Science 344, 488–490 (2014)

Molybdenum disulphide (MoS2) is a popular two-dimensional material, and single layers of MoS2 have previously been shown to support strong second-harmonic generation. Now, Xiaobo Yin and colleagues from the University of California, Berkeley, and Columbia University, New York, claim to have observed one-dimensional nonlinear optical edge states in atomically thin MoS2 sheets. The team fabricated triangles of the monolayer by chemical vapour deposition. When samples were pumped with an excitation wavelength of 1,280 nm, self-assembly occurred uniformly across the samples. In contrast, when the pump wavelength was shifted to 1,300 nm, self-assembly at the edges of the sample was enhanced relative to that elsewhere. The researchers claim that the different electronic structures at the edges of the two-dimensional crystal cause different nonlinear optical susceptibilities. They demonstrate the usefulness of this finding by showing that imaging the sample edges can be used to rapidly determine crystal orientations.