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Two-dimensional materials are an emerging class of new materials with a wide range of electrical and optical properties and potential practical applications. Single layers materials such as semiconducting transition metal dichalcogenides with MX₂stoichiometry, where M is a transition metal element from group VI (M=Mo, W) and X is a chalcogen (X=S, Se, Te) are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Here, we report a type of optical switching based on a single-layer WSe₂ transistor. A side gate controls the polarization of the ions in PEO:CsClO₄, after which we can turn on and off the photoluminescence using a back gate. An on-off ratio of 90 is observed under constant pump light intensity. We believe the “off” and “on” states originate from the net electric field produced by the polarized ions and the back-gate, which aligns the dipole within the WSe₂ either in the same way or in the opposite way.