Negative refraction in hyperbolic hetero-bicrystals

A. J. Sternbach, S. Moore, A. Rikhter, S. Zhang, R. Jing, Y. Shao, B. Kim, S. Xu, A. Rubio, C. Dean, J. Hone, M. M. Fogler, D. N. Basov

Published 2022-09-30Version 1

We visualized negative refraction of phonon-polaritons in hetero-bicrystals of two hyperbolic van der Waals materials: molybdenum oxide ($MoO_3$) and isotopically pure hexagonal boron nitride ($h^{11}BN$). The polaritons - hybrids of infrared photons and lattice vibrations - form collimated rays that display negative refraction when passing through a planar interface between the $h^{11}BN$ and $MoO_3$ crystals. At a special frequency $\omega_0$, 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.