Nanoscale mapping of plasmons, photons, and excitons
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roduction The optical properties of nanoscale objects drastically change at the nanometer scale as a function of their size, morphology, and environment. Indeed, when the structure of a material exhibits variations on the order of or below the typical wavelength of light, new photonic or plasmonic modes emerge. When it exhibits variations of the order of the excitonic Bohr radius—a few nanometers, new excitonic states develop. Such considerations would have probably remained as research curiosities if the growing availability of new materials, such as photonic bandgap crystals, metallic nanoparticles, or quantum confined systems (quantum wells, quantum dots), had not triggered the need for new tools and new concepts for the exploration and understanding of this fascinating new realm. Getting spectroscopic information at the nanoparticle scale, or even better, within individual nanoparticles or unit cells of an artificial material, is not simple because the required spatial resolution is one or two orders of magnitude smaller than the equivalent wavelength in a vacuum or in a medium. This information, however, is crucial in many applications (e.g., biosensors based upon plasmons or photodetectors relying on excitons), but using purely optical microscopy is not usually adequate at these scales. An alternative is to use fast electrons beams, taking advantage of the fact that electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) can be regarded as the nanometer analog of extinction and scattering/
photoluminescence, respectively. These two techniques have for a long time faced similar problems: obtaining, with appropriate spatial and spectral resolutions and signal-to-noise ratio, access to the relevant energy range (IR-visible-UV). After a series of pioneering work both in EELS1–3 and CL,4–7 these problems have been solved in past years4,8,9 for surface plasmons and more recently for quantum emitters.10 The present article is aimed at discussing some of the latest results and applications in mapping and understanding these excitations at the nanometer scale. The interested reader can refer to Reference 11 for a deeper understanding of the theoretical background, Reference 12 for a deeper understanding of technical issues, and References 12 and 13 for information on other spatially resolved electron spectroscopy findings and applications.
Experimental and simulation techniques Figure 1 presents a simple system for performing both EELS and CL experiments that can be realized in a scanning (transmission) electron microscope, S(T)EM. A fast electron beam (1 keV to 30 keV for SEM, 40 keV to 300 keV for a STEM) ranging from a few nanometers to tenths of nanometers in diameter is scanned over the sample of interest. At each position of the beam, basically two different kinds of signals of interest can be detected. The first kind enables the recording of images of the sample: the high angle annular dark-field (HAADF) signal
Mathieu Kociak, Laboratoire de Physique des Solides, Bâtiment 510, CNRS/UMR8502, Université
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