UP2(2): Phase-resolved pump-probe experiments on a QCL

W. Kühn, P. Gaal, K. Reimann, M. Wörner

Fig. 3 (a) Pump-induced change of the transmission through the QCL as a function of pump-probe delay τ for different currents. (b) Fourier transform of the transmission changes shown in (a). (c) Pump-induced shift of the phase of the transmitted pulse as a function of pump-probe delay τ.

We investigated the ultrafast absorption and gain dynamics in a Ga_0.47In_0.53As/Al_0.48In_0.52As quantum cascade laser (QCL) under stationary bias by pump-probe measurements [KPG08]. The detection with electrooptic sampling allows for a clear separation of gain/absorption dynamics from changes of the refractive index. Fig. 3(a) shows the transmission change induced by the pump pulse, plotted as a function of τ for different values of the laser current I. Below threshold, the QCL studied here displays strong absorption on the laser transition. The positive transmission changes observed below threshold [Fig. 3(c), I = 0 and 150 mA] are due to a bleaching of this absorption. The recovery of this absorption requires the depopulation of the upper subband and the repopulation of the lower subband. Accordingly, the time constant for the absorption recovery is determined both by the electron lifetime in the upper subband and by electron heating and cooling within the manifold of states in the injector. The transmission change decays nearly to zero reflecting the complete repopulation of the lower subband. Above threshold, the pump pulse saturates the gain, in this way depleting the quasistationary population inversion and inducing a negative transmission change. Such kinetics is superimposed by oscillations with a frequency of 0.8 THz. The fast initial compo-nent of the gain recovery gives evidence of a very efficient electron supply from the injector through the injection barrier into the active part of the QCL structure. The oscillations originate from coherent electron tunneling through the injection barrier.

UP2(3): Phase-resolved two-dimensional spectroscopy in the mid-infrared spectral range

W. Kühn, K. Reimann, M. Wörner

A novel method for time-resolved two-dimensional spectroscopy has been developed. The combination with field-resolved detection allows for a collinear beam geometry and the measurement of optical nonlinearities of arbitrary order. A first application is shown for an n-type modulation-doped multiple quantum well structure (see Fig. 4). Our approach allows the detailed analysis of all types of nonlinear optical signals, for instance the third-order four-wave-mixing signal, which is in agreement with former results for this homogenously broadened system. As a novel feature, we observe a quantum beat with the LO-phonon frequency of GaAs that is presently being analyzed.