Isotropic fabrication of centimeter-scale, low propagation-loss periodically poled lithium niobate nanophotonic waveguides for efficient second harmonic generation

Abstract: Periodically poled lithium niobate (PPLN) nanophotonic waveguides that simultaneously feature low propagation-loss and uniform periodic poling are essential for a wide range of applications ranging from classical nonlinear frequency-conversion to scalable integrated quantum technology. However, fabrication imperfections have frequently limited the propagation loss of fully domain-inverted PPLN nanophotonic waveguides to a few dB/cm, primarily due to anisotropic etching issue, thereby restricting the absolute conversion efficiency and scale of photonic integration. Here, we present a fabrication approach that overcomes this challenge, yielding a 1.2-cm-long PPLN nanophotonic waveguide with low propagation loss via femtosecond-laser photolithography-assisted chemo-mechanical etching (PLACE). By carrying out domain inversion on a planar thin-film prior to waveguide definition, electric-field distortion is minimized during poling, while isotropic etching of the waveguide is achieved by PLACE with an average surface roughness of only 0.34 nm, resulting in uniform poling of duty cycle of 50% and a record-low propagation loss of 0.042 dB/cm in the telecom band. Under continuous-wave pumping at 1525 nm, the device demonstrates a high normalized quasi-phase-matched SHG conversion efficiency of 2021%/W, and an absolute conversion efficiency of 64% at a pump power of 86 mW which represents the state of the art for single-period PPLN nanophotonic waveguides.