High-purity linearly polarized emission from a compact BIC laser.

Bound states in the continuum (BICs) confer ultrahigh quality factors and, thus, enable low thresholds upon photonic crystal (PhC) lasers. However, BICs feature a momentum-space vectorial polarization vortex, posing a substantial obstacle in achieving high-purity polarized emission. Traditionally, polarization control of a PhC laser can be achieved through various symmetry perturbations. However, the emission suffers from low polarization purity due to the finite-size effect inherent in real laser systems. Achieving a high polarization purity in PhC laser emission without significantly sacrificing other performance metrics has thus been a persistent challenge. Here, we develop a dispersion-assisted polarization control strategy for monolithic polarization engineering of PhC laser emission. As a proof of concept, we have demonstrated a linearly-polarized emission from a compact quasi-BIC laser with a high polarization extinction ratio of ~300:1, while single-mode operation and high beam quality are well-maintained. Our work paves a way for efficient polarization tailoring of on-chip lasers. The resulting devices deliver a miniaturized polarization light source for applications across coherent optical communication, advanced display technology, and diverse precision measurements.