Reconfigurable linear and nonlinear chirality in a quasi-BIC metasurface

Reconfigurable control of optical chirality is highly desirable for active nanophotonic and nonlinear optical systems. Here, we numerically investigate a quasi-bound state in the continuum (quasi-BIC) supporting reconfigurable linear and nonlinear chiroptical responses. An asymmetric all-Si metasurface is first employed as a reference system to elucidate the formation of the chiral quasi-BIC. Combined in-plane displacement and vertical symmetry breaking convert a symmetry-protected BIC into a radiative quasi-BIC, reconstruct the momentum-space polarization topology, and yield a maximum linear circular dichroism (CD) of 0.94. Reconfigurability is subsequently introduced through an equal-height Si/Sb₂Se₃ hybrid configuration, in which the phase-dependent dielectric perturbation of Sb₂Se₃ modifies the helicity-selective coupling to the quasi-BIC. Switching Sb₂Se₃ between its amorphous and crystalline states reverses the linear CD from approximately +0.92 to -0.76. The nonlinear response is further investigated through third-harmonic generation (THG). Under a controlled nonlinear model in which the same effective χ⁽³⁾ is assigned to both Sb₂Se₃ phases, the spatially separated nonlinear polarizations generated in Si and Sb₂Se₃ undergo phase-state-dependent amplitude and phase redistribution, and the model predicts a nonlinear CD reversal from approximately +0.93 to -0.83. Single-source control calculations and effective complex-source analysis consistently support the proposed phase-dependent interference mechanism. These results establish localized phase-change loading and multi-source nonlinear interference as complementary degrees of freedom for reconfigurable chiral nanophotonics.