In this paper, we designed a novel tunable terahertz (THz) absorber that has unique properties of graphene integrated within a dual-layer metasurface structure. The proposed absorber demonstrates excellent performance by achieving eight distinct absorption peaks from 3.9 THz to 9.73 THz with an average absorption efficiency of 99.3%. This is achieved through the tunable surface conductivity of graphene which enables dynamic modulation of resonant frequencies via chemical potential adjustment. The design includes a gold base layer for total reflection, gallium arsenide (GaAs) dielectric spacers for optimum impedance matching and graphene patches to introduce multi-band absorption modes. Simulation results show the absorber's tunability, polarization insensitivity and angular stability which makes it highly adaptable for applications in medical diagnostics, material characterization, security screening and terahertz sensing. The proposed absorber's innovative architecture and simple design offers a versatile solution for the evolving demands of modern terahertz technologies.

Fano resonance presents an asymmetric line shape formed by an interference of a continuum coupled with a discrete autoionized state. In this paper, we show several simple circuits for Fano resonances from the stable-input impedance mechanism, where the elements consisting of inductors and capacitors are formulated for various resonant modes and the resistor represents the damping of the oscillators. By tuning the pole-zero of the input impedance, a simple circuit with only three passive components e.g. two inductors and one capacitor, can exhibit asymmetric resonance with arbitrary Q-factors flexiblely. Meanwhile, four passive components can exhibit various resonances including the Lorentz-like and reversely electromagnetically induced transparency (EIT) formations. Our work not only provides an intuitive understanding of Fano resonances, but also pave the way to realize Fano resonaces using simple circuit elements.

Polarization converters are essential components in a wide range of terahertz applications including imaging, communication and sensing. However, existing polarization converters face significant challenges, such as limited operational bandwidth, low polarization conversion efficiency and the use of expensive or complex materials which hinder their practical implementation. To address these issues, this work presents a dual-functional terahertz metasurface that is capable of achieving ultra-wideband linear cross-polarization conversion along with high sensitivity biosensing. The design is constructed using aluminum and polyimide with a normalized unit cell size of 0.26λ_L × 0.26λ_L × 0.09λ_L (λ_L = wavelength at the lowest operating frequency). It achieves a polarization conversion ratio (PCR) above 90% across 4.442–8.444 THz, covering a bandwidth of 4.002 THz, an FWHM of 5.094 THz and an average PCR of 98.62%. It maintains a PCR above 98% over 3.66 THz and above 99% in two sub-bands totaling 2.598 THz. Moreover, the design sustains an average polarization conversion efficiency of 97.96% at a 20° oblique incidence. As a biosensor, it provides a maximum sensitivity of 1.50 THz per RIU, which enables precise detection of refractive index variations associated with various diseases including cervical and skin cancers, blood disorders and infections such as malaria. The design outperforms existing metasurfaces in terms of polarization conversion efficiency, operational bandwidth and biosensing sensitivity, filling a significant gap in the field of terahertz applications. It provides a cost-effective and fabrication-friendly solution that surpasses all existing designs and provides a reliable platform for multi-disease detection.

Exceptional points, as degenerate points of non-Hermitian parity-time symmetric systems, have many unique physical properties. Due to its flexible control of electromagnetic waves, a metasurface is frequently used in the field of nanophotonics. In this work, we developed a parity-time symmetric metasurface and implemented the 2π topological phase surrounding an exceptional point. Compared with Pancharatnam-Berry phase, the topological phase around an exceptional point can achieve independent regulation of several circular polarization beams. We combined the Pancharatnam-Berry phase with the exceptional topological phase and proposed a composite coding metasurface to achieve reflection decoupling of different circular polarizations. This work provides a design idea for polarimetric coding metasurfaces in the future.