Photonic Doping of Epsilon-Near-Zero Bragg Microcavities.

Epsilon-near-zero (ENZ) photonics provides a powerful route to extreme dispersion engineering, strong field confinement, and unconventional wave phenomena. A closely related concept, photonic doping, employs subwavelength nonmagnetic dielectric inclusions in ENZ media to enable exotic responses such as perfect-magnetic-conductor behavior and simultaneous epsilon-and mu-near-zero states. However, photonic doping has remained limited to microwave and far-infrared regimes due to intrinsic losses in optical ENZ materials. Here, photonic doping is demonstrated at optical frequencies by embedding a periodic array of dielectric Mie resonators within an ultralow-loss all-dielectric ENZ platform based on near-cutoff Bragg microcavities. The resulting structures support spectrally isolated quasi-singular coupled Bragg-Mie resonances spanning electric and magnetic multipolar orders and their overtones. These modes exhibit effective near-zero-index dispersion, with fields confined either within or between the nanoparticles. A representative 18 μ m -scale doped structure exhibits a quality factor of 3.4 × 1 0 4 and a magnetic Purcell enhancement exceeding 1.8 × 1 0 4 in the near-infrared. This platform elevates photonic doping from a microwave-only concept to a fully optical, low-loss, and multipole-resolved system, enabling ultra-narrowband Mie-like resonances, enhanced magnetic-light interactions, and new opportunities in multipolar-selective spectroscopy and lasing, low-threshold nonlinear optics, and engineered spontaneous emission.