Two-photon absorption and photoluminescence of colloidal gold nanoparticles and nanoclusters

This review provides a comprehensive description of nonlinear optical (NLO) properties of gold nanoparticles, which can be used in biological applications. The main focus is placed on two- photon absorption (2PA) and two- photon excited photoluminescence (2PEL) - the processes crucial for multiphoton microscopy, which allows deeper imaging of the material and causes less damage to the biological samples in comparison to conventional (one- photon) microscopy. We present the basics of 2PA measurement techniques and a summary of recent achievements in the understanding of multiphoton excitation and the resulting photoluminescence in gold nanoparticles, both plasmonic ones and small nanoclusters with molecule- like properties. The examples of 2PA applications in bioimaging are also presented, with a comment on future challenges and applications.