resonator graphene structures [24,25] have been proposed to realize broadband absorbers. With small size different multiple resonators within the unit cell, the small frequency different resonances of the absorbers will be merged to form broadband absorption. As another kind of the effective structures, multi-layered graphene absorber structures have been proposed to develop some ultra-broadband graphene absorbers at terahertz frequencies [26,27]. Recently, Zhu et al. have demonstrated a broadband terahertz absorber based on single-layered elliptical graphene ribbons with 43.3% normalized bandwidth of 90% absorbance . By using the isolated ribbon structure with gradient width, it has obviously broader absorption bandwidth compared with the conventional absorbers with fixed ribbon width. Besides, broadband terahertz absorbers based on the periodic structures with graphene-based hyperbolic metamaterials have also been studied . However, most of the above mentioned graphene-based broadband absorbers with multi-resonator, multi-layered, independent ribbon and hyperbolic metamaterial structures have the drawback of incident angle or polarization dependence . Meanwhile, the absorbers with complicated multi-resonator, multi-layered structures not only demand extremely fine device fabrication technique but also result in absorption tuning difficulty via voltage gating structures. Therefore, despite recent progress, novel polarization insensitive and wide-angle broadband absorbers with continuous single-layered graphene still remain to be further investigated.
In this paper, a new net-shaped periodically sinusoidally-patterned single-layered graphene-based absorber is proposed to achieve nearly 100% broadband terahertz absorption. By introducing such a unique gradient width modulation of the unit graphene sheet structure, the continuous plasmon resonances of the absorber can be directly excited and broadband terahertz absorption can be achieved. Under normal incidence in both TE and TM polarizations, over 65% normalized bandwidth of 90% terahertz absorbance can be obtained. As one of the most exciting characteristics, the broadband absorption spectra of this absorber are insensitive to the incident angles and the polarizations. The absorbance remains more than 70% even the incident angles reach 60° for both polarizations. Furthermore, with respect to conventional multi-resonator or multi-layered structures, the continuous net-shaped single-layered graphene structure can greatly simplify the electrostatic gating structure in achieving flexible tunability. By controlling the chemical potential via electrostatic doping of the graphene sheet, the peak absorbance can be continuously tuned from 14% to 100%. This work offers a new perspective on the design of graphene-based tunable broadband terahertz absorbers. It is remarkable that this absorber design scheme can be easily scaled up to other terahertz, infrared or visible regimes for various promising applications in sensing, detecting, and optoelectronic devices.
The paper is organized as follows. Section 2 describes the design and simulation methods of the proposed absorber. Section 3 discusses the key terahertz absorption characteristics of the absorber including absorption spectra, field distributions, geometrical and electrostatic tunabilities. Finally, conclusions are drawn in Section 4.
2. Design and simulation methods
The proposed graphene-based broadband terahertz absorber is composed of a net-shaped periodically sinusoidally-patterned graphene sheet on the top, a lossless dielectric spacer in the middle, and a metallic reflecting plate on the bottom, as depicted in Fig. 1. A thin gating layer is placed beneath the graphene sheet to control the graphene conductivity via electrostatic doping of the graphene sheet by applying a DC voltage Vg. The unit cell of the absorber is shown in Fig. 1 (b), where px and py are the periods in x and y directions, tp, td and tm are the thickness of the gating layer, dielectric spacer and gold plate, respectively. And the sinusoidally modulated width W(y) in x-direction of graphene is expressed as follow:30–34]. Here, we assume the material of the lossless dielectric spacer and the gating layer are polyethylene cyclic olefin copolymer (Topas) with the permittivity of εrt = 2.35 [35,36] and the polysilicon with the permittivity of εrp = 3 [37,38], respectively. The metallic plate is formed by gold with the relative permittivity obtained from the Drude model . The graphene surface conductivity can be expressed as σg = σintra + σinter (Unit: S) with the intraband and interband contributions from the Kubo formula [40–42] below:
In this configuration, the surface plasmon resonances of the single-layered net-shaped graphene can be excited by incident terahertz wave. A tp = 20 nm-thick polysilicon layer located at t = 20 nm beneath the graphene acts as a voltage gating layer to electrostatic dope the graphene, and the metallic plate acts as a reflecting mirror to suppress the transmission. The full terahertz absorption can be achieved by the destructive interference of the reflected fields from the graphene, the thin polysilicon layer embedded dielectric spacer, and metallic plate by adjusting the thickness td. Because of the gradient width modulation of the sinusoidally-pattern graphene, continuous plasmon resonances over a wide frequency range can be excited, opening the possibilities to realize broadband terahertz absorption. In this work, we use the Finite Element Method (FEM) to numerically calculate and analyze the properties of the proposed absorber. In the simulations, Floquet ports in z-direction and unit cell boundaries in both x-direction and y-direction are assigned to the unit cell shown in Fig. 1(b). The initial values of the structure parameters are set to be px = 32 µm, py = 60 µm, td = 26 µm, tm = 0.5 µm, Wmax = 32 µm, and Wmin = 1 µm. The single layered graphene sheet is modeled as an equivalent 2D surface impedance layer without thickness with Zg = 1/σg. Here, we assume the initial values of the graphene chemical potential, relaxation time, and temperature to be μc = 0.7 eV, τ = 0.1 ps, and T = 300 K. The absorbance is obtained from the S parameters in the simulations [8,43]. In addition, due to the narrowest width of the sinusoidally-patterned graphene Wmin >> 50 nm, thus the curved edge and nanoscale effect of the graphene can be neglected [38,44].
