Shedding light on dark plasmons in gold nanorings

We present an experimental and theoretical analysis of the optical properties of gold nanorings of different sizes and cross-sections. We show that for light polarized parallel to the ring, the optical spectrum can depend sensitively on the angle of incidence. For normal incidence, the spectrum is characterized by two dipolar ring resonances. As the angle of incidence becomes more oblique, several previously dark multipolar ring resonances appear in the spectra. We show that the appearance of the multipolar resonances is a consequence of retardation and can be understood in simple general terms.