HIGH-LASER-DAMAGE-THRESHOLD POTASSIUM DIHYDROGEN PHOSPHATE CRYSTALS

Development of solid-state laser drivers of higher operating fluence is dependent upon the damage resistance of the frequency conversion crystals. We have demonstrated the correlation between the purity of the crystal growth solution and the laser damage threshold of single-crystal potassium dihydrogen phosphate (KDP). Impurities introduced in the growth process can be atomic species, inorganic or organic compounds (dissolved or particulate), or bacteria. We have developed a purification process for KDP that minimizes contamination: initial recrystallization of bulk KDP followed by ozonation to remove oxidizable material, ultrafiltration to remove nonoxidized particles, and UV-light exposure to suppress bacterial growth. We have also developed a crystal growth method that excludes load-bearing surfaces, which are a potential source of particulate contamination, from the growth environment. The method, which incorporates continuous filtration and continuous flow of the growth solution has yielded crystals with damage thresholds of 28.6 J/cm2 at 355 nm and greater than 64 J/cm2 at 1064 nm (10-ns pulse length).