  Intensity-dependent enhancement of saturable absorption in a film of PbS-Au4
nanohybrid composites has been observed by femtosecond time-resolved transient
absorption measurement at 780 nm. The nonlinear absorption coefficient of
saturable absorption in PbS-Au4 nanohybrid composites is found to be dependent
on excitation irradiance and it is determined to be -2.9 cm/GW at 78 GW/cm2, an
enhancement of nearly fourfold in comparison with that of pure PbS quantum dots
(QDs). The enhancement is attributed to excitation of surface plasmon by
resonant energy transfer between PbS QDs and Au nanoparticles through Auger
recombination.
