A terahertz polarizer based on bilayer metal subwavelength-grating is designed to achieve high transmission and good extinction ratio. The polarizer is fabricated on the upper and lower surfaces of thin quartz substrate by micro-fabrication technique. Both experimental and simulation results show that the bilayer metal grating has similar transmittance, higher degree of polarization and extinction ratio than the single-layer metal grating. Measured transmittance is between 83.4% and 62.7% in the frequency range of 0.3~2.0 THz, and the degree of polarization and extinction ratio is more than 99.7% and more than 29 dB, respectively. In addition, the two designed bilayer metal gratings are successfully used in the terahertz time-domain spectroscopy (THz-TDS) system, and the degree of polarization of more than 96.2% and the extinction ratio of more than 17.1 dB are obtained. The transmittance, degree of polarization and extinction ratio can be improved by further tuning the parameters (such as the pitch, line width, and metal film thickness) of the metal subwavelength-grating.

In this paper, the packaging technique of terahertz monolithic integrated power amplifier is studied, i.e. waveguide-to-microstrip vertical transition and mode-resonance suppression techniques. Different from the conventional horizontal transition based on rectangular probes, a vertical transition based on a bifurcated probe is proposed, which is suitable for multilayer circuits. To verify the performance, a back-to-back transition is fabricated and measured. In the range of 170~260 GHz, the measured return loss is better than 16 dB, while the single insertion loss, including the loss caused by non-ideal metallic contact, is around 0.42 dB. To further reduce the transition loss, a resonance ring with a slot is proposed to avoid the electromagnetic leakage. As a result, the simulated insertion loss is reduced by half. Moreover, the issue of mode resonance in the amplifier cavity is studied, and electromagnetic band gap structures are set above the plane transmission line to suppress the excitation, transmission and resonance of higher modes. The above techniques are applied to a power amplifier which is operated at 210~230 GHz. In the measurement, the maximum small-signal gain of 20.75 dB and the single packaging loss of 0.8 dB are observed at 210 GHz. At 217 GHz, the maximum output power of higher than 15.6 dBm is achieved, which is consistent with the manual.

In this paper, a robust secure beamforming scheme based on imperfect channel state information (CSI) is proposed for the intelligent reflecting surface (IRS)-aided terahertz satellite communication systems, in order to improve the physical layer security performance of the system. First, we consider a scenario where an IRS-aided terahertz satellite system adopts multicast technique to serve multiple legitimate users in the presence of multiple eavesdroppers in the coverage area, based on which a joint optimization problem is formulated in order to minimize the satellite transmit power under the constraints constructed by the achievable rate (AR) and the achievable secrecy rate (ASR) requirements of each legitimate users. Secondly, considering that AR and ASR are non-convex probabilistic constraints caused by imperfect CSI, an approach associated with the second-order Taylor expansion and S-procedure is proposed to transform the non-convex constraints. Further, the joint optimization scheme of the phase shift of IRS and the transmit power of satellite can be solved by semidefinite programming. Finally, the effectiveness and superiority of the proposed scheme are verified by simulation results.

In this paper, a novel SSPP (Spoof Surface Plasmon Polariton) filter is proposed, fabricated and measured based on 0.18 μm CMOS technology. Meanwhile an extra terahertz SSPP filter is designed and its full-wave simulations are provided to prove feasibility in THz band. This novel SSPP filter has a passband of 11~12.3 GHz (S ₁₁<-10 dB, S ₂₁>-3.5 dB), and its size is compact with only 0.018 4 λ _g × 0.008 4 λ _g in electrical size, making it much smaller than other passive filters designed in IC technology. A designed terahertz frequency band SSPP filter, with a full EM simulated passband of 210.8~241.3 GHz (S ₁₁<-10 dB, S ₂₁>-4.7 dB), possesses an in-band insertion loss less than 2.7 dB and an superior out-of-band suppression. Both types of SSPP filters adopt a new energy transfer method of non-contact electromagnetic coupling, with a novel structural design. Moreover, the miniaturization advantage of the microwave section SSPP coupled filter is obvious, with an electrical size of only 0.019 λ _g × 0.009 λ _g, easy to integrate with chips. Equally scaled novel filters proposed in this work can work in microwave, millimeter wave and terahertz frequency band, which could be a reference for the research of on-chip filters in the future.

In this paper, a reconfigurable terahertz ultra-wideband absorber with umbrella structure based on graphene and vanadium dioxide is proposed, which greatly expands the absorption bandwidth of the terahertz absorber and has the advantages of functional reconfiguration and large modulation depth. The absorber consists of an umbrella shaped VO₂ patch, a polyethylene cycloolefin copolymer (Topas) dielectric layer, graphene layer, and a metallic reflector. When VO₂ is in the insulating state and the Fermi level of graphene is 0 eV, the absorber exhibits total reflection characteristics in the whole terahertz band. When VO₂ is in the metal state and the Fermi level of graphene is 0.75 eV, the absorber achieves ultra-wideband absorption in the frequency range of 3.57~10 THz, with an average absorption rate of more than 94% and an absorption bandwidth of up to 6.43 THz. By adjusting the conductivity of VO₂ and the Fermi level of graphene at the same time, the maximum modulation depth of 97.9% (at 4.3 THz) and 96.8% (at 8.25 THz) can be achieved at the two perfect absorption points, which has good switching characteristics. In addition, the absorber has polarization insensitivity and wide-angle absorption characteristics. In the range of 0~35°, the absorptivity of the absorber is more than 90% in the range of 3.57~10 THz bandwidth. The ultra-wideband reconfigurable terahertz absorber will have potential applications in tunable broadband absorber, stealth devices, thermal detection, terahertz switches and other fields.

