Ultrafast fluorescence upconversion

Underwood, D. E., Kippeny, T. and Rosenthal, S. J. (2001) Ultrafast carrier dynamics in CdSe nanocrystals determined by femtosecond fluorescence upconversion spectroscopy. /. Phys. Chem. B, 105,436-443. 

The apparatus can be divided into three main sections, laser-pulse production, amplification, and fluorescence upconversion. The ultrafast lasers employed for upconversion are those that have been described above. The experiment is a crosscorrelation between the fluorescence pulse and the laser pulse. At time t = 0, the sample is excited by the ultrafast laser pulse. The resulting incoherent fluorescence (frequency = coa) is collected and mixed in a nonlinear crystal (such as KDP) with the gating pulse (frequency = cui) derived from the excitation source and arriving at time t = z. This frequency mixing of the fluorescence and probe beams generates light at the sum frequency, cosum... 

To reveal possible ultrafast processes occurring on a time scale less than 30 ps, femtosecond fluorescence upconversion experiments were performed [30] in toluene under magic angle polarization. To extract complete information of the decay times and their amplitudes in function of detection wavelength, the measurements were performed in three time windows of 5 ps, 50 ps, and 420 ps. 

Ultrashort fluorescence lifetimes are best determined by photon upconversion. An excellent description of the technique for non-specialists wishing to set up an experiment is in preparation as part of a current IUPAC project on ultrafast intense laser chemistry .181 The time resolution is basically limited by the temporal width of the laser pulses that are being used. An intense gate pulse at frequency vq is mixed with the fluorescence at a frequency vp in a nonlinear optical crystal (Special Topic 3.1), to create a short pulse at the sum frequency i j. The gate pulse thus represents a time window for the fluorescence... 

In fluid solvents at room temperature, spectral relaxation is usually comjdete prior to emission and occurs within abont 10 ps. This process is too rapid to be resolved with the usual instrumentation for TD or FD fluorescence. However, advances in laser technology and methods for ultrafast spectroscopy have resulted in an increasing interest in picosecond and femtosecond solvent dynaaiics. Becmise of the rapid timescale, the data on solvent dynamics are usually obtained using flurvescence upconversion. Hiis method is described in Section 4.7.C. lypical data are... 

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