Three-Photon Absorption and Upconversion Fluorescence Properties of Series 1,3,4-Oxadiazole Derivatives

A newly synthesized 1,3,4-oxadiazole derivatives have been studied using a femtosecond Ti：sapphire laser system. The series molecules present strong three-photon absorption and frequency upeonversion fluorescence at wavelengths from 1205 nm to 1575 nm. Furthermore, there is no proportional relationship between three-photon absorption cross sections and the chemical structure transformation from monomer, dimer to trimer. Effective charge-transfer distance by π-conjugation bonds may be the contributing factor. In the experimental design, the far-field intensity distribution of femtosecond laser beam has been taken into account. We give the optimized analytical solution of nonlinear transmission in a three-photon absorption （3PA ） process when the incident beam has a Gaussian transverse spatial profile.