  Increasing fluorescence quantum yields of ligand-protected gold nanoclusters
has attracted wide research interest. The strategy consisting in using bulky
counterions has been found to dramatically enhance the fluorescence. In this
communication, we push forward this concept to the nonlinear optical regime. We
show that by an appropriate choice of bulky counterions and of solvent, a
30-fold increase in two-photon excited fluorescence (TPEF) signal at ~600 nm
for gold nanoclusters can be obtained. This would correspond to a TPEF cross
section in the range of 0.1 to 1 GM.
