Morphology-Engineered MoSe2/WSe2 Heterostructures for Directional Terahertz Emission

Abstract In two-dimensional (2D) heterostructures, ultrafast carrier dynamics at an interface can generate transient dipoles that emit terahertz (THz) radiation. However, deterministic control over the emission direction remains a critical challenge for integrating THz functionality into nanoscale photonic systems. Here, we demonstrate that the interfacial morphology of MoSe2/WSe2 heterostructures provides a direct route to controlling THz-emission directionality. Comparative THz time-domain measurements of a vertically stacked heterostructure (VHS) and a horizontally connected heterostructure (HHS) reveal predominantly in-plane emission from the VHS and surface-normal emission from the HHS. Remarkably, despite its dipole-active region being confined to a nanoscale interfacial width of approximately 3 nm, the HHS generates a THz amplitude comparable to that of the VHS within a factor of approximately four. Transient-current decomposition reveals fundamentally different carrier-transport pathways in the two morphologies. In the VHS, competing diffusion- and drift-associated processes produce opposite-phase THz contributions through dipole buildup and reorientation, resulting in partial cancellation of the emitted field. In contrast, the HHS exhibits a sequential transport process in which built-in-field-driven carrier separation precedes diffusion. Because the resulting current components remain aligned along a common in-plane axis and share the same phase, they add constructively and enable efficient THz emission from the nanoscale heterointerface. These results establish that heterostructure morphology governs not only the direction but also the phase relationship and effective efficiency of THz emission. Morphology-defined in-plane and surface-normal emission therefore provides a compact design principle for directional THz signal generation and routing in heterogeneous electronic–photonic systems.