Can a Nonradiating Mode Be Externally Excited? Nonscattering States versus Embedded Eigenstates

Abstract

In this Letter, we discuss the general problem of exciting radiationless field distributions in open cavities, with the goal of clarifying recent findings on this topic. We point out that the radiationless scattering states, like anapoles, considered in several recent studies, are not eigenmodes of an open cavity; therefore, their external excitation is neither surprising nor challenging (similar to the excitation of nonzero internal fields in a transparent, or cloaked, object). Even more, the radiationless anapole field distribution cannot be sustained without the actual presence of external incident fields. Conversely, we prove that the Lorentz reciprocity theorem prevents the external excitation of radiationless optical eigenmodes, as in the case of embedded eigenstates and bound states in the continuum in open cavities. Our discussion clarifies the analogies and differences between invisible bodies, nonradiating sources, anapole scatterers and emitters, and embedded eigenstates, especially in relation to their external excitation.

Suppressing material loss in the visible and near-infrared range for functional nanophotonics using bandgap engineering

“…However, the Q factor of these nanoresonators is limited by radiative and material losses, and it does not exceed a few tens in the visible range. Although radiative losses can be suppressed with the tailoring of weakly scattering states, such as bound states in the continuum (BIC) [23][24][25][26] , the material (dissipation) loss requires a fundamentally different approach. High-index dielectric resonant NPs look especially promising due to their lower material loss compared to plasmonic nanoparticles [14][15][16][17][18][19][27][28][29][30] , enabling low-loss photonic devices, e.g., nanoantennas, metalenses, and lasers 22,[31][32][33][34][35][36][37] .…”

Up‐And‐Coming Advances in Optical and Microwave Nonreciprocity: From Classical to Quantum Realm

“…The enhancement of magneto‐optical effects can be reached via the excitation of surface plasmon‐polaritons [ 117,118 ] and Mie resonances. [ 119,120 ] Tailoring electromagnetic systems with unusual scattering effects like bound states in the continuum (BICs) with unboundedly large Q ‐factor [ 121–124 ] is another route to efficient nonreciprocal structures and devices. [ 125,126 ]…”

Anapole States and Toroidal Resonances Realized in Simple Gold Nanoplate‐on‐Mirror Structures

“…[ 18,19 ] The anapole state is a resonant field distribution at a particular frequency when the toroidal and electric dipoles cancel the radiation of each other in the far‐field region. [ 20,21 ] A perfect anapole cannot be detected through the emission or absorption of light due to its nonradiating behavior. If the far‐field radiation of the toroidal and electric dipoles is not perfectly balanced, the excitation of the anapole can be observed through the emergence of a narrow dip in the scattering spectrum.…”