Ultra-high quality factor and polarization-sensitive Γ-point unidirectional guided resonances

We present a systematic analysis of the quality (Q) factors and polarization characteristics of unidirectional guided resonances (UGRs) in a symmetry-broken silicon-on-lithium niobate (Si-on-LN) photonic crystal (PhC) slab. Through numerical investigations, we demonstrate ultra-high Q-factor UGRs at the Γ-point, enabled by controlled structural asymmetry. By optimizing the asymmetry parameters, the Q-factor of UGRs can be flexibly tuned across a wide range of 10³ to 10⁶. Furthermore, leveraging the polarization sensitivity and all-pass phase-shift properties of UGRs, we achieve broadband high-efficiency linear-to-circular polarization conversion. The proposed device exhibits exceptional stability in conversion efficiency (>99.5%) over a 20.5 nm wavelength span, 1534.2-1554.7 nm. This work advances the fundamental understanding of UGRs while offering promising applications in PhC-based surface-emitting lasers, optical phased-array radars, and circular dichroism spectroscopy.