▎ 摘　　要

We introduce here a parity-time-(PT) symmetric system using an optically pumped, active graphene metasurface paired with a resistive metallic filament, realizing a unidirectional reflectionless propagation of terahertz (THz) waves. The amplified THz stimulated emission and tailored plasmon resonances in the graphene metasurface may achieve an equivalent negative-resistance converter at THz frequencies. We theoretically demonstrate that the combination of the spectral singularity in a PT -symmetric system and the chemical sensitivity of graphene may give rise to exotic scattering responses, strongly influenced by the presence of charged impurities in graphene at the spontaneous PT -symmetry-breaking point. This graphene-based PT -symmetric device may have broad relevance beyond the extraordinary manipulation of THz waves, as it may also open exciting prospects for detecting gas, chemical, and biological agents with high sensitivities.