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 {} embedded in glass. The third-order optical susceptibility χ⁽³⁾ measured at 80 K increases as a function of radius and subsequently decreases after reaching a maximum value of 2×{}10^-6 esu at R{~}50 {}. 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 χ⁽³⁾ originates from the size-dependent oscillator strength of coherently generated excitons in the QD. The oscillator strength varies as R2.2 for 19R40 {} and is enhanced by a factor of 940 at maximum compared to that of bulk exciton.