Hidden in plain sight: nano-plasmonic signatures in metallic oxides

Infrared nano-imaging from ambient to cryogenic temperatures provides a peerless means to visualize propagation and scattering of polaritons in the highly confined near-field regime. Plasmon polaritons in graphene heterostructures, exciton polaritons in 2D semiconductors, and phonon polaritons in polar crystals have all emerged as rich targets for study by near-field imaging and spectroscopy, supplying insights to their host media and unlocking new photonic functionality. Polaritons in these recently well-studied systems demonstrate that near-field probes resolve not only the local polarizability of materials under study, but also participate in their nonlocal optical response. Wherever it arises, such response is strongly sculpted by the system geometry by tailoring the shape of induced light fields. These facts are well appreciated for “traditional” polaritonic media land can be harnessed to realize near-field cavities and resonators. Nevertheless, evidence for “nonlocal optics” in the context of damped systems like correlated oxides has remained overlooked and under-acknowledged. Here I will showcase observable signatures of damped plasmon polaritons in metallic oxides ranging from quasi-2D MoO$_2$ to phase-change correlated materials like VO$_2$ and NdNiO$_3$. Whether by design or serendipity, system geometry observably molds the plasmonic response in all these cases, producing startling real-space resonances. I will present a unified theoretical framework that serves to recognize and rationalize plasmonic features recorded in near-field imaging studies, and I will demonstrate how these “smoking gun” plasmonic signatures serve to reveal the dynamics of electrons in unconventional metals. I will discuss first realization and future opportunities for strong coupling among these damped oxide polaritons. These findings remind us that polaritons frequently hide in plain sight, and that their recognition awards insight.