Optical properties of semiconductors via pump-probe experiments: statistical-thermodynamical approach, hot plasma and coherent phonon states

We present an analysis of the nonequilibrium thermodynamics and, mainly, a response function theory for the study of optical properties in ultrafast-s pectroscopy pump-probe experiments. These experiments give rise to the form ation of a photoinjected plasma in semiconductors in far-from-equilibrium c onditions. The dissipative processes that evolve in this medium greatly inf luence optical and transport properties. The theory is centred on the appli cation to the study of the phenomenon of modulated changes in the time-reso lved reflectivity spectrum. In particular we show that this phenomenon cons ists in the coupled effect of coherent-LO-phonon and carrier-charge-density motions, which are driven through the action of the coherent photons of th e laser electromagnetic radiation. In the given conditions the modulation e ffect decays in time and has associated a frequency close to the zone-centr e upper LO phonon-optical plasma hybrid mode, as experimentally observed.