Following above band gap photoexcitation of semiconductor materials, a high energy exciton rapidly dissipates its excess energy to end up as a band edge exciton with the electron and the hole occupying the lowest conduction band and highest valence band levels. Very often, hot exciton relaxation is seen as a loss channel, competing with processes that better preserve the free energy of the high energy exciton such as multiple exciton generation or charge transfer of hot charge carriers. On the other hand, the high rate of intraband relaxation may render the process useful for applications relying on a fast change or modulation of materials properties. In both cases, application development would benefit from a better understanding of the properties and dynamics of cooling charge carriers.

In this contribution, we use hyperspectral transient absorption spectroscopy to map the properties of cooling charge carriers in lead sulfide quantum dots. First, we show that, depending on the energy of pump and probe photons and on the pump-probe time delay, different phenomena dominate the transient absorption spectrum. Whereas around the bandgap transition, the well-documented bleach and Coulomb-shift of the bandgap exciton dominate, a short lived transient bleach feature appears at probe photon energies corresponding to the critical point in the energy band diagram along the Σ direction. This temporary slowing-down of the cooling exciton is present in PbS nanocrystals of different sizes, suggesting that this cooling bottleneck is intrinsically related to the PbS band structure. In fact, as the critical point along the Σ direction is a saddle point, the cooling exciton arriving at the critical point must change the direction of its k vector to continue cooling towards the band edges around the L point. Apparently, this results in a temporary accumulation of the exciton around the critical point. Moreover, we argue that this slow cooling via the Σ direction saddle point is the dominant cooling pathway for high energy excitons. This conclusion stresses the importance of the bulk band structure for understanding cooling dynamics of hot charges in semiconductor nanocrystals.

At probe wavelengths below the bandgap, we observe an ultrafast and broadband photo-induced absorption feature that reflects the changing cross section for intraband absorption of the cooling exciton.1 Interestingly, the wavelength at which this photoinduced absorption matches the bleach of the bandgap transition stays put after exciton cooling. Hence, at this matching wavelength, photo-excitation results in a short, 1-2 ps burst of photoinduced absorption after which the sample has the same absorbance as the original, unexcited sample. We find that this burst photoinduced absorption is linear in the excitation power, both in the single and multi-exciton regime. Moreover, excitation pulses separated by only 2.2 ps give rise to a sequence of absorption bursts, showing that PbS quantum dots could be used for the ultrafast (500 Gb/s), all-optical conversion of data signals between two wavelengths.