Distinct peaks associated with atom-cavity polaritons are observed in the transmission spectrum of three-level rubidium atoms in an optically-dense vapor cell inside an optical ring cavity.
We explore the high-density, nonlinear regime of atom-cavity coupling for a gas of three-level atoms at high temperature inside an optical cavity, interacting with the cavity field and a strong driving field. We find a region of parameters where the two side peaks in the transmission spectrum (which, in the linear regime, are associated with ``bright polaritons'') split into two pairs of peaks. Experimental results are presented along with a theoretical model that indicates that this phenomenon is unique to the three-level system in the large Doppler-broadening regime.