Two-Photon Excited Fluorescence and Second-Harmonic Generation of the DAST Organic Nanocrystals

The nonlinear optical properties of the size-controlled trans-4[4-(dimethylamino)-N-methylstilbazolium] p-tosylate (DAST) organic nanocrystals (ONCs) have been investigated. The size and monoclinic crystal structure have been demonstrated by the scanning electron microscope, transmission electron microscope, and X-ray diffraction measurement. The DAST ONCs exhibited 6-fold enhanced fluorescence quantum yield and prolonged fluorescence lifetime compared to DAST molecular solution. The evaluations of two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) microscopes have been implemented for investigating the optical nonlinearities of DAST nanocrystals. The TPEF intensity of DAST ONCs was improved by a factor of 37.3 compared to DAST molecular solution. The extraordinary large two-photon absorption cross section up to 1.72 x 10(6) GM was observed. DAST ONC was demonstrated as a single crystal from the four-lobe polarization response in the SHG microscope. These results would provide DAST ONCs the opportunity for the potential application in photonics and bioimaging as nonlinear optical materials.