Room-Temperature Valley Polarization and Coherence in Transition Metal Dichalcogenide-Graphene van der Waals Heterostructures

van der Waals heterostructures made of graphene and transition metal dichalcogenides (TMDs) are an emerging platform for optoelectronic, -spintronic, and -valleytronic devices that could benefit from (i) strong light-matter interactions and spin-valley locking in TMDs and (ii) exceptional electron and spin transport in graphene. The operation of such devices requires significant valley polarization and valley coherence, ideally up to room temperature. Here, using a comprehensive Mueller polarimetry analysis, we report artifact-free room-temperature degrees of valley polarization up to 40% and, remarkably, of valley coherence up to 20% in monolayer tungsten disulfide (WS2)/graphene heterostructures. At a temperature of 20 K, we measure a record degree of valley coherence of 60%, a value that exceeds the degree of valley polarization (50%) and indicates that our samples are minimally affected by pure dephasing processes. Valley contrasts have been particularly elusive in molybdenum diselenide (MoSe2), even at cryogenic temperatures. Upon interfacing monolayer MoSe2 with graphene, the room-temperature degrees of valley polarization and coherence are as high as 14% and 20%, respectively. Our results are discussed in light of recent reports of highly efficient interlayer exciton and carrier transfer in TMD/graphene heterostructures and hold promise for room-temperature chiral light-matter interactions and opto-valleytronic devices.