Second-harmonic edges

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.