Overcoming High-Quality Limitations in Plasmonic Metasurfaces for Ultrasensitive Terahertz Applications

In photonics, achieving high-quality (Q) resonance is crucial for high-sensitivity devices used in applications, such as switching, sensing, and lasing. However, high-Q resonances are highly susceptible to internal losses of plasmonic devices, impeding their integration into broader systems across terahertz and visible light bands. Here, we overcome this challenge by proposing a low-Q plasmonic metasurface for ultrasensitive terahertz (THz) switching and sensing. Theoretically, we reveal an approach to constructing a low-Q resonator possessing high sensitivity to nonradiative losses. Leveraging this mechanism, we design a highly sensitive plasmonic metasurface induced by strong coupling between a quasi-bound state in the continuum and a dipole mode. By hybridizing with the germanium layer, the metadevice exhibits an ultralow pump threshold of 192 μJ/cm² and an ultrafast switching cycle time of 7 ps. Furthermore, it also shows a high sensitivity of 224 GHz/RIU in refractive index sensing. The proposed paradigm of constructing low-Q and high-sensitivity photonic devices can be applied to biosensing, wide-band filters, and sensitive modulators.