Multipolar Conversion Induced Subwavelength High-Q Supermodes with Unidirectional Radiations

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.