Mie Voids as broadband directional light sources

The Kerker effect arises from the interference between electric and magnetic multipoles, enabling directional light scattering in nanophotonics. However, conventional dielectric and plasmonic nanoparticles can only act as Kerker sources in narrow spectral regions, limiting their applicability. Here, we show that the recently discovered Mie voids overcome this limitation by supporting a broadband generalized Kerker effect spanning the whole visible range. We investigate the optical response of Mie voids under both plane-wave and dipolar excitation. For plane waves, the voids preferentially scatter light in the forward direction. Under dipolar excitation, the resulting radiation emission towards the void and beyond is suppressed due to destructive interference between the dipole field with the directional scattered field of the void. These findings identify Mie voids as versatile broadband directional sources, opening pathways for antenna design and energy harvesting at the nanoscale.

💡 Research Summary

The paper introduces “Mie voids”—air cavities embedded in a high‑index dielectric—as a new class of nanostructures that overcome the narrow‑band limitation of conventional Kerker scattering. The Kerker effect, originally described for a sphere with equal electric and magnetic dipole responses, yields forward‑directed scattering when the electric and magnetic dipoles are in phase (first Kerker condition) and backward suppression when they are out of phase (second condition). In typical high‑index dielectric or plasmonic nanoparticles, these conditions are only met at isolated resonances, resulting in a very limited spectral bandwidth.

Using Lorenz‑Mie theory, the authors analytically calculate the electric (aₗ) and magnetic (bₗ) Mie coefficients for a spherical void of radius R, with n_void≈1 and host index n_host≫1. They show that, unlike solid spheres where aₗ and bₗ are spectrally isolated, the void’s modes are confined to the low‑index region and therefore exhibit low quality factors and broad spectral overlap. Consequently, multiple orders (dipole, quadrupole, octupole, etc.) have comparable magnitudes across the entire visible range (400–800 nm). By inserting these coefficients into the asymmetry parameter g = ⟨cos θ⟩, they demonstrate that g > 0.5 is maintained over a wide wavelength interval, indicating a robust generalized Kerker effect driven by interference among several overlapping multipoles.

The authors compare the scattering efficiency Q_sca and g for a 100 nm sphere with η = n_sphere/n_host = 4 (solid) versus η = 1/4 (void). The solid sphere shows sharp, narrow peaks in Q_sca and g, whereas the void displays an almost flat Q_sca≈2 and a consistently positive g≥0.5. This broadband forward scattering is further validated with full‑wave COMSOL simulations of conical voids (top radius 200 nm, bottom radius 150 nm, height 410 nm) embedded in a silicon‑like substrate (n = 4). Even with the non‑spherical geometry and partial embedding, the far‑field asymmetry remains high (g≈0.3–0.6) across the visible spectrum, confirming that the generalized Kerker mechanism survives realistic fabrication constraints.

Beyond passive scattering, the paper investigates the interaction with quantum emitters modeled as electric dipoles placed either inside or near the void. The Purcell factor F, which quantifies the modification of spontaneous emission, is derived for dipoles oriented parallel (F∥) or perpendicular (F⊥) to the void surface. When the emitter resides inside the void, the low‑index cavity supports resonant modes that boost the local density of states, yielding Purcell enhancements up to a factor of five. Conversely, an emitter placed outside experiences a directional radiation pattern: the dipole field interferes destructively with the forward‑scattered field of the void, suppressing radiation in the forward direction while enhancing emission toward the void (the “backward” side). This directional Purcell effect provides a novel route to control emission directionality without additional external structures.

In summary, Mie voids act as broadband, highly directional nano‑antennas. Their key advantages are: (i) simultaneous excitation of multiple electric and magnetic multipoles across the visible range, (ii) a robust generalized Kerker effect yielding forward‑dominant scattering for both plane‑wave and near‑field excitation, (iii) significant Purcell enhancement for emitters inside the cavity, and (iv) controllable suppression/enhancement of emission direction for external emitters. These properties open pathways for applications such as broadband light harvesting, directional single‑photon sources, and metasurface design where wide‑band, low‑loss, and highly directional scattering is required.