  Since the discovery of high-order harmonic generation (HHG) in solids, much
effort has been devoted to understanding its generation mechanism and both
interband and intraband transitions are known to be essential. However,
intraband transitions are affected by the electronic structure of a solid, and
how they contribute to nonlinear carrier generation and HHG remains an open
question. Here, we use mid-infrared laser pulses to study HHG in CdSe and CdS
quantum dots (QDs), where quantum confinement can be used to control the
intraband transitions. We find that both the HHG intensity per excited volume
and the generated carrier density increase when the average QD size is
increased from about 2 nm to 3 nm. We show that the reduction of the subband
gap energy in larger QDs enhances intraband transitions, and this in turn
increases the rate of photocarrier injection by coupling with interband
transitions, resulting in enhanced HHG.
