  Hybridization of excitons with photons to form hybrid quasiparticles,
exciton-polaritons (EPs), has been widely investigated in a range of
semiconductor material systems coupled to photonic cavities. Self-hybridization
occurs when the semiconductor itself can serve as the photonic cavity medium
resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at
room temperatures which recently were observed in layered two-dimensional (2D)
excitonic materials. Here, we report an extreme version of this phenomenon, an
ultrastrong EP coupling, in a nascent, 2D excitonic system, the metal organic
chalcogenate (MOCHA) compound named mithrene. The resulting self-hybridized EPs
in mithrene crystals placed on Au substrates show Rabi Splitting in the
ultrastrong coupling range (> 600 meV) due to the strong oscillator strength of
the excitons concurrent with the large refractive indices of mithrene. We
further show bright EP emission at room temperature as well as EP dispersions
at low-temperatures. Importantly, we find lower EP emission linewidth narrowing
to ~1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our
results suggest that MOCHA materials are ideal for polaritonics in the deep
green-blue part of the spectrum where strong excitonic materials with large
optical constants are notably scarce.
