High-Q dual-band graphene absorbers by selective excitation of graphene plasmon polaritons: Circuit model analysis

Highlights

• Designing THz dual-band absorbers based on graphene patterned arrays.

• Derivation of closed-form relations for the parameters using an analytical model.

• Simplicity of the structure in comparing to the previous proposed absorbers.

• Higher quality factor in comparison to other graphene-based dual-band absorbers.

• More absorption stability with respect to the oblique incidence angles.

Abstract

This article presents the design of two dual-band graphene-based absorbers for terahertz frequencies. The absorbers are composed of two-dimensional (2D) arrays of ribbons and disks printed on a ground plane backed dielectric spacer. The design is based on the excitation of a specific plasmon polariton of the graphene patterned array at each resonance band. An analytical circuit model is used to derive closed-form relations for the geometrical parameters of the absorber and graphene parameters. The graphene patterned array appears as a surface admittance made of an infinite parallel array of series RLC branches. Each branch is equivalent to a graphene plasmon polariton (GPP) providing a distinct resonance mode. The design procedure is based on selectively exciting the first two GPPs. This means that two of the parallel RLC branches are selectively used in the circuit model analysis. The results obtained using the analytical solution are compared with the full-wave simulations in HFSS. The agreement between the results validates the developed design method.

Introduction

Surface plasmon polaritons (SPPs), the eigenmodes of an interface between a dielectric and metal with strong interaction between light and free electrons, have attracted considerable attention in the past decades [1], [2], [3]. SPPs allow overcoming the diffraction limit to confine light into deep-subwavelength volumes with huge field enhancements [2], [3]. Noble metal-based plasmonics with fascinating properties found promising applications in switches [4], [5], demultiplexers [6], [7], and photovoltaics [8], [9] in visible and near-infrared regimes. However, the noble metals-based plasmonic devices are constrained due to some limitations such as poor confinement, difficulty in controlling their permittivity functions, and high material losses at terahertz (THz) frequencies.

The discovery of graphene emerged new opportunities in optoelectronics and photonics in mid-infrared and terahertz regions [10], [11]. Graphene, a one-atom-thick sheet of carbon, offers a novel design platform due to its attractive electrical and optical properties such as high thermal conductivity [11], Gate-variable optical conductivity [12], and high-speed operation [13]. Like noble metals, graphene is a promising candidate for plasmonic devices. However, graphene plasmon polaritons (GPPs) offer strong confinement and relatively longer propagation distance [14]. In addition, the plasmonic properties of graphene can be controlled electrically or chemically [15], [16], [17], [18]. These features make it a promising candidate for many applications such as optical modulators [19], [20], [21], sensors [22], [23], [24], [25], [26], and absorbers [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41].

Extensive studies are performed so far on the graphene-based absorbers for broadband and multiband applications [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. Absorbers have applications in solar energy harvesting [43], [44], refractive index sensors [45], microbolometers [46], thermal imaging [47], thermal IR emitters [48]. Here, one-dimensional patterned graphene ribbon and two-dimensional patterned disk arrays are used to design dual-band absorbers. In our previous dual-band absorber designs [41], [42], fundamental GPPs were excited at both of the absorption bands. However, the current work discusses the design of dual-band absorbers by selectively exciting a specific GPP at each frequency band. This is performed by developing an analytical procedure that returns the required geometrical parameters and graphene properties such as chemical potential and relaxation time for excitation of a specific GPP. The proposed absorbers show higher absorption quality factor (Q) in comparison with state-of-art graphene-based dual-band absorbers. Furthermore, the new design offers more absorption stability with respect to the oblique incidence angles and provides lower out-of-band absorption values between the adjacent channels.

A circuit model-based procedure is developed for the design of the proposed absorbers providing an analytical solution for the initial design of the unit cell geometrical parameters. This obviates the requirement for time-consuming full-wave simulations for optimization of the absorber response. We demonstrate that the resonance frequencies can be controlled by varying the geometrical parameters or chemical potential of the graphene layer.

The rest of the paper is organized as follows: Section 2 presents an analytical circuit-based design of a dual-band absorber based on an array of graphene ribbons. Closed-form equations are derived for the absorber parameters by exciting two GPPs of the graphene ribbons array. The absorber design examples are presented in this section. The ribbon array-based absorber is polarization sensitive. Thus, Section 3 presents a polarization insensitive dual-band absorber based on an array of graphene disks followed by discussions and comparisons with the state-of-art designs in the literature. Finally, Section 4 presents the main conclusions.