Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino- N ′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from −100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from −80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.