Photonic Doping of Epsilon-Near-Zero Bragg Microcavities

Epsilon-near-zero (ENZ) photonics provides a unique route to extreme dispersion, strong field confinement, and unconventional wave phenomena. Among its most intriguing concepts is photonic doping, where subwavelength dielectric inclusions embedded in ENZ media unlock exotic responses such as perfect magnetic conductor behavior and simultaneous epsilon-and-mu-near-zero properties. While prior demonstrations of photonic doping have been limited to microwave and far-infrared domains due to material losses, we demonstrate the feasibility of photonic doping at optical frequencies. Specifically, we implement photonic doping by embedding nanoparticle Mie resonators into an ultra-low-loss, all-dielectric ENZ platform realized with near-cutoff Bragg-reflection microcavities. We reveal the emergence of hybrid Bragg-Mie, near-zero-index modes whose fields localize within or between the nanoparticles. These modes exhibit substantially higher quality (Q) and Purcell factors than either the standalone nanoparticle array or the bare cavity. This mechanism enables the formation of highly isolated, spectrally pure electric and magnetic modes with dipolar or multipolar character. In representative photonically doped Bragg cavities, we predict Q factors on the order of 10000 and magnetic-dipole Purcell factors exceeding 4000 in the near-infrared region, associated with strong and high-Q magnetic hot spots localized within the nanoparticles. These results establish photonic doping of ENZ Bragg cavities as a powerful paradigm for achieving ultra-narrow-bandwidth, pure electric or magnetic Mie resonances, with exceptional potential for low-threshold nonlinear optics, magnetic dipole spectroscopy in atomic systems, and strong quantum optical light-matter interactions.