3. Results and discussion
To study the absorption properties of the proposed net-shaped periodically sinusoidally-patterned single-layered graphene-based absorber, we simulate the absorption spectra and the field distributions under normal incidence. Figure 2 shows the absorption spectra in TE and TM polarizations under normal incidence when the graphene chemical potential is assumed to be μc = 0.7 eV. As expected, broadband terahertz absorption is obtained for both polarizations. From the absorption spectra, we observe that the 90% absorbance bandwidth reaches 1.32 THz with a central frequency of 1.93 THz for TE polarization, and the 90% absorbance bandwidth reaches 1.23 THz with a central frequency of 1.78 THz for TM polarization. The normalized bandwidth with respect to the central frequency is greater than 65% for both polarizations. It is found that the absorption curve of TM polarization is close to that of TE polarization but with a small red-shift of 0.15 THz, which mainly results from the different effective resonance lengths in x and y directions and may be eliminated by further optimizing the values of td, px and py. Figure 3 displays the simulated electric field amplitude (|E|) distributions of the proposed absorber for both polarizations, where Figs. 3(a) and 3(b) are the |E| distributions for the TE and TM polarizations on the xoz and yoz planes cut in the middle of the unit cell at 2 THz, and Figs. 3(c) and 3(d) are the |E| distributions for the TE and TM polarizations on the xoy plane with z = 0 at the interface between graphene and spacer at 0.6 THz, 2 THz, and 3.5 THz, respectively. It is observed that the absorber exhibits tight field confinement around the graphene sheet on the top layer, leading to strong terahertz trapping and absorption. The electric field confinement characteristics are consistent with the absorption spectra shown in Fig. 2. The stronger the electric field confinement is, the higher of absorbance becomes. For example, very strong electric field confinement is found at 2.0 THz corresponding to the near-unity absorbance, while the electric fields almost vanished at 3.5 THz corresponding to the absorbance of 3%. Furthermore, it is worth pointing out that most of the electric fields are confined to the curved edge of the sinusoidally-patterned graphene due to the localized surface plasmon resonance of the patterned graphene. Specifically, the extremely confined fields of the TE polarization and TM polarization are distributed between two adjacent unit cells in x-direction and y-direction, respectively. The physical origin of this phenomenon is that the electric dipole resonances between two adjacent unit cells in x and y directions are respectively excited by the x-polarized TM and y-polarized TE incident terahertz waves.
As discussed above, the continuously localized plasmon resonance of the sinusoidally-patterned graphene plays a significant role in deciding the broadband absorption of the proposed absorber. The absorption bandwidth is mainly related to gradient width modulation and is expected to be weakly dependent on the incident angle θ. To investigate these characteristics, the absorbance as a function of operating frequency and incidence angle for TE polarization and TM polarization are plotted in Figs. 4(a) and 4(b), respectively. It is found that the absorber exhibits excellent performances with relatively stable absorbance and bandwidth over a wide range of oblique incidence angles for both polarizations. For TE polarization, the peak absorbance decreases very slightly even if the incident angle exceeds 80°. However, the peak absorbance of TM polarization decreases much faster than TE polarization as the incident angle increases. The reason for this phenomenon is mainly related to the electric dipole resonance of the graphene, where the tangential component of the electric field for the TM polarization decreases as θ increases but that of TE polarization remains unchanged. Besides, slightly blue-shifts of the absorption for both polarizations are also observed due to some parasitic resonances occurred at the larger incident angle. Despite slightly blue-shifts and certain amount of absorption decrease, we can still find that the peak absorbance remains more than 70% with a sufficient broadband of 1.4 THz over a wide range of incidence angle up to 60° for both polarizations. From this point of view, the proposed gradient modulated structure is able to compensate the momentum mismatch between the free space terahertz waves and graphene surface plasmons, exhibiting very high terahertz wave coupling efficiency. Compared with some previously proposed broadband absorbers [24,25,28,29], it has the advantages like broader normalized bandwidth or larger incident angle insensitive at terahertz frequencies. The incident angle and polarization insensitive broadband absorption characteristics may have great potential applications in terahertz sensing, detecting, and optoelectronic devices.
