Engineering low-loss phonon polaritons in anisotropic 2D materials

Dr Qingdong Ou, Monash University

Polaritons, hybrid light-matter excitations, enable nanoscale control of light. Particularly large polariton field confinement and long lifetimes can be found in graphene and 2D van der Waals materials, opening new avenues for low-loss photonic applications. Here, we present our recent progress in the engineering of these nanoscale polaritons in anisotropic 2D materials [1]. The observation of anisotropic and ultra-low-loss polariton propagation along the surface of natural 2D material α-MoO₃ will be first introduced, which shows phonon polaritons with elliptic and hyperbolic in-plane dispersion [2]. To better utilize the low-loss directional polaritons, we developed a chemical intercalation method to reversibly control and switch the phonon polaritons [3] . Spatially controlled polariton switching was achieved with various in-plane heterostructure configurations and patterns. Very recently, we show how the hyperbolic polaritons in α-MoO₃ thin slabs were delicately manipulated by controlling the interlayer twist angle [4].

We experimentally observed tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in twisted α-MoO₃ bilayers at a photonic magic twist angle. The theoretical analysis showed these transitions of in-plane hyperbolic polaritons are controlled by a topological quantity. At the transitions, the photonic dispersion of the resulting topological polaritons flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation. We envision such natural anisotropic 2D materials offer an unprecedented platform for controlling the flow of energy at the nanoscale.