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.

范德华材料的扭曲堆叠为带结构工程引入了一种新方法，并引起了许多非同寻常的物理现象。尽管在非门控单层石墨烯中不存在二次谐波生成（SHG），但人工扭曲的双层石墨烯（tBLG）具有更多可能的点组对称性，包括具有反转反转对称性的那些。在这里，我们从tBLG报告了与扭曲角相关的SHG，这是具有固有可调非线性特性的元素材料的首次演示。我们表明，根据扭转角，tBLGs的主要手性张量成分的磁化率可在0到28×10 ⁴ pm ² / V之间变化，与单层MoS ₂的共振灵敏度相同。这些结果揭示了tBLG系统的基本对称性和van Hove奇点附近的电子能带结构。更重要的是，它们在创建有效的中心对称成分的二阶非线性材料时引入了新的自由度，即扭曲。