Elsevier

Carbon

Volume 123, October 2017, Pages 668-675
Carbon

Implementation of selective controlling electromagnetically induced transparency in terahertz graphene metamaterial

https://doi.org/10.1016/j.carbon.2017.08.016Get rights and content

Abstract

A terahertz electromagnetically induced transparency (EIT) metamaterial, consisting of single-layer graphene cut wire resonator arrays with closely placed graphene closed ring resonator arrays, was designed and numerically investigated in this paper. A distinct transparency window resulting from the near field coupling between two resonators can be obtained in the transmission spectrum. More importantly, since two resonator elements of all unit cells connect respectively with the corresponding metallic pads (Pad 1 and Pad 2) by the separated graphene wires, the location and amplitude of the transparency window, and the associated group delay and delay bandwidth product can be actively controlled by the selective doping graphene. Moreover, compared with other separated graphene patterns, a more convenient and fast modulation can be realized by applying gate bias voltage. In addition, a two-particle model was employed to theoretically study EIT behaviors of the graphene metamaterial with different doping states, and the analytic results agree excellently with our numerical results. Therefore, the work could offer a new platform for exploring actively tunable slow light terahertz devices such as modulators, buffers, and optical delays.

Graphical abstract

A terahertz electromagnetically induced transparency (EIT) metamaterial, consisting of single-layer graphene cut wire arrays with closely placed graphene closed ring arrays, was designed and numerically investigated in this paper. A distinct transparency window can be obtained in the transmission spectrum. More importantly, the location and amplitude of the transparency window, and the associated group delay and delay bandwidth product can be actively controlled by the selective doping graphene.

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Introduction

Electromagnetically induced transparency (EIT) was first observed in a three level atomic system, which resulted from the quantum destructive interference between two different excitation pathways [1]. Recently, the EIT-like behavior can be also implemented in metamaterials composed of periodic sub-wavelength unit cell structure array [2]. Currently, a variety of EIT metamaterials have been developed due to potential applications in slow-light devices, sensing, and quantum information storages [3], [4], [5]. Unfortunately, most of these EIT windows obtained can only work at a fixed wavelength range, which severely limits the developments and applications of the EIT-like effect [6]. Therefore, active manipulation of EIT window is highly desirable for practical applications, such as tuning the dispersion and group velocity of light [7].

Generally, dynamic manipulation of EIT window in the coupled resonator systems depends mainly on three parameters: resonant frequency of bright mode, resonant frequency of dark mode, and coupling distance between two modes [8]. According to this principle, currently, many approaches to tuning EIT behavior have emerged in a passive manner by changing the incident angles [9], [10] and in active manners by integrating metamaterials with active materials or components [11], [12], [13], as well as by tuning an external magnetic field [14]. Recently, the Micro-Electro-Mechanical Systems (MEMS) technology is also proposed to realize dynamic controllability of EIT window [15], [16]. However, these approaches can only provide global control because of optical or thermal excitations, which hinders their multifunctional applications due to absence of selective control.

Since discovered in 2004, graphene has attracted considerable attention due to unique electric, mechanical, and thermal properties [17], [18]. More importantly, the graphene conductivity can be dynamically tuned by doping [6]. Recently, different graphene-based tunable metamaterials have been reported by patterning, stacking or integrating graphene structures [19], [20], [21]. These results demonstrate that the active controllability of graphene is an ideal platform for realizing selective control of the coupled resonators in EIT-like systems. In this paper, we proposed a selectively controllable terahertz EIT metamaterial structure, unit cell of which consists of a graphene cut wire resonator with two closely placed graphene closed ring resonators. Moreover, two resonator elements of all unit cells connect respectively with the corresponding metallic pads by the separated graphene wires to realize selectively electrostatic doping. By selectively tuning Fermi energy of two resonators, the bandwidth, intensity and frequency of EIT window can be actively controlled as well as the associated group delay and delay bandwidth product. Therefore, the work would open up new avenues for designing tunable slow light terahertz devices.

Section snippets

Design and simulation of structure

For the traditional metal-based metamaterial EIT system, the amplitude modulation of EIT window can be obtained by changing the resonance strength of the dark mode [22] or tuning the coupling distance between the bright and dark resonators [23]. Here, a terahertz graphene metamaterial, patterned on a low doped silicon substrate with a thin SiO2 layer, is proposed to realize active control of EIT window, as shown in Fig. 1(a). The metamaterial unit cell consists of a graphene cut wire resonator

Forming principle of EIT window

Before studying the EIT-like structure shown in Fig. 1, two individual GCWR and GCRRs arrays in EIT structure are initially investigated to clarify underlying forming process of the EIT window, as shown in Fig. 2. For insolated GCWR array with Fermi energy of 0.2eV, a narrower resonance at 0.465 THz is directly excited by the incident wave when the excitation field is parallel to the GCWR structure (as shown in Fig. 2(a)). Moreover, the direction of the induced surface currents on GCWR

Conclusions

In summary, the proposed graphene metamaterial, composed of the cut wire and closed ring array acting as two bright elements, is numerically demonstrated for active tuning of the EIT window in terahertz region. The transmission spectrum shows a distinct transparency window at 0.469 THz, which is caused by the destructive interference resulting from near-field coupling of two bright modes. By different doping modes, moreover, reconfiguration of the near-field coupling between two bright

Acknowledgements

The work is supported by the National Natural Science Foundation of China (51672062, 51575149 and 51402075), Heilongjiang Province Natural Science Foundation of China (F201309), the Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province (LBH-Q11082), the Youth Academic Backbone Support Plan of Heilongjiang Province Ordinary College (1253G026), Special Funds of Harbin Innovation Talents in Science and Technology Research (2014RFQXJ031) and Science Funds for the Young

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