Plasmons are coupled electromagnetic waves that occur inside and on the surface of metals. What makes plasmons of great interest is their ability to confine light to nanoscale regions – much smaller that the wavelength of light.
The S/TEM is also an ideal tool for mapping optical properties of noble metal nanoparticles and nanostructures. In this rapidly developing field we study nanoobjects by means of energy-filtering TEM and STEM with high energy-resolution EELS. Presently, we are using a monochromated FEI Tecnai F20 with a HR energy-filter (MonoTEM) and a monochromated FEI Titan 60-300 with a Quantum energy-filter (Gatan) (ASTEM).
Our experimental efforts are combined with expertise in nano-optics from the Institute of Physics of the University of Graz (Joachim Krenn, Ulrich Hohenester).
We map the complete plasmonic spectrum of silver nanodisks by EELS and show how the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Since this radial breathing mode does not couple to light, it has previously escaped from observation in optical experiments.
“Dark plasmonic breathing modes in silver nanodiscs”, F.-P. Schmidt, H. Ditlbacher, U. Hohenester, A. Hohenau, F. Hofer, J.R. Krenn, Nano Letters 12 (2012) 5780-2905.