Nonlinear spectroscopy of semiconductor moiré materials

We use time-resolved nonlinear pump–probe measurements to reveal features of semiconductor moiré materials not accessible to linear spectroscopy. With an intense, red-detuned pump pulse, we generate a high density of virtual excitons or exciton–polarons in various moiré minibands. A broadband probe pulse in turn measures the response of all optical resonances induced by the pump-generated excitations. We generically observe a coherent blue shift originating from contact-like exciton–exciton interactions. At charge neutrality, these measurements allow us to assess the spatial overlap between different optical excitations and to observe signatures of a bound biexciton state between two different moiré exciton modes. In contrast to electron doped monolayers, spatially confined moiré attractive polarons behave as an ensemble of non-interacting two-level emitters, exhibiting an electron-density-independent ac-Stark effect. Tuning the pump laser into resonance with the attractive polaron, we demonstrate the filling of the moiré lattice with localized polarons and thereby realize a nonequilibrium Bose–Fermi mixture in moiré flat bands.