Ultrafast transmittance changes induced by exciton or carrier photogeneration in InGaAs/InP quantum wells

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Abstract
Using a widely tunable femtosecond laser source based on an optical parametric amplifier, we performed pump–probe measurements of the dynamics of the nonlinear transmittance on an InGaAs/InP multiple-quantum well sample, with pump and probe beams tuned to the same photon energy as well as with a continuum probe. The pump photon-energy range was 0.8–1 eV. The main results are the following: (i) the ionization time of the n = 1 heavy-hole excitons is 170 fs; (ii) bleaching effects were found to be instantaneous within the temporal resolution of our experiments (50 fs), irrespective of probe photon energy, for excitation within the n = 1 subbands; (iii) the n = 2 to n = 1 intersubband transition time was 1 ps; and (iv) the relative strength of exciton-induced and carrier-induced nonlinearities depends crucially on the photon energy of the probe beam. The transmittance spectra of the multiple-quantum-well system were calculated in the presence of either a cold exciton gas or a quasi-equilibrium carrier plasma. We found that there are conditions in which the ratio of the transmittance variations induced by cold excitons and by thermalized carriers diverges, as experimentally observed.