## 3 Bloch Surface Wave Polaritons
The unavoidable inhomogeneity of the spin coated TDBC aggregates limits the polariton lifetime (because of scattering out of the BSWP mode), and consequently their propagation in the plane of the device. In order to overcome this drawback, a thermally evaporable organic molecule has been used, which guarantees a high homogeneity of the deposition. A 35 nm thick layer of Lumogen Red 305 has been deposited on a photonic crystal (seven couples of TiO\({}_{2}\)/SiO\({}_{2}\), 85 nm/120 nm, respectively) obtaining a Rabi splitting of 186 meV. Instead of the prism-coupler, a leakage radiation microscope is necessary when detecting the spatial distribution of the polaritons in the plane of the device. This detection system consists of an oil immersion microscope objective placed at the substrate side in order to extract signals lying in the total internal reflection regime. An out-of-resonance excitation source (532 nm) is focused on the sample surface by means of a second microscope objective placed at the deposition side (Figure 4a). The sketch of the optical setup is in Figure 4a. The overall in-plane propagation length of the BSWP is about 120 \(\upmu\)m. Both a record quality factor and huge group velocity allow for this extremely long propagation distance, which is comparable to the best inorganic microcavity samples, and on the contrary has been achieved using an easy fabrication process and working at room temperature. In particular, the Q-factor is about 3000, which is about 10-times larger than typical values obtained in organic planar microcavities, where the deposition process of a second DBR on top of the organic layer drastically reduces the quality of the polariton modes. On the other hand, the group velocity (around 150 \(\upmu\)m/ps at the maximum emission intensity) of the BSWP is about 10-times larger than the maximum velocity achievable in typical planar microcavities. It is important to note that the observed propagation length is now limited by the extraction of the signal through the oil immersion objective (i.e., such leakage is contributing to the effective total loss), while--differently from planar microcavity modes--the surface flow can last for longer distances if not extracted. Figure 3: (**a**) Reflectivity (**left**) and photoluminescence (**right**) spectra taken at different angles of incidence with respect to the normal to the sample inside prism. The dashed line is the molecule absorption peak position [38]. (**b**) Experimental data (dots) superimposed to the theoretical calculation of the reflectivity as a function of wavelength and angle of incidence. The anticrossing behavior (distinctive of strong coupled systems) is evidenced in the inset (the horizontal black line is the exciton transition). Reproduced with permission from [PERSON] et al., Optics Letters 39 (7), 2068, © [Optical Society of America], 2014 [38]. Furthermore, we have studied the polariton-polariton interactions via resonant generation of polaritons. The excitation beam (35 fs pulsed laser) is focused on the sample through the oil immersion objective also used for the detection (Figure 4a). Using this configuration and working in total internal reflection regime, the detected signal is the reflection of the sample. The slit (S2 in Figure 4) along the detection line selects the excitation spot area, cutting off the propagating signal. Because of this selection, the BSWP appears as a dip in the dispersion map (Figure 4c, top left panel). When increasing the pump power, the polariton resonance blueshifts. Figure 4c (bottom left panel) shows the TMM calculations (solid lines) of the BSWP dispersion when considering an exciton energy of 2.13 eV (blue) and a blue-shifted (8 meV) exciton (red). They perfectly match the experimental values for 150 \(\upmu\)J cm\({}^{-2}\) and 10 mJ cm\({}^{-2}\), respectively.