MECHANISMS FOR OPTICAL NONLINEARITIES AND ULTRAFAST CARRIER DYNAMICS IN CUXS NANOCRYSTALS

Mechanisms for optical nonlinearities and ultrafast carrier dynamics i n CuxS nanocrystals (NC's) are studied using femtosecond pump-probe te chniques and nanosecond Z-scan measurements. Depending on the copper d eficiency, transient absorption is dominated either by state-filling-i nduce bleaching [for copper sulfide phases with x = 1.8 (digenite), 1. 9, and 1.96 (djurleite)] or by photoinduced absorption [x = 2 (chalcos ite)]. This difference is explained in terms of the change in the domi nant nonlinear-optical mechanism resulting from an indirect-direct-gap transformation accompanying the increase in copper deficiency. Extrem ely fast signal relaxation (on the time scale from 400 fs to 4 ps) obs erved in the initial stage after excitation is attributed to carrier t rapping. The existence of different types of nonlinearities in CuxS NC 's formed by direct- and indirect-gap phases of copper sulfide is conf irmed by results of the Z-scan measurements.