  We show that the third order optical nonlinearity of 15-atom gold clusters is
significantly enhanced when in contact with indium tin oxide (ITO) conducting
film. Open and close aperture z-scan experiments together with non-degenerate
pump-probe differential transmission experiments were done using 80 fs laser
pulses centered at 395 nm and 790 nm on gold clusters encased inside
cyclodextrin cavities. We show that two photon absorption coefficient is
enhanced by an order of magnitude as compared to that when the clusters are on
pristine glass plate. The enhancement for the nonlinear optical refraction
coefficient is ~3 times. The photo-induced excited state absorption using
pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also
show an enhancement by an order of magnitude. These results attributed to the
excited state energy transfer in the coupled gold cluster-ITO system are
different from the enhancement seen so far in charge donor-acceptor complexes
and nanoparticle-conjugate polymer composites.
