Unraveling Dipolar Regime and Kerker Conditions in Mid‐Index Mesoscale Dielectric Materials

Abstract

resonant enhancement of both the electric and magnetic near-fields. [2][3][4] Because of their low losses and strong electromagnetic response, the resonant excitation of dielectric nanostructures offers a unique playground to demonstrate new nanophotonic effects. These include, but are not limited to nonradiating anapole states, [5,6] zero optical backscattering leading to optimum forward scattering, [7,8] magnetic hotspot enhanced Purcell effect, [9,10] surface enhanced Raman scattering, [11] and nanoantenna. [12,13] However, most of these novel nanophotonic effects observed in high refractiveindex lossless dielectric materials are related to the excitation of single dipolar modes-hence so far, they have only been observed in the nanoscale regime at the optical frequencies. In contrast, in the mesoscale regime (where the particle diameter, d is approximately equal to the wavelength of the incident light, λ), these effects are inaccessible due to the contributions from higher order multipolar modes under plane wave (PW) illumination. Recently, there is a growing interest in the study of optical phenomena at mesoscale regime with low refractive index (1< n <2) structures, termed as Mesotronics. [14] In these low-index wavelength-scaled dielectric particles, the electromagnetic fields are enhanced by the interference effects between the different field components generated in or/and near the particle. [14] Even though, the localized field is limited to the particle size of the order of d > λ, a number of interesting phenomena and applications have been observed in these low-index dielectric particles. For example, strong molecule-cavity coupling, [15] optical singularities that form two extreme hotspots near the particle poles, [16] higher order Fano resonances that provide giant magnetic field leading to super-oscillation effects, [17,18] the whispering gallery mode effect overcoming the diffraction limit leading to super-resolution imaging, [19] structured fields in the form of photonic hook and loops allowing new class of "on-chip" optical traps for anisotropic nanoobjects, [20] etc., indicate promising new directions of research enabled by low-index mesoscale dielectric particles.Recently, it was theoretically predicted, [21,22] one can unravel dipolar regimes in a homogenous lossless dielectric sphere with a wide range of size parameter and several refractive indices (n) under illumination with a pure dipolar field (PDF). More specifically, it was shown that the sectoral and propagating Nanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes-hence so far, they have only been observed within a few relatively high-index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close-to-zero backscattering is demonstrated, and optical anapoles are excited in mid-index dielectric spheres (titanium di-oxide, TiO 2 ; n ≈ 2.6) at the mesoscale reg...

Spheres of maximum electromagnetic chirality

“…In fact, the purple circle represents a chiral sphere that behaves as a non-radiating source for both helicities of the incident field. In other words, a chiral sphere satisfying a = b = c = 0 meets the hybrid anapole condition [82][83][84]. In short, an hybrid anapole emerges when the electric and magnetic scattering coefficients simultaneously vanish regardless of the incident helicity.…”

“…In short, an hybrid anapole emerges when the electric and magnetic scattering coefficients simultaneously vanish regardless of the incident helicity. In Refs [82][83][84], all illuminated samples are achiral (χ = 0) and satisfy the hybrid anapole condition a = b = 0. In our case, we have a very different sce-m x FIG.…”

Optical Mirages from Spinless Beams

“…To get more insight into the physics behind the interference terms between the electric and magnetic dipoles, it is convenient to represent η and g in the scattering phase-shift notation . That is, by rewriting the electric and magnetic polarizabilities as α E = −(6π/ k 3 )sin α 1 e –iα 1 and α M = −(6π/ k 3 )sin β 1 e –iβ 1 , we arrive from eq to From eq , we can infer that besides a Rayleigh sphere, the pure magnetic (or electric) scattering regimes, given by the so-called electric (or magnetic) anapoles, − i.e., sin α 1 = 0 (sin β 1 = 0), give rise to . As a result, according to eq , the streamlines of the scattered Poynting point radially.…”