Mechanisms for optical nonlinearities and ultrafast carrier dynamics in CuxS nanocrystals.

Mechanisms for optical nonlinearities and ultrafast carrier dynamics in Cu{sub {ital x}}S nanocrystals (NC{close_quote}s) are studied using femtosecond pump-probe techniques and nanosecond {ital Z}-scan measurements. Depending on the copper deficiency, transient absorption is dominated either by state-filling-induced bleaching [for copper sulfide phases with {ital x} = 1.8 (digenite), 1.9, and 1.96 (djurleite)] or by photoinduced absorption [{ital 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{sub {ital x}}S NC{close_quote}s formed by direct- and indirect-gap phases of copper sulfide is confirmed by results of the {ital Z}-scan measurements. {copyright} {ital 1996 The American Physical Society.}