Relativistic effects in EELS of nanoporous alumina membranes

Electrochemically prepared porous alumina membranes with a pore diameter of 58 nm have been investigated by electron energy-loss spectroscopy in a transmission electron microscope operated at 120 keV and 200 keV. Energy-loss spectra are recorded for electrons traveling along the pores at different impact parameters. The dominant spectral features near the pore wall are shown to originate in the excitation of surface plasmons. Additional loss features at 7 eV (8 eV) for 200 keV (120 keV) are observed, whose origin is associated to Cherenkov radiation modified by the sample nanostructure. Maxwell equations are solved for different geometrical models of the target, providing a good qualitative description of the experimental spectra. Quantitative agreement is obtained by considering multiple plasmon excitation, suggesting a promising tool to investigate pore surfaces.Support from the Natural Sciences and Engineering Research Council (Canada), the Basque Departamento de Educacion, Universidades e Investigacion, the University of the Basque Country UPV/EHU (Contract No. 00206.215-13639/2001), and the Spanish Ministerio de Ciencia y Tecnologia (Contract No. MAT2001-0946) is gratefully acknowledged.Peer reviewe

Cherenkov radiation effects in EELS for nanoporous alumina membranes

Swift electron energy losses have been investigated in nanoporous alumina, taking into account retardation effects. A simple analytical expression is obtained for the energy loss probability. The electron energy loss spectra observed with the Scanning transmission electron microscope are reasonably well reproduced using a simple model consisting of a cylindrical shell of alumina surrounded by an effective porous medium. The Cherenkov radiation is responsible for energy losses below 8 eV. These losses are very sensitive to the shape of the nanostructures, up to distances much larger than the adiabatic length v/ω.Support from the Basque Government and the Basque Country University is gratefully acknowledged.Peer reviewe

Cherenkov effect as a probe of photonic nanostructures

Electron energy-loss spectroscopy (EELS) is shown to be an excellent source of information both on photonic crystal bands and on radiation modes of complex nanostructures. Good agreement is reported between measurements and parameter-free calculations of EELS in porous alumina films, where Cherenkov radiation is scattered by the pores to yield a strong 8.3-eV (7-eV) feature for 120-keV (200-keV) electrons. The latter is related to the bands of two-dimensional photonic crystals formed by air cylinders in an alumina matrix with similar near-range ordering. Finally, the band structure is proved to be directly mapped by angle-resolved EELS.Peer reviewe

Stopping power and Cherenkov radiation in photonic crystals

Electron energy loss spectroscopy (EELS) induced by fast electrons in electron microscopes are used to probe photonic structures. Some of the loss features are shown to be associated to the excitation of radiative modes in the samples (Cherenkov radiation), from where information on photonic bands is extracted. The case of a 1D crystal is qualitatively discussed to explain the physics behind Cherenkov radiation in photonic crystals. For an electron beam collimated on one of the pores of 2D crystal consisting of a porous alumina film with 100-nm lattice constant, theory predicts a Cherenkov feature at around 6–9 eV, which is in excellent agreement with experiment. Finally, the features of the loss spectra are shown to be strongly correlated with the density of photonic states, suggesting the potential application of this technique to probe the quality and actual performance of photonic crystals.The authors acknowledge support from the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (contract no. 00206.215-13639/2001), and Spanish Ministerio de Ciencia y Tecnologa (contract no. MAT2001-0946 and MAT2002-04087-C02-01).Peer reviewe