Time delayed four wave mixing

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.

The forward folded boxcar four-wave mixing technique is illustrated in Fig. la and lb. Three pulsed laser beams are directed, through variable delay lines, toward the sample. Two time-coincident short excitation pulses and a third delayed probe beam cross in the sample. Figure 1 shows the beam geometries on the surface of the sample and at the aperture plane. The three incident beams are labeled 1, 2, and 3. Beam 4 is the signal beam, generated by the input beams via The... 

Fig. 13. Schematic for backward scattered (phase conjugate) degenerate four-wave mixing. (BS = beam splitter, M = mirror.) The pump beams (1) and (2) are collinear and counterpropagating. The probe beam (3) when incident on the sample leads to the creation of the phase conjugate beam (4), which is detected and compared to a suitable reference. The delay lines are included so that the timing of pulsed light arriving at the sample via paths (1), (2), and (3) can be varied.

Dynamic four wave mixing experiments in the nematic phase were performed by Eichler and Macdonald [150] and Khoo et al. [151, 152] using picosecond lasers. They have observed that the short excitation pulse is followed by a delayed reorientation process, indicating a large inertial moment. The observed dynamics were explained by flow-alignment theory, taking into account translational motion of the molecules under the action of the optical field. Build-up and decay times of the diffraction grating were... 

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