Theory, Observation, and Ultrafast Response of the Hybrid Anapole Regime in Light Scattering

Abstract

Modern nanophotonics has witnessed the rise of “electric anapoles” (EDAs), destructive interferences of electric and toroidal electric dipoles, actively exploited to resonantly decrease radiation from nanoresonators. However, the inherent duality in Maxwell equations suggests the intriguing possibility of “magnetic anapoles,” involving a nonradiating composition of a magnetic dipole and a magnetic toroidal dipole. Here, a hybrid anapole (HA) of mixed electric and magnetic character is predicted and observed experimentally via dark field spectroscopy, with all the dominant multipoles being suppressed by the toroidal terms in a nanocylinder. Breaking the spherical symmetry allows to overlap up to four anapoles stemming from different multipoles with just two tuning parameters. This effect is due to a symmetry‐allowed connection between the resonator multipolar response and its eigenstates. The authors delve into the physics of such current configurations in the stationary and transient regimes and explore new ultrafast phenomena arising at sub‐picosecond timescales, associated with the HA dynamics. The theoretical results allow the design of non‐Huygens metasurfaces featuring a dual functionality: perfect transparency in the stationary regime and controllable ultrashort pulse beatings in the transient. Besides offering significant advantages with respect to EDAs, HAs can play an essential role in developing the emerging field of ultrafast resonant phenomena.

High-Q Localized States in Finite Arrays of Subwavelength Resonators

“…We notice that for other separation distances there are dips in the radiative emission rate provided by simultaneous suppression of smaller numbers of multipoles. Similar to other collective effects, such as superscattering and zero scattering with high-order anapole states that rely on the interference of the several high-order multipoles, here the suppression of the radiation can be so strong because a simple one-dimensional array of dipoles naturally exhibits high-order multipole response.…”

Special scattering regimes for conical all-dielectric nanoparticles

“…Anapoles are semi-nonradiating sources that arise due to the destructive interference of the quasistatic electric dipole moment and the toroidal dipole in the far field. Alternatively, in a more general picture, they can be understood as being originated by the destructive interference of symmetry-compatible quasinormal modes 33 . However, the energy stored by the quasinormal modes within the nanoparticle is nonzero, leading to counterintuitive light-matter interaction processes in the absence of elastic scattering.…”

“…The experimental demonstration of hybrid anapoles (HA) following the pioneering theoretical proposal 70 , has evidenced the possibility to simultaneously overlap the zeros of all the dominant multipolar channels through a careful design of the nanoparticle geometry. These novel states are much more promising than their dipolar counterparts for a number of reasons 33 , 34 ; despite the larger volume of the nanoresonator required to obtain them, the scattering suppression is improved by more than 20 times, while the excited quasinormal modes store approximately 10 times more energy. Such values exceed by far the performance of anapoles and 2 nd order anapoles 71 arising in homogeneous disk nanoresonators.…”

“…In nanocylinders pertaining to the cylindrical symmetry, it was shown in 33 how the anapoles from different multipolar orders but equal parity were connected. Counterintuitively, such connection allows to simultaneously overlap four anapoles with just the two geometrical degrees of freedom of the nanoparticle.…”

“…Another intriguing possibility is an access to the so-called anapole (cancellation of the dipole radiation via the toroidal dipole one) 32 or hybrid anapole regimes (HA) 33 —simultaneous fulfillment of anapole conditions for all the main multipoles in a scatterer, giving rise to the far-field scattering suppression and strong near-field energy localization regardless of a substrate. Metasurfaces based on such meta-atoms feature many useful properties such as a controlling phase shift of transmitted light, almost perfect transparency, strong nearfield enhancement useful for nonlinear and Raman applications, complex and unusual transient properties, etc., for various types of substrates and shapes 34 – 36 .…”