Semiconductor nanowires and nanoparticles have the potential to strongly restrict light, similar to their metallic counterparts. In light of their intrinsic properties, they can be easily incorporated into optoelectronic semiconductor devices such as LEDs and solar cells where they can act as a light outcoupling layer or an anti-reflection coating . In addition, they serve as effective nanoantennas that can be used in metasurfaces and metamaterials that mold the flow of light at the nanoscale .
Semiconductor particles support Mie-like geometry-dependent optical modes. Compared to metallic nanoparticles, most of the electromagnetic field is restricted within the particle and not at the surface. This makes it difficult to investigate these modes with near-field methods as those depend on the evanescent fields outside of the structure. Cathodoluminescence (CL) spectroscopy does not have this drawback as the electrons can easily penetrate such nanoparticles, enabling a user to directly visualize the internal modal structure.
Figure 1 shows how spatially resolved CL spectroscopy can be employed to image a set of eigenmodes that is supported by a single silicon nanocylinder. The angular emission profiles of the particles can also be measured using the SPARC system, which is highly relevant for their performance as an antenna .
Figure 1. (a) Schematic of the silicon nanoparticle geometry. The particle was fabricated using a combination of electron beam lithography and reactive ion etching on a silicon-on-insulator substrate. (b) CL spectrum (magenta dots) averaged over all excitation positions on the particle. The spectrum is fitted with a sum of five Lorentzians (black curve) that are also plotted individually (gray curves). An SEM micrograph of the particle is shown as inset (scale bar is 150 nm). (c) Spatial CL maps showing the modal excitation profiles for the peaks in (b) derived from a single hyperspectral CL dataset .
Mixing particles in more difficult geometries allows more tunability of the optical response and is key for their incorporation into a macroscopic device, such as a solar cell or an LED [1, 2, 4]. Electromagnetic coupling between the particles has a vital role in governing the optical properties in these systems and results in mode hybridization, for example. CL imaging can be used for visualizing such coupling effects. Figure 2 illustrates an example where the modal hybridization is apparent from the spatial CL map. The experimental results can be compared with accurate full-wave simulations as a verification [3,4].
Figure 2. (a) SEM micrograph of a silicon nanoparticle dimer. (b) Spatial CL distribution at λ = 450 nm, derived from a hyperspectral CL dataset. The mode hybridization manifests itself in the CL measurement as an enhanced emission probability at the outer edges of the dimer. (c) Vertical electric field intensity from a COMSOL eigenmode simulation. The simulated field distribution matches the measured CL distribution in (b) .
In this article, the approach offered is general and can be easily applied to other particle geometries and materials .
 M. L. Brongersma, et al., Light management for photovoltaics using high-index nanostructures, Nat. Matter., 13, 451-460 (2014).
 D. Lin et al., Dielectric gradient metasurface optical elements, Science 345, 298-302 (2014).
 T. Coenen et al., Resonant modes of single silicon nanocavities excited by electron irradiation, ACS Nano 7, 1689-1698 (2013).
 J. van de Groep et al., Direct imaging of hybridized eigenmodes in coupled silicon nanoparticles, Optica 3, 93-99 (2016).
 M. A. van de Haar et al., Controlling magnetic and electric dipole modes in hollow silicon nanocylinders, Opt. Express 24, 2047-2064 (2016).
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