Abstract
Strong coupling between a single quantum emitter and an electromagnetic mode is one of the key effects in quantum optics. In the cavity QED approach to plasmonics, strongly coupled systems are usually understood as single-transition emitters resonantly coupled to a single radiative plasmonic mode. However, plasmonic cavities also support non-radiative (or "dark") modes, which offer much higher coupling strengths. On the other hand, realistic quantum emitters often support multiple electronic transitions of various symmetry, which could overlap with higher order plasmonic transitions -- in the blue or ultraviolet part of the spectrum. Here, we show that vacuum Rabi splitting with a single emitter can be achieved by leveraging dark modes of a plasmonic nanocavity. Specifically, we show that a significantly detuned electronic transition can be hybridized with a dark plasmon pseudomode, resulting in the vacuum Rabi splitting of the bright dipolar plasmon mode. We develop a simple model illustrating the modification of the system response in the "dark" strong coupling regime and demonstrate single photon non-linearity. These results may find important implications in the emerging field of room temperature quantum plasmonics.
Highlights
Interaction of a quantum emitter (QE) with an optical cavity is at the heart of modern quantum optics
We have presented a scheme for realizing strong lightmatter coupling with use of a high-energy electronic transition of a large oscillator strength quantum emitter
Exploiting the nonradiative modes of a plasmonic cavity, the high-energy transition can be tuned to lower energies, where it can couple with the bright plasmon cavity mode leading to observable vacuum Rabi splitting in the scattering spectrum
Summary
Interaction of a quantum emitter (QE) with an optical cavity is at the heart of modern quantum optics. Strong coupling between the QE and the so-called “dark,” weakly radiative modes of conventional Ag and Au nanoparticles has been explored [30,31,32,33,34,35,36,37] Despite the fact these dark modes are not observable using traditional optical techniques ( they can be observed by electron energy loss spectroscopy (EELS) [38,39,40,41,42], or in scattered light by large clusters [43] and anapoles [44]), it might be. In the strong coupling regime, Rabi splitting is relatively robust with respect to quenching when the emitter is spectrally tuned to the bright dipole mode of a plasmonic nanoparticle, as was shown recently [31]. We demonstrate theoretically that by coupling a high-energy transition of a QE to a cavity dark mode, it is possible to achieve observable Rabi splitting between two bright polariton modes. Our results could potentially help in understanding the microscopic behavior of experimental observations of QE-plasmon systems such as those shown in Refs. [19,20], where a single emitter strong coupling was demonstrated but modeling the system with a single mode for the plasmonic response yields unrealistic values for the dipole moments of the QE [23]
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