Multi-scale second harmonic generation microscopy of ferroelectric domains in x-cut thin-film lithium niobate

Thin-film lithium niobate (TFLN) is a widely used platform for nonlinear frequency conversion, as its strong nonlinear susceptibility and enhanced modal confinement intensify nonlinear interactions. Frequency doubling from NIR to visible wavelengths necessitates fabrication of quasi-phase matching (QPM) gratings with minimal period variation (<20nm) and control of ferroelectric domain inversion at the micron-scale along centimeter-long waveguides. Second harmonic generation microscopy (SHM) is a powerful tool for optimizing domain engineering (E-field poling), and it enabled the fabrication of near-ideal QPM gratings. Here, we show that increasing the SHM raster scan step size from 200nm to 400nm results in a 4x imaging speedup without sacrificing the accuracy of QPM grating characterization. To that end, Monte Carlo simulation of the coupled rate equations agreed with experimental measurements of second harmonic output power. We also employed a statistical subsampling scheme to characterize 5.6 mm-long waveguides (poling period = 3.240um) in approximately 5 minutes (30 seconds per field x 10 fields per waveguide). Each field is 100 microns in length, so our results indicate that sampling only ~300 periods of a QPM grating is sufficient to accurately predict its second harmonic output. For the device characterized, this corresponds to ~20% of the total grating length. Together, discretization and device-length subsampling speed up SHM imaging by an order of magnitude.