Terahertz Electronic and Spin Currents in Wafer-Scale Van der Waals Bi₂Se₃/WSe₂ Heterostructures and Polymorphs

Van der Waals heterostructures and polymorphs have promised the realization of artificial materials with multiple physical phenomena such as giant optical nonlinearities, spin-to-charge interconversion in spintronics and topological carrier protection, through an infinitely diverse set of 2D quantum materials and their stacking order in a single layered device. However, their exploitation for the terahertz range has been limited with most investigations based around the optical domain, owing to the use of exfoliated material that inherently limits both the dimensions of the materials and the scalability for applications. Here, the combination of terahertz electronic and spin currents is demonstrated through the realization of large area complex crystalline heterostructures of topological insulators, transition metal dichalcogenides (TMDs) and ferromagnets. This is demonstrated through down-conversion of optical beams into coherent terahertz currents, where the terahertz phase permits the decoupling of the physical phenomena, and bringing novel functionalities beyond those achievable in simple homostructures. In particular, the role of different TMD polymorphs (stacking orders - 1T', 2H, and 3R) is shown with the simple change of one atomic monolayer of the material stack entirely changing the terahertz responses - both electrical and magnetic - of the artificial material. This allows to highlight ultrafast phenomena that combine both the electronic- and spin-based processes in these structures. The importance of the crystal symmetry on the magnetic properties through proximity effects is further demonstrated, showing a nonlinearity of magnetic origin as a result of the 1T' polymorph. As well as control of the terahertz currents through different polymorphs, this scalable integration of a set of highly diverse materials establishes a platform for next-generation 2D heterostructures that integrate photonic, electronic, and spintronic properties into device architectures.