Hyperbolic Phonon Polaritons in Suspended Hexagonal Boron Nitride

Abstract

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.

Phonon Polaritons and Hyperbolic Response in van der Waals Materials

“…Due to the unique material structure and properties related to the reduced size/dimension, low‐dimensional systems offer new advantages for phonon polaritons. In the following sections, we will discuss the representative properties of phonon polaritons in low‐dimensional materials, including anisotropy and hyperbolic response, high optical confinement down to the atomic scale, structure engineering for tunability, and polariton hybrids …”

“…a) The s‐SNOM image reveals the difference in wavelength and damping from polaritons in suspended hBN (denoted as “sus,” red curve) and hBN on SiO 2 substrate (denoted as “sup,” blue curve). Reproduced with permission . Copyright 2019 American Chemical Society.…”

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…In principle, these hyperbolic polariton modes can have arbitrarily large wave vectors and infinitely many transverse branches regardless of the material thickness . The experimentally observed confinement factor of h‐BN hyperbolic phonon polaritons reaches up to n eff ≈ 50 . Phonon polaritons exhibit relatively low losses compared to plasmon polaritons because their lifetime is not affected by the electronic losses, but mainly dictated by the crystal quality of the atomic lattice.…”

“…Phonon polaritons exhibit relatively low losses compared to plasmon polaritons because their lifetime is not affected by the electronic losses, but mainly dictated by the crystal quality of the atomic lattice. The losses in natural h‐BN crystals are demonstrated to be lower than that of most of the plasmonic materials (γ −1 ≈ 15–30), and can be further reduced through isotopic enrichment (γ −1 ≈ 40) …”

Scaling and Reflection Behaviors of Polaritons in Low‐Dimensional Materials

“…For nanophotonic device applications, low‐damping polariton propagation is required. To eliminate polariton reflection by this strain‐induced dielectric function fluctuation, choosing an atomically flat substrate or a suspended configuration for photonic media is essential …”