Nonlinear optical spectrum of bilayer graphene in the terahertz regime

We demonstrate that the nonlinear optical response in bilayer graphene is among the strongest, especially in the important frequency regime of terahertz to far-infrared. Furthermore, we show that both the single frequency and frequency tripled nonlinear response become comparable to the linear response at very moderate electric field. The field strength for the onset of nonlinear effect in bilayer graphene is well within the experimental achievable range in laboratories. Our result suggests that bilayers are preferred structures for developing graphene-based nonlinear photonics and optoelectronics devices.