Stability and nonlinear optical properties of Cu nanoparticles prepared by femtosecond laser ablation of Cu target in alcohol and water

By using PVP of different concentrations as the protective agent, Cu nanoparticles (NPs) of different mean sizes are fabricated through laser ablation of Cu target in alcohol, water, and their mixtures. As-grown Cu NPs show good stability in alcohol. The nonlinear absorptions of Cu NPs with different mean sizes are investigated. With the increase of laser intensity, the nonlinear absorption of small NPs changes from saturable absorption (SA) to reverse saturable absorption (RSA) and two-photon absorption (TPA), whereas SA dominates the nonlinear absorption of the large NPs. The modulations of occupied and unoccupied density of states (DOS) in conduction band are proposed to be the key factors for the size-dependence of the nonlinear absorption. For the large Cu NPs, there are sufficiently occupied DOS in the ground state and unoccupied DOS in the first excited state to sustain the increase of SA, whereas for the small Cu NPs, the corresponding DOS is insufficient, so the SA is surpassed by the RSA and TPA at high laser excitation intensity.