Characterization of the third-order nonlinearity of CdSe and CdSe/ZnS quantum dots

The third-order nonlinear optical properties of CdSe and CdSe/ZnS core-shell quantum dots in solution with toluene were studied over a spectrum ranging from 450 to 680 nm using 225-fs pulses. The electronic and thermal contributions to the nonlinearity, leading to positive or negative nonlinear refractive indices, are investigated. Although the sign and magnitude of the nonlinearity vary with wavelength, it was found to be of the same order of magnitude as that of bulk CdSe for the same photon energy-to-band gap ratio. However, because the absorption edge of CdSe quantum dots is blue-shifted relative to bulk, :it permits fast optoelectronic applications in the visible spectrum by avoiding slow thermal effects normally associated with linear absorption.