Melting-induced enhancement of the second-harmonic generation from metal nanoparticles

We report on the generation of second-harmonic signals by irradiating monolayers of high purity Ga nanoparticles, embedded in a ${\mathrm{S}\mathrm{i}\mathrm{O}}_{x}$ matrix, with femtosecond laser pulses at 800 nm. A remarkable melting-induced enhancement of the second-harmonic generation is observed in correspondence of the phase transition. In addition, the hysteresis cycle of nonlinear transmittance is shown to be amplified a factor of 80--100 with respect to the linear response and interpreted in the framework of a nonlinear effective-medium model.