Cylindrical vector beams reveal radiationless anapole condition in a resonant state

Abstract

Nonscattering optical anapole condition is corresponding to the excitation of radiationless field distributions in open resonators, which offers new degrees of freedom for tailoring light-matter interaction. Conventional mechanisms for achieving such a condition relies on sophisticated manipulation of electromagnetic multipolar moments of all orders to guarantee superpositions of suppressed moment strengths at the same wavelength. In contrast, here we report on the excitation of optical radiationless anapole hidden in a resonant state of a Si nanoparticle utilizing a tightly focused radially polarized (RP) beam. The coexistence of magnetic resonant state and anapole condition at the same wavelength further enables the triggering of resonant state by a tightly focused azimuthally polarized (AP) beam whose corresponding electric multipole coefficient could be zero. As a result, high contrast inter-transition between radiationless anapole condition and ideal magnetic resonant scattering can be achieved experimentally in visible spectrum. The proposed mechanism is general which can be realized in different types of nanostructures. Our results showcase that the unique combination of structured light and structured Mie resonances could provide new degrees of freedom for tailoring light-matter interaction, which might shed new light on functional meta-optics.

Optical nanoresonators

“…In [134], scattering spectra in a system based on a silicon dielectric cylinder were studied experimentally using highly focused axisymmetric Bessel beams with radial and azimuthal polarizations (see Fig. 22).…”

“…Another interesting feature of this geometry is that the same sample, when irradiated with an azimuthally polarized beam, strongly radiates a magnetic quadrupole spherical harmonic. scattered power spectrum for different polarizations of the excitation beam [134].…”

“…Fig.22. Excitation of a Si disk by strongly focused beams with radial (a) and azimuthal (b)polarizations[134].…”

Boosting second-harmonic generation in the LiNbO3 metasurface using high- Q guided resonances and bound states in the continuum

“…Consequently, all-dielectric nanoparticles with high refractive indices, large nonlinear coefficients, and low losses have been used in building enhanced optical nonlinear processes over the past few years. Such nanoparticles support Mie resonances which enable the enhanced harmonic generations, including second-harmonic generation (SHG), third-harmonic generation (THG), and four-wave mixing (FWM) [14][15][16][17][18][19][20][21][22]. Mie-type resonances can provide strong local field confinement inside the nanostructure and thus enhance the nonlinear conversion efficiency from the resonant nanostructures [23,24].…”

Anapole States in Gap-Surface Plasmon Resonators

“…It is important to emphasize that the far-field excitation of a pure nonradiating anapole state is problematic due to the reciprocity principle unless some specially designed illumination is used as, for instance, a combination of counter-propagating strongly focused radially polarized beams. , In practice, the formation of an anapole state is usually referred to when the suppression of a dominant (radiative) multipole moment is achieved. Because multipole moments are associated with charge and current distributions, the size and shape of the structure have a strong impact on the type and order of the multipole moments determining the far-field distributions of generated fields.…”