Giant Out-of-Plane Second Harmonic Generation Susceptibility in Janus Group III Chalcogenide Monolayers

Using first-principles calculations, we first reported that Janus group III chalcogenide monolayers have giant out-of-plane second harmonic generation susceptibility, and it is even 1-2 orders of magnitude larger than that of the in-plane components. In the Ga2SSe monolayer, the χzxx(2)/χxxy(2) and χzzz(2)/χxxy(2) are up to 125:1 and 86:1, respectively. The large intraband\interband velocity transition matrix and charge transfer are responsible for the strong second harmonic response of Janus group III chalcogenide monolayers. The present work provides a general protocol to design two-dimensional nonlinear optical materials with strong out-plane response for applications in optoelectronic and nonlinear optical devices.