Comparative features of optical limiting in monolayer protected gold, silver and alloy nanoclusters under picosecond and nanosecond laser excitation

Au, Ag and Au:Ag alloy nanoclusters have been synthesized with an average core size of 3-4 nm. Octanethiol and octadecanethiol are used as capping agents. Strong optical limiting is exhibited by these samples when excited by 532 nm pulses of both 35 ps and 7 ns duration. It is found that some of these samples can behave as saturable absorbers also, provided certain input intensity conditions are fulfilled. For all the samples studied, the limiting efficiency is found to be consistently better with ns excitation. Similarly, octanethiol capped clusters are found to be better limiters than octadecanethiol capped clusters. In addition, it is seen that the alloy clusters are less efficient in limiting and are less photostable than their monometal counterparts. It is possible to explain the observed effects in terms of the electron dynamics of the excited state species.