Tailoring the wave function of electron probes for the selective detection of plasmonic modes.
European Microscopy Conference (EMC), Lyon, France, 2016
Electronic spectroscopies are important in the study of localised surface plasmon resonances of metallic nanostructures, allowing to detect and image the strong spatial variations in the electrical field of the induced resonances of a single nanoparticle . These techniques do however present some drawback when compared to their optical counterparts. While optical spectroscopies can make use of polarisation to directionally probe the response of a nanoparticle, an electronic beam can’t discriminate between energy-degenerate eigenmodes and is also blind to optical activity and dichroism. Here we present a way to expand the applicability of EELS to the characterisation of plasmonic resonances by exploiting the recently developed methods of electron beam shaping through phase manipulation . This radically new approach is based on the idea of tailoring the electronic probe to fit the properties under investigation that can then be selectively detected, allowing to perform new measurements that were previously impossible . In particular, we first show how the phase in the electron beam's complex wave function couples to the electric potential of the plasmonic excitation (see fig1), allowing to selectively detect localised plasmonic excitations that possess the same symmetry as the electron probe . While this concept is entirely general and potentially applicable to any plasmonic resonance, we decide to focus a first experimental demonstration on detecting the dipolar mode of a nanorod. The ideal probe for this purpose is formed by two intensity lobes opposite in phase as shown in figure 1a, which we successfully generate in the TEM by applying state of the art phase manipulation techniques. Finally, we show experimental proof of the method’s effectiveness on purposefully made test sample, demonstrating the viability of this new approach and opening to a new generation of plasmon oriented TEM experiments, that will expand in parallel with the availability of wave manipulation methods.  J. Nelayah et al., Nat. Phys. 3, 348 (2007).  F.-P. Schmidt et al., Nano Lett. 12, 5780 (2012).  G. Guzzinati et al., Ultramicroscopy 151, 85 (2015).  H. Lorenço-Martins, M. Kociak, in preparation. Acknowledgments: GG, AB and JV acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC Starting Grant No. 278510-VORTEX.