Population-induced nonlinear optical properties near the E1 and E1+Δ1 critical points in Ge quantum dots

We here report on an investigation of the differential transmittance of Ge quantum dots; with average radius between 4 and 16 nm, in the subpicosecond time domain. Pump pulses of 390 nm have been used, and the response has been probed in the spectral range 450 to 750 nm, including in this way the E-1 and E-1 + Delta(1) spectral structures, which have a partially excitonic nature. It is shown that the observed features can be essentially explained in terms of (i) occupancy of the conduction states by the photoexcited carriers and (ii) their correlation-exchange interactions. A comparison with the ultrafast response of Ge bulk is also carried out. Finally the screening of the excitonic interaction and the presence of quantum confinement effect are discussed.