Moiré Photonic Crystal enabled Optical Vortex Generation through Bound States in The Continuum

The twisted stacking of van der Waals crystals has led to the emerging moiré physics as well as intriguing chiral phenomena such as chiral phonon and photon generation. Along with the surge of research into moiré physics of two-dimensional materials, moiré patterns in photonic systems have also drawn broad attention. In this work, we identified and theoretically formulated a non-trivial twist-enabled coupling mechanism in twisted bilayer photonic crystal (TBPC), which connects the bound state in the continuum (BIC) mode to the free space through a twist-enabled “moiré channel”. Moreover, the radiation from TBPC hosts an optical vortex in the far-field radiation with both odd and even topological orders. We quantitatively analyzed the twist-enabled coupling between the BIC mode and other non-local modes in the photonic crystals, giving rise to radiation carrying orbital angular momentum. The optical vortex generation is robust against geometric disturbance, making TBPC a promising platform for stable vortex generation. Furthermore, the incorporation of TBPC and micro/nanoelectromechanical systems (MEMS/NEMS) could lead to the creation of tunable vortex beams with adjustable topological orders, beam center positions, divergence angles, etc. As a result, TBPCs not only provide a new approach to manipulating the angular momentum of photons, but may also enable novel applications for integrated optical information processing and optical tweezers. Due to the intrinsic similarities between photonic systems and condensed matter systems, this work may also guide the exploration of orbital angular momentum and spin-orbit coupling in moiré van der Waals structures. Our work broadens the field of moiré photonics and paves the way toward the practical application of moiré physics.