Tunable optical chirality of twisted light with graphene

Twisted light carrying orbital angular momentum is inherently chiral. To promote the application of such twisted light in chiral science, it is highly desirable to control its optical chirality in a flexible way. We report here an approach to control the chirality of twisted light by taking advantage of tunable optical properties of graphene. A full vector theoretical model is developed to investigate the chirality of twisted light with Laguerre-Gaussian mode reflected from a graphene-substrate interface. It is revealed that the chirality of twisted light upon reflection is very sensitive to the incident angle. The greater the incident angle, the greater the maximum value of the chirality density. Moreover, the chirality of twisted light can be flexibly tuned by modulating the Fermi energy of the graphene and the substrate refractive index. In addition, the chirality of twisted light can be tuned from positive to negative values by changing the sign of the topological charge. Such tunable optical chirality has potential applications in chiral molecular recognition, chiral structure detection, and chiral micromanipulation.