Localized field enhancements in fractal shaped periodic metal nanostructures.

Fractal shaped structures formed with a 100-nm-period square lattice of gold nanoparticles placed on a gold film are characterized by using far-field nonlinear scanning optical microscopy, in which two-photon photoluminescence (TPL) excited with a strongly focused laser beam (in the wavelength range of 730 - 790 nm) is detected. The TPL images recorded for all wavelengths exhibit diffraction-limited (~ 0.6 mum) bright spots corresponding to the field intensity enhancement of up to 150, whose positions are dictated by the incident light wavelength and polarization. We relate these field enhancements to the occurrence of constructive interference of surface plasmons (SPs), which are excited by the incident radiation (due to scattering by nanoparticles) and partially reflected by fractal shaped boundaries due to a difference in the SP effective index at a flat and periodically corrugated gold surface. The conjecture on SP index difference is verified with observations (using leakage radiation microscopy) of SP focusing by circular and waveguiding by rectangular areas filled with periodically arranged nanoparticles.