Investigation of multipolar interference in silicon disks for on-chip photonics

Abstract

High-index nanoparticles support electromagnetic multipoles that determine their response to an incident wave. When different multipoles are excited, they can interfere, giving rise to surprising phenomena. For example, from the antiphase oscillation of the Cartesian toroidal and electric (or magnetic) dipole or the corresponding higher-order multipoles arise the so-called anapole states, characterized by a substantial reduction in the far-field scattering and a strong localization of energy inside the disk. One of the simplest high-index structures supporting multipolar interference is the disk, which can be easily built on a silica substrate using standard silicon nanofabrication tools. Most studies of anapole states in high-index dielectric disks have addressed anapoles that can be excited under normal illumination, but the in-plane incidence is necessary for building silicon photonic integrated circuits (PICs) when light is completely bound to the chip plane. In this thesis, we investigate via numerical simulations annex experimental measurements the appearance of multipolar interferences in silicon disks when we excited in-plane through waveguides. First, we investigate the effects on isolated subwavelength-sized disks and then extend our investigation to one-dimensional (1D) periodic chains. Under the in-plane excitation of a silicon subwavelength-sized disk, we observe magnetic and electric anapoles of various orders, changing the geometry of the system. Interestingly, we observed a decoupling of the minimum in the far-field scattering and the maximum of energy localization in the disk, which takes place at well-separated wavelengths for in-plane excitation of the anapole as compared to the usual normal incidence case. On the other hand, through the excitation of the toroidal dipole, we demonstrate the efficient transmission above the light cone in a periodic structure formed by silicon subwavelength-sized disks. Finally, we predict the closure of the Bragg bandgap due to the interaction between electric and magnetic dipoles in a periodic structure formed by silicon nanobricks. Our results highlight significant differences between multipoles interferences when the particles are illuminated from different directions and have direct implications for the use of wavelength-size disks in high-index PICs for applications ranging from biosensing and spectroscopy to nonlinear signal processing.