Hyperbolic Phonon Polaritons in Suspended Hexagonal Boron Nitride.

Highly confined and low-loss hyperbolic phonon polaritons in hexagonal boron nitride possess properties analogous to surface plasmon polaritons, but with enhanced confinement and lower loss. Their properties have been so far mostly studied on dielectric substrates, which provide an asymmetric environment for polariton propagation, and add to damping. In this work, we investigate hyperbolic phonon polaritons over suspended hexagonal boron nitride, showing remarkable properties, including elongated polariton wavelength and reduced damping, up to 18% lower compared to dielectric-backed samples. We use real-space nanoimaging of the polaritons in hexagonal boron nitride to demonstrate and visualize these effects. Our results indicate that suspended boron nitride offers better figures of merit for polariton transport, which are generalizable to other polaritonic materials, and they may be explored in heterostructures for advanced nanophotonic applications.