We present an analysis of the nonequilibrium thermodynamics and, mainly, a response function theory for the study of optical properties in ultrafast-spectroscopy pump-probe experiments. These experiments give rise to the formation of a photoinjected plasma in semiconductors in far-from-equilibrium conditions. The dissipative processes that evolve in this medium greatly influence optical and transport properties. The theory is centred on the application to the study of the phenomenon of modulated changes in the time-resolved reflectivity spectrum. In particular, we show that this phenomenon consists in the coupled effect of coherent-LO-phonon and carrier-charge-density motions, which are driven through the action of the coherent photons of the laser electromagnetic radiation. In the given conditions the modulation effect decays in time and has associated a frequency close to the zone-centre upper LO phonon-optical plasma hybrid mode, as experimentally observed.