Mesoscopic enhancement of optical nonlinearity in CuCl quantum dots: Giant-oscillator-strength effect on confined excitons

We investigate size-dependent nonlinear polarization due to Wannier excitons confined in CuCl quantum dots (QD’s) with a radius R of 15–80 Å embedded in glass. The third-order optical susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ measured at 80 K increases as a function of radius and subsequently decreases after reaching a maximum value of 2×${10}^{\mathrm{-}6}$ esu at R∼50 Å. Homogeneous widths and longitudinal relaxation times were also measured and the results were analyzed taking into account a two-level atomic model. We obtain evidence that the size-dependent ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ originates from the size-dependent oscillator strength of coherently generated excitons in the QD. The oscillator strength varies as ${\mathit{R}}^{2.2}$ for 19<R<40 Å and is enhanced by a factor of 940 at maximum compared to that of bulk exciton.