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

The absorptive optical nonlinearity in nanoclusters of Au@hexanethiol and Au@dodecanethiol has been investigated. From the mass spectra, each cluster is estimated to contain about 140 atoms of gold. The optical absorption spectra of the clusters show a gradual increase in absorbance towards the UV region, over which an extremely weak surface plasmon resonance (SPR) is superposed. No plasmon bleach effects are seen when the samples are excited with nanosecond laser pulses at 532 nm. Instead, a reduced transmission behavior is observed, which fits to a three-photon absorption mechanism. We propose that this nonlinearity is caused by two-photon induced excited state absorption occurring in the nanoclusters. Femtosecond pump–probe studies show that when excited and probed at 400 nm, the nonlinearity is very fast with a lifetime of less than 2 ps.