PT-symmetric metasurfaces: wave manipulation and sensing using singular points

Weinvestigate here active metasurfaces obeying parity-time (PT) symmetry and their sensing applications, taking advantage of singularities unique to non-Hermitian systems, such as the spontaneous PT-symmetry-breaking point (exceptional point or EP) and the coherent perfect absorber-laser (CPAL) point. Weshow theoretically that a PT-symmetric metasurface sensor may provide enhanced sensitivities compared to traditional passive sensors based on metamaterial/metasurface resonators, because the singular point of one-way zero reflection arising from the EP or the CPAL-related sharp resonance may result in dramatically modulated scattering responses or resonance offsets. Wedemonstrate the proposed concept with realistic metasurface sensors based on photopumped graphene metasurfaces that simultaneously offer terahertz optical gain and (bio) chemical sensing functions. The proposed PT-symmetric metasurfaces may impact not only loss compensation and extraordinary manipulation of electromagnetic waves, but also practical sensing and detection applications.