A tunable hybrid graphene-metal metamaterial absorber for sensing in the THz regime

Highlights

• Design and study a high-Q tunable hybrid graphene-metal metamaterial absorber.

• Propose an accurate circuit model for the structure based on the transmission-line theory.

• Investigation of the proposed structure as a potential label-free sensor in the THz regime.

Abstract

In this paper, a graphene-based metamaterial absorber is proposed and investigated numerically, in which the interaction between a split ring resonator (SRR) and graphene results in a high-Q absorption. To make a better understanding of the resonance mechanism, the electric and the magnetic fields, and the surface currents at the resonance frequency are investigated. In order to ease the analysis of the structure, an equivalent circuit model is introduced using the transmission line theory, and the accuracy of the proposed model is verified by the full-wave simulation. Finally, different aspects of the designed metamaterial are discussed as a potential label-free sensor for chemical and biomedical sensing. It is shown that by using this structure, a sensor with a sensitivity of 597 GHz/RIU can be achieved.

Introduction

Metamaterials are artificially manufactured materials designed to obtain unique electromagnetic properties, which can not be found in conventional materials existing in nature. Metamaterials have enabled many amazing applications, such as negative refraction [1], super-resolution lenses [2,3], and cloaking [4]. One of the most beneficial features of metamaterials is the capability of electromagnetic absorption, which makes them a strong candidate for being used in EMC/EMI shielding [5]. Recently, using 2D materials has opened up the possibility of creating metamaterials in much smaller dimensions than conventional ones, having the capability of being dynamically tuned by biasing them using optical or electrical sources [6]. Graphene, as a 2D material, due to its high electron mobility, has a great thermal and electrical conductivity [7,8]. By controlling the electron mobility, one can manage a graphene sheet to have the desired conductivity, which can be achieved either by controlling the chemical doping or the gate voltage of graphene. Using graphene to create metamaterials, due to the compressed surface plasmon wavelength of graphene, has opened the way of absorption in the THz frequencies and fabricating compact devices in this frequency range, which was not possible with conventional materials [[9], [10], [11]].

THz absorbers are illustrated to have a great potential to be used in emitters [12], stealth applications [13], photodetectors [14], and photovoltaic [15]. Sensing is one of the most important applications of metamaterial absorbers [[16], [17], [18]], which is the basic concept of this paper. Sensing is a crucial procedure in order to provide information for biology and chemistry; hence, introducing a novel method with high sensitivity, robustness, and low cost has always been a matter of interest in recent research. A marker is required in conventional sensing methods, either physical or chemical, potentially altering the original properties of the material under test [19]. The response of a metamaterial strongly depends on the refractive index of the environment; this property can be used to develop label-free sensors. Defining an unknown analyte on the surface of the absorber and measuring the shift in the resonance frequency resulted from the change in the effective refractive index of the environment is the measurement procedure in metamaterial label-free sensors. Therefore, to reach a high sensing rate, the metamaterial is supposed to have a sharp response with a high quality factor.

Recently, there has been numerous research on the concept of label-free sensors [[20], [21], [22]], with different aspects of limitations [23]. Graphene, as discussed earlier, due to its strong dispersion, can be used to reach a narrow-band impedance matching between the free-space and the metamaterial structure, which is the main absorption procedure in this paper. A graphene-based structure is much smaller than those introduced in Refs. [[24], [25], [26]] and can be dynamically tuned to change the frequency range of the measurement. Furthermore, hybrid graphene-metal structures, due to their unique offered characteristics, hold noticeable research attention on different applications [[27], [28], [29]]. To give more illustration, based on the combination of graphene and gold slits, a tunable Fano resonator with a high quality factor is introduced in Ref. [30]. Besides, a tunable graphene-metal hybrid chiral metamaterial is proposed in Ref. [31] to generate mid-infrared circular dischroism with an intensity of more than 10%. This paper introduces a hybrid graphene-metal narrowband absorber with miniaturization in size and improvement in quality factor. The small size of the absorber brings out a localized surface plasmon resonance (LSPR), guaranteeing the appropriate function as a label-free sensor. Due to its small size, the proposed sensor provides a reliable sensing rate in the presence of small amounts of analytes. Besides, a precise circuit model is derived from the transmission line theory, showing a good agreement with the full-wave simulation results. The point in using a circuit model for the structure is that one can analyse and modify the introduced structure expeditiously by adding different components to the derived circuit model [32].

This paper is structured as follows: The properties of graphene are studied in Section 2, the structure of the absorber is introduced in Section 3, the circuit model is derived in Section 4, and the structure is investigated as a potential biosensor in Section 5.