Femtosecond nonlinear absorption of Ag nanoparticles at surface plasmon resonance

Nonlinear absorption of Ag nanoparticles was investigated using open aperture Z-scan method with femtosecond laser pulses at 400 nm. It was found that, at low intensities, Ag nanoparticles behave as saturable absorbers but at higher intensities a shift from saturable absorption to reverse saturable absorption occurs. And open aperture Z-scan curves become asymmetric when the repetition rate of pulse laser is high. The transition process and asymmetry were analyzed in terms of plasmon bleaching, free carrier absorption and migration of Ag nanoparticles at surface plasmon resonance.