Nonlinear response and optical limiting in inorganic metal cluster Mo_2Ag_4S_8( PPh_3)_4 solutions

We describe a series of experiments on acetonitrile solutions of an inorganic cluster molecule ${\mathrm{Mo}}_2{\mathrm{Ag}}_4{\mathrm{S}}_8({\mathrm{PPh}}_3{)}_4$ and compare them with data on a suspension of carbon particles in liquid (dilute ink). The optical-limiting behavior is measured by single-picosecond 532-nm pulses and nanosecond-long trains of these picosecond pulses. Nonlinear loss measurements are also performed with pulse trains at 1064 nm. Both materials show reduced transmittance for increasing fluence (energy per unit area). We also perform picosecond time-resolved pump–probe measurements at 532 nm, and we find that the observed pump–probe behavior is identical for the metal-cluster solution and the carbon-black suspension. We believe that the nonlinear mechanisms are the same for the two materials. Our previous studies of carbon-black suspension indicate that the primary nonlinear losses are due to scattering and absorption by microplasmas formed after thermionic emission from heated carbon black augmented by scattering from subsequently created bubbles. The conclusion of a similar limiting mechanism for the two materials is confirmed by time-resolved shadowgraphic images taken on both samples; however, a definitive conclusion concerning the role of microplasmas versus bubbles in either material is still under investigation.