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While chiral plasmonic nanostructures are often characterized through optical circular dichroism, retrieving local chiroptical information from individual structures in the far field remains difficult [1, 2]. Using a transmission electron microscope, electron energy-gain spectroscopy (EEGS), and consequently photon-induced near-field electron microscopy, allows us to probe optically driven near fields with nanometric spatial resolution [3, 4].
The growing ability to structure both light and free electrons has paved the way for symmetry- and angular-momentum-resolved spectroscopies [5, 6, 7, 8]. This renders SAM- and OAM-resolved EEGS highly desirable for chirality sensing, given that the handedness of the optical source and the phase structure in the azimuth of a vortex beam can, in principle, have different couplings to the chiral nanostructure. However, a transparent analytical description of how these two contributions enter spatially resolved EEGS signals is still missing. In particular, it is desirable to retain an explicit connection between the measured dichroic response, the geometry of the nanostructure, and the angular momentum carried by the applied field.
Here, we investigate spin- and orbital-angular-momentum dichroism in EEGS of the plasmonic Born–Kuhn model system [9], formed by two laterally shifted gold nanorods rotated with respect to each other. Using an analytical model together with full retarded boundary-element simulations (MNPBEM [10]), we show (and prove) that the dichroic signal can be written in a form that explicitly separates the SAM- and OAM-dependent contributions. In this formulation, the SAM-dependent part is controlled by the polarization helicity of the driving field, whereas the OAM-dependent part is governed by the azimuthal phase sampled by the dimer with respect to the vortex center.
One of the key results is that, after summing over opposite circular polarizations, the SAM-dependent contribution is eliminated and the remaining signal isolates a quantity that can be interpreted as a discrete OAM density [11]. In this sense, the Born–Kuhn system acts as a local analyzer of the orbital structure of the driven near field. The same analytical approach is also able to reproduce the dominant spatial features observed in the simulations (see Fig. 1).
Beyond the present model system, these results point toward experimentally accessible SAM- and OAM-resolved electron spectroscopies in optically driven near-field platforms, and toward the study of genuinely three-dimensional chiral geometries, such as metallic helices, where the relation between structural chirality and orbital angular momentum can be explored further [12].
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