Furthermore, the geometrical and electrostatic tunabilities of the proposed absorber are investigated. The absorption properties of the proposed absorber directly depend on the geometry and material parameters, especially on the dielectric spacer thickness td and the graphene conductivity σg. To briefly demonstrate these properties, below we only discuss the absorption analysis under normal incidence for the TE polarization. Figure 5 illustrates the relation between the absorbance and the spacer thickness td with the chemical potential μc fixed as 0.7 eV. It is found that the absorption is related to phase-matching conditions of reflection cancellation, which is sensitive to the spacer thickness td variation. As td continuously increases from 16 µm to 34 µm, the absorbance enhances, the bandwidth decreases, and the central frequency red-shifts from 2.2 THz to 1.6 THz. And the absorption enhancement can be directly predicted from the stronger terahertz field confined to patterned graphene between adjacent unit cells when td is bigger. It is remarkable that the absorption tuning via td, is very useful in designing an absorber with specific requirement, but such tuning is no longer possible since the absorber is fabricated. However, absorption tuning via controlling the graphene conductivity can be used to solve this problem and make the fabricated absorber with flexible tunability. As shown in Kubo formulas Eq. (2) and Eq. (3), the surface conductivity of graphene is directly dependent on the chemical potential μc, which can be easily tuned from 0 to 0.8 eV in experiments by the electrostatic doping [45–47]. Figure 6 shows the absorption spectra as a function of terahertz frequency and the chemical potential μc under normal incidence for the TE polarization when td is fixed as 26 µm. It is obvious that, as μc increases, the absorption increases without changing the operating frequency range. And the absorbance increases from 14% to nearly 100% as μc increases from 0 to 0.8 eV. This absorber with flexible tunability may be used as a tunable broadband attenuator or spatial modulator at terahertz frequencies. In addition, unlike the conventional absorbers consisting of isolated graphene ribbons, disks and multi-resonators, the proposed absorber has net-shaped sinusoidally-patterned graphene layer with inherent continues electrical connection, which may greatly simplify the gating structure for achieving flexible tunability of the absorption properties by applying just one voltage gate, as shown in Fig. 1(a). And the design scheme of this tunable broadband terahertz absorber can be easily scalable to infrared or visible regimes.
In conclusion, we have demonstrated a new efficient route to achieving broadband terahertz absorber with nearly 100% absorption using a net-shaped sinusoidally-patterned graphene sheet placed on a thin polysilicon layer embedded dielectric spacer supported on a metallic reflecting plate. It is found that, by introducing such a unique gradient width modulation of the unit graphene sheet structure, the continuous plasmon resonances of the absorber can be excited, and over 65% normalized bandwidth of 90% terahertz absorbance can be achieved under normal incidence for both TE and TM polarizations. The numerical results show the broadband absorber has excellent absorption properties with weak dependence on the incident angles and the polarizations. Its peak absorbance remains more than 70% with a sufficient broadband of 1.4 THz over a wide range of incidence angle up to 60° for both polarizations. Furthermore, the geometrical and electrostatic tunabilities of the absorber have also been explored under normal incidence of TE polarization. It is found that the absorption is sensitive to the spacer thickness variation. As the spacer thickness increases from 16 µm to 34 µm, the absorbance enhances, the bandwidth decreases, and the central frequency red-shifts from 2.2 THz to 1.6 THz. Compared to conventional multi-resonator or multi-layered structures, the continuous net-shaped single-layered graphene structure can greatly simplify the electrostatic gating structure in achieving flexible tunability. By controlling the chemical potential via electrostatic doping of the graphene sheet, the peak absorbance can be continuously tuned from 14% to 100%. This work offers a new perspective on the design of graphene-based tunable terahertz broadband absorbers. And the absorber design scheme can be easily scalable to other terahertz, infrared or visible regimes for various promising applications in sensing, detecting, and optoelectronic devices.
National Natural Science Foundation of China (61601393, 11604276, 11501481, 11504305); Natural Science Foundation of Fujian Province of China (2016J01321); Natural Science Foundation of Guangdong Province of China (2015A030310009, 2016A030310372).
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