Subpicosecond time-resolved fluorescence spectrometer using a Ti:sapphire laser

In this paper a subpicosecond time-resolved fluorescence spectra's measurement apparatus is presented. The principle of measurement is based on sum frequency in nonlinear crystal between the pump laser pulses and the fluorescence from sample stimulated by excitation laser pulses. The fluorescence decay can be obtained by varying time delay of the excitation laser pulses to the gating pulses. Adjusting the phasematching angle of nonlinear crystal records the fluorescence decay at various wavelengths. In this spectrometer, a CW mode-locked Ti:sapphire tunable laser is used, with approximately 100 fs pulse width and 82 MHz repetition. The IR light from the lasers is frequency doubled to excite the sample with the remained unconverted light used as gating pulses. A very thin BBO (500 micrometers ) is selected to be a sum frequency nonlinear crystal. The time resolution of the apparatus mainly depends on the pulse width of laser and the effects of group velocity in the crystal. The upconversion UV light is detected by a single photon counting system. The delay line driven by stepper motor is controlled by computer. This computer controls the course of experiments and accumulates data meanwhile. Some experimental results are also presented in study of dynamics of photosystem II reaction center which is isolated and purified from higher plants.