Phase-Matched Coupling and Frequency Conversion of Terahertz Waves in a Nonlinear Graphene Waveguide
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In this paper, we propose a novel technique for the phase-matched coupling and frequency conversion of terahertz (THz) waves. The principle of operation is based on nonlinear THz wave interaction in a graphene parallel plate waveguide filled with lithium niobate. The THz waves induce nonlinear polarization that modulates the effective permittivity of the waveguide by modifying the graphene conductivity. To model the evolution of the THz waves, small modulation amplitudes are presumed and a perturbation analysis is performed. First, two THz inputs (the pump and the signal) with distinct frequencies, that is, f1 and f2, are considered. The two waves are coupled in a phase-matched fashion and show significant signal gain. Second, a third THz input with low intensity is considered at frequency f3. Frequency conversion from f3 to a new frequency, that is, f4, is attainable, assisted by the permittivity modulation induced by the co-propagating waves f1 and f2. The generated frequency has a Ω = f1 - f2 frequency detuning from the f3 frequency, that is, f4 = f3 - (f1 - f2). Thus, tunable frequency conversion can be achieved by controlling f1, f2, or f3. Our numerical estimations show viable gain and conversion efficiency over the entire THz range considered, from 0.1 to 2 THz. The proposed technique is compact, tunable, and (in principle) spans the entire THz range.