Second-harmonic generation in metal island films consisting of oriented silver particles of low symmetry

A new two-step evaporation technique has been developed by which the particles in a silver island film grow with parallel alignment and low symmetry. In contrast with rotationally symmetric particles prepared in the usual way of island evaporation, films of such particles shown second-harmonic generation (SHG) in transmission and reflection when irradiated normally to the substrate plane. Although silver atoms arrange in a centrosymmetric crystal lattice, such specially prepared island films behave like a film of a non-centrosymmetric material. Owing to localized plasma oscillation resonances in the particles, the SH is strongly enhanced. The measured SH efficiency exceeds the SH from an α-quartz sheet of equal thickness by a factor of order 104.