Tunable Bound States in the Continuum in All-Dielectric Terahertz Metasurfaces.

Xu Chen,Wenhui Fan

doi:10.3390/nano10040623

Abstract

In this paper, a tunable terahertz dielectric metasurfaces consisting of split gap bars in the unit cell is proposed and theoretically demonstrated, where the sharp high-quality Fano resonance can be achieved through excitation of quasi-bound states in the continuum (quasi-BIC) by breaking in-plane symmetry of the unit cell structure. With the structural asymmetry parameter decreasing and vanishing, the calculated eigenmodes spectra demonstrate the resonance changes from Fano to symmetry-protected BIC mode, and the radiative quality factors obey the inverse square law. Moreover, combining with graphene monolayer and strontium titanate materials, the quasi-BIC Fano resonance can be tuned independently, where the resonance amplitude can be tuned by adjusting the Fermi level of graphene and the resonance frequency can be tuned by controlling the temperature of strontium titanate materials. The proposed structure has numerous potential applications on tunable devices including modulators, switches, and sensors.

Highlights

Terahertz (THz) waves have attracted significant interest in their promising applications in chemical identification, security screening, and sensing [1,2]
bound states in the continuum (BIC) can be realized as quasi-BIC by introducing the structural asymmetry in the unit cell structure, where both the Q-factor and the resonance linewidth become finite [24]
It was revealed that dielectric metasurfaces with broken in-plane symmetry of unit cells can support high Q-factor resonance arising from the distortion of symmetry-protected BIC [25]

Summary

Introduction

Terahertz (THz) waves have attracted significant interest in their promising applications in chemical identification, security screening, and sensing [1,2]. It was revealed that dielectric metasurfaces with broken in-plane symmetry of unit cells can support high Q-factor resonance arising from the distortion of symmetry-protected BIC [25] Such a BIC-inspired mechanism allows a general strategy to access extremely high Q resonances and giant enhancement of electromagnetic fields [26], realizing many useful functionalities including lasing and biosensing [27,28]. In this paper, inspired by the dynamic characteristics of graphene and STO materials, a tunable BIC-inspired THz dielectric metasurface combined with graphene monolayer and STO is proposed, where its amplitude and resonance frequency can be tuned independently. To the best of our knowledge, this is the first time to study the tunable quasi-BIC Fano resonance metasurface with multidimensional and independently manipulation of resonance amplitude and frequency of THz waves, offering great prospects for designing tunable THz devices

Structure and Methods

Results and Discussions

Conclusions