Bound states in the continuum of double resonant ring metal metasurfaces

Abstract

Metasurfaces have found extensive application in microwave, terahertz, and optical range, serving purposes such as filters, sensors, slow light devices, and nonlinear devices due to their distinctive electromagnetic response characteristics. Recent advancements demand metasurface devices to exhibit enhanced monochromaticity and stronger light interaction. Consequently, there is a growing interest in designing metasurfaces with high-quality factor (<i>Q</i>-factor) resonances, considering their crucial role in achieving sharp resonances through constructing bound states in the continuum (BIC) mode. The utilization of BIC has emerged as a prominent method for designing metasurfaces with high <i>Q</i>-factor resonances. Due to the fact that changes in the structural parameters of metasurfaces can simultaneously affect the resonance of two components of q-BICs, it is difficult to achieve on-demand design of operating frequency, bandwidth, and <i>Q</i>-factor. In this work, we have investigated a novel THz metasurfaces supporting q-BIC resonance. We optimize the geometric parameters of two split ring resonators (SRRs) to tailor the operating frequencies of the intrinsic resonance, and tuning the coupling between different resonance modes to form the q-BIC mode resonance. The dominant modes are demonstrated by the results of multipolar decomposition calculations of the electromagnetic field distributions and scattered power at different resonant operating frequencies. In the incident electromagnetic wave along <i>x</i> and <i>y</i> polarization, the normalized coupling strength ratio between the two modes is 0.54% (<i>x</i> polarization) and 4.42% (<i>y</i> polarization) respectively calculated through the Jaynes-Cummings model, which explains the law of the resonant frequency of different modes with the change of structural parameters of SRRs device. In order to analyze the refractive index sensing capability of our designed metasurfaces under the incident electromagnetic waves with different polarizations. We investigated the variation of the transmitted spectra of the metasurfaces under different refractive index of matters. The calculated results show that the sensitivity of the metasurface is 151 GHz/RIU when the incident wave is <i>y</i>-polarized and 108 GHz/RIU when the incident wave is <i>x</i>-polarized. We realize the effective control of the operating frequency, bandwidth, and <i>Q</i>-factor of the q-BIC mode resonance in the transmission spectrum of the metasurface, which provides a new idea for the practical design of terahertz metasurfaces with high <i>Q</i>-factor.