Direct Imaging of Fermi Polaron Blockade

Strong optical nonlinearity in solid-state systems is crucial for advancing many-body physics and quantum optical technologies. In two-dimensional transition-metal dichalcogenides (TMDCs), excitons interacting with a Fermi sea form quasiparticles known as exciton–Fermi polarons, which exhibit enhanced nonlinear interactions. These arise from phase-space filling and Coulomb coupling, modifying exciton energy and mass. Yet the microscopic nature of the polaron—its spatial extent and interaction strengths governing nonlinear response—remains poorly understood. We combine experiment and theory to investigate Fermi polarons in molybdenum disulfide (MoSe₂). Ultrafast pump–probe microscopy reveals optical saturation of the polaron resonance at excitation densities an order of magnitude below that of the bare exciton—corresponding to interparticle separations of 55 nm versus 5 nm. This giant nonlinearity indicates a large spatial footprint and strong many-body correlations. To explain its microscopic origin, we develop a variational model based on the Chevy ansatz that truncates the hierarchy of particle–hole excitations while retaining key correlations. A simplified blockade framework captures spatial correlations and phase-space filling from impurity–Fermi sea coupling. Together, these results establish exciton–Fermi polarons in TMDCs as a platform for exploring strongly correlated optical quasiparticles and realizing ultra-low-power nonlinear devices.