Direct Three-Dimensional\nObservation of the Plasmonic\nNear-Fields of a Nanoparticle with Circular Dichroism

Characterizing the spatial distribution of the electromagnetic\nfields of a plasmonic nanoparticle is crucial for exploiting its strong\nlight–matter interaction for optoelectronic and catalytic applications.\nHowever, observing the near-fields in three dimensions with a high\nspatial resolution is still challenging. To realize efficient three-dimensional\n(3D) nanoscale mapping of the plasmonic fields of nanoparticles with\ncomplex shapes, this work established autoencoder-embedded electron\nenergy loss spectroscopy (EELS) tomography. A 432-symmetric chiral\ngold nanoparticle, a nanoparticle with a high optical dissymmetry\nfactor, was analyzed to relate its geometrical features to its exotic\noptical properties. Our deep-learning-based feature extraction method\ndiscriminated plasmons with different energies in the EEL spectra\nof the nanoparticle in which signals from multiple plasmons were intermixed;\nthis component was key for acceptable 3D visualization of each plasmonic\nfield separately using EELS tomography. With this methodology, the\nelectric field of the plasmon that induces far-field circular dichroism\nwas observed in 3D. The field linked to this chiroptical property\nwas strong along the swirling edges of the particle, as predicted\nby a numerical calculation. This study provides insight into the correlation\nbetween structural and optical chiralities through direct 3D observation\nof the plasmonic fields. Furthermore, the strategy of implementing\nan autoencoder for EELS tomography can be generalized to achieve competent\n3D analysis of other features, including the optical properties of\nthe dielectrics and chemical states.