Transient Photomodulation Spectroscopy

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample, hi addition, the pump and probe pulses are synchronized so that the time interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

A critical difference between the transient and CW measurements is that while the CW probe source uniformly illuminates the sample, both the transient pump and probe beams have Gaussian distributions. Equation (7.7) can be rewritten for the transient case as  

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nm (or 2.0 eV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used [12]. Although their time resolution was not better than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 eV to 2.2 eV. In addition, a color center laser (CCL) can be easily 

The probe portion of the beam, on the other hand, goes to the cube mirror on the translation stage, which can move and thereby change the path length of the probe. The displacement of the translation stage determines the time delay between the pump and probe pulses. The time delay can be varied from 1 to 3 ns with the resolution of less than 1 fs. 

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