Insights into hyperbolic phonon polaritons in h−BN using Raman scattering from encapsulated transition metal dichalcogenide layers

Alternative techniques for probing hyperbolic phonon polaritons (HPPs) in two-dimensional materials will support the development of the emerging technologies in this field. Previous reports have shown that it is possible for $\mathrm{W}{\mathrm{Se}}_{2}$ monolayers in contact with the hexagonal boron nitride ($h\text{\ensuremath{-}}\mathrm{BN}$) to generate HPPs in the $h\text{\ensuremath{-}}\mathrm{BN}$ via Raman scattering. In this paper, we set out our results on HPP Raman scattering induced in $h\text{\ensuremath{-}}\mathrm{BN}$ by $\mathrm{W}{\mathrm{Se}}_{2}$ and $\mathrm{Mo}{\mathrm{Se}}_{2}$ monolayers including multiple resonances at which the Raman scattering is enhanced. Analysis of the observed Raman line shapes demonstrates that Raman scattering allows HPPs with wave vectors with magnitudes significantly in excess of $15\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ to be probed. We present evidence that the Raman scattering can probe HPPs with frequencies less than the expected lower bound on the reststrahlen band, suggesting new HPP physics still waits to be discovered.