Tunable Second Harmonic Generation in Twisted Bilayer Graphene

Summary Twisted stacking of van der Waals materials introduces a new way in band-structure engineering and has given rise to numerous extraordinary physical phenomena. Despite the absence of second harmonic generation (SHG) in non-gated monolayer graphene, artificially twisted bilayer graphene (tBLG) possesses more possible point-group symmetries, including those with broken inversion symmetry. Here, we report twist-angle-dependent SHG from tBLG, which is the first demonstration of an elemental material with intrinsically tunable nonlinearity. We show that depending on the twist angle, the susceptibility of the dominant chiral tensor component of tBLGs can vary from 0 to 28 × 104 pm2/V, which is at the same order of magnitude as on-resonance susceptibility of monolayer MoS2. These results shed light on the underlying symmetry of tBLG systems and electronic band structure near van Hove singularities. More importantly, they introduce a new degree of freedom, the twisting, in creating efficient second-order nonlinear material form centrosymmetric constituents.