Mechanisms of Bound States in the Continuum-Induced Terahertz Circular Dichroism Enhancement and Dynamic Reversal Imaging Characteristics

Abstract We present a tunable terahertz circular dichroism (CD) metasurface based on the bound states in the continuum (BIC) mechanism. The metasurface is composed of rectangular resonant units made of the phase-change material Ge2Sb2Te5 (GST). By breaking the in-plane C2 symmetry, quasi-bound states in the continuum (q-BIC) are excited, enabling nonmonotonic evolution of the resonance frequency, multipolar interference modulation, and independent tuning of both CD and the quality factor Q. A maximum dichroism contrast (CDT) of 0.96 is achieved at 1.371 THz. Furthermore, leveraging the reversible phase-transition characteristics of GST between amorphous and crystalline states, reversible flipping of the chiral response is realized. Based on the above physical mechanisms, we construct a terahertz imaging array, which shows polarization-dependent multigray-scale imaging and dynamic switching of imaging contrast via numerical simulations.