Optical second-harmonic generation in few-layer MoSe2

We show that optical second-harmonic generation (SHG) from few-layer MoSe 2 can sensitively probe external symmetry perturbations such as chemical doping. The SHGs from few-layer MoSe 2 are enhanced after AuCl 3 chemical doping, especially more so for the thicker MoSe 2 samples. This enhancement is due to an electric-field-induced SHG contribution originating from the charge transfer between the adsorbed chemical dopants and the MoSe 2 . 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 MoSe 2 is counter-circularly polarized for a circularlypolarized fundamental beam due to the three-fold rotational symmetry of MoSe 2 . 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.