Ultrafast switching of a metasurface quasi-bound state in the continuum via transient optical symmetry breaking

In photonic structures, bound states in the continuum (BICs) have recently attracted huge interest in both fundamental and applied research. Quasi-BIC leaky modes resulting from in-plane symmetry breaking in metasurfaces are particularly relevant to applications, due to their high quality factor, which scales as the squared inverse of the asymmetry parameter. Here, we theoretically propose an innovative approach to switch on quasi-BICs on sub-picosecond timescales via optically induced symmetry breaking in semiconductor metasurfaces. The desired effect is granted by exploiting the spatial inhomogeneities in the distribution of photo-excited hot carriers at the single meta-atom nanometric scale. In our simulations, the quasi-BIC state manifests itself as an ultra-sharp dip in transmission, emerging upon pump arrival, and disappearing completely within the carriers' diffusion timescale. Our strategy allows to envision reconfigurable platforms with switchable high-Q resonances, with ultrafast recovery beyond the limits of carrier relaxation, typical of previous approaches.