Hybrid Cavity from Tunable Coupling between Anapole and Fabry‐Pérot Resonance or Anti‐Resonance

ABSTRACT Enhancing light‐matter interactions depends critically on the ability to tailor photonic modes at subwavelength scales, and combining distinct resonant modes has shown remarkable potential unattainable by individual resonances alone. Despite recent advances in anapole metasurfaces for energy confinement and Fabry‐Pérot (FP) cavities for spectral control, their synergistic coupling and resulting opportunities remain largely unexplored due to challenges such as precise nanoscale assembly in experiments. Here, we demonstrate that embedding a terahertz (THz) anapole metasurface within a tunable FP cavity results in a hybrid cavity that demonstrates exotic properties as the anapole transitions between coupling to FP resonances and anti‐resonances via cavity‐length tuning. At room temperature, we observe ultrastrong coupling (> 30% of the anapole frequency) between anapoles and FP resonances, generating tunable‐dispersion polaritons that blend favorable properties of both modes. Meanwhile, anapole spectrally aligns with FP anti‐resonances, leading to weak coupling that narrows anapole's transmission peak linewidth by one order of magnitude and enhances its local density of states (LDOS) near the metasurface correspondingly. With exceptional capabilities, including the formation of polaritons and significant enhancement of LDOS, the hybrid cavity enables strong interaction with functional materials, paving the way for exploration of quantum optics, molecular sensing, and ultrafast nonlinear photonics.