A new twist
By stacking layers of graphene on top of each other and twisting them, a research team led by associate professor of materials science and engineering Jie Yao has converted a common linear material into one with nonlinear optical capabilities crucial to everyday technology — from spectroscopy and material analysis to communications and computing.
In the study of optics, scientists distinguish between linear and nonlinear materials, which are rare. Unlike linear materials, nonlinear materials can combine multiple photons into one; for example, in the second-order nonlinear process, the frequency of the resulting photon is double that of the original, so it has twice the energy. But by twisting two single-atom-thick sheets of graphene in opposite directions, the team showed that it’s possible to fine-tune a linear material’s ability to combine photons, which practically converts a linear material into a nonlinear one.
To twist the layers of graphene, the researchers relied on a seemingly low-tech approach. Fuyi Yang, a Ph.D. student, used tape to peel layers of graphene off a large graphite crystal. She then used a piece of hexagonal boron nitride, which can attract sheets of graphene like a magnet, to lift and place the graphene layers on top of each other at an angle, producing samples of bilayer graphene at varying twist angles. By tuning the twist angles of the graphene samples, the team was able to achieve resonant second-harmonic generation, which combines photons into one with much greater efficiency.
While the research showed that graphene can be turned into a nonlinear material, Yao said it’s just a milestone toward the ultimate goal: to make two layers of graphene twist on command. The discovery might one day be used to enhance the performance of the silicon microchips found in nearly all laptops, tablets and phones, as faster photonic circuitry is increasingly combined with traditional electronic circuitry.