Frequency-domain approach to evaluation of data obtained in optical pump-terahertz probe experiments

Describes the increasing effort to develop optical pump-terahertz probe (OPTP) experiments as a tool for studying transient processes following excitation by an optical pulse, e.g. in semiconductors, superconductors and solvated chromophores. If the timescale of the investigated process is much longer (hundreds of ps) than the duration of the THz probing pulse, the experimental data can be evaluated using the formalism for steady-state cases: the measured THz waveforms provide complex spectra as a function of the pump-probe delay. However, if the relaxation occurs within picoseconds or faster, this approach will fail, since different parts of one temporal waveform correspond to different states of the sample, resulting in a frequency mixing. Thus, a method is necessary to find the unknown non-equilibrium part of the response function. We have developed a general frequency domain theory which can serve as a guide for performing the experiment.