Figure 1

(a) Four phase stabilized linearly polarized beams obtained from the MONSTR instrument described in Ref. 5 are focused on the sample, which is held in the cryostat at 5 K. The cryostat is mounted at a small angle with the perpendicular to the excitation beams in order to allow for the FWM signal to be collected in reflective geometry. A replica of the focus is monitored with the CCD camera. The interference pattern between the beams is used to obtain the phase difference between laser pulse pairs. (b) The sequence of the laser pulses used in the experiments, where $A^{*}$ corresponds to the phase conjugate pulse. (c) Top view of the experimental setup. In order to maintain phase stability between the FWM signal and the reference beam, a HeNe laser propagates the same path. The HeNe light is reflected back by dichroic mirrors labeled as DCM and the interference pattern is detected with the photodiode (PD). Similarly, as implemented in the instrument (Ref. 5) any mechanical drifts are compensated using a piezoelectric transducer (PZT) driven by a high speed loop filter.Reuse & Permissions

Figure 2

(a) Time integrated FWM for $T=0$ fs. Black squares are the experimental data whereas the red line is a single exponential fit with $T_2\sim 440$ fs. The dashed line corresponds to the instrumental response assuming a 200 fs autocorrelation width. (b) Temperature dependence of the homogeneous width obtained from the cross diagonal profile of the 2DFT spectra. The black squares are obtained from experiment, whereas the red line is fitting using Eq. (1).Reuse & Permissions

Figure 3

Representative two-dimensional FT spectra of PbS quantum dots at 5 K for different excitation densities, (a) 1 mW, (b) 0.050 mW, and (c) 0.025 mW per excitation beam. On top the amplitudes (A) are shown, whereas the bottom corresponds to the real parts ($R$). The spectra were collected with $T=0$ fs. Blue (0) and red (1) correspond to the normalized intensity.Reuse & Permissions

Figure 4

Time integrated FWM for different $T$ delays (red) as compared to the $T=0$ fs FWM decay (black) (a) $T=200$ fs and (b) $T=250$ fs. Squares represent experimental data, whereas lines are the exponential fits. (c) For $T=1$ ps (black squares) a longer dephasing component starts to emerge and the dephasing curve exhibits periodic oscillations. The blue squares are the experimental dephasing for $T=0$ ps shown for comparison. The dominating rapid component observed decays at the same rate of $\sim 440$ fs as for $T=0$ ps, whereas the longer component has a dephasing rate of at least four picoseconds. Furthermore, the periodic quantum oscillations for $T=1$ ps are shown together with the fit obtained using Eq. (2) (red line). The time integrated FWM data was collected at an excitation density of 1 mW per excitation beam. (d) Schematic of the excitonic ground state showing the two split state by the intervalley coupling (IVS) which are again split into dark (D) and bright (B) states by the electron-hole exchange (ES).Reuse & Permissions