Experimental observation of higher-order anapoles in individual silicon disks under in-plane illumination

TL;DR

This paper reports the experimental observation of higher-order anapole states in silicon disks under in-plane illumination, expanding understanding of non-radiating electromagnetic modes for potential applications in photonic circuits.

Contribution

It demonstrates the existence of magnetic and second-order electric anapoles in silicon disks at telecom wavelengths, advancing the study of complex non-radiating modes.

Findings

Detection of far-field scattering dips at expected wavelengths

Observation of differences between normal and in-plane excitation

Discussion of non-cancellation of electric and toroidal moments

Abstract

Anapole states - characterized by a strong suppression of far-field scattering - naturally arise in high-index nanoparticles as a result of the interference between certain multipolar moments. Recently, the first-order electric anapole, resulting from the interference between the electric and toroidal dipoles, was characterized under in-plane illumination as required in on-chip photonics. Here, we go a step further and report on the observation of higher-order (magnetic and second-order electric) anapole states in individual silicon disks under in-plane illumination. To do so, we increase the disk dimensions (radius and thickness) so that such anapoles occur at telecom wavelengths. Experiments show dips in the far-field scattering perpendicular to the disk plane at the expected wavelengths and the selected polarizations, which we interpret as a signature of high-order anapoles. Some differences between normal and in-plane excitation are discussed, in particular the non-cancellation of the sum of the Cartesian electric and toroidal moments for in-plane incidence. Our results pave the way towards the use of different anapole states in photonic integrated circuits, either on silicon or other high-index dielectric materials.