Multipolar Conversion Induced Subwavelength High-Q Kerker Supermodes with Unidirectional Radiations

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. This fundamental model is revisited from a different perspective of multipolar expansions, and a hidden dimension of it is revealed, by establishing a subtle connection between the two seemingly unrelated properties of Q-factors and far-field angular radiation patterns. It is discovered that in both regimes of negative and positive couplings, significant Q-factor enhancement can be attributed to dramatic redistribution of radiation that originates from multipolar conversions from lower to higher orders. Relying on this connection and generalized Kerker effects of interferences among different multipoles, the two outstanding features of high-Q factor and unidirectional radiation are synchronized into one subwavelength supermode. The implications of this 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.