Three-photon-absorption-induced fluorescence and optical limiting effects in an organic compound.

Three-photon-absorption-induced frequency-upconversion f luorescence emission has been observed in a solution of 2,5-benzothiazole 3,4-didecyloxy thiophene (BBTDOT) in tetrahydrofuran, pumped with 10-ns Q-switched 1.06-microm laser pulses. The spectral peak of the observed fluorescence emission is located in the 450-480-nm range, and the intensity dependence of the visible emission on the IR excitation obeys the cubic law. At a higher solute concentration (0.18 M/L) and moderate IR excitation intensity levels (50-200 MW/cm(2)), obvious optical limiting behavior has been observed as a result of three-photon absorption. The measured nonlinear absorption coeff icient and the corresponding molecular three-photon-absorption cross section are gamma= 2.7 x 10(-18) cm(3)/W(2) and sigma'(3) = 8.8 x 10(-76) cm(6) s(2), respectively.