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Low-symmetry van der Waals materials have enabled strong confinement ofmid-infrared light through hyperbolic phonon polaritons (HPhPs) at thenanoscale. Yet, the bottleneck persists in manipulating the intrinsicpolaritonic dispersion to drive further progress in phonon-polaritonics. Here,we present a thermomechanical strategy to manipulate the HPhPs in$\alpha$-MoO$_3$ using high pressure and temperature treatment. The hotpressing engineers the stoichiometry of $\alpha$-MoO$_3$ by controllablyintroducing oxygen vacancy defects (OVDs), which cause asemiconductor-to-semimetal transition. Our density functional theory andfinite-difference time-domain results, combined with experimental studies showthat the OVDs induce a metastable metallic state by reducing the bandgap whilemodifying the intrinsic dielectric permittivity of $\alpha$-MoO$_3$.Photo-induced force microscopy confirms an average dielectric permittivitytunability of $|\Delta\varepsilon/\varepsilon|\approx0.35$ within aReststrahlen band of $\alpha$-MoO$_3$, resulting in drastic shifts in the HPhPdispersion. The polariton lifetimes for pristine and hot-pressed flakes weremeasured as $0.92 \pm 0.06$ and $0.86 \pm 0.11$ ps, respectively, exhibiting aloss of only 7%, while the group velocity exhibited an increase of $38.8 \pm0.2$%. The OVDs in $\alpha$-MoO$_3$ provide a low-loss platform that enablesactive tuning of mid-infrared HPhPs and have a profound impact on applicationsin super-resolution imaging, nanoscale thermal manipulation, boosted molecularsensing, and on-chip photonic circuits.

Vacancy-Engineered Phonon Polaritons in $\alpha$-MoO$_3$