Switchable broadband/narrowband absorber based on a hybrid metasurface of graphene and metal structures.

This paper proposes a switchable broadband/narrowband absorber based on a hybrid metasurface comprising graphene and metal in the millimeter-wave regime. The designed absorber achieves broadband absorption when the surface resistivity of graphene Rs = 450 Ω/◻ and narrowband absorption when Rs = 1300 Ω/◻ and 2000 Ω/◻. The physical mechanism behind the graphene absorber is explored by analyzing the distributions of power loss, electric field, and surface current densities. An equivalent circuit model (ECM) based on transmission-line theory is derived to theoretically investigate the performance of the absorber, with ECM results in good agreement with simulation results. Furthermore, we fabricate a prototype and evaluate its reflectivity by applying various biasing voltages. The results obtained from the experiment are also consistent with those obtained from the simulation. When the external bias voltage is changed from +1.4 V to -3.2 V, the proposed absorber has an average reflectivity ranging from -5 dB to -33 dB. The proposed absorber has potential applications in radar cross-section (RCS) reduction, antenna design, electromagnetic interference (EMI) shielding, and EM camouflage techniques.

Broadband electrically tunable linear polarization converter based on a graphene metasurface.

In this study, a broadband tunable reflective graphene-based linear polarization converter (GLPC) is proposed based on the graphene-ionic liquid-ITO structure (GIIS) integrated with a periodic double split ring resonator (DSRR) in the millimeter-wave regime. The tuning characteristic of the designed GLPC is analyzed using full-wave simulations and the equivalent circuit model method (ECM), which is based on multi-section transmission lines. There is a good agreement between ECM and simulation results. A comprehensive physical mechanism for the proposed broadband GLPC is then achieved by analyzing the surface current distributions. After manufacturing, the GLPC prototype's co- and cross-polarized reflection coefficients were measured using various bias voltages. The reflectivity can be controlled from -4.5 to -20 dB by changing the bias voltage in the range of +1.1 to -3.3 V. The designed GLPC can provide a tunable polarization conversion within the frequency range of 15.5∼35 GHz and shows a more than 75% conversion efficiency. The results of the simulation and the measurement are also in good agreement. The designed GLPC has potential applications in radar cross-section reduction, antenna design, and stealth technology by reconfiguring its polarized reflection characteristic dynamically.

Accurate Measurement of Human Vital Signs With Linear FMCW Radars Under Proximity Stationary Clutters.

Vital sign detection using linear frequency-modulated continuous-wave (LFMCW) radar may be subject to the proximity stationary clutters. This paper presents a novel technique to synthesize the slow-time I/Q signals, which are equivalent to those in a single tone quadrature CW radar, from a single-channel LFMCW radar. It correlates the two types of radars in such a way that the proximity stationary clutters are translated to direct current (DC) offsets in the synthesized I/Q signals across slow-time. The circle-fitting based DC offsets calibration (DCcal) technique, which was developed for CW radar, can now be applied to eliminate the impact of the proximity stationary clutters in LFMCW radars for accurate vital sign detection. Moreover, the modified differentiate and cross-multiply (MDACM) algorithm can also be leveraged to eliminate the phase ambiguity issue. Thorough theoretical analysis and working principles are presented. Simulations are performed to validate the proposed technique. Moreover, exhaustive experiments are carried out with a millimeter-wave 79 GHz FMCW radar in the office environment. Mechanical vibration and vital signs are extracted with micrometer-level accuracy in the existence of proximity stationary clutters.

Orbital angular momentum radiator multiplexing electromagnetic waves in free space.

Orbital angular momentum (OAM) modes of electromagnetic (EM) waves have been extensively studied to obtain more than two independent channels at a single frequency. Thus far, however, multiple radiators have been used to achieve this goal in wireless communications. For the first time, a single radiator was designed to simultaneously transmit three OAM waves in free space at the same frequency. Our design makes use of the radiating resonant modes of a dielectric resonator antenna (DRA). For demonstration, a wireless communication system consisting of a pair of transmitting and receiving OAM DRAs was setup and measured. Three EM waves carrying three different signals were transmitted and received successfully, increasing the system throughput without requiring any complex signal processing algorithms. It confirms that a single radiator can wirelessly transmit more than two independent EM waves at a single frequency by using multi-OAM modes. The work is useful for the future high-speed wireless communication systems.

Overlapped optics induced perfect coherent effects.

For traditional coherent effects, two separated identical point sources can be interfered with each other only when the optical path difference is integer number of wavelengths, leading to alternate dark and bright fringes for different optical path difference. For hundreds of years, such a perfect coherent condition seems insurmountable. However, in this paper, based on transformation optics, two separated in-phase identical point sources can induce perfect interference with each other without satisfying the traditional coherent condition. This shifting illusion media is realized by inductor-capacitor transmission line network. Theoretical analysis, numerical simulations and experimental results are performed to confirm such a kind of perfect coherent effect and it is found that the total radiation power of multiple elements system can be greatly enhanced. Our investigation may be applicable to National Ignition Facility (NIF), Inertial Confined Fusion (ICF) of China, LED lighting technology, terahertz communication, and so on.