Anisotropic optical properties of semitransparent coatings of gold nanocaps

An ordered array of cap-shaped gold nanoparticles has been prepared by vapor deposition onto polystyrene nanospheres supported on a glass substrate, The method of fabrication used imparts a significant anisotropy to the geometric and optical properties of the coating. The optical-absorption properties of these deposits have been measured using UV-vis spectrometry and simulated using a code based on the discrete dipole approximation. Because the nanocaps are not interconnected, they interact with incident light as individual particles with a plasmon resonance that depends upon wavelength and the polarization vector of the light. The resulting extinction peaks manifest in the upper visible and near-infrared regions of the electromagnetic spectrum. Surprisingly, varying the angle of incidence of the light (for a fixed polarization) has no effect on the optical properties of individual nanocaps. Calculations show that these phenomena may be readily interpreted in terms of dipole resonances excited across the longitudinal, transverse, and short-transverse directions of the nanocaps. Coatings comprised of arrays of these particles have the potential to serve as angularly and spectrally selective filters.