Polarization-Filtered Cathodoluminescence Imaging

Light is a transverse electromagnetic wave: the electric and magnetic fields that compose the light wave always oscillate transversely to the propagation direction.

Image courtesy of www.schoolphysics.co.uk

Besides color (energy) and momentum (propagation direction), light is also characterized by a polarization which describes in what direction these electromagnetic fields oscillate. If the electromagnetic oscillations remain in the same plane, the polarization is referred to as linear. This plane can also rotate while the wave is propagating. In that case, the polarization is elliptical which can be either left – (anti-clockwise) or right-handed (clockwise) depending on the direction of rotation (circular polarization is a special case of elliptical polarization).

Angle-Resolved Polarimetry

The figure below is a schematic representation of the angle-resolved polarimetry imaging mode, currently only available on the Delmic SPARC. The cathodoluminescence is collected by the paraboloid mirror and then filtered by the polarization analyzer which consists of a quarter-wave plate (QWP) and a linear polarizer (LP).

Pictorial representation of Angle-Resolved Cathodoluminescence

The CCD camera records the polarization filtered image. When images are captured over six different analyzer settings, the full polarization state is retrieved for every emission angle. The original emission of the sample can then be reconstructed by applying a correction for the distorting effect of the paraboloid. This correction is also wavelength dependent, so color filters are used to increase the accuracy of the correction and to achieve spectral sensitivity. It is also possible to perform polarization filtered hyperspectral imaging without angle-resolution. In this imaging modality, it is possible to obtain polarization-filtered nanoscale hyperspectral images.  This technique was previously unavailable on commercial systems because it requires very high precision in mirror alignment, a high collection efficiency and a detailed understanding of the mirror distortion.

Polarization Visualization

Polarization plays a key role in light-matter interactions and can be used to study coherence, scattering, birefringence/birefractive, and chirality. Additionally, it can be used to block spurious background radiation and to correct for aberrating effects in the collection optics. When light is emitted from a (nano)material the polarization is not necessarily the same for every emission angle. Fully comprehensive polarization studies have to be performed in the Fourier-plane, otherwise known as angle-resolved mode. The SPARC is the only commercially available tool that can perform angle-resolved imaging to study polarization effects in this manner.

An example of what can be done with this technique is shown in below. This image shows the radial and azimuthal electric field amplitudes for different emission angles on a gold plasmonic bullseye grating, measured with CL polarimetry.

Images of a Plasmonic Bullseye Grating
(a) An SEM of a plasmonic bullseye grating milled into a single-crystal of gold using focused-ion-beam milling. (b) Radial and (c) azimuthal field amplitudes as a function of emission angles θ and φ for central electron beam excitation of the bullseye.

The data above was generated by using an electron beam to launch a circular plasmon wave in the center of the bullseye which is converted by the structure into a radially polarized coaxial beam. The azimuthal component is negligibly small for this geometry. In this case, the light is linearly polarized, but in principle, the handedness can also be determined if the emission is elliptically polarized.