Observation of resonance-enhanced high-order harmonics from direct excitation of metal nanoparticles with femtosecond pulses

We report on the observation of resonance enhanced high-order harmonic generation (HHG) from a dense medium composed of indium or tin nanoparticles. Unlike previous studies of resonance enhanced high harmonics from laser produced plasma plume from a prepulse, we generate HHG directly, thus eliminating the undesired contributions from various components in the plasma. We discuss the mechanisms for our observed resonance effect in the absence of preionized particles and the implications of the experimental results on the HHG mechanism in metal nanoparticles.