Microscopy of localized second-harmonic enhancement in random metal nanostructures

Using second-harmonic (SH) scanning optical microscopy in reflection we study the effect of localized SH enhancement in random metal nanostructures. With a tightly focused tunable (750–830 nm) laser beam, we obtain SH images of a 55-nm-thick gold film surface covered with randomly distributed 70-nm-high gold bumps showing small $\left(\sim 0.7\mu \mathrm{m}\right)$ and very bright $(\sim {10}^3$ times the background) spots. Wavelength and polarization dependencies of both positions and intensities of these SH bright spots as well as their statistics are investigated and compared for two areas having different nominal densities of scatterers, i.e., 25 and $50\mu {\mathrm{m}}^{-2}.$ For relatively large signals, it is found that, the probability density function of the SH signal follows the power-law dependence with the index being in the range of 2.5–3 for both values of the scattering density. We observe that, for incident laser powers in the range 3–40 mW, the SH bright spots exhibit, as expected, quadratic dependencies of the maximum signal on the incident power. However, for higher power levels, the interrogating laser beam causes nonlocal modifications of SH images, i.e., some SH bright spots disappear and others emerge even outside of the area that was exposed to a high power radiation. We relate the observed feature to surface plasmon polariton contribution in the process of multiple scattering, resulting in the formation of resonant eigenmodes (at fundamental and SH frequency) whose excitation in turn gives rise to the SH bright spots.