Based on the theory of Jones matrix and the principle of Pancharatnam-Berry phase, a dual-band terahertz circularly-polarized (CP) absorbing and anomalous reflecting chiral metasurface is proposed in this paper. The CP chiral absorption and conversion function with anomalous reflection angles are realized independently in the two frequency bands. The metasurface unit consists of a combination of two chiral structures, which has an absorption rate of 96.3% for the incident left-hand circularly polarized (LHCP) waves, and realizes co-polarized reflection for the incident right-handed circularly polarized (RHCP) waves at a low frequency of 2.53 THz; meanwhile, the co-polarized reflection will be achieved for the LHCP waves at a high frequency of 3.43 THz, and the absorption rate of the RHCP waves is 90.9%. According to the principle of Pancharatnam-Berry phase, the full coverage of the 360° reflection phase can be realized by rotating the chiral metasurface unit. This chiral metasurface array can absorb the incident CP waves with specific handedness at two operating frequencies (low frequency 2.53 THz and high frequency 3.43 THz), and generate the handedness-preserving anomalous reflections with -26° and +19° angles for the corresponding orthogonal CP waves, respectively. This terahertz multi-function beam control device based on the chiral metasurface has numerous potential in electromagnetic energy harvesting, polarization converters, chiral sensing, radar and other fields.

In the microwave frequency band, switches applied to reconfigurable metasurface units often have good on-off performance, and can achieve 1-bit phase quantization characteristics with low loss and 180° phase difference. As the frequency increases to the millimeter wave or terahertz band, the switching performance will degrade due to parasitic effects, which makes the design of reconfigurable metasurface units difficult. In this work, the equivalent model of the reconfigurable metasurface unit is a microwave two-port network. By analyzing the impedance parameters and scattering parameters of the network, the transfer relationship between the unit reflection coefficient and the switching reflection coefficient is pointed out. The proposed relationship is verified by simulation of different switches and unit structures in different frequency bands. This work provides the upper limit of the performance of a 1-bit unit according to arbitrary switching parameters. The proposed design procedure can guide the design and optimization of terahertz reconfigurable metasurface element.

Metamaterials, having important theoretical research and application value in terahertz band, are artificial electromagnetic materials with special properties, which can regulate the frequency, amplitude, phase and polarization of electromagnetic waves. Due to the complexity of design process of metamaterials and the limitation of simulation time, the design of metamaterials is always involved in great challenges. In view of the fact that terahertz metamaterial devices have made certain achievements in biomedicine, broadband communication, security screening and other fields, this paper firstly describes the research progress and problems existing in the design process of terahertz metamaterial devices in traditional design methods, and sums up in detail the research results of encoded metamaterials, especially encoded hypersurfaces and programmable hypersurfaces. Additionally, the application of deep learning algorithms in THz (TeraHertz) metamaterial structure design is summarized. Finally, the challenges and expanding research directions of intelligent method in THz metamaterial structure design are discussed. This research not only provides a reference for people to fully grasp the simple, fast and intelligent design methods, but also puts forward some novel ideas for the development and application of intelligent design methods in terahertz metamaterials.

In this paper, the recent research progresses on terahertz metasurface lenses (THz ML) based on artificial microstructure are introduced. The interaction between artificial microstructure unit and THz wave is analyzed by numerical simulations. And the three main existing phase modulation mechanisms, including resonance phase, geometric phase and transmission phase, are described. From the spatially phase distribution of lens, the manipulation of incident THz wavefront can be realized by the suitable artificial microstructure layout, so as to realize the focusing and imaging of THz ML. Due to the advantages of flexible design, ultra-thin thickness and multi-functions, THz ML will have great potential applications in the fields of nondestructive detection, high-speed wireless communication and public security. Considering the key technical parameter of focusing efficiency, the research works of THz ML based on single-layer structure, multi-layer structure, all dielectric structure and tunable materials are introduced, and the trends of future development of THz ML are prospected.

The terahertz band lies between the microwave-millimeter wave band and the optical band, and its related technologies are of great significance for the development of next-generation high-speed communication, high-resolution imaging, intelligent sensor-communication integration systems and other information fields. The development of terahertz application technology requires terahertz beam manipulation and information loading. As a new realization scheme and device, active metasurface is one of the important ways to realize these functions. In this paper, the terahertz dynamic metasurface is classified according to the regulation function of the metasurface, the amplitude regulation, phase regulation, polarization regulation, beam regulation and high-order nonlinear regulation of the terahertz dynamic metasurface are introduced from the complementary materials technology, structure setup, and working mechanisms and their performance in the application demonstrations. The similarity and uniqueness of different types of dynamic metasurface are introduced, and some of the performances are compared. At the end of this paper, the development trend of the terahertz dynamic metasurface is forecasted. It is hoped that this paper helps more scholars get knowledge of terahertz dynamic metasurface hence promoting the development of this field.

Reconfigurable intelligent surface (RIS) is one of the potential key technologies for sixth generation (6G) communications, which has the characteristics of low cost, low complexity, and easy deployment. By applying control signals to adjustable elements on the electromagnetic unit, it has the ability to adjusting the wireless communication environments, which provides a new opportunity to improve the high energy-efficiency performance of wireless communication systems. This paper provides a comprehensive overview of channel modeling and characteristics analysis for RIS-assisted unmanned aerial vehicle (UAV) high energy-efficiency communications. Firstly, based on the research basis of the UAV communications technologies, we clarify the necessities of introducing the RIS into UAV communications. Then, we summarize the key technologies for channel modeling and characteristics analysis for RIS-assisted UAV communications. Finally, we point out some future research directions in RIS-assisted UAV channel modeling and characteristics analysis.