Green frequency upconversion fluorescence of symmetrical 1,3,4-oxadiazole derivative by femtosecond three-photon excitation

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Multiphoton-absorption properties of π -conjugated symmetrical A - π - D - π - A chromophore, 3,6-Bis[2-{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenyl}-vinyl]-N-ethylcarbazolyl, has been experimentally studied by using femtosecond laser pulse. At the wavelengths from 1205nm to 1570nm, the molecule shows large three-photon absorption (3PA) properties. The 3PA cross section reaches 4.5 * 10^-24cm⁶·GW^-2 at 1255nm. The output/input characteristic curve measured by nonlinear transmission method demonstrated a good optical power limiting performance based on 3PA mechanism. Luminous blue-green (about 477nm) up-conversion fluorescence excited by infrared laser pulse from 1205nm to 1570nm was easily observed in the quartz cuvette along the laser propagation direction. The material was also theoretically studied using Gaussian 03' software. The geometry, electronic structure, ground state charge distribution and excited state charge distribution were systematically calculated by HF/6-31G(d) and CIS/6-31G(d) method. Calculation results show that the obvious three-photon absorption and frequency up conversion fluorescence of this chromophore is attributed to its rigidity of chemical structure and effective charge transfer from the excited terminal units to the π -conjugated bridging units. The 1,3,4-oxadiazole is considered to be a good electron-acceptor and the key part in this charge transfer route. Particularly, for the first time we give the analytical solution to nonlinear transmission in a 3PA process when the incident beam has a Gaussian spatial profile.