Hybrid Graphene-Plasmonic Gratings to Achieve Enhanced Nonlinear Effects at Terahertz Frequencies

High input intensities are usually required to efficiently produce nonlinear optical effects in ultrathin structures due to their extremely weak nature. This problem is particularly critical at low terahertz frequencies because high-input-power terahertz sources are not available. The demonstration of enhanced nonlinear effects at terahertz frequencies is particularly important since these nonlinear mechanisms promise to play a significant role in the development and design of new reconfigurable planar terahertz nonlinear devices. In this work, we present an alternative class of ultrathin nonlinear hybrid planar terahertz devices based on graphene-covered plasmonic gratings exhibiting very large nonlinear response. The robust localization and enhancement of the electric field along the graphene monolayer, combined with the large nonlinear conductivity of graphene, can lead to boosted third-harmonic-generation and four-wave-mixing nonlinear processes at terahertz frequencies. These interesting nonlinear effects exhibit very high nonlinear conversion efficiencies and are triggered by realistic input intensities with relatively low values. In addition, the third-harmonic-generation and four-wave-mixing processes can be significantly tuned by the dimensions of the proposed hybrid structures, the doping level of graphene, or the input intensity, whereas the nonlinear radiated power remains relatively insensitive to the incident angle of the excitation source. The nonlinear hybrid graphene-covered plasmonic gratings presented have a relative simple geometry and, as a result, can be used to realize efficient third-order nonlinear terahertz effects with a limited fabrication complexity. Several new nonlinear terahertz devices are envisioned on the basis of the proposed hybrid nonlinear structures, such as frequency generators, all-optical signal processors, and wave mixers. These devices are expected to be useful for nonlinear terahertz spectroscopy, noninvasive terahertz subwavelength imaging, and terahertz communication applications.