Plasmon resonances have long been used to locally enhance the rate of optical processes such as spontaneous emission.  The great majority of experiments probing the coupling between emitters and plasmons have operated in this weak-coupling regime, where rates are increased but basic spectral properties are unchanged.  Recently, however, experiments have explored the qualitative changes in spectral properties that can arise for stronger, coherent emitter-plasmon coupling.  In the strong-coupling regime, the coupled emitter and plasmon form new normal modes (labeled “plexcitons”); this is observable as Rabi splitting in the scattering spectrum of the plasmonic nanostructure and in the photoluminescence spectrum of the emitter.  Intermediate between these two regimes, Fano-like interference between the coupled emitter and plasmon dipoles leads to an induced transparency dip in the (coherent) scattering spectrum of the plasmonic nanostructure, without a corresponding change in the (incoherent) photoluminescence spectrum.  Most observations of Fano interference and Rabi splitting for coupled emitter-plasmon systems have involved large numbers of emitters, because the collective coupling strength increases as more emitters are coherently coupled to the same plasmon resonance.  Achieving the same effects at the level of a single emitter requires highly localized plasmonic near fields. <br/> <br/>We show that Fano interference and Rabi splitting can be observed for a single semiconductor nanocrystal, or quantum dot, in the gap between a metal nanoparticle and a metal film.  We create the gap plasmons in two ways: (1) by linking quantum dots to metal nanoparticles and depositing the decorated nanoparticles on a metal film, or (2) by scanning a sharp metal tip over quantum dots that have been deposited on a metal film.  In both cases, we observe Rabi splitting in the photoluminescence spectra of individual quantum dots, providing an unambiguous demonstration of strong coupling.  In the metal-nanoparticle case, we also clearly observe induced transparency in the absence of strong coupling.  Large variations in coupling strength are observed from one quantum dot to another, which we attribute to local structural differences in the metal nanoparticles and metal film.  These observations open up the possibility of coherently controlling scattering and emission spectra at ultralow energies, potentially down to the level of single photons. <br/>

Strong coupling between single quantum dots and plasmon resonances: From Fano interference to Rabi splitting

256th National Meeting (2018)

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