Spectroscopy of the nonlinear refractive index of colloidal PbSe nanocrystals

A spectroscopic study of the optical nonlinearity of PbSe colloidal solutions was performed with the Z-scan technique at wavelength intervals of 1200–1350 and 1540–1750nm. While nonlinear absorption remains below the detection threshold, the third order nonlinear refractive index n2 shows clear resonances, somewhat blueshifted relative to the exciton transitions in the absorbance spectrum. The occurrence of thermal effects is ruled out by time-resolved measurements. At 1.55μm, measured (resonant) n2 values exceed typical bulk semiconductor values by two orders of magnitude. At high optical intensity, the refractive index change saturates, indicating that statefilling lies at the origin of the observed effect.