Thermal enhancement of defect motion for optimizing periodic poling of x-cut thin-film lithium niobate

Patterning of stable, spatially tailored ferroelectric domains in thin-film lithium niobate enables efficient nonlinear optical interactions through quasi-phase matching. The engineering of domain structure is limited by the uncontrolled distribution of defects, which disrupt domain wall motion. Here, we fabricate quasi-phase matching gratings in thin-film lithium niobate with sub 20 nm of period variation. We demonstrate that annealing processed samples at 350 or 500 °C for 48 h, prior to E-field poling, can dramatically reduce the duty cycle variation. We show that maintaining an elevated temperature of 200 °C during poling enhances defect mobility, which leads to more rectangular inverted domains. Moreover, poling at elevated temperatures also increases inversion depth without sacrificing the periodic domain pattern's accuracy or precision. Elevating the temperature prior to and during poling resulted in near-ideal square wave patterning of ferroelectric domains (50% mean duty cycle, sub 10% domain width variation, and 100% depth inversion). This enables effective quasi-phase matching for second harmonic generation in 5.6 mm-long waveguides fabricated from MgO-doped x-cut thin-film lithium niobate.