Switchable mode-conversion and optical limiting by bandgap tuning in 2D materials

We demonstrate a novel graphene-based switchable optical mode converter, to our knowledge, that transforms Laguerre-Gaussian mode to Hermite-Gaussian mode and vice versa. This was achieved by creating a microlens in functionalized graphene by nonlinear thermo-optic effect at low pump powers of ∼1<i>m</i><i>W</i>. Enhanced nonlinear refraction of graphene on functionalization with a strong electron-donating ligand was found to be responsible for this capability. The mode conversion is theoretically understood by considering the microlens as a Fourier-transforming element with each component undergoing different phase shifts. The optical limiting response of graphene has also been found to scale up by an order of magnitude on functionalization. The enhancement has been attributed to the systematic tuning of the bandgap due to charge transfer interaction between the functionalizing ligands and the base moiety.