Two-photon excitation and optical spatial-profile reshaping via a nonlinear absorbing medium

Two-photon processes have recently received considerable attention, as they offer opportunities for both fundamental research and technological applications. In this paper, we illustrate both of these opportunities by reporting on a study of two-photon properties and discussing one specific application of a new chromophore, tris[4-(7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)phenyl]amine (AF-350). This new compound exhibits a large two-photon absorptive cross section and, more importantly from-the application point of view, a high photochemical/photothermal stability. The nonlinear optical properties of an AF-350 solution were studied with ∼800-nm laser pulses in both nanosecond and femtosecond regimes. The two-photon excited fluorescence spectrum and temporal behavior were compared with the corresponding results obtained for one-photon excitation. There is an ∼11-ps delay between an ultrashort pump pulse and the first peak of the two-photon induced fluorescence signal, whereas no delay was measured between the pump pulse and the first peak of the one-photon induced fluorescence. The measured effective two-photon absorption (TPA) cross section is σ₂ = (151 ± 23) × 10^-20 cm⁴/GW for 7-ns, 810-nm laser pulses and σ₂ = (0.61 ± 0.02) × 10^-20 cm⁴/GW for 135-fs, 796-nm laser pulses. One specific application reported here is the spatial-profile reshaping and smoothing of a focused laser field.