Optical limitation induced by gold clusters:: Mechanism and efficiency

The optical limiting effect induced by gold clusters was measured as a function of excitation wavelength. The limiting effect is most efficient below 530 nm, and decreases towards the red. Two different behaviors are seen in the time-resolved signals in the nanosecond and picosecond ranges. These behaviors have different time delays for the amplitude maxima and for the fluence thresholds where nonlinear effects are observed. This suggests that two types of scattering centers are responsible for the optical limitation. The fast mechanism, which reaches a maximum amplitude in less than 1 ns, occurs at relatively high fluence and for short pulses and is assigned to the vaporization of metal particles. It is more pronounced for large clusters where the absorbed light energy is primarily highly confined. The slow mechanism, which develops in a few nanoseconds, is assigned to the energy transfer from the gold particles to the surrounding solvent and to the formation of solvent bubbles. At lower fluence and for smaller size clusters, only the second mechanism is observed in the nanosecond range, because the efficient dissipation of energy from the small clusters to the solvent, which produces bubbles, precludes metal-particle vaporization.