Time-resolved nonlinear fluorescence spectroscopy using femtosecond multiphoton excitation and single-photon timing detection

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Abstract
We have developed a time-correlated single-photon timing nonlinear fluorometer for recording the fluorescence decay times and rotational correlation times of molecular probes using 120 fs regeneratively amplified Ti:Sapphire laser excitation via simultaneous non-resonant absorption of two or more near infrared photons. A microchannel plate photomultiplier giving 70 ps impulse response is used for detection. Studies on 1,6-diphenylhexatriene, rhodamine 6G and p-terphenyl in propylene glycol demonstrate two- and three-photon induced fluorescence characteristics. The radiative properties for one- and multiphoton excitation were found to be identical. However, the time-zero anisotropy observed for multiphoton excitation was larger than for one-photon excitation, indicating an increased degree of orientation of excited molecules after multiphoton absorption. The results reveal the potential of multiphoton-induced fluorescence anisotropy in the study of the structure and dynamics of microheterogeneous systems (i.e. biomembranes, porous matrices etc) by selecting the excitation wavelength and class of probe molecule.