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Nonlinear optics: When graphene ‘does the twist’ Modifying the angle between the two layers of twisted bilayer graphene can considerably improve how photons interact with it to generate higher-energy photons. Dong-Il Yeom of Ajou University and colleagues in Korea shined laser light onto twisted bilayer graphene stacked at various angles, and measured the outcoming optical signals. Twisted bilayer graphene is made by stacking two monolayer graphenes at an angle. The interlayer interaction in this material leads to intriguing properties. The scientists found that changing the angle could enhance a nonlinear optical property, called third-harmonic generation, by 60 times compared to monolayer graphene. The team then conducted Raman measurements and modeling calculations to understand how this process occurs. They say their findings could pave the way towards novel designs for enhancing optical nonlinearity in two-dimensional stacked materials.