The molecular fingerprint sensing technology based on metasurface has unique attraction in the biomedical field. However, in the terahertz (THz) band, existing metasurface designs based on multi-pixel or angle multiplexing usually require more analyte amount or possess a narrower tuning bandwidth. Here, we propose a novel single-pixel graphene metasurface. Based on the synchronous voltage tuning, this metasurface enables ultra-wideband ( $$\sim$$∼ 1.5 THz) fingerprint enhancement sensing of trace analytes, including chiral optical isomers, with a limit of detection (LoD) ≤ 0.64 μg/mm². The enhancement of the fingerprint signal ( $$\sim$$∼ 17.4 dB) originates from the electromagnetically induced transparency (EIT) effect excited by the metasurface, and the ideal overlap between the light field constrained by single-layer graphene (SLG) and ultra-thin analyte. Meanwhile, due to the unique nonlinear enhancement mechanism in graphene tuning, the absorption envelope distortion is inevitable. To solve this problem, a universal fingerprint spectrum inversion model is developed for the first time, and the restoration of standard fingerprints reaches R_max² ≥ 0.99. In addition, the asynchronous voltage tuning of the metasurface provides an opportunity for realizing the dynamic reconfiguration of EIT resonance and the slow light modulation in the broadband range. This work builds a bridge for ultra-wideband THz fingerprint sensing of trace analytes, and has potential applications in active spatial light modulators, slow light devices and dynamic imaging equipments.

A magneto-metasurface with nonreciprocal terahertz (THz) transmission has been proposed to form a THz isolator. Importantly, we have discussed the two necessary conditions for THz nonreciprocal transmission in the metasurface: (1) There should be magneto-optical responses for THz waves in the metasurface; (2) The transmission system of the metasurface needs to be asymmetric for forward and backward waves. These two conditions lead to the time reversal symmetry breaking of system, and the magnetoplasmon mode splitting and nonreciprocal resonance enhancement can be observed in the asymmetry magneto-metasurface. Moreover, the isolation dependences and tunability on the external magnetic field and temperature have also been investigated, which shows that the best operating state with a high isolation can be designed. The numerical simulations show a maximum isolation of 43 dB and a 10 dB operating bandwidth of 20 GHz under an external magnetic field of 0.3 T, and the insertion loss is smaller than 1.79 dB. This low-loss, high isolation, easy coupling THz isolator has broadly potentials for THz application systems.

The nonreciprocal circular dichroism and Faraday rotation effect for terahertz (THz) waves in longitudinally magnetized InSb were investigated by theoretical and experimental studies in the THz regime, which indicated its ability for a THz circularly polarized isolator, THz circular polarizer, tunable polarization converter, and polarization modulator by manipulation of different magnetic fields. Furthermore, we demonstrated the InSb plasmonics based on its magneto-optical effects combined with artificial microstructure. We found the magneto-optical enhancement mechanisms in this magneto-plasmonic structure, achieving broadband near-perfect orthogonal linear polarization conversion modulated by the weak magnetic field in an experiment with an extinction ratio of 33 dB. Moreover, the magneto-optical modulation with an amplitude modulation depth of 95.8% can be achieved by this device under a weak magnetic field of 150 mT. InSb and its magneto-plasmonic device have broad potential for a THz isolator, magneto-optical modulator, and polarization convertor in THz application systems.

A terahertz modulator based on the insulator–metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (${\mathrm{VO}}_2$) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of ${\mathrm{VO}}_2$, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8–1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare ${\mathrm{VO}}_2$ film. Extensive investigation demonstrates that the thickness of ${\mathrm{VO}}_2$ film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

The state conversion and terahertz (THz) wave modulation based on a plasmonic device composed of silicon column arrays with vanadium dioxide (${\mathrm{VO}}_2$) coating were experimentally demonstrated. For double 45° tilted optical pumping, a state conversion from dielectric photonic crystal (PC) to metallic PC was demonstrated due to the insulator–metal transition (IMT) of ${\mathrm{VO}}_2$ with the pump power increasing. In this process, a broadband intensity modulation with 70% modulation depth was achieved. Furthermore, for normally incident optical pumping, another state conversion from dielectric PC to plasmonic device was also demonstrated due to the partial IMT of ${\mathrm{VO}}_2$, and the out of plane PC resonance gradually changed to be plasmonic resonances. This device and its modulation scheme will be of great significance for potential THz applications.

The combination of graphene and a metasurface holds great promise for dynamic manipulation of the electromagnetic wave from low terahertz to mid-infrared. The optical response of graphene is significantly enhanced by the highly-localized fields in the meta-atoms, and the characteristics of meta-atoms can in turn be modulated in a large dynamic range through electrical doping of graphene. Graphene metasurfaces are initially focused on intensity modulation as modulators and tunable absorbers. In this paper, we review the recent progress of graphene metasurfaces for active control of the phase and the polarization. The related applications involve, but are not limited to lenses with tunable intensity or focal length, dynamic beam scanning, wave plates with tunable frequency, switchable polarizers, and real-time generation of an arbitrary polarization state, all by tuning the gate voltage of graphene. The review is concluded with a discussion of the existing challenges and the personal perspective of future directions.