Observation of a fifth order optical nonlinearity in 29 kDa Au@alkanethiol clusters

Restricted Access.The optical nonlinearity in nanoclusters of Au@hexanethiol and Au@dodecanethiol has been investigated. From the mass spectra, each cluster is estimated to contain between 29 to 40 atoms of gold. The optical absorption spectra of the clusters are devoid of the plasmon absorption band, and show a gradual increase in absorbance towards the UV region. When excited with nanosecond laser pulses at 532 nm, an optical limiting behavior is observed, which fits to a threephoton absorption mechanism. We propose that this nonlinearity is caused by free carrier absorption subsequent to twophoton absorption taking place in the medium. The temporal dynamics of the nonlinearity reveals a fast process, with a lifetime of less than 2 picosecond