## 3 Bloch Surface Wave Polaritons  
In this second section, we will review the recent results on Bloch surface wave exciton-polaritons (BSWPs). The photon-exciton strong coupling leads to the hybridization of the bare components with the consequent generation of two hybrid eigenmodes, the upper and lower polariton branches. A first theoretical proposal in 2011 [40], predicted the generation of BSWP using GaAs quantum wells deposited on top of a DBR. The first experimental demonstration of the formation of a BSWP was simultaneously obtained in 2014 from two different research teams [38, 41]. In both these experimental papers--different from the first theoretical proposal--the excitonic medium is an organic layer. This choice comes with the benefit to work at room temperature, where the oil immersion techniques (useful for the extraction of the signals beyond the critical angle) are available. Indeed, Wannier-Mott excitons (typical of III-V semiconductors, including GaAs) dissociate at room temperature because of their weak electron-hole binding energy, while excitons in organic materials (Frenkel excitons) have a strong binding energy, making them stable also at high temperature. Furthermore, considering that the exciton-photon coupling strength increases with increasing exciton oscillator strength, the huge absorption coefficient (>100,000 cm\({}^{-1}\)) of some of these organic materials--makes them perfect candidates for the obtainment of the strong coupling regime. In the experiments performed by our team [38], a 20 nm thick layer is deposited on top of a DBR--identical to the DBR described in the previous section--via spin coating technique starting from a blend of TDBC:PVA (1:1 weight ratio) dissolved in a mixture of methanol:water (1:1 volume ratio). Using the same setup reported in the previous experiments on MoS\({}_{2}\), emission and reflection signals lying in total internal reflection regimes are collected. The pinhole positioned at the back focal plane of the detection lens allows for the scan of the dispersion map with an angular resolution of 0.2\({}^{\circ}\). The TDBC is an organic dye with extremely huge extinction coefficient deriving from its J-aggregating property. The PVA is a polymer with a large energy bandgap (about 5.3 eV), used as a matrix to obtain an homogeneous distribution of the aggregates within the excitation area and to avoid the formation of big TDBC aggregates--which can drastically reduce the quality of the BSW mode. Scanning the back focal plane of the detection lens with the pinhole, two polaritonic branches are clearly observed when approaching the TDBC exciton transition. Figure 3a shows the typical anticrossing behavior of a polariton system in both reflection and emission measurements. The Rabi splitting is about 45 nm and the FWHM of the polariton modes at the anticrossing point is 14 meV. Figure 3b shows experimental data (black dots) superimposed to the theoretical dispersion map (TMM calculations). The interest for these particular exciton-polariton modes derives from the fact that these hybrid states exhibit macroscopic propagation. Indeed, both metal-based surface and planar microcavity modes are affected by intrinsic losses, which limit the quasi-particle propagation length. The main drawback of the first one is the metal dissipation, while the low group velocity (about 1% of light speed at maximum) is the main limitation in the propagation of planar microcavity polaritons. On the contrary, BSWPs exhibit group speed which can be higher than 50% speed of light--which is also much larger than the light speed in the excitonic medium--and very low dissipations--thanks to the fact that the supporting structure is almost completely made by dielectric materials. These properties have been accurately investigated and reported in the paper \"High Speed Flow of Interacting OrganicPolaritons\" ([PERSON] et al. Light: Science & Applications 2017, 6, e16212) [42]. Hereafter, we will reassume the main findings.