Optical Properties of Photobleached DAST Molecular Crystals in Terahertz Domain

Highly nonlinear optical 4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST) crystals have been successfully grown. The photobleaching effect was demonstrated by irradiation with an UV light at 405 nm during 120 h. The UV-Vis and terahertz absorption spectra of DAST in crystalline states before and after photobleaching were measured and compared. Results of photobleaching modification of DAST monocrystal have been demonstrated with the help of differential interference contrast microscopy (DIC). A possible explanation of photobleaching effect has been suggested. Since absorption in the terahertz range of crystalline materials is associated with vibrational modes of the crystal lattice, we can conclude that photobleaching leads to its destruction. Taking into account the obvious change of crystalline spectra in the UV-Vis and terahertz frequency range and the absence of changes in the molecular spectra from irradiated DAST in ethanol solution, we can conclude that the photobleaching process may be a photoactivated destruction of chemical in DAST molecules. The photobleaching technique was proposed to implement quasi-phase matching (QPM) structures for efficient nonlinear conversion by optical rectification.