The advancements in top-down and bottom-up nanofabrication techniques as well as the ability to synthesize metallic and semiconducting quantum dots with size- and shape-tunable optical properties have been enabling us to precisely control and deeply understand light–matter interaction at nanoscale dimensions [1–3]. In order to engineer light–matter interactions at nanoscale dimensions, plasmonic systems have recently been combined with excitonic systems. A new hybrid plasmon–exciton mode called plexciton has been observed in the strong coupling regime where the hybrid system shows new optical modes as upper and lower polariton branches. Separation between the branches at zero-detuning is called a Rabi splitting energy [3,4]. A strong coupling regime is appealing because it has direct applications in molecular and material science by changing the optical properties of the emitter or absorber. However, in a weak coupling regime, optical modes are not perturbed, resulting in modification only of emission or absorption of the material (the Purcell effect) [5].

Recently, we have shown colloidally stable plexcitonic nanoparticles demonstrating ultrastrong coupling between the localized surface plasmon polaritons (LSPPs) of Ag nanoprisms (NPs) and excitons of J-aggregates [6]. Herein we show coupling of plexcitons of J-aggregate-coated Ag NPs with the propagating surface plasmon polaritons (PSPPs) of the Ag thin film. The plexcitonic nanoparticles are controllably placed at a distance nanometers away from a silver thin film. Reflection measurements reveal that the strong coupling of plasmon polaritons and plexcitons results in a newly formed hybridized state that we call here for the first time as a plexcimon state. The group velocity of the newly formed optical state is calculated from the dispersion curve. In addition, the new band demonstrates waveguiding behavior similar to a coupled resonator optical waveguide (CROW). The schematic representations of plexciton and plexcimon formations are shown in Fig. 1. In the strong coupling regime, the plexcimon state [hybridized plasmon–exciton–plasmon (PEP)] is observed.

 

Fig. 1. Schematic representations of (a) plexciton and (b) plexcimon formations in a strong coupling regime.

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Ag nanoprisms were rapidly synthesized using reaction conditions similar to the previously reported synthesis [6]. In a typical experiment, the seed nanoparticles were synthesized by combining 5 ml trisodium citrate solution (2.5 mM) with 0.25 ml poly(sodium 4-styrenesulfonate) (500 mg/L), and then with freshly prepared 0.3 ml NaBH4 (10 mM). Under the stirring condition, 5 ml ${\mathrm{AgNO}}_3$ (0.5 mM) was added drop by drop at a rate of $\sim 2\mathrm{ml}/\min$.

Anisotropic Ag nanoparticles were synthesized by varying the amount of seed colloidal solution added to the reaction solution, thereby affecting the morphology or the localized plasmon resonance of the Ag NPs. For example, in a typical experiment, 5 ml of Millipore water was combined with 75 μl ascorbic acid (10 mM) and 60 μl of seed solution to synthesize Ag NPs having an LSPP wavelength of $\sim 750\mathrm{nm}$ [Fig 2(a)].

 

Fig. 2. (a) Extinction spectrum of Ag NPs. The inset indicates the Ag NPs in a reaction vessel, (b) absorption spectrum of J-aggregate, (c) extinction spectrum of plexcitonic nanoparticles. The dispersion curve obtained from (d) bare 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$–40 nm Ag film, (e) Ag film covered with J-aggregates in a PVA matrix, and (f) 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$–40 nm Ag film covered with bare Ag NPs. The blue and red regions indicate the low and high reflectivity, respectively.

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Plexcitonic nanoparticles are chemically synthesized by self-assembling a J-aggregate dye on their surface [6]. Recently, similar plexcitonic nanoparticles have been synthesized using another kind of J-aggregate dye [7]. Freshly synthesized Ag NPs are effectively isolated from the reaction by-products by centrifugation. A cyanine dye [5,$5^{\prime }$,6,$6^{\prime }$-tetrachloro-di-(4-sulfobutyl) benzimidazolocarbocyanine] (TDBC) is purchased from FEW Chemicals. In order to self-assemble TDBC molecules on the nanoprism surfaces, 1 mM TDBC containing 1 mM KBr is prepared [Fig. 2(b)]. The isolated nanoprism solution is combined with the TDBC in KBr solution. Immediately, the color of the nanoprism sol changes [Fig. 2(c)]. Plexcitonic nanoparticles are uniformly and entirely coated on dielectric surfaces by an electrostatic self-assembly process [6]. Prior to the self-assembly process, the dielectric surfaces are modified with 3-aminopropyl-triethoxysilane (APTES) (10 mM in ethanol) and allowed to react for an hour [6]. The APTES modified glass substrates are inserted in the as-prepared Ag NP sol or plexcitonic nanoparticle sol and then the nanoparticles are allowed to electrostatically self-assemble on the functionalized glass surface for 24 h.

Ag thin films were grown on cleaned glass substrates by thermal evaporation of Ag. A dielectric spacer layer of 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$ was deposited by RF magnetron sputtering. The absorption measurements of the bare Ag NPs and plexcitonic nanoparticles were performed using an Agilent Technologies’ Cary Series UV–Vis spectrophotometer. Reflection measurements in the Kretschmann configuration (KC) were obtained using a spectroscopic ellipsometer (VASE). The details of the reflection measurements and the coupling of incident light to the surface plasmons have been reported and extensively discussed in our earlier works [8–10]. Figure 2(d) demonstrates the experimentally obtained PSPP dispersion curve of flat Ag film. To study plasmon–exciton coupling on flat 40-nm-thick Ag film, 1.25 mM TDBC in a 0.75% polyvinyl alcohol (PVA) solution was prepared and then spin-coated at 3000 rpm onto the Ag film [Fig. 2(e)]. When the Ag film is covered with a J-aggregate containing $\sim 30$-nm-thick PVA film, the coupling of PSPPs of the metal film and excitons of J-aggregates occurs, resulting in formation of plexcitons [8]. In a similar way, when bare Ag NPs are placed at a distance that is nanometers away from the Ag thin film, plasmon–plasmon (PP) coupling is observed [Fig. 2(f)] [10].

