Observation of subwavelength localization of cavity plasmons induced by ultra-strong exciton coupling

In condensed matter systems, there exists a class of exotic localized electronic states wherein the localization is induced, not by a disorder or a defect, but by extremely strong interactions, for example, Kondo-insulator and Mott-insulator. In this work, we investigate and experimentally implement the photonic analog of localization induced by ultra-strong interactions in a coupled three-mode system. We show that the localization of a propagating mode can be achieved without the aid of an underlying spatial disorder, a defect, or even periodicity. We demonstrate the same by realizing ultra-strong coupling between a highly dispersive cavity plasmon mode and dimer excitons of Rhodamine B. Using a photon tunneling arrangement, we map the dispersion of the hybrid modes and provide evidence for the existence of a quasi-dispersionless hybrid mode with the sub-wavelength localization length and cavity plasmon-like characteristics.