Dual-band plasmon induced transparency metamaterial based on multi-quasi-bright modes

Highlights

• A dual-band PIT based on a bright mode and multi-quasi-bright modes is proposed.

• An extensive group index over 680 can be obtained in the PIT structure.

• The PIT properties are studied by equivalent circuit and coupled oscillator model.

Abstract

In general, we have proposed an innovative dual-band plasmon-induced transparency (PIT) metamaterial (MM) consists of a metal cut-wire (CW) and a parallel cut-wire-pair (CWP) separated by a dielectric spacer, which behaves as a bright mode and multi-quasi-bright modes separately. The dual-band transparency widow can be observed in the microwave region due to the near-field coupling among the bright mode and multi-quasi-bright modes, numerically and experimentally investigated. It is found that a strong magnetic resonance has been introduced in our PIT structure due to the local magnetic interaction between the double-layered CWs; therefore, an extensive group index over 680 can be obtained. Finally, the dual-band PIT MM properties are investigated numerically and theoretically, using equivalent circuit model (ECM) and coupled oscillator model (COM), respectively.

Introduction

Plasmon-induced transparency (PIT) is a quantum phenomenon that usually appears in a three-level atomic medium [1], [2], [3], [4], [5]. Mainly, in metamaterial (MM), the PIT effect is realized based on the near-field coupling between two artificial atoms, a bright mode, which couples strongly to the incident light, and a dark mode, which can only be excited by the near-field coupling with the bright mode [6], [7], [8]. Besides, the PIT response also originates from the coupling between the super-radiant and sub-radiant modes, both of them can couple directly to the incident light [9], [10]. Using such as CWs and split-ring resonators (SRRs), serving as the bright and dark modes, various PIT MMs have been investigated in the gigahertz [11], [12], [13], near-infrared [14], [15], [16], and terahertz frequency regimes [17], [18], [19].

With the rapid development of PIT MMs, the dual-band PIT effect in MMs has attracted particular attention in recent years. In general, the dual-band PIT effect in MMs can be obtained by the coupling between different resonance modes such as bright, dark, and quasi-bright modes [20], [21]. The quasi-bright mode can couple weakly to the incident light compared with the bright mode and exhibits a higher quality factor. In the gigahertz frequency regime, He et al. [22] have proposed the dual-spectral PIT MM consists of a metal bar as bright mode and two metal stripe pairs with different sizes as double dark modes. The interference between the asymmetric structures results in two transparency windows. The dual-band PIT effect has been reported by Hu et al. [23] in a planar MM consists of two metal wires with different sizes and double split-ring resonators. This paper shows the double transparency windows result from the near-field coupling among two bright atoms and a dark atom in the microwave frequency regime. A flat MM composed of a CW and two asymmetric C resonators, which serve as a bright mode and double-dark mode, has been reported by R. Sarkar et al. [24]. This configuration realizes the dual-band PIT effect in the terahertz frequency regime. Yu et al. [25] proposed a planar plasmonic MM composed of an SRR and a pair of stripe to realize the dual-band PIT effect at the optical region. The two transparency windows result from the coupling between a bright–bright mode in the symmetry and a bright–dark mode in the asymmetry structure. Zhang et al. [26] provided a new approach to obtaining dual-band PIT by introducing a meta-molecule consisting of a bright atom, a quasi-dark atom, and a dark atom simultaneously. This report shows that the near-field coupling between the bright atom and quasi-dark atom leads to a pronounced transparency window; meanwhile, the interference between the bright atom and dark atom induces the other transparency window in the terahertz region. Nevertheless, it is worth mentioning that dual-band PIT in the report mentioned above can be achieved using at least one dark mode, not to mention these structures could not provide high group indices (used to describe the ability of slow light). Therefore, designing a dual-band PIT MM consists of bright and multi-quasi-bright modes that possess an extensive group index with many potential applications in optical switching [27], [28], multi-band filters [29], [30], and slow-light systems [31], [32].

In this paper, based on the kinds of literature mentioned above, we introduce a new scheme for generating PIT spectral response at microwave frequency in planar MM by introducing a bright atom and multi-quasi-bright atoms simultaneously. Since the coupling of the bright and multi-quasi-bright modes induces highly-dispersive transparency windows, the dual-band slow light effect can be obtained. Furthermore, our proposed PIT MM possesses an extensive group index with low losses due to high dispersion caused by introducing the strong magnetic resonance compared with some previous works. The resonance excitation mechanism of the dual-band PIT effect is also explored by theory and experiment thoroughly. Eventually, the slow light effect of the proposed MM is investigated and proved by utilizing CST Microwave studio software.