Nonlinear nano-imaging of interlayer coupling in graphene-WSe₂ heterostructures

Two-dimensional heterostructures of graphene and transition metal dichalcogenides exhibit enhanced photonic, electronic, spin, and other novel quantum properties. These emergent phenomena are controlled by the underlying interlayer coupling and associated charge and energy transfer and their dynamics. However, these processes are sensitive to interlayer distance and relative crystallographic orientation, which are in turn affected by defects, grain boundaries, bubbles, local strain, and other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer and their competition with other relaxation processes, further amplified by spatial averaging across sample heterogeneities in conventional spectroscopy techniques. Here we combine nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) with nano-cavity clock spectroscopy to resolve the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, we show that energy as opposed to charge transfer dominates the interlayer coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by nano-cavity enhancement and quenching, we derive an interlayer energy transfer time of τ_ET ∼ 0.35 ps consistent with recent reports. As a local probe technique, our approach highlights the ability for both nano-scale imaging and control of interlayer coupling to engineer new nonlinear nano-optical devices.