Observation of multiple-harmonic radiation induced from a gold surface by picosecond neodymium-doped yttrium aluminum garnet laser pulses

Illuminating a gold surface by strong (5 GW/${\mathrm{cm}}^2$) picosecond neodymium-doped yttrium aluminum garnet laser pulses at grazing incidence, we observed a generation of coherent beams of both even and odd harmonics up to fifth order in the reflected direction with efficiencies ${10}^{\mathrm{-}10–}$ ${10}^{\mathrm{-}13}$. The observed decrease of the harmonic efficiencies with increasing harmonic order is much weaker than predicted by perturbative theories.