Ultrafast intraband relaxation in colloidal quantum dots

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots using time-resolved luminescence and time-resolved TeraHertz spectroscopy. The rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes a direct proof of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 ± 0.15 ps time scale. This is only marginally slower than the timescale of intraband relaxation of electrons in single crystal CdSe. In bulk CdSe, sequential multi-phonon emission allows for cooling rates of (0.4 ps)^-1 at room temperature.