Strong plasmon-exciton coupling in a hybrid system of gold nanostars and J-aggregates
Hybrid materials formed by plasmonic nanostructures and J-aggregates provide a unique combination of highly localized and enhanced electromagnetic field in metal constituent with large oscillator strength and extremely narrow exciton band of the organic component. The coherent coupling of localized plasmons of the multispiked gold nanoparticles (nanostars) and excitons of JC1 dye J-aggregates results in a Rabi splitting reaching 260 meV. Importantly, broad absorption features of nanostars extending over a visible and near-infrared spectral range allowed us to demonstrate double Rabi splitting resulting from the simultaneous coherent coupling between plasmons of the nanostars and excitons of J-aggregates of two different cyanine dyes.
KeywordsGold Nanostars Organic compounds Plasmons Rabi splitting Fano effect
In the strong coupling regime, the value of Rabi splitting depends on the oscillator strength of the exciton as well as on the increase in the local density of the electromagnetic modes and field enhancement both provided by noble metal nanostructures. To date, Rabi splitting arising from coherent coupling between electronic polarizations of plasmonic systems and molecular excitons in J-aggregates of cyanine dyes has been demonstrated for a variety of metal constituents, such as Au, Ag, and Au/Ag colloidal nanoparticles [4, 5], core-shell Au and Ag nanoparticles [6, 7], Ag films , spherical nanovoids in Au films , Au nanoshells , Au nanorods [11, 12], and arrays of Ag nanodisks .
Among different plasmonic nanostructures, multispiked gold nanoparticles with a star-like shape [14, 15, 16, 17] are of particular interest for the development of photonic devices and sensors based on the strong coupling phenomenon. These nanoparticles consist of a core with typically five to eight arms , whose sharp tips give rise to the strong spatial confinement of the electromagnetic field, with enhancement factors similar to those in metallic nanoshell dimers [19, 20]. The coexistence of different plasmon resonances resulting from the hybridization of the core and the individual tips results in the increased number of localized plasmonic modes [19, 21] (as compared to spherical nanoparticles or nanorods) available for the coherent interaction with quantum emitters. Moreover, the hybridization of plasmons localized at the core and the tips of the stars results in the increased effective dipole moment of the tip plasmons and the enlarged cross section for plasmon excitation . In this study, we use these advantages of gold nanostars to develop their hybrid structures with J-aggregates of different organic dyes operating in the strong coupling regime.
J-aggregates were formed from the following two dyes: JC1 (5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethyl-imidacarbocyanine iodide) and S2165 2-[3-[1,1-dimethyl-3-(4-sulfobutyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-propenyl]-1,1-dimethyl-3-(4-sulfobutyl)-1H-benzo[e]indolium hydroxide. J-aggregates of the JC1 dye form spontaneously upon dissolution of this dye in deionized water at pH7, while the formation of J-aggregates of S2165 required the addition of polyethyleneimine (PEI). The reason why we choose these particular dyes was that upon aggregation they develop very narrow absorption bands (J-bands) both located very close to the maximum of nanostar absorption which favors the regime of strong plasmon-exciton coupling in hybrid systems.
The formation of the hybrid structures of two constituent compounds has been further confirmed by surface-enhanced Raman scattering (SERS) measurements using a confocal Raman microscopy setup (Alpha300, 600 mm−1 grating, 3 cm−1 spectral resolution, continuous wave laser excitation at 532 nm, WITec, Ulm, Germany), as the hot spots provided by sharp tips of agglomerated Au nanostars are expected to enhance Raman scattering response of the attached organic compounds . Indeed, the SERS spectrum of the hybrid nanostructures of gold nanostars and the JC1 J-aggregates (red curve in Figure 3) shows identical but by more than an order of magnitude enhanced features as compared to the conventional Raman spectrum of J-aggregates (black curve in Figure 3). Raman micromapping of hybrid gold nanostars/J-aggregate (JC1) complexes dispersed over a glass slide (Figure 3, right inset) directly demonstrates the strong enhancement of the Raman signal at the location of agglomerates.
