Angle‐resolved cathodoluminescence polarimetry on plasmonic nanostructures
Authors: Coenen Toon , Osorio Clara , Brenny Benjamin , Polman Albert , Koenderink Femius 
Published: December 2016

Plasmonic metamaterials and metasurfaces have the ability to influence the propagation, confinement, and emission of light on a deep‐subwavelength scale. Many of the optical properties of such materials are encoded in the spectrum, the angular intensity distribution, and the polarization of the far‐field emission. Angle‐resolved cathodoluminescence (CL) imaging spectroscopy (ARCIS) is a powerful platform for studying these properties, as it combines nanoscale excitation resolution, with the capability to measure both spectra and the angular emission intensity distribution. In particular, we use a 30 keV electron beam as a well‐defined broad‐band excitation source which is sensitive to the optical density of states. This method has been used to study the spectral and angular optical properties of a large variety of dielectric and plasmonic nanostructures. However, thus far we were only able to measure emission intensities and had to disregard the vectorial polarization nature of the light emission. The emission polarization contains valuable information, which can be used to identify multipoles, separate TM and TE modes in waveguides, characterize the coherence of an emission source
etc
.

Here, we demonstrate a novel CL polarimetry technique in which we retrieve the Stokes vector,
i.e
. the full polarization state of the far‐field emission, as function of angle [1]. To that end, we extend our setup to include a quarter‐wave plate (QWP) and a linear polarizer in the beam path (see Figure 1 for a schematic representation of the setup). By taking six measurements with the appropriate combinations of QWP and polarizer angle we retrieve the polarization distribution in the detection plane. By applying a correction for the aluminum parabolic mirror optic we then find the emission polarization distribution.

This approach is applied to gold plasmonic bull's eye gratings which were fabricated using focused‐ion‐beam milling in a single‐crystal gold substrate (see Figure 2(a) for an SEM image). These bull's eyes can coherently couple out the Surface Plasmon Polaritons (SPPs) that are excited by the electron beam. Because the electron beam can be positioned at will, we can study the effect of exciting the bull's eye at different positions. For central excitation, the grating is driven in phase leading to an azimuthally symmetric pattern and a radial polarization distribution, as expected from symmetry (see Figure 2(b‐c)). However, when we excite off‐center the patterns become significantly more complex, showing multiple lobes and alternating regions in angular space in which the polarization goes from circular to linear. To demonstrate the applicability to chiral structures, we move to spiral bull's eyes with different handedness, and show that their chirality is reflected in the field distributions. The validity of the polarimetry technique is verified by measuring transition radiation which has a characteristic radial polarization distribution, similar to a vertical dipole source. This work paves the way for polarimetry measurements on a myriad of metallic and semiconductor nanophotonic geometries for characterization and better understanding of their optical properties.