Optical properties of spherical and oblate spheroidal gold shell colloids

The surface plasmon modes of spherical and oblate spheroidal core-shell colloids composed of a 312 nm diameter silica core and a 20 nm thick Au shell are investigated. Large arrays of uniaxially aligned core shell colloids with size aspect ratios ranging from 1.0 to 1.7 are fabricated using a novel ion irradiation technique. Angle- and polarization-resolved extinction spectroscopy is performed on the arrays. Extinction spectra on spherical particles reveal dipole, quadrupole, and octupole resonances in good agreement with calculations using Mie theory. Optical extinction measurements on oblate spheroidal core-shell show strong red- and blue-shifts for polarizations along major and minor axes, respectively, that increase with size anisotropy. The measured spectral shifts of the dipole, quadrupole, and octupole resonant modes with angle and anisotropy are in good agreement with T-matrix calculations. The data provide insight into the tunable resonant behavior of surface plasmons in oblate spheroidal core-shell particles.