Nonlinear optics is a key enabling technology of our modern society, such as in imaging and high-speed data communication. But the traditional devices suffer from relatively small nonlinear optical coefficients of conventional optical materials. An interdisciplinary team of scientists from Aalto University, University of Eastern Finland, University of Arizona, Cambridge University, University of Ottawa, Italian Institute of Technology, and National University of Singapore, discovered that monolayer molybdenum disulfide, a unique two-dimensional (2-D) layered material similar to graphene, has an extremely large nonlinear optical response, which can efficiently convert low-energy photons into coherent high-energy photons.