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Optical second harmonic generation (SHG) has been experimentally proven to be sensitive to the electronic excitation in Cu nanowire arrays on the NaCl(110) substrate lacking an inversion center. These nanowires were produced by a shadow deposition method and their nonlinear optical behavior was examined in air by using an optical parametric generator and amplifier system producing the fundamental photon energy from 1.2 to 2.3eV. The excitation beam was focused on the sample surface at the incident angle of 45°. The SH signal from the sample was detected by a photomultiplier, and the absolute magnitudes were obtained by normalizing the SH response to that of the quartz single crystal. Spectral dependence of the SH signal was investigated as a function of the SH photon energies from 2.4eV to 4.6eV. The SH intensity spectra showed steady increase above the SH photon energy of 3.0eV in the p-in/p-out (p-polarized input/p-polarized output) and s-in/p-out polarization configurations. The SH response exhibited a peaked resonance near the SH photon energy of 4.4eV. At this photon energy the SH response due to the resonant coupling between the fundamental field and the surface plasmon in the Cu nanowires has a dominant contribution. This characteristic plasmon resonance was previously observed in the linear absorption spectra at the fundamental photon energy of 2.2eV. On the other hand, the spectrum taken from Cu nanodots showed a nearly flat dependence of SH intensity on the photon energy below the SH photon energy of 4.0V. The intensity abruptly increased above the SH photon energy of ~ 4.2eV, and reached a maximum at 4.5eV. We have calculated the local field enhancement factor for Cu nanowires and Cu nanodots by a quasi-static theory. The spectral shape was consistent with the observed SH spectra for Cu nanowires, but it did not reproduce the one for Cu nanodots. The overall agreement of the spectra of interest between theory and experiment became good when the dynamic depolarization and the radiation damping terms were taken into account for Cu nanodots because they reduced enhancement substantially and shifted the plasmon resonance maximum to the higher photon energy. |