Dynamically tunable strong coupling <i>via</i> insulator–metal phase transition in VO ₂ coupled to mid-infrared SrTiO ₃ phonons

Dynamically tunable strong light-matter coupling in the mid-infrared (MIR) regime holds significant promise for next-generation photonic and thermal devices, especially in the long-wave infrared (LWIR) range of 19-23 μm, where vibrational and thermal signatures are most prominent. In this work, we demonstrate strong coupling between the phase-transition-tunable Fabry-Pérot (FP) cavity modes of vanadium dioxide (VO₂) and the phonon polaritons (PhPs) of strontium titanate (SrTiO₃), a polar dielectric with prominent optical phonon resonances in the LWIR region. The insulator-to-metal transition (IMT) of VO₂ enables dynamic tuning of the coupling strength and mode hybridization. At low temperatures, the coupled system exhibits a pronounced Rabi splitting exceeding 1.5269 eV, unambiguously confirming the regime of strong coupling. At elevated temperatures, VO₂ transitions to a metallic state, introducing substantial absorption losses and thereby reducing the quality factor (<i>Q</i>-factor) of the resonant mode. The phase transition in VO₂ enables dynamic control over the coupling strength, resulting in tunable Rabi splitting and the modulation of the coupled system's resonance frequency. This tunability allows for the manipulation of MIR photonic signals, offering potential for novel applications in active optical modulators, tunable filters, and sensors.