Aligned metallic nanoparticles on rippled surfaces: Optical characterization using generalized ellipsometry

Oblique-angle deposition of metallic island films onto self-organized rippled surfaces results in arrays of particles and wires with characteristic dimensions below 50 nm. Films of the noble metals exhibit distinct optical properties due to localized plasmon resonances. Here the films are characterized using generalized spectroscopic ellipsometry. Due to the small characteristic dimensions we can describe the optical properties using an effective medium approach. The dielectric properties are highly anisotropic and a biaxial dielectric tensor description is required. The plasmonic resonances provide a means to tune the form birefringence by adjusting the metal fraction, from aligned nanoparticles with anisotropic localized plasmon resonances, to arrays of nanowires with orthogonal metallic and dielectric properties. The deposition of the particles on one side of the ripple ridges causes additional anisotropy that shows a 2-fold symmetry upon rotation in the substrate plane. This is associated with a longitudinal plasmon resonance in the out-of-plane direction.