Development of optical second-order nonlinear optical material made of metallic nanowire arrays

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We have observed optical second harmonic generation (SHG) from metallic nanowire arrays for the first time. We used Au nanowires of 60nm width on facted NaC1 (110) faces fabricated by shadow deposition method. From the polarization and photon energy dependence, we have found that Au behaves as an ideal metal and its optical property is dominated by the effect of the depolarization field for the photon energy below 1.5eV.We have fabricated Pt nanowire arrays of 9nm and 20nm widths and have observed their SH response. The SH response was anisotropic due to the anisotropic shapes of the cross sections of the nanowires caused by the low mobility of Pt atoms during wire growth. The second-order nonlinear susceptibility element X^<(2)>_<222> was found to be large, where 2 represents the direction perpendicular to both the nanowire axes and the array planes. The specific feature of the Pt nanowires is that the cross section has an anisotropic shape and the poor dielectric shielding due to the large imaginary part of the dielectric function.We have observed SHG from Cu nanowire arrays with the wire widths of 14 to 19nm, in order to generalize this study to other metallic species. The dielectric and nonlinear optical properties of Cu nanowires could be explained by using a local plasmon model and were reproduced by electromagnetic numerical calculation incorporating the retardation effect. The response of the nanowires is that of a local plasmon when the incident field is perpendicular to the nanowire axes, while it is described by lightning-rod effect when the incident field is parallel to the nanowire axes.From the results of these works we could pick up at least three types of nonlinear effect of metallic nanowires, and have contributed considerably to the understanding of the nonlinear optical effect of metallic nanostructures.