Toon Coenen

Determining the emission polarization properties of sub-wavelength structures like optical nanoantennas, nanocavities and photonic crystals is important to understand their physical properties and to optimize their use in applications. Recently we have shown that angle-resolved cathodoluminescence imaging spectroscopy (ARCIS), which uses a 30 keV electron beam as an excitation source, is a useful technique to study the far-field properties of such structures. Here we extend the technique with polarization-sensitive angular detection. As proof-of-principle, we experimentally probe the emission polarization properties of three orthogonal dipolar emitters of which the polarization is well-known and find excellent agreement between experiment and theory. We access these dipole orientations by exciting an unstructured gold surface and a ridge nanoantenna with an in-plane dipolar plasmon resonance. The light emission is collected with an aluminum half paraboloid mirror. We show how to take the effect of the paraboloid mirror on the emission polarization into account and how to predict the polarization-filtered pattern if the emission polarization is known. Furthermore, we calculate that by introducing a slit in the beam path the polarization contrast in cathodoluminescence spectroscopy can be strongly enhanced. Finally, we reconstruct the emission polarization from the experimental data and show that from these field patterns we can infer the orientation of the induced dipole moment. The ability to measure the emission polarization, in combination with the sensitivity to the local density of optical states, broad spectral range and high excitation resolution, can be employed to study photonic nanostructures in great detail.

We use spatially and angle-resolved cathodoluminescence imaging spectroscopy to study, with deep subwavelength resolution, the radiation mechanism of single plasmonic ridge antennas with lengths ranging from 100 to 2000 nm. We measure the antenna’s standing wave resonances up to the fifth order and measure the dispersion of the strongly confined guided plasmon mode. By directly detecting the emitted antenna radiation with a 2D CCD camera, we are able to measure the angular emission patterns associated with each individual antenna resonance. We demonstrate that the shortest ridges can be modeled as a single point-dipole emitter oriented either upward (m = 0) or in-plane (m = 1). The far-field emission pattern for longer antennas (m > 2) is well described by two interfering in-plane point dipoles at the end facets, giving rise to an angular fringe pattern, where the number of fringes increases as the antenna becomes longer. Taking advantage of the deep subwavelength excitation resolution of the cathodoluminescence technique, we are able to determine the antenna radiation pattern as a function of excitation position. By including the phase of the radiating dipoles into our simple dipole model, we completely reproduce this effect. This work demonstrates how angle-resolved cathodoluminescence spectroscopy can be used to fully determine the emission properties of subwavelength ridge antennas, which ultimately can be used for the design of more complex and efficient antenna structures.

We experimentally study the local density of optical states of a plasmonic meta-molecule exhibiting plasmon induced transparency. We identify the potential of narrow bright sub-radiant modes for spontaneous emission control with optical antennas.

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi–Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.