$KNbO_{3}$ is one of the best materials for efficient nonlinear frequency conversion, due to its large nonlinear optical coefficient ($d_{33}$ = 20.6 pm/V), wide transparency range (0.4 ~ 5 $\mu m$) and freedom from photorefractive effects. But wide practical application of $KNbO_{3}$ crystals has been prohibited because it is difficult to fabricate single crystals of large size and single domain. In this thesis, growth of $KNbO_{3}$ crystals and fabrication of periodically poled $KNbO_{3}$ are studied.
$KNbO_{3}$ crystals were grown by the top seeded solution growth (TSSG) technique with pulling using the [110] seed. Crack-free, large $KNbO_{3}$ crystals of good quality was obtained with 40 × 40 × 32 $mm^3$ in size and 99.8 g by weight. For fabrication of single-domain $KNbO_{3}$ crystals, the direction of crystal axes of the largest domain was determined utilizing optical method and the polarity of the c-axis was examined with pyroelectricity. By applying an electric field of 420 V/mm-450 V/mm along the c-axis at 120-190 ℃ for 12 hours, single domain $KNbO_{3}$ crystals of 7 × 13 × 12 (a × b × c) $mm^3$ and 12 × 16 × 7 (a × b × c) $mm^3$ were obtained.
To investigate the characteristics of polarization reversals of single domain $KNbO_{3}$ crystals, A hysteresis loop was measured and dynamical behavior of coercive fields was understood with relaxation of internal field. The analysis of electric field for backswitching showed that the internal field relaxed according to a stretched exponential function of time, and the relaxation times for forward and reverse polings were 35.1 ms and 45.8 ms, respectively. As the poling field rose higher than 600 V/mm, 60$^\circ$ domains (or 120$^\circ$ domains) were generated in the surface layer of 7.8 $\mu m$ thickness, which might be ascribed to a neutralizing process of space charges that were placed in the surface layer of the 180$^\circ$ domains.
Based on the findings above, periodically poled structures were fabricated using a single domain $KNbO_{3}$ crystal plate. To avoid generating 60$^\circ$ domains in the surface layer and to facilitate the formation of 180$^\circ$ domains along the field direction, a modified triangular pulse was designed with a peak intensity of 525 V/mm and a pulse width of 250 msec. A periodically poled $KNbO_{3}$ of very high quality was fabricated with a dimension of 3 × 4 × 0.925 (a × b × c) $mm^3$ and a period of 32.5 $\mu m$ to utilize the $d_{31}$ component of the second-order nonlinear optical coefficient tensor.
An efficient second-harmonic generation of a mode-locked Nd:YAG laser, operated at 1064 nm with a pulse width of 35 ps, was observed in the periodically poled $KNbO_{3}$ with a maximum conversion efficiency of 34.8%.
