Sensing at exceptional point in terahertz metasurfaces

The peculiar properties of the exceptional point (EP) in a non-Hermitian metasurface have attracted tremendous attention in many fields: optics, acoustics, photonics, quantum computing, and biomedicine, among others. The integration of terahertz spectroscopy's high-resolution and non-destructive attributes with perturbation-induced frequency shift responses near the EP demonstrates well the potential applications of EP sensing in various detection scenarios. Herein, we propose a non-Hermitian terahertz metasurface design with a unit cell composed of metallic elements (a strip and a split ring) and a conductivity-tunable vanadium oxide (VO₂) strip. By tuning the geometrical parameters and modulating the VO₂ conductivity, we systematically investigate the dynamic responses of the metasurface at the EP. Importantly, a metasurface-based refractive index sensing scheme using EP is demonstrated via the frequency shift and temperature variation after EP reconfiguration. Our work on sensing using EP could facilitate the design of tunable on-chip optical sensors for the terahertz range.