Negative refraction in hyperbolic hetero-bicrystals

Abstract

We visualized negative refraction of phonon-polaritons in hetero-bicrystals of two hyperbolic van der Waals materials: molybdenum oxide (MoO3) and isotopically pure hexagonal boron nitride (h11BN). The polaritons - hybrids of infrared photons and lattice vibrations - form collimated rays that display negative refraction when passing through a planar interface between the h11BN and MoO3 crystals. At a special frequency ω0, these rays can circulate along closed diamond-shaped trajectories. We show that polariton eigenmodes display regions of both positive and negative dispersion interrupted by multiple gaps resulting from polaritonic level repulsion