Terahertz (THz) radiation has received much attention during the past few decades for its potential applications in various fields, such as spectroscopy, imaging, and wireless communications. To use terahertz waves for data transmission in different application systems, the efficient and rapid modulation of terahertz waves is required and has become an in-depth research topic. Since the turn of the century, research on metasurfaces has rapidly developed, and the scope of novel functions and operating frequency ranges has been substantially expanded, especially in the terahertz range. The combination of metasurfaces and semiconductors has facilitated both new opportunities for the development of dynamic THz functional devices and significant achievements in THz modulators. This paper provides an overview of THz modulators based on different kinds of dynamic tunable metasurfaces combined with semiconductors, two-dimensional electron gas heterostructures, superconductors, phase-transition materials, graphene, and other 2D material. Based on the overview, a brief discussion with perspectives will be presented. We hope that this review will help more researchers learn about the recent developments and challenges of THz modulators and contribute to this field.

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.

A low radar cross section (RCS) antenna designed with band-notched absorber is described. Firstly, a dual-polarization absorber with relative bandwidth more than 80% is designed using loaded-resistors. Pairs of circular slot resonators and metal strip array resonators are introduced in the absorber to realize a notch band with full reflectance in the vertical polarization while a wide absorption band in the horizontal polarization is maintained. The proposed band- notched absorber is thus realized. Within the notch band, the absorber can be served as a metal ground for antenna; while a great RCS reduction is obtained out of the notch band and in the horizontal polarization band. Then, a dipole antenna rigorously designed is mounted above the band-notched absorber, whose operating frequency is exactly in accordance with that of the notch band. The proposed low RCS antenna is established based on assembling the dipole antenna and the band-notched absorber together. The measured results demonstrate that the proposed antenna has fairly good radiation patterns. Added to that, more than 10dB RCS reduction in two polarizations is realized simultaneously compared with that one of a conventional dipole antenna.

A two-port structure-shared planar ultra-wideband antenna with high isolation is presented in this paper. The proposed antenna consists of a planar monopole and two back-to-back tapered slots etched on the monopole. The monopole and the tapered slots are fed by microstrip line and differential feeding network, respectively. The differential feeding network of the tapered slots excites modes which are orthogonal to those of the monopole. Therefore, high isolation between the two ports can be achieved without additional decoupling structures. The measured results show that the proposed antenna can operate from 3 to 10 GHz with reflection coefficient of the two ports lower than -10 dB and isolation higher than 38 dB. The measured realized peak gains are above 2.6 dB and the measured envelope correlation coefficients are less than 9×10-5. The proposed UWB MIMO antenna can be applied in the MIMO communication systems, especially those requiring high isolation.

This paper presents a low-profile coplanar du-al-polarized dual-band base station antenna array. A slotted patch antenna operating from 0.82 to 0.99 GHz with four shorting strips is proposed and analyzed. The four shorting strips are employed not only to improve the impedance matching in the lower band, but also to remove the center part of the patch, thus making room for higher band dipoles operating from 1.68 to 2.86 GHz. The height of the proposed array is nearly half of the conventional base station array while other performances are guaranteed. Measured results show that the proposed antenna array has wide impedance bandwidths of 18.7% (0.82 ~ 0.99 GHz) and 57.9% (1.68 ~ 2.86 GHz) (VSWR 24.6 dB) and high isolations (> 30 dB and > 25 dB in lower and upper bands, respectively) are achieved. The proposed antenna can be easily built with printed circuit board fabrication technique and applied in low profile multiband base stations for 2G/3G/4G and the next generation systems.

A low-profile planar Yagi-Uda antenna with exact endfire radiation and vertical polarization is presented in this paper. The concept is the combination of an aperture element and a novel vertical I-shaped resonator (VISR). The aperture acts as the driven antenna, which can be equivalent of a virtual magnetic dipole. The VISR array constructed with VISR unit cell is employed as a director or a reflector element depending on the resonant frequency of the unit cell, which consists of two identical rectangular metal conductors printed on both sides of a substrate and their electrical connection using via holes. Two prototype antennas are then designed, fabricated, and measured to test the validity of the operating principle. Antenna I with one director and one reflector achieves an endfire realized gain of 9.9-10.3 dBi and a front-to-back ratio (FBR) of more than 17.4 dB across 6.36-6.70 GHz. With four directors and one reflector, an endfire realized gain of 12.9-13.6 dBi and an FBR of more than 17.9 dB are accomplished by antenna II across 6.38-6.71 GHz. Moreover, a sensitivity analysis is implemented for the proposed antennas. Simulated results show that the standard PCB process is a reliable method for antenna fabrication.