Observation of Mode Splitting in Photoluminescence of Individual Plasmonic Nanoparticles Strongly Coupled to Molecular Excitons

Abstract

Plasmon-exciton interactions are important for many prominent spectroscopic applications such as surface-enhanced Raman scattering, plasmon-mediated fluorescence, nanoscale lasing, and strong coupling. The case of strong coupling is analogous to quantum optical effects studied in solid state and atomic systems previously. In plasmonics, similar observations have been almost exclusively made in elastic scattering experiments; however, the interpretation of these experiments is often cumbersome. Here, we demonstrate mode splitting not only in scattering, but also in photoluminescence of individual hybrid nanosystems, which manifests a direct proof of strong coupling in plasmon-exciton nanoparticles. We achieved these results due to saturation of the mode volume with molecular J-aggregates, which resulted in splitting up to 400 meV, that is, ∼20% of the resonance energy. We analyzed the correlation between scattering and photoluminescence and found that splitting in photoluminescence is considerably less than that in scattering. Moreover, we found that splitting in both photoluminescence and scattering signals increased upon cooling to cryogenic temperatures. These findings improve our understanding of strong coupling phenomena in plasmonics.

Nano‐Cavity QED with Tunable Nano‐Tip Interaction

“…Furthermore, TESC allows direct comparison of coupling parameters for individual emitters coupled to the same plasmonic cavity. This increases experimental flexibility enabling, for example, measurement of anticrossing curves through addressing different emitters with intrinsically varying cavity detuning δ instead of changing fundamental experimental parameters like laser power or temperature that can impact the underlying system properties . One can envision a broad range of new experiments based on this flexibility, such as TESC imaging in 2D Van der Waals materials, the investigation of randomly located single photon emitting defects in solid state systems that are difficult to probe with stationary plasmonic cavities despite their exceptional properties for quantum information and technology, and coupling of multiple single emitters for entanglement and superradiance .…”

“…Recently, plasmonic nano‐antenna cavities in the form of highly polarizable metallic nanostructures with dimensions down to just a few nanometers, which allow for deep sub‐diffraction limited mode volumes, have been broadly employed to generate hybrid quantum states of plasmons and emitters . The increased interaction rate facilitated by these nanoscopic cavity mode volumes have enabled strong coupling to both ensembles and single emitters at room temperature …”

“…Many plasmonic cavity strong coupling studies have relied on multiple ( n em ) emitters in the form of, for example, J‐aggregates to increase the coupling strength through a large collective effect (). However, experiments involving multiple emitters are impracticable for some applications in quantum gates and entanglement because they exhibit an equidistant energy spectrum that is identical to that of a classical system.…”

“…The observation of strong couping of single emitters to plasmonic cavities at room temperature was first demonstrated through Rabi splitting in scattering spectra . However, observation of multiple peaks in the scattering signal is not sufficient to ensure strong coupling, as other effects like Fano‐like interference can lead to nearly identical spectral features . Over the past two years, strongly coupled plexciton states were observed in the emission of single emitters coupled to plasmonic cavities in several experiments at room temperature.…”

Strong Plasmon–Exciton Interactions on Nanoantenna Array–Monolayer WS2 Hybrid System

“…As shown in Figure b, we note that the spectral dip in the PL response coincides with that in the dark field spectra, but with upper polariton emission (ω + ) much less pronounced than the lower one (ω − ). We found that the splitting in the PL response is narrower than the dark field scattering counterparts, which appears to be in agreement with other works that investigated PL splitting in J‐aggregates coupled to silver nanoprisms . Our numerical calculations also yield a smaller splitting in the absorption cross section than in the scattering one (Figure S6, Supplementary Information).…”