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II3.02 - Probing Coherent and Incoherent Radiation from Semiconductor and Plasmonic Nanostructures and Surfaces 
Date/Time:
April 22, 2014   4:00pm - 4:15pm
 
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We demonstrate a novel way to distinguish coherent and incoherent radiation from silicon and metallic nanostructures by using a nanoscale electron probe to generate light emission. We use angle-resolved cathodoluminescence imaging spectroscopy (ARCIS), in which a 30 keV electron beam in a scanning electron microscope is used as a broadband point-like excitation source. The transient electric field about the electron trajectory excites coherent transition radiation from the surface which is a direct measure of the local dielectric constant. In addition, incoherent radiation is excited by the recombination of electron-hole pairs generated by electrons penetrating into the silicon. In our experiments we use a half-parabolic mirror placed between the electron column and the silicon sample to collect the electron beam generated coherent and incoherent emission. We collect angle-resolved radiation profiles over a spectral range from 400-900 nm. The data are fitted to a model that assumes dipolar radiation profiles for the transition radiation component and Lambertian profiles for the incoherent emission. Excellent agreement between the angular data and the theory is observed. The analysis enables partitioning the collected emission spectrum in coherent and incoherent parts. The coherent part serves as a nanoscale probe of the local dielectric constant, a property that can not be detected using far-field techniques at nanoscale resolution. Moreover, we reconstruct the incoherent radiation spectrum over the entire 400-900 nm spectral band. The measurements are complemented with studies on thin-film and bulk samples of Au, Ag, Cu and Al. These plasmonic metals are all efficiently excited by the electron beam and their transition radiation spectrum is collected. Good agreement with theory is observed, based on dielectric constants measured using ellipsometry.Overall, the ARCIS technique serves as a nanoscale probe of optical resonances in dielectric and plasmonic nanostructures, and enables measurements of optical constants at length scales that are inaccessible with any other technique.
 


 
 
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