{'fraction_non_alphanumeric': 0.03467879476975554, 'fraction_numerical': 0.017055144968732235, 'mean_word_length': 5.219081272084805, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Periodically poled vapor transport equilibrated lithium niobate for visible light generation

The ability to achieve high quality periodic poling in lithium niobate (LN) has allowed quasi-phase-matching to be used for second-order nonlinear optics, leading to experimental demonstration of efficient optical frequency generation throughout its wide transparency range (0.35-4.5 microns). Applications of congruent lithium niobate involving visible or ultraviolet wavelengths are limited to low power or high temperature operation due to the effects of photorefractive damage (PRD) and green-induced infrared absorption (GRIIRA). The standard methods of suppressing PRD include doping with 5 mol-% MgO or ZnO and varying crystal stoichiometry. More recent methods employ a combination of lower doping level and near-stoichiometric composition. We use vapor transport equilibration (VTE) and significantly lower MgO doping (<0.5% in the melt) to obtain near-stoichiometric PRD-resistant crystals with improved parameters for periodic poling compared to the commercially available 5% MgO-doped congruent crystals. An efficient process for periodic poling at room temperature using baked photoresist as a patterned dielectric on one crystal surface with LiCl-solution electrodes was developed for periods as short as 8.3 microns for 0.5% and 7 microns for 0.3% MgO-doped VTE:LN. The quality of periodic poling improves as the MgO concentration is lowered. Stable second harmonic generation of 1.3-W continuous-wave 532-nm radiation was observed near room temperature (43 degrees Celsius, as determined by the phase matching condition) with no sign of degradation in a 1.5-cm long crystal of 0.3-% MgO-doped VTE:LN periodically poled with a period of 7.06 microns.