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 angstrom embedded in glass. The third-order optical susceptibility chi(3) measured at 80 K increases as a function of radius and subsequently decreases after reaching a maximum value of 2 X 10(-6) esu at R is similar to 50 angstrom. 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 chi(3) originates from the size-dependent oscillator strength of coherently generated excitons in the QD. The oscillator strength varies as R2.2 for 19 < R < 40 angstrom and is enhanced by a factor of 940 at maximum compared to that of bulk exciton.