Photonic Doping of Epsilon-Near-Zero Bragg Microcavities

Abstract

Epsilon-near-zero (ENZ) photonics provides a powerful route to extreme dispersion engineering, strong field confinement, and unconventional wave phenomena. A closely related concept is \textit{photonic doping}, where subwavelength nonmagnetic dielectric materials embedded in ENZ media 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 the intrinsic losses of optical ENZ materials. Here, photonic doping is demonstrated at optical frequencies by embedding a periodic array of dielectric Mie resonators into 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 \(14\,μ\mathrm{m}\)-scale doped structure exhibits quality factors approaching \(10^{4}\) and magnetic-dipole Purcell enhancements exceeding \(5\times10^{3}\) in the near-infrared. The demonstrated platform elevates the photonic doping from a microwave-only concept to a fully optical, low-loss, and multipole-resolved platform, enabling ultra-narrowband Mie-like resonances, enhanced magnetic-light interactions, and new opportunities in multipolar-selective spectroscopy and lasing, low-threshold nonlinear optics and efficient single-photon emission.

Figures (14)

• Figure 1: Real parts of Mie coefficients $a_0$, $a_1$, and $a_2$ as functions of wavelength for an infinite cylinder of radius $R=150~$nm, $n_i=3.5$ and (a) $n_e=1$, (b) $n_e=0.1$. The inset illustrates normal incidence of a plane wave with its electric field polarized perpendicular to the cylinder axis.
• Figure 2: Real parts of Mie coefficients $b_0$, $b_1$, and $b_2$ as functions of wavelength using the same parameters as in Fig. 1 and for (a) $n_e=1$, (b) $n_e=0.1$.
• Figure 3: Real parts of Mie coefficients as functions of wavelength for a spherical NP of radius $R=85$ nm and refractive index $n_i=3.4$, inside a medium with refractive index of (a) $n_e=1$, (b) $n_e=0.1$.
• Figure 4: Simulation domain of the ENZ Bragg cavity loaded with a periodic array of infinitely-long silicon cylinders of radius $R = 150~\mathrm{nm}$. The domain is enclosed by periodic and scattering boundary conditions (BCs).
• Figure 5: Transmittance spectra and corresponding field profiles. (a) Bare Bragg cavity (cutoff wavelength $\lambda_c = 1500\,\mathrm{nm}$) consisting a half-wave SiO$_2$ core, sandwiched between 10 pairs of quarter-wave cladding layers of SiO$_2$ and SiN. (b,c) The same cavity loaded with arrays of infinitely-long silicon NCs, placed at the core center, with lattice periods (b) $p = 500\,\mathrm{nm}$ and (c) $p = 700\,\mathrm{nm}$. All structures are illuminated by a normally incident plane wave (along $y$-axis) with the magnetic field $H$ along $z$-axis (TE, $H_z$).