Ultrathin Twisted Bilayer Photonic Crystal with Tunable Micron-Level Near-Field Coupling for Multimode Polarization Manipulation

Moiré chiral metasurfaces hold great promise for polarization-sensitive applications. However, achieving multimode tunability remains challenging and requires further efforts. This paper presents an on-site tunable twisted bilayer photonic crystal (TBPhC) composed of two ultrathin silicon nitride photonic crystal films stacked into moiré patterns, each only 50 nm thick. These ultrathin photonic crystals support guided-mode resonances and form an optical resonator through interlayer coupling, simultaneously generating twist-tunable moiré chiral resonances. Notably, compared to hundred-nanometer scale counterparts, this deep-subwavelength photonic crystal thickness enhances near-field coupling, enabling the tuning of multiple resonant modes at micron-level spacing, thus allowing for more flexible manipulation. Experimental results demonstrate that this ultrathin resonant configuration enables active control of resonant chiroptical responses by adjusting the interlayer distance and twist angle, thereby modulating the polarization state of light at the telecom region and opening new possibilities for dynamic polarization control in advanced optical devices.