Size-dependent enhancement of nonlinear optical susceptibilities due to confined excitons in CuBr nanocrystals

We have investigated the third-order nonlinearity on resonance with the confined exciton level in CuBr nanocrystals with radii in the range (Formula presented) nm embedded in glass by means of degenerate four-wave mixing, time-resolved luminescence, and resonant luminescence measurements. The third-order optical susceptibility (Formula presented) exhibits resonant behaviors at (Formula presented) and (Formula presented) excitons, which are weakly confined in nanocrystals. The figure of merit (Formula presented) increases with increasing radius (Formula presented) in the whole range studied here. We have measured homogeneous widths and lifetimes of (Formula presented) excitons and obtained size dependences of those relaxation parameters. Assuming a two-level atomic model for the confined exciton, we have deduced the size dependence of the oscillator strength of (Formula presented) excitons from the measured lifetimes, homogeneous widths, and (Formula presented). The oscillator strength exhibits the (Formula presented) dependence in the whole range studied here, which reveals the giant oscillator strength effect on the confined exciton that is coherently generated in the nanocrystal. The oscillator strength per nanocrystal is enhanced by a factor (Formula presented) for (Formula presented) nm compared to that of bulk excitons. We also discuss the derivation of (Formula presented) in the stationary regime from the (Formula presented) in the transient regime where the nonlinear time response shows a multiexponential decay.