Coherent steering of nonlinear chiral valley photons with a synthetic Au-WS2 metasurface

Two-dimensional transition metal dichalcogenides (TMDCs) present extraordinary nonlinearities and direct bandgaps at the K and K' valleys. These valleys can be optically manipulated through, for example, plasmon-valley-exciton coupling with spin-dependent photoluminescence. However, the weak coherence between the pumping and emission makes exploring nonlinear valleytronic devices based on TMDCs challenging. Here, we show that a synthetic metasurface, which entangles the phase and spin of light, can simultaneously enhance and manipulate nonlinear valley-locked chiral emission in monolayer tungsten disulfide (WS2) at room temperature. The second-harmonic valley photons, accessed and coherently pumped by light, with a spin-related geometric phase imparted by a gold (Au) metasurface, are separated and routed to predetermined directions in free space. In addition, the nonlinear photons with the same spin as the incident light are steered owing to the critical spin-valley-locked nonlinear selection rule of WS2 in our designed metasurface. Our synthetic TMDC-metasurface interface may facilitate advanced room-temperature and free-space nonlinear, quantum and valleytronic nanodevices.