Interplay Between Strong Coupling and Radiative Damping in Hybrid Excitonic-Plasmonic Nanostructures

Abstract

Surface plasmon polaritons (SPPs), optical excitations at the interface between a metal and a dielectric, carry significant potential for guiding and manipulating light on the nanoscale (Bozhevolnyi et al. Nature 440(7083):508–511, 2006; Lal et al. Nat Photonics 1(11):641–648, 2007; Maier et al. Nat Mater 2(4):229–232, 2003). Their weak optical nonlinearities, however, hinder active device fabrication, e.g., for all-optical switching (Chang et al. Nat Phys 3(11):807–812, 2007; Dintinger et al. Adv Mater 18(13):1645, 2006; MacDonald et al. Nat Photonics 3(1):55–58, 2009; Vasa et al. ACS Nano 4:7559–7565, 2010) or information processing (Engheta Science 317(5845):1698–1702, 2007; Gonzalez-Tudela et al. Phys Rev Lett 106:020501, 2011). Recently, strong optical dipole coupling between SPPs and nonlinear quantum emitters with normal mode splittings of up to 700 meV has been demonstrated (Aberra Guebrou et al. Phys Rev Lett 108:066401(5p), 2012; Bellessa et al. Phys Rev Lett 93:036404, 2004; Dintinger et al. Phys Rev B 71:035424, 2005; Fofang et al. Nano Lett 8:3481–3487, 2008; Hakala et al. Phys Rev Lett 103(5):053602, 2009; Sonnefraud et al. ACS Nano 4(3):1664–1670, 2010). The predicted ultrafast energy transfer between quantum emitters and SPP fields could be a crucial microscopic mechanism for switching light by light on the nanoscale. Here, we present the first real-time observation of ultrafast Rabi oscillations in a J-aggregate/metal nanostructure, evidencing coherent energy transfer between excitonic quantum emitters and SPP fields. We demonstrate coherent manipulation of the coupling energy by controlling the exciton density on a 10-fs timescale, a step forward towards coherent, all-optical ultrafast plasmonic circuits and devices. We report on the interplay between strong coupling and radiative damping of strongly coupled excitons (Xs) and surface plasmon polaritons (SPPs) in a hybrid system made of J-aggregate and metal nanostructures. The optical response of the system is probed at the field level by angle-resolved spectral interferometry. We show that two different energy transfer channels coexist: coherent resonant dipole-dipole interaction and an incoherent exchange due to the spontaneous emissions of a photon by one emitter and subsequent reabsorption by another. The interplay between both pathways results in a pronounced modification of the radiative damping due to formation of super- and sub-radiant polariton states. This is confirmed by probing the nonlinear response of the polariton system and explained within a coupled oscillator model. Such a strong modification of the radiative damping opens up new directions in coherent active plasmonics.