阴极发光在纳米光子学的极化成像

来自荷兰delmic公司的先进阴极发光系统SPARC，实现角分辨成像同时实现极化成像，解决纳米光子学、表面等离激元研究中的问题。 

Light is a transverse electromag- netic wave: the electric and mag- netic fields that compose the light wave always oscillate trans- versely to the propagation direc- tion. Besides color (energy) and momentum (propagation direc- tion), light is also characterized by a polarization which describes in what direction these electro- magnetic fields oscillate. If the electromagnetic oscillations re- main in the same plane, the po- larization is referred to as linear but 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).

Polarization plays a key role in light-matter interactions and can be used to study coherence, scattering, birefringence, and chirality, for example. Addi- tionally, it can be used to block spurious background radiation and to cor- rect 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 therefore to be performed in the Fourier-plane, i.e. angle-resolved mode. As the SPARC can perform angle-resolved imaging it is ideal to also study polarization effects.

The Stokes formalism provides a complete description of the polarization state (i.e. linear, elliptical, circular) of the light which is cast in the form of a Stokes vector. This stokes vector can be retrieved for every emission angle by using a polarization analyzer in conjunction with a 2D CCD or CMOS cam- era. The analyzer is composed of a quarter wave plate (QWP) and linear

polarizer (LP). Figure 1 shows a schematic representation of this setup [1-3]. To gain full polarization information the measurement has to be per- formed for six analyzer settings. The measured Stokes vector comprises the polarization state at the detector. However, this polarization has been altered with respect to the polarization coming from the sample by the paraboloid collection optic inside of the SEM. By applying the appropriate correction the polarization distribution from the sample can be retrieved. Wavelength sensitivity can be included through bandpass filters.

An example of what can be done with this technique is shown in Figure 2 where we show the radial and azimuthal electric field amplitudes for differ- ent emission angles on a gold plasmonic bullseye grating, measured with CL polarimetry. With the electron beam we 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. The light is linearly polarized in this case but in principle the handedness can also be determined if the emission is ellipti- cally polarized [2].

In addition to angle-resolved polarimetry it is also possible to perform polarization filtered hyperspectral imaging as is described in References [1] and [4]. In this imaging modality it is possible to obtain polarization-filtered nanoscale hyperspectral images. In conclusion, the modularity, the sensitiv- ity, and the ability the measure the angular profile makes the SPARC the ultimate platform for versatile polarization studies at the nanoscale.