A Tunable Dual-Band and Polarization-Insensitive Coherent Perfect Absorber Based on Double-Layers Graphene Hybrid Waveguide
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A suspended monolayer graphene has only about 2.3% absorption rate in visible and infrared band, which limits its optoelectronic applications. To significantly increase graphene’s absorption efficiency, a tunable dual-band and polarization-insensitive coherent perfect absorber (CPA) is proposed in the mid-infrared regime, which contains the silicon array coupled in double-layers graphene waveguide. Based on the FDTD methods, dual-band perfect absorption peaks are achieved in 9611 nm and 9924 nm, respectively. Moreover, due to its center symmetric feature, the proposed absorber also demonstrates polarization-insensitive. Meanwhile, the coherent absorption peaks can be all-optically modulated by altering the relative phase between two reverse incident lights. Furthermore, by manipulating the Fermi energies of two graphene layers, two coherent absorption peaks can move over a wide spectrum range, and our designed CPA can also be changed from dual-band CPA to narrowband CPA. Thus, our results can find some potential applications in the field of developing nanophotonic devices with excellent performance working at the mid-infrared regime.
KeywordsGraphene Absorption Surface plasmons
Coherent perfect absorber
Indium tin oxide
Surface plasmon polaritons
As a crucial issue for nanophotonics and optoelectronics, efficient light-matter interaction has widely caused concerns in recent years [1, 2], particularly in the atomically thin two-dimension (2D) materials. Many reports have been demonstrated, such as transition-metal dichalcogenides (TMDCs) [3, 4], graphene [5, 6, 7, 8, 9], hexagonal boron nitride , black phosphorus , and so on. As a prototypical 2D material, graphene can interact with light in a wide (ultraviolet to terahertz) wavelength range. However, due to its natural gapless and conical electronic band structure , the absorption efficiency of light in graphene is as low as about 2.3%. Fortunately, the optical bandgap of graphene can be opened up by doping or using the other special methods, which results in the excitation of surface plasmon polaritons (SPPs) in the terahertz and infrared bands . Then, the absorption and confinement of light in graphene can be remarkably strengthened because of the excited SPPs, which can prolong the interaction time between graphene and light [14, 15, 16, 17, 18, 19]. Therefore, graphene plasmonic devices have become an interesting and significant topic, and extensive researches have been demonstrated in various fields, such as absorbers [17, 18], optical filters , sensors , modulators , and photodetectors [23, 24].
More specifically, among these devices based on graphene, optical absorber takes an important role in the field of developing advanced optoelectronic devices, such as solar energy-trapping devices and emitters. Recently, due to the unique attributes of graphene, some absorbers based on graphene have been reported. Moreover, as mentioned above, most of these absorbers are focused on the terahertz and infrared regimes, because graphene with special processes can excite SPPs, leading to the strong light-graphene interactions in these wavelengths . For instance, based on graphene, Luo et al.  proposed a tunable perfect absorber with ultra-narrowband, which can maintain satisfactory performances under broad-angle incidence. In Ref. , by embedding monolayer graphene to the metamaterials, Xiao et al. demonstrated that the EIT analog was realized in the terahertz regime, and its resonance intensity could be flexibly manipulated over a wide range. Jiang et al.  designed, fabricated, and investigated a broadband absorber based on patterned graphene in the terahertz regime, and the absorption above 90% is achieved from 1.54 to 2.23 THz. In order to manipulate the surface plasmon of graphene in an effective and feasible way, Xia et al. suggested that it could be realized by using a conductive sinusoidal grating with sub-wavelength size .
Importantly, coherent perfect absorber (CPA), which is another way to control and strengthen the optical absorption of graphene, has attracted great attention due to the all-optical modulation features [27, 28]. Depending on the interference effects and interplay of absorption, CPA provides a potential method to manipulate light with light without nonlinearity. Y. D. Chong et al. theoretically investigated the CPA with the scattering matrix . Before long, two kinds of CPA were successively reported in the silicon slab  and planar metamaterial . Recently, CPA has also been intensively studied in graphene-based devices. For example, combined with centrosymmetry metal-graphene nanostructure, Y. Ning et al.  investigated a tunable polarization-insensitive CPA and showed that the absorption could be flexibly and all-optically modulated by the Fermi energy of graphene and the relative phase between the incident lights. By trapping the guided-mode resonance in a subwavelength dielectric grating, X. Feng et al.  realized a tunable graphene-based CPA, which can be applied in a wide spectrum coverage from visible to infrared regimes. Y. C. Fan et al.  exploited graphene nanoribbon-based metasurface to CPA in the mid-infrared regime, and demonstrated that this CPA can be manipulated flexibly by changing the properties of graphene and structural parameters of the metasurface. However, the dual-band graphene-based CPA is also of great significance to the nanophotonics and optoelectronics devices, but seldom investigated in the mid-infrared regime. Furthermore, how to improve its adjustability is also a challenge facing the dual-band CPA.
