Monitoring the excited state of a fluorophore in a microscope by stimulated emission

A two-pulse experiment is described, using stimulated emission to reduce the fluorescence of 1-ethyl-4-(4-(p-dimethylaminophenyl)-1,3-butadienyl)-pyridinium perchlorate (Pyridine 2) in a microscope of high numerical aperture. The experiment employed a 130 fs pulse at 375 nm for excitation and a 20 ± 5 ps pulse at 750 nm for stimulated emission. The pulses were provided by a mode-locked Ti:sapphire laser. The 375 nm excitation pulse was obtained by frequency-doubling and the 20 ± 5 ps infrared pulse by optical grating dispersion. The population of the excited state was monitored by varying the temporal delay between the excitation and stimulating pulse. This method enabled the measurement of the lifetime of the dye. For Pyridine 2 in glycerol, we determined a lifetime of 0.86 ± 0.2 ns. The decrease of fluorescence was due to stimulated emission, as was established by measurements of the temporal behaviour of the fluorescence signal upon amplitude modulation of the stimulating beam. We present a theoretical analysis of the temporal behaviour of depletion by stimulated emission in a two-pulse experiment.