  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 $L$ the length of the
weakly confined direction, $a_B$ the exciton Bohr radius and $N$ the
dimensionality of the nanostructure -- while the rate of intraband and
inter-valence-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
opto-electronic 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 inter-subband transition rates in quantum
cascade lasers.
