  We report here a giant $\left|\chi_{baa}^{2}\right|=$780 pm/V second harmonic
and $\left|\chi_{aaaa}^{3}\right|=$1.4$\times$10$^{-17}$ m$^{2}$/V$^{2}$ 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 of non-linear signal generations.
