This matching provides a perfect condition for strong coupling. It is well known that the presence of charged polyelectrolytes enhances the tendency

of cyanine dyes to form J-aggregates [28, 30, 31]. Moreover, as demonstrated above (Figure 4), the value of the Rabi splitting and therefore the strength of exciton-plasmon coupling can be increased by raising the concentration of J-aggregates, which, in turn, can be controlled by an addition of charged polyelectrolytes. For these reasons, the PEI polyelectrolyte LEE011 manufacturer has been used to induce the formation of J-aggregates of both dyes bound to gold nanostars. The absorption spectrum of the resulting complex hybrid system shows two pronounced

dips at 590 and 642 nm (Figure 5, red curve), which correspond to the maximum absorption wavelengths of the J-aggregates of JC1 and S2165, respectively. Thus far, the double Rabi splitting was observed with the energies of 187 and 119 meV. Figure 5 Absorption spectra of gold nanostars, pristine J-aggregates of JC1 and S2165, and their hybrid structure. Absorption spectra of gold nanostars (black curve) and their hybrid structure with J-aggregates of both JC1 and S2165 dyes (red curve). Absorption spectra of pristine J-aggregates of JC1 and S2165 dyes are shown in magenta and blue, respectively, together with their RAD001 chemical structures. 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) [32]. 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 for goes from a symmetric dip to an asymmetric lineshape, respectively [33]. In line with this, the observed asymmetric profiles of both dips in Figure 5 can be interpreted as results of slight mismatch check details between main resonance in the spectrum of the nanostars and spectral positions of J-aggregate excitonic transitions. The observed lineshape can be theoretically reproduced using the model of a hybrid nanostructure consisting of a gold nanostar core surrounded by two layers of different J-aggregates [10]. Because direct modeling of nanostar shape is very challenging, we used a more simple approach approximating their shape as an ellipsoid with three different radii and tried to match the experimental plasmon spectra of the nanostars.