PhD Thesis Defense- Andrea Konečná

Published: May 3, 2019

Theoretical description of low-energy excitations in nanostructures as probed by fast electrons

Candidate: Andrea Konečná

Supervisors: Javier Aizpurua and Rainer Hillenbrand

Research group: Theory of nanophotonics (CFM) and Nanooptics (CIC nanoGUNE)

When: May 17, 2019 (11:00h)

Where: CFM Auditorium

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Figure: Schematics of surface-enhanced EELS: the electron beam excites a localized surface plasmon (LSP) in the rod which couples via the induced electromagnetic field with molecular excitations (either molecular excitons or vibrations). The spectrum of the coupled system (blue) exhibits a broad plasmonic peak with a dip arising due to the coupling. The signal contrast (dip depth) in the coupled scenario is much larger than if the beam probes only the molecules (the peak in the red spectrum).

Electron energy loss spectroscopy (EELS) with a focused electron beam is a well-established technique to characterize excitations in nanostructured materials. EELS in the so-called low-loss energy region is particularly interesting in connection with the field of nanophotonics, which deals with optical properties of various materials at the nanoscale. Characterizing the optical excitations and electromagnetic near-field of photonic nanostructures, which are naturally accessible by fast electron probes, is often crucial for understanding their functionality and optical behavior.

In this thesis, we use classical electrodynamics to describe the interaction of fast electrons with low-energy excitations in the infrared and visible energy range, and calculate spatially-resolved EEL spectra in different configurations that are of fundamental and practical interest. In the first part we analyze the coupling of fast electrons with phonon polaritons in infinite and truncated thin films. We describe different phonon-polariton modes excitable in slabs made of materials with either isotropic or anisotropic optical properties. We also demonstrate how finite temperature affects the spectra in the vibrational region.

Apart from characterizing excitations in inorganic samples, we show that EELS can be used to probe molecular excitations in sensitive organic samples with an electron beam in “aloof” geometry. Also, we suggest the concept of “surface-enhanced EELS” exploiting electromagnetic coupling between the so-called plasmonic nano-antennas and molecular samples. Such a scheme of spectroscopy could help to enhance the sensitivity of the technique to small molecular amounts or enable remote probing of molecular samples.

Finally, we explore the excitation of resonant optical cavity modes in high-refractive-index nanoparticles by conventional and vortex electron beams (VEBs). We show how to exploit the use of (vortex) electron beams to distinguish modes of electric and magnetic nature from the EEL spectra in dielectric nanoparticles. We also analyze nanostructures exhibiting dichroic response, and show that VEBs are naturally capable of performing chiral-specific spectroscopy at the nanoscale.