Trapping waves with tunable prism-coupling terahertz metasurfaces absorber

Passive trapping of biomolecules in hotspots with all-dielectric terahertz metamaterials

Electromagnetic metamaterials feature the capability of squeezing photons into hotspot regions of high intensity near-field enhancement for strong light-matter interaction, underpinning the next generation of emerging biosensors. However, randomly dispersed biomolecules around the hotspots lead to weak interactions. Here, we demonstrate an all-silicon dielectric terahertz metamaterial sensor design capable of passively trapping biomoleculars into the resonant cavities confined with powerful electric field. Specifically, multiple controllable high-quality factor resonances driven by bound states in the continuum (BIC) are realized by employing longitudinal symmetry breaking. The dielectric metamaterial sensor with nearly 15.2 experimental figure-of-merit enabling qualitative and quantitative identification of different amino acids by delivering biomolecules to the hotspots for strong light-matter interactions. It is envisioned that the presented strategy will enlighten high-performance meta-sensors design from microwaves to visible frequencies, and serve as a potential platform for microfluidic sensing, biomolecular capture, and sorting devices.

THz Broadband Absorber Based on MoS2 with Split Rings and Archimedean Spiral Structures

The MoS2 surface plasmon resonance structure is proposed as a THz absorber in this work. The absorber adopts a double layer structure of Archimedean spirals stacked with split rings. In 1.2–3.0 THz, the absorption is greater than 92%, and the relative absorption bandwidth reached the value of 85.7%. Due to the circular-like symmetry of the unit, the polarization of the absorber is less sensitive to the incident angle within a certain range. When the incident angle is within 60°, the absorption in the bandwidth is still greater than 85%. The design efficiency is also significantly improved by the combined method of the equivalent circuit and finite difference time domain. Our work provides new directions for the design of terahertz devices, which is of great importance for various fields including terahertz imaging, detection and sensing, and especially in 6G communication systems.

Terahertz hybrid optical-plasmonic modes: tunable resonant frequency, narrow linewidth, and strong local field enhancement

Hybrid optical-plasmonic modes have the characteristics of low loss and small mode volume, which will result in the strong localization and enhancement of electromagnetic field. Such advantages of hybrid optical-plasmonic mode are important for the enhancement of light-matter interactions. Here, terahertz (THz) hybrid modes of Fabry-Perot resonances (FPRs) and spoof surface plasmon polaritons (SSPPs) in the modified Otto scheme are investigated both in theoretical and experimental aspects. The device structure is composed of a metal grating silicon waveguide (MGSW) and a metal slit grating (MSG). The two components are vertically stacked with a variable air gap between them. The THz hybrid modes are originated from the far-field coupling of the FPRs and the SSPP supported by the air gap and the MSG, respectively. By changing the thickness of the air gap, the resonant frequency of the FPR-SSPP modes can be tuned in a frequency range of about 0.1 THz. An anti-crossing behavior between two reflection dips corresponding to the guided-mode resonance in the MGSW and the FPR-SSPP mode is observed, which leads to the narrowing of the reflection dips in the anti-crossing region. Numerical simulations show that at the resonant frequencies of FPR-SSPP mode, there is a huge volume-averaged electromagnetic energy enhancement of about 1600 times in the grooves of the MSG, which is around 8.7 times larger than that induced by the SSPP directly launched by free-space electromagnetic field. The hybrid FPR-SSPP modes can be used to construct THz sensors and detectors with high sensitivity.

Terahertz smart dynamic and active functional electromagnetic metasurfaces and their applications

The demand for smart and multi-functional applications in the terahertz (THz) frequency band, such as for communication, imaging, spectroscopy, sensing and THz integrated circuits, motivates the development of novel active, controllable and informational devices for manipulating and controlling THz waves. Metasurfaces are planar artificial structures composed of thousands of unit cells or metallic structures, whose size is either comparable to or smaller than the wavelength of the illuminated wave, which can efficiently interact with the THz wave and exhibit additional degrees of freedom to modulate the THz wave. In the past decades, active metasurfaces have been developed by combining diode arrays, two-dimensional active materials, two-dimensional electron gases, phase transition material films and other such elements, which can overcome the limitations of conventional bulk materials and structures for applications in compact THz multi-functional antennas, diffractive devices, high-speed data transmission and high-resolution imaging. In this paper, we provide a brief overview of the development of dynamic and active functional electromagnetic metasurfaces and their applications in the THz band in recent years. Different kinds of active metasurfaces are cited and introduced. We believe that, in the future, active metasurfaces will be combined with digitalization and coding to yield more intelligent metasurfaces, which can be used to realize smart THz wave beam scanning, automatic target recognition imaging, self-adaptive directional high-speed data transmission network, biological intelligent detection and other such applications. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.

HR-Si prism coupled tightly confined spoof surface plasmon polaritons mode for terahertz sensing

We report a high-resistivity silicon (HR-Si) prism coupled terahertz (THz) spoof surface plasmon polaritons (SSPPs) on flat subwavelength metasurface. Using a high refractive index prism as an external coupler, a more tightly confined SSPPs mode can be excited in a smaller resonant cavity, leading to strong light-matter interaction. Besides, theoretical analysis and experimental results have both indicated that the SSPPs resonance response to the filling patterns of analyte in the resonant cavity are quite different. In particular, we have found that the interaction between analyte and SSPPs wave can be maximized when the analyte filled with the whole resonant cavity and a higher sensitivity for THz sensing can be obtained. A high sensitivity varied from 0.31 THz/RIU to 0.85 THz/RIU is predicted. Furthermore, these SSPPs modes exhibit high Q-factor, and characteristic spectra of water caused by surface plasmon resonance (SPR) are observed, which is significant in promoting the THz-SPR sensing of polar liquids or aqueous analytes with THz metasurfaces.