Excitonic signatures of ferroelectric order in parallel-stacked MoS2

Interfacial ferroelectricity is a phenomenon arising in a plethora of parallel-stacked layered materials, 1-11 in which out-of-plane ferroelectric order is switched by in-plane sliding of adjacent layers. 4 Unlike conventional ferroelectrics, interfacial ferroelectricity is stable against doping, 7 potentially enabling next-generation devices with storage and computational capabilities. 12 However, the field has been limited to indirect sensing or visualizing the ferroelectricity in basic bilayer units employing surface-sensitive probes. In the case of transition metal dichalcogenides, this leaves a void of in-depth knowledge about the influence of ferroelectric order on their intrinsic valley and excitonic properties. Here, we report direct far-field probing of ferroelectricity in few-layer 3R-MoS 2 using reflectance contrast spectroscopy. Contrary to a simple electrostatic perception, the intralayer excitons with strictly in-plane dipole orientation are sensitive to out-of-plane ferroelectric ordering. On the other hand, despite possessing an out-of-plane electric dipole component, 13 layer-hybridized momentum-indirect excitons remain decoupled from such ordering. Ab initio calculations elucidate the microscopic origin of the correspondence between ferroelectric order and optical response. Using a field-effect device, we demonstrate room temperature control and optical readout of multi-state polarization and its hysteretic switching. Time-resolved Kerr ellipticity measured in different ferroelectric domains reveals a direct correspondence between spin-valley dynamics and ferroelectric order.