Organic nonlinear optical DAST crystals for electro-optic measurement and terahertz wave generation

We review the recent developments of crystal growth, electro-optic (EO) measurements and terahertz (THz) wave generation of organic nonlinear optical (NLO) 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). DAST crystals are expected to apply high-performance electric-field sensors, THz wave generators, and so on. We had developed a new technique called the slope nucleation method (SNM) for the growth of high-quality DAST crystals. Using SNM, we could effectively grow high-quality DAST crystals as compared to conventional spontaneous nucleation method. The EO sensitivity of DAST crystals depended on the crystal thickness and exhibited an increase of more than 45-fold that of the commercial KTiOPO₄ (KTP) crystals. The high-quality DAST crystals could also generate a coherent, widely tunable THz wave in the range of 2-31.5 THz by using the difference frequency generation (DFG) based on the KTP-optical parametric oscillator (OPO). The authors also demonstrated the efficient crystallization by irradiating femtosecond laser pulse into the supersaturated solution. The laser irradiation increased the probability of nucleation. Aging the solution prior to laser irradiation could enhance the probability of nucleation. These results indicate that the laser irradiation could promote DAST nucleation in the supersaturated solution.