Saturation and recovery of the direct interband absorption in semiconductors

We report observation of dynamical saturation of the optical absorption in direct band-gap semiconductors using mode-locked pulses from a Nd: glass laser. The alloys were of the types InAsxP1-x and Ga 1-yInyAs, with x or y chosen so that the direct band gap at room temperature was about 10 meV less than the laser photon energy. Increases in the transmission coefficient of epitaxial thin films by more than 4 orders of magnitude above the equilibrium value have been observed. Using a probe pulse with variable delay with respect to the strong bleaching pulse, we have studied the time dependence of the recovery of the absorption following bleaching and find it to be as short as 30 psec. Thus such alloys may be useful as saturable absorbers for mode locking visible and infrared lasers.