Saturable absorption and reverse saturable absorption in platinum nanoparticles

The optical nonlinear absorption of the aqueous solution of platinum nanospheres protected by poly (N-vinyl-2-pyrrolidone) was investigated using open aperture Z-scan method with nanosecond pulse laser at the wavelength of 532 nm. A shift from saturable absorption to reverse saturable absorption was observed at higher input pump intensities. The transition process was analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity.