Collective near-field coupling in infrared-phononic metasurfaces for nano-light canalization

TL;DR

This paper explores the topological transition in infrared-phononic metasurfaces, demonstrating how near-field coupling of nanoribbons induces a shift from hyperbolic to elliptical dispersion, enabling diffraction-less polariton propagation.

Contribution

It provides the first experimental observation of a synthetic phonon resonance causing a topological transition in hBN metasurfaces, and visualizes subwavelength canalization modes.

Findings

Observation of synthetic transverse optical phonon resonance

Visualization of subwavelength canalization mode

Identification of topological transition from hyperbolic to elliptical dispersion

Abstract

Polaritons, coupled excitations of photons and dipolar matter excitations, can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion. At the transition from hyperbolic to elliptical dispersion (corresponding to a topological transition), various intriguing phenomena are found, such as an enhancement of the photonic density of states, polariton canalization and hyperlensing. Here we investigate theoretically and experimentally the topological transition and the polaritonic coupling of deeply subwavelength elements in a uniaxial infrared-phononic metasurface, a grating of hexagonal boron nitride (hBN) nanoribbons. By hyperspectral infrared nanoimaging, we observe, for the first time, a synthetic transverse optical phonon resonance (that is, the strong collective near-field coupling of the nanoribbons) in the middle of the hBN Reststrahlen band, yielding a topological transition from hyperbolic to elliptical dispersion. We further visualize and characterize the spatial evolution of a deeply subwavelength canalization mode near the transition frequency, which is a collimated polariton that is the basis for hyperlensing and diffraction-less propagation. Our results provide fundamental insights into the role of polaritonic near-field coupling in metasurfaces for creating topological transitions and polariton canalization.