Transient hole burning in exciton-phonon systems

The imaginary part of the nonlinear optical susceptibility experienced by a test field is calculated for a coupled exciton-phonon system in the transient regime when a strong pump field is directed on the system. Hole burning is found in the absorption spectrum when the pump field frequency is either on resonance with the exciton frequency or one phonon frequency lower than the exciton frequency. The results strongly indicate the saturation of excitons in the system excited either directly or phonon-assisted by the pump field. Other characteristic features of the bleaching in the absorption spectrum are found and their physical implications discussed.