Properties of nonlinear optical absorption and refraction of rapidly grown KDP crystals

Potassium dihydrogen phosphate (KDP) single crystals grown under different conditions were obtained by the “point seed” technique. Third-order nonlinear optical properties of type-II samples which were prepared from the pyramidal and prismatic sections of crystal, respectively, were systematically investigated using the Z-scan technique at λ = 532 nm. Some heat-treated samples have been studied simultaneously. The results show that the nonlinear optical absorption and nonlinear optical refraction have been reflected, meanwhile, the nonlinear refractive index (n2) is positive, suggesting the generation of self-focusing effect. Furthermore, the results indicate that the nonlinear absorption and nonlinear refraction were associated with the growth conditions of the crystal. The prismatic sample grown under the relatively low growth temperature range (45∼25 °C) and small supersaturation (4 %) possessed a smaller nonlinear absorption coefficient (β) and n2 (β=4.73 × 10−13 m/W, n2=2.82 × 10−13 esu) than that of the others. In addition, the values of nonlinear absorption and nonlinear refraction of the samples can be further reduced after thermal annealing. These results may provide an assist to understand the mechanism of laser-induced damage of KDP crystals.