Photocurrents and Electroluminescence at gate-defined edges in monolayer $p$--$n$ junctions

Charge carriers in single layer transition metal dichalcogenides (TMDs) possess coupled spin and valley degrees of freedom arising from strong spin-orbit coupling and the absence of inversion symmetry. The valley-contrasting Berry curvature and magnetic moment of these carriers enables valley currents to be created with circularly polarized light and measured electrically. However, due to a high binding energy, excitons only efficiently dissociate into free carriers in the depletion region of metal contacts, strongly impacting coupled spin, valley, and charge dynamics. To overcome this difficulty, we utilize electrostatic gating to define artificial edges with large electric field changes far from metal-semiconductor boundaries\footnote{B. W. H. Baugher, H. O. H. Churchill, Y. Yang, and P. Jarillo-Herrero. \emph{Nature Nanotech.}, \textbf{39}, 262–267 (2014).}. We explore the spatially-resolved photocurrent and electroluminescence properties of dual-gated WSe$_2$ $p$--$n$ junctions in a geometry enabling detection of transverse and longitudinal currents and discuss applications of this geometry to measuring valley-polarized opto-electronic processes of electrons and holes.