Contact photocurrent measurements or contactless microwave photoconductivity can be used for this purpose. The key aspect is to create optically excited free carriers and monitor their transport in the applied electric field. One can do steady-state or kinetics, single wavelength or spectrally-resolved measurements. One of the useful textbooks may be "Photoelectronic Properties of Semiconductors" by R. Bube:
http://www.amazon.com/Photoelectronic-Properties-Semiconductors-Richard-Bube/dp/0521406811
P.S. Adding optical excitation to Hall measurements is also possible. The resulting method is known as a photo-Hall technique.
To my opinion, the frequency tunable two-photon exitation as known spectroscopy direction would be quite new and usefull for study of photoconductivity because the essential differences in relation of free carrier generation in compare two one photon exitation. The differences will result from other transition rules for two photon absorption and due to another distribution of generated carrier in semiconductor volume in compare to distribution at one photon exitation.
Pump-probe spectroscopy will help you to measure the optical conductivity/
Let me clearly understand your question. What you are interested in is the frequency dependence of photoconductivity in which photogenerated carriers undergo relaxation (and maybe trapping) before contributing to conductivity or optical conductivity as proportional to the imaginary part of the dielectric susceptibility at optical frequencies?
In the latter case, one possible way to go (if feasible) is to measure reflectivity of your samples and perform Kramers-Kroenig transformation of your data. I would suggest reading the classical textbook: F. Wooten, Optical Properties of Solids (Academic Press, 1972).
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