Evanescent quadrupole polariton

In this work we demonstrate the formation of a new type of polariton on the interface between a cuprous oxide slab and a polystyrene microsphere placed on the slab. The evanescent field of the resonant whispering gallery mode (WGM) of the microsphere has a substantial gradient, and therefore effectively couples with the quadrupole 1S excitons in cuprous oxide. This evanescent polariton has a long lifetime, which is determined only by its excitonic and WGM component. The polariton lower branch has a well-pronounced minimum. This suggests that this excitation is localized and can be utilized for possible BEC. The spatial coherence of the polariton can be improved by assembling the microspheres into a linear chain.