Millimeter wave difference-frequency mixing by mobile carriers in gated monolayer graphene with broken centrosymmetry

Abstract

Motivated by the device application potential of microwave difference-frequency generation (DFG) in graphene, we develop a theory of this second-order non-linear phenomenon in individual graphene layers lacking center-of-inversion symmetry due to their interaction with a substrate such as SiC or h-BN. Using the Boltzmann equation techniques, we calculate the microwave DFG conductivity of the graphene layer where the number of free carriers is controlled by an external gate voltage. The results obtained shed light on how the doping level of graphene, as well as a substrate-induced bandgap in the graphene’s electronic spectrum, affects the efficiency of the graphene-based frequency downconversion from the millimeter wave band to the microwave one. In particular, it is shown that there are some possibilities to maximize the output power of the microwave DFG by selecting graphene/substrate pairs with smaller substrate-induced bandgap values and by tuning, in the proper way, the Fermi energy of charge carriers via electrostatic gating. The gate controllable DFG enhancement can be achieved in gapped and doped graphene samples at the electron concentration level of the order of cm−2, thus paving the way to electrically tunable frequency downconvertors for millimeter wave applications in high data rate communication systems and signal processing.