"Nanoscale characterization of optical chirality"

Abstract:

In this talk, we will report recent results on nanometer-scale circular dichroism in canonical 3D chiral systems and discuss their interpretation. We will first present nanoscale, circularly polarized cathodoluminescence (pCL) experiments performed in a scanning transmission electron microscope on plasmonic Born–Kuhn systems.

Using a novel detection scheme, we observe highly localized and strongly dichroic signals , in excellent agreement with simulations . At first glance, such strong dichroism could be attributed to the well-known large geometrical chirality of BKS. However, a systematic experimental and numerical study as a function of BKS parameters reveals that this contribution is unexpectedly weak. Instead, we show that a continuous pseudo-scalar quantity—the plasmonic chirality characteristic—can be rigorously defined to characterize the chirality of individual plasmonic modes. Remarkably, we demonstrate that electron spectroscopy, unlike optical spectroscopy, provides direct experimental access to this quantity.

We will then extend cathodoluminescence excitation spectroscopy (CLE)—i.e., spatially resolved, time-correlated measurements of EELS and CL —to the polarized CL case using experiments on BKS. If time permits, we will discuss how combining such experiments with phase shaping paves the way to probing entanglement between electrons and photons mediated by plasmons.