Doping-driven topological polaritons in graphene/{\alpha}-MoO3 heterostructures

TL;DR

This paper predicts and demonstrates how doping levels in graphene/0-MoO3 heterostructures can induce topological transitions in hybrid polaritons, enabling tunable subwavelength focusing for advanced nanooptical applications.

Contribution

It introduces a method to control topological transitions in polaritons via doping in 2D heterostructures, with experimental validation and potential for on-chip nanoimaging.

Findings

Doping changes polariton isofrequency contour topology from open to closed.

Substrate modification allows further dispersion engineering.

Achieved subwavelength focusing using a negative refraction lens.

Abstract

Controlling the charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties in natural materials. This approach could be used to induce topological transitions in the optical response of photonic systems. Here, we predict a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and $\alpha$-phase molybdenum trioxide ($\alpha$-MoO3). By chemically changing the doping level of graphene, we experimentally demonstrate that the contour topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, by changing the substrate medium for the heterostructure, the dispersion contour can be further engineered into a rather flattened shape at the topological transition, thus supporting tunable polariton canalization and providing the means to locally control the topology. We demonstrate this idea to achieve extremely subwavelength focusing by using a 1.2-$\mu$m-wide silica substrate as a negative refraction lens. Our findings open a disruptive approach toward promising on-chip applications in nanoimaging, optical sensing, and manipulation of nanoscale energy transfer.