▎ 摘　　要

In this letter, a novel technique for terahertz (THz) wave amplification based on subjecting a nonlinear graphene layered medium to two intensive optical waves is proposed. The principle of operation is based on generating a backward-propagating THz wave at the expense of the THz input wave, by properly designing the medium dispersion, graphene nonlinear response, and optical frequency spacing. The generated THz wave interacts, in turn, with the co-propagating (THz and optical) waves, enabling an energy exchange regime. It is shown that, via proper design of the propagation length, a giant THz gain can be achieved by means of optical parametric down-conversion. The presented numerical evaluations show a possible 50-dB-THz gain for a few centimeters of propagation lengths with reasonable optical input intensities. The proposed scheme is tunable, simple, and operates at room temperature.