Vacancy-Engineered Phonon Polaritons in a van der Waals Crystal

≈ 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.