In the recent decade, optical microcavities have attracted a great deal of interest in a broad range of research fields [1]. The monolithic structure of most microcavities, such as Fabry-Perot (FP) or whispering-gallery-mode (WGM) resonators, lack the capability of large-range spectral and spatial tuning. As an alternative, an open-access microcavity system combines several advantages [2]: realization of small mode volume, large-range tunability, flexible structural engineering, and easiness of integration with external emitters. Thus, this fiber-based cavity system results from an outstanding scheme to implement a new kind of semiconductor devices which are promising candidates for the future development of the new and exciting field of quantum polaritonics [3].

From the material perspective, tin-based perovskites have become attractive materials for light-matter interaction. Their advantages include straightforward fabrication by chemical synthesis, and Tin-based perovskites have accentuated because of their low toxicity and the optimal optical bandgap (1.2-1.4 eV) close to the Shockley-Queisser (SQ) limit under 1 sun illumination. Tin (II) halide perovskite has drawn attention because of the hybrid structure. The hybrid term refers to the combination of organic cations (in our case 2-tiopheneethylammonium, TEA+) and an inorganic layer of tin iodide octahedral layer. Despite this, TEA2SnI4 perovskite exhibits high exciton binding energy and reduced exciton-phonon interactions. In our work, the cavity consists of a first planar DBR mirror and a second microscale concave DBR mirror embedded at the tip of an optical fibre. These two mirrors are facing each other with a micron-sized gap, building a semi-concave Fabry-Perot cavity. The relative position between each mirrors can be tunned independently by nano-positioners. Here, we  study the coupling of the open cavity modes with the TEA2SnI4 exciton optical emission at room temperature (RT).  We observe an avoided crossing between the bulk exciton and the cavity mode levels, which is the principal signature of the strong coupling regime of the light-matter interaction, and hence of the formation of new dressed states at RT, i.e excitons-polaritons (Upper Polaritons- UP & Lower Polaritons-LP). The use of fibre based open cavity facilitates the tuning of the cavity mode as a function of  the cavity length, thus obtaining indepent control of the cavity resonance with respect to the exciton light emission. As it is shown in Figure 1, the avoided crossing is characterized by a  vacuum Rabi splitting of 15 meV. The development of lead-free perovskite polaritons are new candidates to be included in the quantum material database for the new exciting polariton condensates and quantum topology applications at RT.