Biomedical Engineering Reference
In-Depth Information
in Fig. 12.3 is similar to experimental results (Boudarham et al.,
2010).
Of particular note is that EELS allows for mapping out modes
that do not couple well to the radiative field, so-called “dark
modes” (Chu et al., 2009). Near-field scanning optical microscopy
is another technique that allows for accessing the “dark modes,” but
it requires an invasive probe and it does not have as good spatial
resolution (Imuraet al., 2005).
It is also possible to detect radiative photons from the electron
energy loss process. This has been used in many cathodolumines-
cence studies (e.g., see (Vesseur et al., 2007)), as well as studies
of tunneling-based light emission (e.g., see (Bharadwaj et al., 2011;
Lambe and McCarthy, 1976)). Tunneling-based light emission may
prove interesting as a metal-insulator source of light, but so far, the
emission e
ciency has remained low.
12.4 Vortex-EELS and the Magnetic Plasmon
Recently, advances in electron beam optics have allowed for the
creation of vortex beams with orbital angular momentum. As these
beams act like an effective magnetic current density, they may be
used to probe the magnetic response of nanostructures through
energy-loss spectroscopy. In the following sections, the vortex
electron beam will be introduced, as well as the effective magnetic
charge that it allows. This will enable the calculation of vortex-
EELS, for which it is shown that the scattering probability can be
comparable to conventional EELS, opening up the door for a new
probe for the local magnetic response.
12.4.1
Vortex Electron Beams
In 2010, it was demonstrated that point dislocations in crystal
lattices allow for the production of diffracted electron beams with
orbitalangularmomentum(UchidaandTonomura,2010).Thepoint
defect creates diffraction planes that are the superposition of a
phase singularity with a linear grating. Later, man-made vortex
diffraction structure was created in a thin platinum film to produce
