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Optical second-harmonic generation in few-layer MoSe2

We show that optical second-harmonic generation (SHG) from few-layer MoSe2 can sensitively probe external symmetry perturbations such as chemical doping. The SHGs from few-layer MoSe2 are enhanced after AuCl3 chemical doping, especially more so for the thicker MoSe2 samples. This enhancement is due to an electric-field-induced SHG contribution originating from the charge transfer between the adsorbed chemical dopants and the MoSe2. By using this sensitivity, we demonstrate an in-situ monitoring of the thermal desorption of adsorbed chemical dopants. Moreover, we find that the SHG generated from monolayer MoSe2 is counter-circularly polarized for a circularlypolarized fundamental beam due to the three-fold rotational symmetry of MoSe2. Our work shows that SHG can probe not only the symmetry properties of atomically-thin 2D semiconductors, but also the external symmetry perturbations on them.