Effects of Carrier Cooling and Carrier Heating in Saturation Dynamics and Pulse Propagation Through Bulk Semiconductor Absorbers

Numerical modeling has shown that carrier cooling and carrier heating strongly influence saturation dynamics and pulse shaping in bulk semiconductor absorbers. With no electric field in the absorbing region, carrier cooling leads to strong additional fast saturation of absorption. The saturation causes a substantial decrease in the saturation energies for subpicosecond pulses in comparison with picosecond pulses. Comparison of bulk and quantum-well absorbers shows that fast saturation can be stronger in a bulk absorber, so bulk saturable absorbers may be interesting for usage in mode-locked solid-state lasers. Applying a nonzero electric field to a bulk absorber leads to strong carrier heating, which in turn suppresses absorption saturation. In this case, transition to absorption saturation involves new mechanisms such as screening of the electric field by photogenerated carriers, as well as carrier cooling due to carrier–phonon interaction and by generated cold carriers. Carrier heating by the electric field causes the saturation energy of the absorber to increase with the applied electric field. The increased saturation energy allows one to shorten high-energy picosecond and subpicosecond pulses without increasing the length of the saturable absorber, which could be useful for the generation of high energy pulses with mode-locked and Q-switched semiconductor lasers.