Time-delayed four-wave mixing with

Fig. 2. Geometry of degenerate four-wave mixing (BOXCARS geometry) for short-pulse, time-resolved measurements of the nonlinear response. Beams 1, 2, and 3 are derived from a single laser beam by the use of beam splitters and the beam paths are adjusted for the pulses to arrive simultaneously at the sample. By delaying one of the beams with respect to the others, the time-resolved measurements can be performed.

The differential transmission spectra DT(X)=[T(A,)-To(X)]/To(A,), where T(Z) and T0(X) are the transmission spectra of excited and unexcited films of QDs, were measured. Wide-band picosecond pulse of white light (picosecond continuum) obtained by four-wave mixing of the part of pump radiation focused in the cell with heavy water was used as a probe pulse. The central part of the QD sample s excited area was probed. The application of the optical delay line has allowed to measure the differential transmission spectra of QDs at different time delay with respect to the excitation by ultrashort pulses. 

Fig. 2 Typical correlation traces of transient degenerate four-wave mixing calculated by analytical equations for idealized light sources t (A) coherent short pulses with zero pulse width (tp 0), and (B) incoherent long pulses with zero correlation time (Tq 0). Here, x is the delay time, and Tj and T2 denote the longitudinal (energy) and transverse (phase) relaxation times, respectively.

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