We theoretically and experimentally demonstrate a label-free terahertz biosensor with ultrahigh sensitivity and distinctive discretion. By constructing a metal-air-metal (MAM) metamaterial perfect absorber (MPA) with a metallic paired-ring resonator array, a hollow microfluidic channel, and a backed reflector, a novel dual-band absorptive sensing platform is proposed in the THz range. The near field coupling by dipole-induced trapped modes and the magnetic momentum caused a vertical to transverse power flux that dramatically enhanced the electromagnetic field on top of the metasurface and in the microfluidic channel, respectively. Both the resonant modes exhibit perfect absorption and produce ultrahigh normalized sensitivities of 0.47/RIU (refractive index unit, RIU) and 0.51/RIU at 0.76 THz and 1.28 THz, respectively. Compared with conventional microfluidic sensors, the salient advantages of our design are the perfect spatial overlap for light-matter interaction and polarization insensitivity. Characterized by THz time domain spectroscopic absorption quantification measurements with different concentrations of bovine serum albumin (BSA), the proposed sensor exhibits promising applications in microfluidic biosensing.

In this work, we experimentally demonstrate a bifunctional terahertz (THz) metasurface that can achieve broadband linear-to-circular polarization conversion (LCPC) and linear polarization conversion (LPC) in two different bands, respectively. The measured 3-dB axial ratio relative bandwidth is 34.2% for the LCPC in the lower band from 0.34 THz to 0.48 THz, and the measured polarization conversion ratio is over 80% for the LPC in the higher band from 0.53 THz to 0.85 THz. The physical mechanism of the LCPC and LPC is explained by the surface current distribution and is further discussed by employing the multiple interference model. This bifunctional THz metasurface polarization converter has promising applications in THz systems, communications, spectroscopy, etc., and the design strategy can also be extended to microwave and millimeter bands.

Boasting superior flexibility in beam manipulation and a simpler framework than traditional phased arrays, terahertz metasurface-based phased arrays show great promise for 5G-A/6G communication networks. Compared with the reflective reconfigurable intelligent surface (reflective RIS), the transmissive RIS (TRIS) offers more feasibility for transceiver multiplexing systems to meet the growing demand for high-performance beam tracking in terahertz communication and radar systems. However, the terahertz TRIS encounters greater challenges in phase shift, beam efficiency, and complex circuitry. Here, we propose a sub-terahertz TRIS based on the phase shift via Pancharatnam-Berry (PB) metasurface and self-on-off keying (OOK) modulation via Schottky diodes. The electrically reconfigurable unit cell consists of a column-wise phase resonator and a rectangular slot. An experimental retrieved equivalent lumped-element circuit model is implemented in joint field-circuit simulations and is validated by experiments. A fabricated prototype demonstrates excellent performance of TRIS with the minimum insertion loss of 2.8 dB for operational states, large bandwidth nearly covering the entire W-band for 1-bit phase shift, deep OOK amplitude modulation of 12 dB, and wide scanning range of ±60° with low specular transmission. We further implement an integrated platform combining high-speed beam steering and spatial-light modulation, verifying the point-to-point signal transmissions in different directions using the TRIS platform. The proposed TRIS with high-performance and cost-effective fabrication makes it a promising solution to terahertz minimalist communication systems, radar, and satellite communication systems.

High-speed logic modulation of terahertz (THz) waves is crucial for future communications, yet a technology gap persists. Here, we report a dual gate–controlled two-dimensional electronic gas logic modulation metasurface that enables symmetric and asymmetric electron distribution states through independent control of the two electron transport channels. The transition between these two states leads to various response modes in the metasurface and notably increases the diversity of the spectrum transformation, resulting in a multivalued relationship in which each output corresponds to more than one input signal, thus establishing the logical modulation. Our results demonstrate the common logical functions of AND, OR, XOR, XNOR, NOR, and NAND at different frequencies with a modulation speed faster than 250 picoseconds. This work offers a unique avenue for the high-speed, free-space logical operation of THz waves and increases the security of secure communication.

Due to the weak energy storage characteristics of the monolayer metamaterial, how to increase the sub-wavelength scale interaction between biological or chemical samples and terahertz wave on sufficient sensitivity premise becomes the key scientific problems in the study of terahertz sensing technology. In this paper, an ultrasensitive dual-band spectral terahertz sensor with polarization insensitive metamaterial perfect absorber (MPA) is demonstrated. A double-ring shaped metal microstructure integrated with microfluidic channel is applied in the polarization sensitive terahertz sensor. The numerical results show that the normalized sensitivity of refractive index unit 0.638/RIU with mode A and 0.582/RIU with mode B are obtained at 1.04THz and 0.506THz, respectively. The absorptive sensing mechanism is elucidated by the electric field and surface current distributions. Compared to other metamaterials (MMs) based terahertz sensors, the proposed MPA sensor with significantly confined field, enhanced sensitivity and polarization insensitivity is much more suitable for label-free terahertz probing of fluidic matter detection.

Polarization manipulation plays a pivotal role in integrated multifunctional devices. Various metasurface-based polarization converters successfully demonstrated high efficiency and broad bandwidth. However, new mechanisms to aggressively enhance performance are still in dire need. Herein, theoretical and experimental evidence corroborates an efficient ultra-wideband transmissive polarization converter based on a topological design method. The triple-layer meta-device consists of a layer of anisotropic zigzag-split resonator array amid two orthogonal wire-grating layers. By splitting double zigzag lines into compound 90°V-shaped resonators, the intra-unit cross-coupling extends to strong inter-unit cross-coupling, giving birth to the multi-resonances enhancement and significant bandwidth broadening. This polarization converter can efficiently convert linearly polarized incident waves into 90° cross-polarized transmitted waves, with a conversion efficiency above 80% over 3.98-22.71 GHz, reaching a fractional bandwidth of 140.4%. The proposed meta-device enables strong cross-coupling in mutual transition from intra-unit to inter-unit. From a physical viewpoint, the novel mechanism is elucidated by the surface current and electric field distributions upon constructive interferences stemming from V-shape-resonator combinations and the Fabry-Pérot-like cavity effect. With both spectrum and function extensions, the proposed polarization conversion strategy finds essential applications in polarization-related systematic scenarios, such as 6G communication, radar imaging, anti-interference, chiral sensing, etc.