Exciton-driven giant nonlinear overtone signals from buckled hexagonal monolayer GaAs

We report here a giant χbaa(2)=780 pm/V second harmonic and χaaaa(3)=1.4×10-17m2/V2 third harmonic signal from single atomic sheet of buckled hexagonal GaAs. We demonstrate this through the solution of an ab initio real-time Bethe-Salpeter equation by including the electron-hole screened-exchange self-energy. The coupling between time-dependent external electric field and correlated electrons is treated within the modern theory of polarization. The result of our calculation envisage monolayer GaAs to be a prominent member in the material library for nonlinear signal generations.