Directional emission from a single plasmonic scatterer.

Directing light emission is key for many applications in photonics and biology. Optical antennas made from nanostructured plasmonic metals are suitable candidates for this purpose but designing antennas with good directional characteristics can be challenging, especially when they consist of multiple elements. Here we show that strongly directional emission can also be obtained from a simple individual gold nanodisk, utilizing the far-field interference of resonant electric and magnetic modes. Using angle-resolved cathodoluminescence spectroscopy, we find that the spectral and angular response strongly depends on excitation position. For excitation at the nanodisk edge, interference between in-plane and out-of-plane dipole components leads to strong beaming of light. For large nanodisks, higher-order multipole components contribute significantly to the scattered field, leading to enhanced directionality. Using a combination of full-wave simulations and analytical point scattering theory we are able to decompose the calculated and measured scattered fields into dipolar and quadrupolar contributions.