  Low-symmetry van der Waals materials have enabled strong confinement of
mid-infrared light through hyperbolic phonon polaritons (HPhPs) at the
nanoscale. Yet, the bottleneck persists in manipulating the intrinsic
polaritonic 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 hot
pressing engineers the stoichiometry of $\alpha$-MoO$_3$ by controllably
introducing oxygen vacancy defects (OVDs), which cause a
semiconductor-to-semimetal transition. Our density functional theory and
finite-difference time-domain results, combined with experimental studies show
that the OVDs induce a metastable metallic state by reducing the bandgap while
modifying the intrinsic dielectric permittivity of $\alpha$-MoO$_3$.
Photo-induced force microscopy confirms an average dielectric permittivity
tunability of $|\Delta\varepsilon/\varepsilon|\approx0.35$ within a
Reststrahlen band of $\alpha$-MoO$_3$, resulting in drastic shifts in the HPhP
dispersion. The polariton lifetimes for pristine and hot-pressed flakes were
measured as $0.92 \pm 0.06$ and $0.86 \pm 0.11$ ps, respectively, exhibiting a
loss of only 7%, while the group velocity exhibited an increase of $38.8 \pm
0.2$%. The OVDs in $\alpha$-MoO$_3$ provide a low-loss platform that enables
active tuning of mid-infrared HPhPs and have a profound impact on applications
in super-resolution imaging, nanoscale thermal manipulation, boosted molecular
sensing, and on-chip photonic circuits.

Optics;Applied Physics;Chemical Physics