UC Irvine

Low-symmetry van der Waals (vdW) 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 HPhPs in α-MoO 3 using high-pressure and temperature treatment. The hot pressing engineers the stoichiometry of α-MoO 3 by controllably introducing oxygen vacancy defects (OVDs), which cause a semiconductor-to-semimetal transition. Our density functional theory (DFT) and finite-difference time-domain (FDTD) 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 α-MoO 3 . Photo-induced force microscopy (PiFM) confirms an average dielectric permittivity tunability of |𝚫𝜺/𝜺|≈𝟎.𝟑𝟓 within a Reststrahlen band of α-MoO 3 , resulting in drastic shifts in the HPhP dispersion. The polariton lifetimes for pristine and hot-pressed flakes were measured as 0.92±0.06 and 0.86±0.11 ps, respectively, exhibiting a loss of only 7%, while the group velocity exhibited an increase of 38.8±0.2%. The OVDs in α-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.