Optical power limiting in ensembles of colloidal Ag2S quantum dots

The effect of power limiting for optical radiation at a wavelength of 660 nm with a pulse duration of 10 ms and operation threshold of 2.2–3.1 mJ cm^-2 is observed in ensembles of colloidal Ag₂S quantum dots (QDs). Using the z-scanning method in an open-aperture scheme it is found that the power is limited mainly due to reverse saturable absorption caused by two-photon optical transitions that involve energy levels of Ag₂S photoluminescence centres, related to structural impurity defects in colloidal Ag₂S QDs. At the same time, the z-scanning in a closed-aperture scheme demonstrates the formation of a thermal dynamic lens.