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We show that electron-hole correlation can be used to tune interband andintraband optical transition rates in semiconductor nanostructures with atleast one weakly confined direction. The valence-to-conduction band transitionrate can be enhanced by a factor $(L/a_B)^{N}$ -- with $L$ the length of theweakly confined direction, $a_B$ the exciton Bohr radius and $N$ thedimensionality of the nanostructure -- while the rate of intraband andinter-valence-band transitions can be slowed down by the inverse factor,$(a_B/L)^{N}$. Adding a hitherto underexplored degree of freedom to engineerexcitonic transition rates, this size dependence is of interest for variousopto-electronic applications. It also offers an interpretation of thesuperlinear volume scaling of two-photon absorption (TPA) cross-sectionrecently reported for CdSe nanoplatelets, thus laying foundations to obtainunprecedented TPA cross sections, well above those of conventional two-photonabsorbers. Further, our concept explains the background of the validity of theuniversal continuum absorption approach for the determination of particleconcentrations via the intrinsic absorption. Potential applications of ourapproach include low excitation intensity confocal two-photon imaging,two-photon autocorrelation and cross correlation with much higher sensitivityand unprecedented temporal resolution as well as TPA based opticalstabilization and optimizing of inter-subband transition rates in quantumcascade lasers.

Tuning intraband and interband transition rates via excitonic correlation in low-dimensional semiconductors