Chiral spectral singularities spawning from quasi-bound states in the continuum in PT-symmetric dielectric metasurfaces

We investigate the chiral spectral singularities, i.e., laser threshold modes, in PT-symmetric dielectric metasurfaces originating from quasi-bound states in the continuum. The poles, referred to as the quasi-bound states in the continuum, of scattering matrix can move to the upper complex frequency plane from the lower half-plane by increasing the balanced gain and loss. The maximal intrinsic optical chirality of laser threshold mode has been theoretically demonstrated via the large transmission circular dichroism spectra. Our work paves the way for studying the enhanced optical chirality in non-Hermitian nanophotonics empowered by quasi-bound states in the continuum, together with various intriguing applications, such as chiral coherent perfect absorber and laser.