Frequency-dependent dephasing in an inverted semiconductor

Recent theoretical calculations have shown a strong k dependence of the dephasing time across the gain region at the transparency point and in the absorption region of an inverted semiconductor. In the presence of a dense plasma, it is predicted that the dephasing rate approaches zero at the transparency point and increases in both the gain and absorption regions. We have obtained the first complete experimental verification of this phenomenon by using femtosecond-DFWM and spectral-hole-burning measurements in an optically excited semiconductor. In this study we prepared a low-temperature semiconductor in a state of optical gain and then measured the dephasing time at, below, and above the Fermi edge.