Transient Absorption Dynamics and Nonlinear Optical Response in Colloidal Ag2S Quantum Dots

The mechanism of the nonlinear optical response of colloidal solutions of Ag2S quantum dots passivated with thioglycolic acid molecules is considered. Appearance of nonlinear absorption and nonlinear refraction is shown by the Z-scan method using 10-ns pulses of the second harmonic of a Nd:YAG laser. The absorption coefficient dependence was found to saturate with increasing incident radiation intensity. It is suggested that nonlinear absorption occurs with participation of the localized states of structural defects in Ag2S, including the defect luminescence center, which is confirmed by femtosecond transient absorption spectroscopy. A broad structureless transient absorption band is observed in the range of 500-1000 nm, which decays on a time scale of picoseconds. It is concluded that the transient absorption signal is caused by fast (fractions of picoseconds) capture of charge carriers by localized states, which then induce the nonlinear absorption of 10-ns pulses.