Mechanisms for optical nonlinearities and ultrafast carrier dynamics in CuxS nanocrystals

Mechanisms for optical nonlinearities and ultrafast carrier dynamics in Cu_xS nanocrystals (NC's) are studied using femtosecond pump-probe techniques and nanosecond Z-scan measurements. Depending on the copper deficiency, transient absorption is dominated either by state-filling-induced bleaching [for copper sulfide phases with x = 1.8 (digenite), 1.9, and 1.96 (djurleite)] or by photoinduced absorption [x = 2 (chalcosite)]. This difference is explained in terms of the change in the dominant nonlinear-optical mechanism resulting from an indirect-direct-gap transformation accompanying the increase in copper deficiency. Extremely fast signal relaxation (on the time scale from 400 fs to 4 ps) observed in the initial stage after excitation is attributed to carrier trapping. The existence of different types of nonlinearities in Cu_xS NC's formed by direct- and indirect-gap phases of copper sulfide is confirmed by results of the Z-scan measurements.