Tunable and Anisotropic Dual-Band Metamaterial Absorber Using Elliptical Graphene-Black Phosphorus Pairs
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We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated.
KeywordsMetamaterial absorber Two-dimensional material Dual-band absorber Surface plasmons
Finite element method
Hexagonal boron nitride
Perfect electric conductor
Graphene is a two-dimensional material with carbon atoms arranged in a honeycomb lattice [1, 2]. Various graphene-based photonic devices have been developed in the recent years due to their ultracompact size and unique light-graphene interaction [3, 4, 5, 6]. As one of its most significant applications, metamaterial absorbers based on graphene have attracted burgeoning amount of interest due to their strong and tunable plasmonic response [7, 8, 9, 10]. However, several applications that require high on-off ratio are restricted due to the zero or near-zero band gap of graphene . As an alternative two-dimensional material, black phosphorus (BP), a monolayer of phosphorus atoms arranged in a hexagonal lattice with a puckered structure , has also received a surge of research interest recently. It possesses exceptional optical and electronic properties, such as in-plane anisotropy, thickness-dependent tunable band gap , and high carrier density and mobility . Over the past few years, in the infrared region, researchers have investigated numerous structures to enhance the light-BP interaction strength in the metamaterial based on BP [15, 16, 17]. Nevertheless, the plasmonic resonance of BP-based absorber is hardly to be tuned flexibly and effectively, and they normally suffer from relatively low absorption rate with moderate doping level. This is attributed to the fact that the resonance strength in monolayer BP is rather weak, limiting its anisotropic potentials. Thus, graphene-BP-based plasmonic absorbers have been proposed utilizing the hybridization of graphene and BP to achieve strong and anisotropic plasmonic absorption [18, 19, 20]. However, the previous reported graphene-BP-based absorbers generally require relatively complicated fabrication technique or possess single absorption band, impeding their further applications for imaging, biosensing, and communication systems.
In our work, an anisotropic dual-band infrared absorber is numerically proposed using periodic elliptical graphene-BP pairs, which is ease of fabrication. The independent tunability of resonance by geometric size and doping level is demonstrated. Electric field distributions are plotted to reveal the physical mechanism. The incident angle tolerance and polarization sensitivity are also illustrated.
In the simulation, both graphene and BP are treated as two-dimensional surface with surface conductivities instead of bulk materials with permittivity tensors. This assumption solves the problems of thickness definition for ultrathin materials and low computational efficiency .
According to Eq. 1, σ(ω) consists of the intraband and interband counterparts, namely σintra and σinter. ω is the radian frequency, μc is the chemical potential, Г is the scattering rate, and T is the Kelvin temperature. ħ, e, ξ, and kB are the reduced Planck constant, electron charge, electron energy, and Boltzmann constant, respectively.
Results and Discussion
We next analyze the absorption spectra with different geometric configurations to demonstrate the tunable dual-band absorption property in Fig. 2b–d. In Fig. 2b, the first absorption peaks have redshifts as a increases from 42 to 52 nm for both polarizations, while second resonant frequencies are almost unchanged. On the other hand, as shown in Fig. 2c, by increasing the long axis length b, the second resonances are redshifted as well, while the first absorption peaks remain constant for TE and TM polarization. Therefore, the dual absorption peaks can be tuned independently by varying the corresponding axis length in the elliptical graphene-BP pairs. Moreover, the thickness of dielectric layer also plays a critical role in the performance for the proposed device, which acts as a Fabry-Perot resonator formed by the graphene-BP metasurface and the PEC substrate. Thus, the absorption spectra with different td are plotted in Fig. 2d. As td increases from 0.95 to 1.75 μm, the first absorption peaks for TE and TM polarization have a dramatic drop, while the second peaks increase at first then decrease sharply. As a consequence, there is an optimal thickness td that maximizes the dual absorption peaks of the proposed absorber.
Absorption spectra under normal incidence with different polarization angles φ are presented in Fig. 5c to investigate the polarization dependence of the proposed absorber. We assume the polarization angle of TE polarization to be 0°. One can see from Fig. 5c that, as φ increases from 0 to 90°, the absorption spectrum turns out to be the same as the TM polarization in Fig. 2a. When 0° < φ < 90°, the incidence will excite electrons in BP to oscillate in both armchair and zigzag directions due to its x- and y- components of the incident electric field. Consequently, surface plasmon resonances can be induced simultaneously in armchair and zigzag directions of BP.
In conclusions, we have proposed an anisotropic dual-band infrared absorber consisting of periodic transverse and longitudinal graphene-BP ellipses. The maximum PER at each resonance can reach up to 23 dB and 25 dB. The dual anisotropic resonances are attributed to the induced electric dipoles located at the ends of short and long axes. By adjusting the lengths of short axis and long axis, the first and second absorption peaks can be independently tuned, respectively. Moreover, the resonant absorption bands can also be tuned by changing the corresponding doping level of graphene and BP. Besides, high absorption rates at both peaks can be achieved under oblique incidence for any polarization. The proposed absorber can be utilized as a tunable reflective polarizer and novel infrared sensor.
CY and LS conceived the idea and wrote the manuscript. ZY and WX undertook the simulations. XK analyzed the data. GR and JW supervised the project. All authors read and approved the final manuscript.
This work was supported by the National Natural Science Foundation of China (51702271), the Young and Middle-aged Teachers Education and Scientific Research Foundation of Fujian Province, China (JAT170407), and the High Level Talent Project of Xiamen University of Technology, China (YKJ16016R).
The authors declare that they have no competing interests.
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