Nonlinear absorption of PbS nanocrystals in silicate glass

Nonlinear transient absorption of glasses doped with PbS nanocrystals was investigated by means of picosecond pump-probe absorption spectroscopy. The entire lowest energy absorption band was bleached on glass excitation at any spectral position inside this band, indicating that homogeneous broadening determines the bandwidth. Three different induced absorption mechanisms taking place at different incident light intensities were distinguished: absorption to the two-electron-hole pair states limits absorption bleaching to about 50% of the steady-state value at moderate excitation intensity; short-lived absorption from the two-electron-hole pair states appears at higher intensity when these states are populated; and two-photon absorption occurs at very high light intensities. The two-photon absorption populates high-energy exciton states which relax bypassing the lowest energy exciton state. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.