Multipolar Conversion Induced Subwavelength High-Q Supermodes with Unidirectional Radiations

TL;DR

This paper reveals a fundamental link between multipolar radiation patterns and Q-factor enhancement in coupled resonator systems, enabling the creation of subwavelength supermodes with high quality factors and unidirectional radiation.

Contribution

It introduces a novel perspective connecting multipolar expansions with two-mode coupling, demonstrating how multipolar conversions lead to high-Q supermodes with directional radiation.

Findings

Significant Q-factor enhancement via multipolar redistributions.

Unidirectional radiation achieved through multipolar interference.

Applicable to various physical resonance systems beyond optics.

Abstract

The two-mode coupling model with energy splitting and formation of supermodes with different life times has been pervasive in almost every discipline of physics. We revisit this fundamental model from a different perspective of multipolar expansions, and manage to reveal a hidden dimension of it, by establishing a subtle connection between two seemingly unrelated properties of Q-factors and far-field angular radiation patterns. We discover that, in both regimes of negative and positive couplings, significant Q-factor enhancement can be attributed to dramatic redistributions of radiations that originate from multipolar conversions from lower to higher orders. Relying on this connection and generalized Kerker effects of interferences among different multipoles, we synchronize both outstanding features of high-Q factor and unidirectional radiation into one subwavelength supermode. The implications of our study are not confined to optics and photonics, and can potentially shed new light on coupling between resonances of mechanical, phononic, electronic or other hybrid natures.