Femtosecond pulse compression and adiabatic following in semiconductor amplifiers

The authors report on their studies of a plane wave femtosecond pulse propagating in a semiconductor amplifier. Preliminary results on the extension of the study to include transverse spatial effects necessary to model realistic waveguiding and broad area amplifier structures is also discussed. The overall conclusion of the study for a plane wave pulse is that, after initial linear amplification and saturation, the pulse undergoes strong intensity and spectral deformation before settling into a strongly compressed intense superluminal pulse undergoing adiabatic following (AF) with off-resonant noninverted states, well above the chemical potential. In this AF regime the pulse continues to sharpen and grow in intensity. This scenario appears to hold irrespective of the initial pulse intensity or its carrier frequency offset from the gain peak. However, the transient evolution of the pulse intensity and spectrum is very sensitive to the initial detuning of the carrier frequency from the linear gain peak. Dynamic bandgap renormalization leads to a downshift of the peak gain as the carrier density is driven down on the leading edge of the pulse as it saturates. This in turn leads to linear amplification of that part of the pulse spectrum that lies under the maximum of the gain