Light trapping and manipulation of quasibound states in continuum Ge2Sb2Se4Te metasurfaces

Abstract

Actively tunable nanodevices play an important role in modern photonics systems. Metasurfaces based on phase-change materials (PCMs) provide a new way for the realization of such devices. At present, the PCM is usually integrated with the resonant metasurfaces with low $Q$ factor or as a resonator to achieve the optical switches. Note that less work has focused on the switching characteristics of strong light trapping of in high-$Q$-factor metasurfaces. The main reason is that PCM has a significant intrinsic loss. Recently, the PCM ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Se}}_{4}{\text{Te}}_{1}$ (GSST) with ultralow loss has been proved by in experiment ($k\ensuremath{\approx}0$), which makes it is possible to achieve the active metasurfaces with high $Q$ factor. Moreover, the symmetry-protected bound states in the continuum (BICs) provides an effective method to achieve the high-$Q$-factor metasurface. The perturbation that breaks the in-plane inversion symmetry of a structure can transform such a BIC mode into a leaky quasi-BIC with high $Q$ factor. In this work, the BIC and the light trapping characteristics of quasi-BIC in PCM GSST metasurfaces are studied numerically. Through moving the relative position of nanobars, the structure symmetry of the amorphous GSST metasurface is disturbed, which makes an ideal BIC convert to a quasi-BIC with high $Q$ factor. The average field-enhancement factor (EF) of quasi-BIC is evaluated, which is proportional to the $Q$ factor of resonance mode. Meanwhile, the effects of structure parameters, polarization angle and incident angle of light on EF are also discussed. In addition, by calculating the EF values under different disturbance parameters $d$ and crystallization degrees $m$. The switching characteristic of light trapping of quasi-BICs is discussed. It is worth noting that the asymmetric parameters can be precisely controlled in the process of the excitation of BICs in this way, which provides a way for the realization of ultrahigh $Q$ factors and makes it possible to strongly trap light and manipulate it dynamically. Our work provides a route for actively tunable and reconfigurable micro or nano optical devices.