Strong coupling of two-dimensional semiconductor excitons with plasmonic resonators enables control of light-matter interaction at the subwavelength scale. Here we develop strong coupling in plasmonic nano-gap resonators that allow modification of exciton number contributing to the coupling. Using this system, we not only demonstrate a large vacuum Rabi splitting up to 163 meV and splitting features in photoluminescence spectra, but also reveal that the exciton number can be reduced down to single-digit level (N < 10), which is an order lower than that of traditional systems, close to single-exciton based strong coupling. In addition, we prove that the strong coupling process is not affected by the large exciton coherence size that was previously believed to be detrimental to the formation of plasmon-exciton interaction. Our work provides a deeper understanding of storng coupling in two-dimensional semiconductors, paving the way for room temperature quantum optics applications.Introduction-Two-dimensional (2D) transitional metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS 2 ) and tungsten disulfide (WS 2 ) have attracted tremendous attention recently [1,2]. These semiconductor nanosheets, when thinned down to monolayers (MLs), become direct bandgap, hosting excitons having ultralarge binding energy[3-5] and very high oscillator strength [2,6], which arise from the strong coulomb interaction and reduced dielectric screening in atomically thin structures. As a result, excitons in TMDC MLs can be tighly bound even at room temperature, producing strong light absorption and photoluminescence (PL). Integrating TMDC MLs with an optical resonator enables fast energy exchange between electromagnetic (EM) resonances and semiconductor excitons, i.e. the strong lightmatter interaction or strong couling, allowing the formation of half-light half-matter quasiparticles, known as polaritons. The strong coupling process not only is of interest for fundamental quantum optics, e.g. Bose-Einstein condensation[7] with superfluid characteritics, but also exhibits a great potential for many compelling applications, e.g. quantum computing [8] and thresholdless semiconductor lasing [9,10].

Plasmonic cavities can be used to control the atom-photon coupling process at the nanoscale, since they provide ultrahigh density of optical states in an exceptionally small mode volume. Here we demonstrate strong coupling between molecular excitons and plasmonic resonances (so-called plexcitonic coupling) in a film-coupled nanocube cavity, which can induce profound and significant spectral and spatial modifications to the plasmonic gap modes. Within the spectral span of a single gap mode in the nanotube-film cavity with a 3-nm wide gap, the introduction of narrow-band J-aggregate dye molecules not only enables an anti-crossing behavior in the spectral response, but also splits the single spatial mode into two distinct modes that are easily identified by their far-field scattering profiles. Simulation results confirm the experimental findings and the sensitivity of the plexcitonic coupling is explored using digital control of the gap spacing. Our work opens up a new perspective to study the strong coupling process, greatly extending the functionality of nanophotonic systems, with the potential to be applied in cavity quantum electrodynamic systems. † These authors contributed equally to this work 2 Strong light-matter interactions build upon fast energy exchange [1] between excitonic quantum emitters and electromagnetic (EM) modes in a cavity, which leads to cavity-exciton mode hybridization, manifesting as a split in the cavity spectral response, known as vacuum Rabi splitting.Understanding these phenomena is very important for cavity quantum electrodynamics applications [2,3]; for example, quantum computing requires repeated manipulation of excitonic states with photons before atomic or molecular excitons lose their coherence. On the other hand, once strongly coupled to a resonant cavity, electronic states of molecules can be greatly reshaped [4] from those in free space, enabling a variety of extraordinary effects, such as modification of molecular thermodynamics [5] and chemical landscape [6], mediation of energy transfer between multiple molecules [7] and even alternation of photosynthetic processes in bacteria [8].Strong coupling (SC) requires a high atomic cooperativity (, where g is the coupling strength, γ and κ are the spectral width of the excitonic transition of emitters and EM modes respectively. To achieve high C, traditional investigations [9][10][11][12] have used cavities with high Q performance (quality factor κ ω / = Q , where ω is the oscillation frequency of the cavity).Another solution is to reduce the cavity mode volume V, sincewhere N is the number of excitons contributing to the coupling process. In this context, plasmonic cavities have been proposed to investigate strong coupling, because these cavities can concentrate light energy within deep subwavelength volumes, and thus are capable of providing high cooperativity by compensating their moderate Q values with ultra-small V [13][14][15]. Specifically, plasmonic cavities are specially designed metallic nanostructures, in w...

Spectral and directional reshaping of fluorescence from dye molecules embedded in self-assembled hybrid plasmonic-photonic crystals has been examined. The hybrid crystals comprise two-dimensional hexagonal arrays of dye-doped dielectric nanospheres, capped with silver semishells. Comparing the reshaped fluorescence spectra with measured transmission/reflection spectra and numerical calculations reveals that the spectral and directional reshaping of fluorescence is the result of its coupling to photonic crystal Bloch modes and to void plasmons localized inside the silver caps.