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Femtosecond transient absorption spectrometer  



Our femtosecond transient absorption spectrometer is a homemade pump-continuum-probe setup. The main femtosecond laser source is a Ti:Sapphire system from Spectra-Physics (Tsunami oscillator followed by a Spitfire regenerative amplifier). It delivers 50-fs ~0.7 mJ pulses at 1 kHz around 800 nm. Tunable pump pulses in the visible are produced by a noncollinear optical parametric amplifier (NOPA, Clark-MXR). A broadband continuum probe is obtained by self-phase modulation of a 800-nm beam in a CaF2 plate. Its time delay with respect to the pump is adjusted in a motorized optical delay line. The pump and probe beams are focused with parabolic mirrors and overlapped in the sample, kept in a 1-mm cell. The sample may be constantly displaced in two directions to avoid the re-excitation of a previously excited area. The sample may also be thermostated with the help of a water bath circulated through the sample holder. Fig 1 gives a schematic view of the setup. Its time resolution of the order of 100 fs and it allows broadband probing over the entire visible region.



Fig. 1 : Schematic view of the transient absorption setup. Ch: chopper. CR: corner reflector. l/2: half-wave plate. PM: parabolic mirror. CaF2: fluorite plate. F : filter. BS: beam splitter. Ph: photodiode.



Femtosecond fluorescence up-conversion setup



The same femtosecond laser source as above is used to power a commercial fluorescence up-conversion spectrometer (FOG100-DA, CDP Systems), operating à 1 kHz. In brief, the pump-induced fluorescence is collected and mixed with an ultrashort pulse (the gate) at 800 nm in a nonlinear optical crystal (BBO). An up-converted fluorescence beam is generated when the gate and fluorescence pulses temporally overlap. Fig 2 gives a schematic view of the setup. Its time resolution of the order of 200 fs. Spectral resolution is achieved by tuning the phase-matching condition of the nonlinear crystal.



Fig. 2 : Schematic view of the fluorescence up-conversion setup.