Light-switchable quasi-BIC optical vortex generators

Optical vortices, the spatial modes of an electromagnetic wave carrying orbital angular momentum (OAM), have attracted increasing interest because of their potential for applications in optical communication with enhanced security and channel capacity. A unique optical vortex (OV) generation method has been recently proposed based on the Pancharatnam-Berry (PB) phase induced by the winding topology of polarization around a vortex singularity at bound states in the continuum (BIC). Compared with the recently emergent metasurface-based OV generators, which rely on spatial variations, the BIC-based OV generators have yielded advances in terms of design feasibility, fabrication complexity, and robustness. However, their applications in practical photonic systems are currently limited because OV generations from BIC-related devices originate from the topological property of the photonic bands and cannot be dynamically altered. Here, by leveraging the vortex topology in momentum space together with the nonlinear dynamics of silicon, we demonstrate that a silicon photonic crystal slab can realize optically switchable OV generation. In particular, the spatial tunability and the switching effects in the picosecond scale are studied using nonlinear modeling at near-infrared wavelengths. The demonstrated nontrivial topological nature of the active generators can expand the application of BIC-based devices to include ultrafast vortex beam generation, high-capacity optical communication, and mode-division multiplexing.