AC-Stark Spectroscopy of Interactions between Moir\'e Excitons and Polarons

Abstract

We use nonlinear pump-probe spectroscopy to study optical excitations in a charge-tunable MoSe$_2$\WS$_2$ moir\'e heterostructure. An intense red-detuned laser pulse creates a photonic dressing of the material by introducing a large virtual population of excitons or exciton polarons in a deep moir\'e potential. By measuring the resulting ac Stark effect with a weak resonant laser pulse, we gain access to the nature and mutual interactions of the elementary optical excitations. At charge neutrality, our measurements reveal that different exciton resonances, associated with confinement of their center-of-mass motion in the moir\'e potential, have a significant spatial overlap. The resulting short-range interactions manifest themselves as a density-dependent blueshift for same-valley excitons and bound biexciton states for opposite-valley excitons. The attractive polaron resonance that appears upon injection of electrons into the heterostructure shows a contrasting behavior: Here, we observe an electron-density-independent light shift and a clear pump-power-dependent saturation. These features are equivalent to that of an ensemble of independent two-level emitters and indicate a breakdown of the Fermi-polaron picture for optical excitations of electrons subject to a strong moir\'e potential. Our work establishes an experimental approach to elucidate the elementary optical excitations of semiconductor moir\'e heterostructures, providing a solid ground for the spectroscopy of correlated electronic and excitonic states in such materials.