Two-dimensional transition-metal dichalcogenides (TMDCs) exhibit extraordinary nonlinearities and direct bandgaps at K (K') valleys. Those valleys could be optically manipulated through the plasmon-valley-exciton coupling, for example, with spin-dependent photoluminescence. However, the weak coherence between the pumping and emission due to intervalley scattering poses formidable challenges in exploring the valley-contrasting physics and applications. Here we show that a synthetic metasurface entangling the phase and spin of light can simultaneously enhance and manipulate the nonlinear valley-locked chiral emission in monolayer tungsten disulfide (WS 2) at room temperature. The second-harmonic valley photons, accessed and coherently pumped by the light with spin-related geometric phase imparted by Au-metasurface, are separated and routed to predetermined directions in free space. Besides, the nonlinear photons with the same spin of incident light can be steered into any predefined direction thanks to the nonlinear optical selection rule of WS 2 in our synthetic metasurface. Our synthetic TMDCs-metasurface interface may facilitate advanced roomtemperature and free-space nonlinear, quantum and valleytronic nanodevices.

The growing demand for tailored nonlinearity calls for a structure with unusual phase discontinuity that allows the realization of nonlinear optical chirality, holographic imaging, and nonlinear wavefront control. Transition-metal dichalcogenide (TMDC) monolayers offer giant optical nonlinearity within a few-angstrom thickness, but limitations in optical absorption and domain size impose restriction on wavefront control of nonlinear emissions using classical light sources. In contrast, noble metal-based plasmonic nanosieves support giant field enhancements and precise nonlinear phase control, with hundred-nanometer pixel-level resolution; however, they suffer from intrinsically weak nonlinear susceptibility. Here, we report a multifunctional nonlinear interface by integrating TMDC monolayers with plasmonic nanosieves, yielding drastically different nonlinear functionalities that cannot be accessed by either constituent. Such a hybrid nonlinear interface allows second-harmonic (SH) orbital angular momentum (OAM) generation, beam steering, versatile polarization control, and holograms, with an effective SH nonlinearity χ2 of ~25 nm/V. This designer platform synergizes the TMDC monolayer and plasmonic nanosieves to empower tunable geometric phases and large field enhancement, paving the way toward multifunctional and ultracompact nonlinear optical devices.

High-quality chirality-selected second-harmonic holography is achieved based on a Au–WS₂ interface by combining geometry phase and binary amplitude control.