Results and discussion
In the hybrid structure of both nanostars and J-aggregates, the pronounced dip at 590 nm (which corresponds to the absorption wavelength of the J-aggregates) appears as a result of strong coupling of the excited states of J-aggregates and plasmon modes of the nanostars (Figure 4a, blue curve). The wavelength separation between the two peaks in this spectrum (indicated by arrows in Figure 4) is 61 nm, giving the value of Rabi splitting of 213 meV. This value depends on the total absorbance or, in other words, on the concentration of J-aggregates , which, for cyanine dye molecules used in this work, can be influenced by the addition of charged polyelectrolytes . This is demonstrated in Figure 4a (green curve), where positively charged polyelectrolyte PEI has been added to gold nanostars and to the JC1 molecules. As a result, Rabi splitting energy increased to 260 meV, which is 13% of the total transition energy (which corresponds to spectral position of the dip), indicating the strong coupling regime between the plasmons and the J-aggregate excitons.
To demonstrate the advantage of using Au nanostars for the strong coupling with J-aggregates, it would be instructive to compare the values of the achieved Rabi splitting with that of a hybrid system consisting of J-aggregates and gold nanorods  of similar volume as nanostars. Based on the TEM image (Figure 2), the effective volume of nanostars was estimated approximating their inner core part by a sphere to which the spikes are attached.
The absorption spectrum of Au nanorods used here (Figure 4b, violet curve) exhibits two main resonances: the red-shifted peak at 766 nm corresponds to the longitudinal surface plasmon resonance, whereas the spectral position of the two other bands spanning over the region between 450 and 650 nm is consistent with the wavelengths of the transverse plasmon modes. The absorption band of J-aggregates of JC1 dye (Figure 4c) falls within the spectral region of the blue-shifted band of the nanorods. In the hybrid system of Au nanorods and J-aggregates, which was fabricated in a similar fashion as that of the gold nanostars, a dip at 595 nm (Figure 4b, cyan curve) with Rabi splitting of 185 meV is observed, which is a much smaller value than that demonstrated above for the nanostar-based hybrid system.
It is well known that in the strong coupling regime, the spectral lineshapes of the hybrid system can be interpreted interchangeably as a result of the plasmon-exciton hybridization (leading to the formation of two distinct mixed states (Rabi effect)) and also by the interference of different excitation pathways (Fano interference) . In the last case, one of the paths is a discreet excitonic state and the other is a quasi-continuum plasmonic state (Figure 1). Depending on whether or not the plasmonic and excitonic resonances are exactly matching, the profile of Fano resonances goes from a symmetric dip to an asymmetric lineshape, respectively . In line with this, the observed asymmetric profiles of both dips in Figure 5 can be interpreted as results of slight mismatch between main resonance in the spectrum of the nanostars and spectral positions of J-aggregate excitonic transitions.
where f n is the reduced oscillator strength, γ n is the line width, ω0n is the transition frequency, and ε∞jn is the high-frequency component of dielectric function of the first (n = 1) and second (n = 2) types of J-aggregates.
In conclusion, we introduced hybrid structures consisting of Au nanostars and J-aggregates of the cyanine dyes, where the coherent coupling between the localized plasmons of the metal component and the excitons of the J-aggregates reveals itself in Rabi splitting with the energy up to 260 meV. Owing to the remarkably broad features in the absorption spectra of gold nanostars, we were able to realize double Rabi splitting through their surface plasmon coupling to the excitons of two different dyes. This experimental finding paves the way towards the development on advanced hybrid systems and further investigations of the interaction between multiple emitters mediated by localized plasmons of different metallic nanostructures in the quantum electrodynamics regime. Alongside with the other multicomponent hybrid plexcitonic structures [32, 34], hybrid systems realized and studied here offer a platform for the practical development of nanoscale optoelectronic and quantum information devices.
This work was supported by the ETORTEK 2011–2013 project ‘nanoIKER’ from the Department of Industry of the Basque Government and by the Visiting Fellowship program of Ikerbasque Foundation. Helpful discussions with Dr. J. Aizpurua and Prof. A. Chuvilin are gratefully acknowledged.
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