Tuning Intraband and Interband Transition Rates via Excitonic Correlation in Low-Dimensional Semiconductors

We show that electron-hole correlation can be used to tune interband and intraband optical transition rates in semiconductor nanostructures with at least one weakly confined direction. The valence-to conduction band transition rate can be enhanced by a factor (L/a(B))(N), with L being the length of the weakly confined direction, a(B) is the exciton Bohr radius, and N is the dimensionality of the nanostructure, while the rate of intraband and intervalence-band transitions can be slowed down by the inverse factor, (a(B)/L)(N). Adding a hitherto underexplored degree of freedom to engineer excitonic transition rates, this size dependence is of interest for various optoelectronic applications. It also offers an interpretation of the superlinear volume scaling of two-photon absorption (TPA) cross-section recently reported for CdSe nanoplatelets, thus, laying foundations to obtain unprecedented TPA cross sections, well above those of conventional two-photon absorbers. Further, our concept explains the background of the validity of the universal continuum absorption approach for the determination of particle concentrations via the intrinsic absorption. Potential applications of our approach include low excitation intensity confocal two-photon imaging, two-photon autocorrelation, and cross correlation with much higher sensitivity and unprecedented temporal resolution, as well as TPA-based optical stabilization and optimizing of intersubband transition rates in quantum cascade lasers.