Absorption spectra of PbS nanoparticles created by fs laser at different times after irradiation. Left, sample irradiated with 40 mW, mean NP size 8 nm. Right, sample irradiated with 10 mW, mean NP size 4 nm. (Curve a) Just after irradiation, (curve b) 50 days after irradiation, and (curve c) 100 days after the initial irradiation. Adapted from .