Quantum-sized gold clusters as efficient two-photon absorbers.

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.