The utilization of nonlinear optical crystals for optical communications and signal processings requires single crystals of high perfection and basic understandings of their structural and optical properties. In this study the single crystal growth and physical properties of an organic material, meta-nitroaniline(mNA) and an inorganic material, $KNbO_3$ were investigated.
Crystals of organic nonlinear optical material, mNA were grown for the first time using the seeded supercooled melt technique. The perfection of the crystals were assessed by using the synchrotron X-ray topography. the UV cutoff wavelength of mNA crystal was measured to be 510nm, and the conversion efficiency of the 5 mm thick crystal for type-I phase matching at θ =90˚ and $\phi$ = 55˚, is 13.7 % when the energy of the incident 1064nm Nd:YAG laser beam was 1.2mJ.
$KNbO_3$ crystals were grown by the top seeded solution growth (TSSG) technique using the [100] and [110] seeds. It was found that crystals grown by using the [110] seed yielded better results. Multi-domains develop in $KNnO_3$ crystals through the two structural phase transitions during the cooling process from about 1060℃ to room temperature. However a single domain single crystal was successfully fabricated by electric poling technique at about 200℃ with the applied field of 4 kV/cm. Further a single domain single crystal of a size 6 × 9 × 11 ㎣ could be grown without after-growth electric poling by careful control of temperature lowering.
The cutoff wavelength of $KNbO_3$ was measured to be 327nm, and the laser damage threshold was 234 MW/㎠ for the incident energy of 3.68mJ. The second harmonic conversion efficiency of $KNbO_3$ crystals at the phase matching angle of θ = 90˚ and $\phi$ = $45˚ was 15.7% when the incident 1064nm bean energy was 2.97mJ, while the second harmonic efficiency goes up as high as 19.7% at the phase matching angle of θ = 19˚ and $\phi$ = 0˚ when the incident beam energy was 1.8mJ with pulse width of 8 ns and repetition rate of 10 Hz.
Raman spectra for the $B_1$ mode was measured as a function of temperature in the orthorhombic phase. It was found that one of four $B_1$ modes, which has a wavenumber of $249cm^-1$, was a good indicator for probing the phase transition from orthorhombic to tetragonal.
The domain structure of pseudocubic {100} plane of $KNbO_3$ single crystal was investigated by using polarizing microscope, and two types of domains, 60˚ and 90˚ were observed. The 60˚ domain walls were inclined to {100} plane with an angle of 47˚ and the domain width was ranged from 5 to 20 microns. The 90˚ domains were distributed inbetween 60˚ domain walls with a uniform width of 5μm. After the phase transition from orthorhombic to tetragonal the 60˚ domains were disappeared and new 90˚ domains were generated.
The dielectric measurements of $KNbO_3$ single crystals showed that the piezoelectric resonances occurred in the frequency range of 00 kHz to 10 MHz in the ferroelectric orthorhombic and tetragonal phases. The resonance was also observed in the paraelectric cubic phase in the frequency range of 4 to10 MHz. Very interesting dielectric dispersions were observed near the phase transition temperature in the cubic phase. As temperature approaches the transition point the resonance dispersion was gradually overtaken by the relaxation dispersion. This could be explained by the piezoelectric resonance and the dipole reversals of the changes which are distributed noncentro symmetrically in the grain boundaries.
