Vacancy-Engineered Phonon Polaritons in a van der Waals Crystal

Phonon polaritons (PhPs) in low-symmetry van der Waals (vdW) materials enable deep-subwavelength control of mid-infrared light for nanoscale optics and sensing. However, intrinsically reconfiguring their dispersion without external fields, lithography, or chemical intercalation has remained elusive. Here, we introduce a thermomechanical approach that tunes PhPs in α-molybdenum trioxide (α-MoO₃) through controlled oxygen vacancy formation and lattice strain. Near-field nanoimaging reveals an average polariton wavevector shift of Δk/k ≈ 0.13 within the lower Reststrahlen band. Stoichiometric analysis, density functional theory, and finite-difference time-domain simulations indicate vacancy concentrations of 1-2% and ≈-1.2% compressive strain, resulting in a dielectric permittivity modulation of up to ≈15%. Despite these structural perturbations, polariton lifetimes remain high (1.15 ± 0.29 ps). This work offers thermomechanical vacancy engineering as a robust route for reprogrammable polaritonic response in vdW crystals for nonvolatile nanophotonic architectures.