Size dependent optical second harmonic generation from surface bound Na clusters: comparison between experiment and theory

Optical second harmonic (SH) generation from large Na clusters bound to dielectric surfaces is reported for laser excitation with λ = 660 nm. A pronounced maximum of SH yield is found as a function of increasing (via growth) and decreasing (via thermal desorption) cluster size, which results from SH enhancement by collective surface plasmon excitation. Experimental measurements, performed both at λ = 660 nm and λ = 1064 nm are found to be in good agreement with predictions derived from the electromagnetic field enhancement theory, thereby suggesting that χ(2) for large Na aggregates is independent of cluster size. The observation of SH as a function of surface temperature shows that thermal desorption results in a strongly changing size distribution for clusters remaining on the surface that differs significantly from the distribution imposed by adsorption processes.