Vacancy-Engineered Phonon Polaritons in a van der Waals Crystal

Abstract: Phonon-polaritons (PhPs) in low-symmetry van der Waals materials confine mid-infrared electromagnetic radiation well below the diffraction limit for nanoscale optics, sensing, and energy control. However, controlling the PhP dispersion at the nanoscale through intrinsic material properties$-$without external fields, lithography, or intercalants$-$remains elusive. Here, we demonstrate vacancy-engineered tuning of PhPs in $\alpha$-phase molybdenum trioxide ($\alpha$-MoO$_3$) via oxygen vacancy formation and lattice strain. Near-field nanoimaging of PhPs in processed $\alpha$-MoO$_3$ reveals an average polariton wavevector modulation of $\Delta k/k \approx 0.13 $ within the lower Restrahlen band. Stoichiometric analysis, density functional theory, and finite-difference time-domain simulations show agreement with the experimental results and suggest an induced vacancy concentration of $1\% - 2\%$ along with $(1.2\pm 0.2)\%$ compressive strain, resulting in a non-volatile dielectric permittivity modulation of up to $\Delta \varepsilon / \varepsilon \approx 0.15$. Despite these lattice modifications, the lifetimes of thermomechanically tuned PhPs remain high at $1.2 \pm 0.31$ ps. These results establish thermomechanical vacancy engineering as a general strategy to reprogram polaritonic response in vdW crystals, offering a new degree of freedom for embedded, non-volatile nanophotonics.