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Second harmonic microscopy of monolayer MoS<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>
We show that the lack of inversion symmetry in monolayer MoS${}_{2}$ allows strong optical second harmonic generation. The second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10${}^{\ensuremath{-}7}$ m/V. The susceptibility reduces by a factor of seven in …