  Light-matter coupling in excitonic materials has been the subject of intense
investigation due to emergence of new excitonic materials. Two-dimensional
layered hybrid organic/inorganic perovskites (2D HOIPs) support strongly bound
excitons at room-temperatures with some of the highest oscillator strengths and
electric loss tangents among the known excitonic materials. Here, we report
strong light-matter coupling in Ruddlesden-Popper phase 2D-HOIPs crystals
without the necessity of an external cavity. We report concurrent occurrence of
multiple-orders of hybrid light-matter states via both reflectance and
luminescence spectroscopy in thick (> 100 nm) crystals and near-unity
absorption in thin (< 20 nm) crystals. We observe resonances with quality
factors > 250 in hybridized exciton-polaritons and identify a linear
correlation between exciton-polariton mode splitting and extinction coefficient
of the various 2D-HOIPs. Our work opens the door to studying polariton dynamics
in self-hybridized and open cavity systems with broad applications in
optoelectronics and photochemistry.
