Circularly Polarized THz Wave Generation Using Graphene Nanostructures

Building on the strong nonlinear response of graphene nanostructures and Neumann’s symmetry principle, we theoretically study the difference-frequency generation (DFG) of a circularly polarized THz field with controllable handedness (or spin angular momentum, SAM). Using the tight-binding description of graphene and the time-dependent density matrix approach, we perform real-time calculations of the nonlinear conversion produced by the interaction of the two contra-rotating, circularly polarized infrared electromagnetic pulses with individual graphene nanoflakes and nanoribbon compounds. We observe high conversion efficiency for all studied systems, as evidenced by their performance when arranged into the metasurface. In such configurations, the efficiency of the DFG of the SAM-carrying THz field can be above 10–3 W/W2. We further address the possibility of mitigating the effects of the unavoidable in practical situations symmetry distortions at the atomic or macroscopic level, which degrade the SAM selectivity of DFG. Our work extends theoretical studies of the THz manipulation with graphene nanomaterials toward spin-carrying radiation.