Low-loss thin-film periodically poled lithium niobate waveguides fabricated by femtosecond laser photolithography.

Periodically poled lithium niobate on insulator (PPLNOI) ridge waveguides are essential photonic components for both classical and quantum information processing. However, dry etching of PPLNOI waveguides frequently generates rough sidewalls and variations in the etching rates of oppositely poled lithium niobate ferroelectric domains, leading to relatively high propagation losses (0.25-1 dB/cm), which significantly limits net conversion efficiency and hinders scalable photonic integration. In this work, a low-loss PPLNOI ridge waveguide with a length of 7 mm was fabricated using ultra-smooth sidewalls through photolithography-assisted chemo-mechanical etching followed by high-voltage pulse poling with low cost. The average surface roughness was measured to be only 0.27 nm, resulting in a record-low propagation loss of 0.11 dB/cm in PPLNOI waveguides. Highly efficient second harmonic generation was demonstrated with a normalized efficiency of 1643% W·cm without temperature tuning, corresponding to a conversion efficiency of 805%/W, which is close to the best conversion efficiency reported in nanophotonic PPLNOI waveguide fabricated by expensive electron-beam lithography followed by dry etching, and the absolute conversion efficiency reached 15.7% at a pump level of 21.6 mW. The normalized efficiency can be even improved to 1742% W·cm at the optimal temperature of 59C.