Spatiotemporal Visualization of Long-Range Anisotropic Plasmon Polaritons in Hyperbolic MoOCl2

TL;DR

This paper demonstrates direct nanoscale visualization of long-range anisotropic plasmon polaritons in hyperbolic MoOCl2, revealing their propagation characteristics and establishing the material as a promising platform for low-loss nanophotonic applications.

Contribution

It provides the first direct imaging and spatiotemporal analysis of long-range anisotropic plasmon polaritons in MoOCl2 using time-resolved microscopy, highlighting their extended propagation and low-loss features.

Findings

Plasmon polaritons propagate over 10 μm with low optical loss.

LRAPPs exhibit approximately three times longer propagation than short-range polaritons.

Direct observation of polariton reflections at flake edges.

Abstract

Manipulating light at the nanoscale with minimal loss remains a central challenge for nanophotonic technologies that can be tackled by using the direction-dependent polariton modes supported by anisotropic materials. Although best known for their highly confined polaritons, hyperbolic materials can also host long-range directional polaritons, whose direct observation has remained challenging as it requires experimental techniques that combine nanometre and femtosecond spatial and temporal resolution, respectively. Here, we use time-resolved photoemission electron microscopy for direct nanoscale visualization of long-range anisotropic plasmon polariton (LRAPP) dynamics on a flake of the van der Waals hyperbolic material molybdenum oxydichloride. We directly image plasmon polaritons with propagation lengths larger than 10 $\mu$m, exhibiting an approximately three times longer propagation length and intrinsically lower optical loss than short-range polaritons previously reported on the same material. By tracking the spatiotemporal evolution of LRAPPs, we determine their phase and group velocities at the nanoscale and directly observe their reflections at flake edges. These results establish molybdenum oxydichloride as a versatile platform for integrated nanophotonics, supporting both low-loss directional transport and deeply subwavelength field confinement within a single natural material, in the visible spectral range.