Nardeep Kumar, Qiannan Cui, Frank Ceballos, Hui Zhao
We show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. Monolayer and few layer MoS2 samples are fabricated by mechanical exfoliation. Strong second harmonic of a normal incident 810-nm pulse is observed, with the central wavelength of 405 nm and the expected quadratic dependence on the fundamental power. The magnitude of the second order nonlinear susceptibility is on the order of 1E-7 m/V. The parallel and perpendicular components of the second harmonic are measured as we rotate the crystal with respect to the fundamental polarization. The results are consistent with the lattice structure of monolayer MoS2. The susceptibility reduces by a factor of seven in trilayers, and reduced by about two orders of magnitude in flakes with even numbers of layers. Since the second harmonic generation is very sensitive to crystalline orientation and layer number, it can be used as a non-invasive tool to characterize MoS2 thin films.
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http://arxiv.org/abs/1302.3935
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