Second harmonic generation from resonantly excited arrays of gold nanoparticles

We show that second harmonic generation from lithographically prepared arrays of symmetric gold nanorods can be increased by two orders of magnitude by choosing the nanoparticle size to be resonant with the 800-nm wavelength of the 50-fs pump laser. The angular variation of the second-harmonic yield, which is defined by the pitch of the nanorod array, can be predicted using standard diffraction theory. This in turn makes it possible to bound approximately the relative contributions of dipole and quadrupole oscillations to the total second-harmonic yield; the two contributions appear to be of similar magnitude. Resonant ultrafast irradiation also changes the nanorod morphology, apparently due to surface melting and refreezing. At higher fluence, the intensity dependence of the second-harmonic yield changes from quadratic to cubic, an indication that the reshaping influences the mechanism of second-harmonic generation.