Optical investigation of the band structure-dependent many-body interactions in monolayer and bilayer transition metal dichalcogenides

Transition metal dichalcogenide (TMD) heterostructures allow the investigation of many-body interactions between the exciton and a tuneable Fermi-sea, which can be described by the Fermi-polaron model. Experimental and theoretical work has mainly focused on direct bandgap monolayer TMDs, where carriers are doped into the conduction and valence band edges at the ±K valleys. However, there has been recent interest in natural homobilayer TMDs as a host of interlayer excitons and a platform for correlated physics. The band extrema in these systems are often predicted to be located at other high symmetry points (elsewhere to ±K) in the Brillouin zone. Despite this, there is a lack of optical characterisation of the doping-dependent properties in these systems. Here we perform reflection contrast measurements at 4 K on a high-quality dual-gate tuneable device that contains regions of monolayer and bilayer WSe₂ and MoSe₂. We explain the doping-dependent peak dispersions and magnetic properties of the attractive exciton-polarons that form in the bilayer regions through their distinctive band-structures. We use our observations from the natural bilayers to gain insight into the poorly understood feature that emerges at high carrier concentrations in monolayer WSe₂ and other TMD systems, including monolayer MoS₂ . Our results further the understanding of monolayer and bilayer TMDs which can help to engineer future optoelectronic devices using these materials.