After synthesizing plexcitonic nanoparticles, the mixed solution of plexcitonic nanoparticles and dye solution is centrifuged to remove excess dye molecules or J-aggregates not bound to the nanoprisms (Fig. 3). It should be noted here that the J-aggregates also self-assemble on an Ag NP surface without adding KBr. The nanoparticle sol only contains plexcitonic nanoparticles, as shown in Fig. 3.

 

Fig. 3. (a) Extinction spectrum of the plexcitonic nanoparticles and uncoupled (free) TDBC dye molecules before centrifugation. (b) Extinction spectrum of the plexcitonic nanoparticles after centrifugation.

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In both cases (PP and plasmon–exciton), a classical description explains the strong coupling occurring between plasmon polaritons and Lorentzian oscillator that are generating two newly formed optical modes as [3,6–9]

 

Fig. 4. Strong PP and weak PEP couplings. (a) PSPP reflection curves of 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$–40 nm Ag film. (b) With strong PP coupling, the polariton reflection curve obtained from bare Ag NPs separated from the 40-nm-thick Ag film by a 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$ film. The upper and lower polariton bands are at $\sim 550\mathrm{nm}$ and $\sim 675\mathrm{nm}$, respectively. (c) With weak PEP coupling, the polariton reflection curve obtained from plexcitonic nanoparticles separated from the 40-nm-thick Ag film by 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$.

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In order to observe strong PEP coupling, the thickness of the Ag film has been increased to 60 nm [8]. A reflection curve of 60-nm-thick Ag film coated with 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$ is indicated in Fig. 5(a). The large plasmonic bandgap has been observed when the bare Ag NPs are randomly self-assembled on dielectric-coated 60-nm-thick Ag film [Fig. 5(b)] [10]. The PP coupling is in the ultrastrong coupling regime since the Rabi splitting energy ($\sim 700\mathrm{meV}$) is very large compared with the plasmon resonance energy of Ag NPs (2.07 eV). In a similar way, plexcitonic nanoparticles are placed onto the dielectric-coated 60-nm-thick Ag films and, subsequently, reflection measurements have been made. The dispersion curve indicates that PEP coupling has now entered into a new regime, i.e., a strong coupling regime because there are newly formed optical modes that we’ve called plexcimon states (PEP hybrid states) in the dispersion curve [Fig. 5(c)]. The raw data are also shown in Figs. 5(d)–5(f), corresponding to the processed data in Figs. 5(a)–5(c), respectively.

 

Fig. 5. (a) SPP reflection curves of 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$–60 nm Ag film. (b) Polariton reflection curve obtained from the Ag NPs separated from the 60-nm-thick Ag film by a 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$ film. The upper and lower polariton bands are at $\sim 475\mathrm{nm}$ and $\sim 700\mathrm{nm}$, respectively. (c) Reflection curve obtained from plexcitonic nanoparticles separated from the 60-nm-thick Ag film by a 10 nm ${\mathrm{Si}}_3{\mathrm{N}}_4$ film. The raw experimentally obtained data are shown in (d), (e), and (f), which correspond to the processed data in (a), (b), and (c), respectively.

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For further analysis of the plexcimon state in the dispersion curve, we have zoomed in on the plexcimon band in Fig. 6(a), which is obtained from the reflectivity map in Fig. 5(c). It should be noted here that when the bare Ag NPs are coated onto the same film structure, large plasmonic bandgap has been observed [Fig. 5(b)]. In the presence of plexcitonic nanoparticles, a new and weakly dispersive localized mode appears inside the bandgap region. The new band suggests that there is a waveguiding mode in the bandgap region.

 

Fig. 6. (a) Dispersion curve (wavelength versus angle) of the plexcimons, (b) dispersion curve (frequency versus momentum) of the plexcimons, (c) calculated group velocity of the plexcimons.

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The waveguiding band can be modeled by using the tight binding approach as also applied to the CROWs [Fig. 6(b)] [18]. Dispersion of the weakly dispersive plexcimonic waveguiding mode by using tight binding approximation can be described as [18,19]

It is noteworthy to mention here that with $n$ resonances in the coupled system, $n-1$ transparency dips arise in the dispersion curve as theoretically predicted [20]. In the PEP coupling, for example, three resonances (PSPP, LSPP, and exciton) produce two transparency dips in the dispersion curve.

In summary, we have extensively studied the ternary system of PSPPs of thin metal film, the excitons of J-aggregates, and the LSPPs of metal nanoprisms mutually interacting with each other. In the strong coupling regime, we have observed a new optical state called the plexcimon state, and group velocity of the plexcimons has been calculated. The coupled plexcimons shows CROW-like waveguiding behavior where plasmon loss, dielectric thickness, and plexcitonic nanoparticle density control the dispersion of the plexcimon band. The newly observed plexcimon state is appealing and may find a variety of fundamental and practical applications in understanding light–matter interaction at nanoscale dimension. Therefore, more experimental and theoretical works are urgently needed to fully understand the optical properties of this newly formed optical state and to find interesting applications in areas such as spectroscopy, lasers, and quantum optics.