Saturable and reverse saturable absorption in aqueous silver nanoparticles at off-resonant wavelength

Silver nano-colloid was prepared by chemical reduction method and its nonlinear absorption properties were investigated by using open aperture z-scan experiment with nanosecond laser pulses operating at 532?nm. Interestingly, a switch over from saturable absorption to reverse saturable absorption was observed when the input intensity is increased from 28.1 to 175.8?MW/cm². The underlying mechanism responsible for the observed switching behaviour is the interplay between ground state plasmon band bleaching and excited state absorption. Theoretical fitting was done by using a model in which nonlinear absorption coefficient as well as saturation intensity are incorporated.