In this paper, we design and study a tunable dual-band and polarization-insensitive CPA in the mid-infrared band, which contains a silicon array coupled in double-layers graphene waveguide. The physical mechanism of the designed CPA is analyzed by the scattering matrix. Meanwhile, the features of proposed CPA are demonstrated by the finite-difference time-domain (FDTD) simulations. When the incident light is illuminated into the silicon array, since the plasmonic resonances on the double continuous graphene films can be emerged due to the mechanism of guided-mode resonance, then the coupling effect between them results in the perfect dual-band absorption peaks, which are achieved in 9611 nm and 9924 nm, respectively. Moreover, due to its center symmetric feature, the proposed absorber also demonstrates polarization-insensitive. Furthermore, most of the reported graphene-based absorbers are manipulated by only changing the properties of graphene through an electrostatic field, magnetic field, or chemical doping, which are the causes of additional losses and also make the devices more complicated. For our proposed CPA, the coherent absorptions can be all-optically modulated by altering the relative phase between two reverse incident lights, which improves the absorber’s adjustability and does not increase the complexity of the structure. Meanwhile, by manipulating the Fermi energies of two graphene layers, two coherent absorption peaks can move over a wide spectrum range, and our designed CPA can also be changed from dual-band CPA to narrowband CPA. Therefore, our work provides a very promising way with convenience and sensitivity for potential applications include switches, all-optical logical devices, and coherent photodetectors.
In the simulation, we utilize the 3D FDTD method for the numerical calculation. Meanwhile, periodic boundary conditions are applied along the x- and y-directions, and perfectly matched layer is applied along the z-direction including both the top and bottom of the proposed device. Moreover, we utilize the non-uniform mesh to compute the simulation results, where the minimum mesh size inside the graphene layer equals 0.1 nm and gradually increases outside the graphene film to cut down on the storage space and computing time.
Results and Discussion
Thus, according to Eq. (3), Aco can be manipulated by changing α and φ. In particular, if α = 1, Aco can be tuned from the minimum Aco − min = 0 to the maximum Aco − max = 1 when φ varies from (2N + 1)π to 2Nπ.
As mentioned before, most reported graphene-based perfect absorbers are polarization-sensitive and focused on the narrowband or broadband perfect absorbers, dual-band graphene-based perfect absorbers are seldom investigated in the mid-infrared regime. In this paper, we have designed a tunable dual-band and polarization-insensitive CPA in the mid-infrared regime, and the corresponding absorption features are discussed by using the scattering matrix and FDTD simulation, which illustrate that dual-band perfect absorption peaks are achieved in 9611 nm and 9924 nm, respectively. Moreover, due to its center symmetric feature, the proposed CPA also exhibits polarization-insensitive. Meanwhile, the coherent absorption peaks can be all-optically modulated by altering the relative phase between two reverse incident lights. Furthermore, by manipulating the Fermi energies of two graphene layers, two coherent absorption peaks can move over a wide spectrum range, and our designed CPA can also be changed from dual-band CPA to narrowband CPA. On the other hand, for the proposed CPA, subwavelength metamaterials based on silicon squares can be integrated for the current CMOS technology, and chemical vapor deposition (CVD) grown graphene can be transferred over the silica layer using standard transfer techniques . Moreover, compared with the devices based on patterned graphene, our structure keeps graphene in the continuous form, which has the benefit of preserving the high mobility of graphene and simplifies the fabrication processes as well as the doping configuration. In recent years, some research groups have tried to design some graphene-based devices in an experiment based on the above methods [39, 40, 41]. Therefore, we believe it is possible to fabricate our proposed structure with similar processing, and our proposed graphene-based CPA can find some potential applications in the field of developing nanophotonic devices at the mid-infrared regime.
XL, ZQC, XZ, and ZML designed the study and analyzed the data. SQL, JPL, LLW, LQ, and YHZ helped in data analysis and manuscript modification. All the authors have read and approved the final manuscript.
This work was supported by the National Natural Science Foundation of China (Grant nos. 61775055, 61505052, 61764005, and 11804093); Natural Science Foundation of Jiangxi Province (nos. 20181BAB201013 and 20192BAB212003).
The authors declare that they have no competing interests.
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