Giant effective Zeeman splitting in a monolayer semiconductor realized by spin-selective strong light–matter coupling

Strong coupling between light and the fundamental excitations of a two-dimensional electron gas (2DEG) is of foundational importance both to pure physics and to the understanding and development of future photonic nanotechnologies(1-7). Here we study the relationship between spin polarization of a 2DEG in a monolayer semiconductor, MoSe2, and light-matter interactions modified by a zero-dimensional optical microcavity. We find pronounced spin-susceptibility of the 2DEG to simultaneously enhance and suppress trion-polariton formation in opposite photon helicities. This leads to observation of a giant effective valley Zeeman splitting for trion-polaritons (g-factor of >20), exceeding the purely trionic splitting by over five times. Going further, we observe clear effective optical nonlinearity arising from the highly nonlinear behaviour of the valley-specific strong light-matter coupling regime, and allowing all-optical tuning of the polaritonic Zeeman splitting from 4 meV to >10 meV. Our experiments lay the groundwork for engineering topological phases with true unidirectionality in monolayer semiconductors, accompanied by giant effective photonic nonlinearities rooted in many-body exciton-electron correlations.