Cathodoluminescence System
Applications T e SPARC system can be used in a variety of fi elds. Several practical examples from diff erent fi elds are given below. Nanophotonics and plasmonics . T e fi eld of nanopho- tonics is concerned with the confinement, generation, detection, guiding, and switching of light at the nanoscale. Plasmonics is a branch of nanophotonics that has received signifi cant interest in the last decade. Plasmons are elementary excitations of free electrons in metals in which light is confi ned to a metal-dielectric interface. Plasmonic nanostructures have the ability to strongly confi ne light for spontaneous emission enhancement in antennas, to create optical hot spots for sensing, for enhancement of light absorption in photodetectors and thin-fi lm solar cells, for photothermal therapy, and as transparent conducting nanowire meshes [ 11 ]. Figure 3a (inset) shows an SEM image of a single Au nanodisk on a Si substrate, made using electron beam lithog- raphy and lift-off techniques [ 12 ]. The Au nanoparticle is 180 nm in diameter and has a height of 80 nm. In addition, Figure 3a shows a CL spectrum taken by averaging the CL emission collected from a raster scan over the particle with 7.5 nm resolution. T e spectrum refl ects the resonant plasmon modes in the Au nanoparticle. T e linewidth of the resonant peak in the spectrum is determined by radiative and Ohmic damping. A spatial map of the CL emission, also shown in Figure 3a , shows that the nanoparticle plasmon is excited most effi ciently at the edge, where the electron beam couples efficiently to in-plane dipolar and quadrupolar plasmon modes.
Figure 3 : (a) Normalized CL spectrum from an Au nanodisk on Si with a diameter of 180 nm and a height of 80 nm. A SEM image is shown as an inset, scale bar = 50 nm. The spectrum is taken by averaging the CL spectrum over the particle. A 2D CL excitation map taken at the peak wavelength λ 0 = 550 nm is shown as an inset (top right). The dashed line indicates the edge of the structure taken from the SEM image. (b) Experimental normalized angular CL emission patterns at λ 0 = 600 nm for excitation at the center (1) and near the edge for four orthogonal azimuthal angles: 0° (2), 90° (3), 180° (4), and 270° (5). The edge excitation positions are also indicated by the schematic illustrations in the corners. The patterns have been normalized to the maximum intensity value for all fi ve patterns to show the relative brightness for different excitation positions [ 13 ].
angle-resolved measuring mode, band-pass color filters can be used. In the beam path there is space for additional optical components, such as polarizers. This allows reconstruction of the emission polarization for every emission angle and/or polarization-sensitive hyperspectral imaging [ 8 – 10 ].
14
Figure 3b shows angular distributions of the radiation emitted by the single Au nanoparticle for central excitation and excitation at four edge positions (as indicated by the illustra- tions). The CL emission intensity is shown as function of zenithal angle θ and azimuthal angle φ . T e black region on the top of the patterns corresponds to the opening in the paraboloid mirror (see Figures 2 b and 2 c) where no light is collected. While the nanoparticle excited in the center shows a symmetric angular profi le refl ecting the doughnut-type emission from an upward-pointing dipole, the angular distributions on the edge show a clear asymmetry. For instance, with excitation on the leſt side of the particle the CL emission is more intense toward the right side of the angular pattern. T is asymmetry arises from the fact that off -center electron beam excitation leads to the simultaneous excitation of in-plane and out-of-plane multipole components, which interfere in the far-fi eld [ 12 ]. T is example demonstrates how angle-resolved CL can be used to study such nanophotonic systems in great detail on length scales that are inaccessible with conventional light microscopy. It should be noted that the technique is not limited to plasmonic systems and can be applied also to dielectric/semiconductor nanophotonic geometries (for example, nanoparticles, photonic crystals) [ 4 , 5 ].
Imaging in 3D: CL tomography . Imaging with CL
usually provides information in 2D. However, it is known from seismic, X-ray, and TEM imaging that tomographic analysis can provide 3D information. In tomography a series of 2D images is made under diff erent tilt angles after which mathematical algorithms are applied to reconstruct the 3D structure of the data. Such tomographic reconstruction was recently performed with EELS in the context of plasmonic nanostructures [ 13 ], but it can also be done with CL
www.microscopy-today.com • 2016 May
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