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Excitation, dynamics and dephasing in quantum dots

This paper concerns the femtosecond dynamical relaxation of excited states in quantum dots or clusters. The clusters are typically polar, consisting of 150-200 atoms, and are of interest for potential photonic devices. Our work identifies certain novel effects, `dynamic dilation' and `ringing', associated with excited states of non-metallic dots of this size. Dynamic dilation occurs for dots at constant pressure, rather than constant volume. Dilation induces a modest but significant energy shift on a picosecond timescale, slower than the characteristic vibrational period of the configuration coordinate. The magnitude of the shift is clearly size dependent, tending to zero in the limit of a bulk system. This shift can be equivalent to dephasing, and may explain one component of dephasing already experimentally observed. Such dephasing may also affect the usefulness of dots in ultrafast optical switching. The second novel effect associated with the clusters is the relatively long-lived vibrational excitation of the dot leading to `ringing'. This is strongly sensitive to the acoustic mismatch with the environment. Neither `dynamic dilation' nor `ringing' are especially sensitive to the composition of the dots, or to whether the excited states correspond closely to bulk excitons or to defect states, and hence need to be taken into account in the excitation of quantum dots.