Anisotropy‐Engineered Ultrabroadband Anti‐BIC Polarizer Based on Thin‐Film Lithium Niobate

Thin‐film lithium niobate (TFLN) has emerged as a revolutionary platform for photonic integrated circuits due to its outstanding electro‐optic efficiency, strong nonlinearity, and low loss. However, polarization crosstalk in TFLN PIC remains a critical challenge. Conventional polarizers cannot simultaneously achieve both low insertion loss (<0.5 dB) and large operational bandwidth (>300 nm). In this work, a waveguide orientation‐engineering strategy that manipulates the coupling between TM bound mode and TE continuous modes in the anisotropic TFLN platform is proposed and demonstrated. By aligning the extraordinary optical axis parallel to the waveguide direction, an ultrabroadband polarizer with enhanced anti bound state in the continuum has been achieved. The device achieves an insertion loss below 0.18 dB and an extinction ratio exceeding 17 dB across 1.24–1.7 µm wavelength range.