A tunable dual-band terahertz sensor based on graphene is proposed. The sensor consists of a metal bottom layer, a middle dielectric layer, and single-layer graphene patterned with four strips on the top. The numerical simulations results show that the proposed sensor exhibits two significant absorption peaks at 2.58 THz and 6.07 THz. The corresponding absorption rates are as high as nearly 100% and 98%, respectively. The corresponding quality factor (Q) value is 11.8 at 2.58 THz and 29.6 at 6.07 THz. By adjusting the external electric field or chemical doping of graphene, the positions of the dual-frequency resonance peak can be dynamically tuned. The excitation of plasma resonance in graphene can illustrate the mechanism of the sensor. To verify the practical application of the device, the terahertz response of different kinds and different thicknesses of the analyte is investigated and analyzed. A phenomenon of obvious frequency shifts of the two resonance peaks can be observed. Therefore, the proposed sensor has great potential applications in terahertz fields, such as material characterization, medical diagnosis, and environmental monitoring.

A polarization-insensitive broadband terahertz absorber with a sandwich structure of metal–dielectric-graphene is designed and simulated. The graphene is patterned as an array of graphene square blocks with circular apertures. The results of the simulations and theoretical analysis show that the absorption exceeds $99\%$ from $0.93\mathrm{to}1.65\mathrm{THz}$ while $90\%$ from $0.80\mathrm{to}1.87\mathrm{THz}$, and a broad relative bandwidth of 80.2% is achieved. The absorption performance is passively enhanced by altering physical dimensions of the graphene pattern and actively adjusted by changing the chemical potential of graphene. When the chemical potential increases from $0.1 \mathrm{eV}$ to $0.7 \mathrm{eV}$, the corresponding terahertz absorption increases from $59.1\%$ to $99\%$. The mechanism of absorption is disclosed by analyzing the impedance matching theory and distribution of electric-field intensity. In addition, different polarization modes and incident angles are also studied. The proposed absorber has the superiorities of broad relative bandwidth, high absorption rate, polarization insensitivity, and a wide incident angle, which offers some potential applications in the field of terahertz technology such as imaging, detection, and cloaking.

A promising solution has been proposed to overcome blackout using the magnetic field component of low-frequency electromagnetic waves to transmit a small amount of critical data. The principle of the method is that the magnetic-field attenuation is less than the electric-field attenuation for low-frequency electromagnetic waves in electrically small plasma. This paper presents an experiment based on a shock tube to verify the theory. Analytical expressions of S_E and S_H suitable for a shock-tube cylindrical model are derived using an equivalent circuit, which is used to describe the different forms of attenuation of electric and magnetic fields. For comparison, numerical simulations are conducted with varying plasma parameters. In the experiment, low-frequency electromagnetic waves penetrate the plasma produced by the shock tube from the test section, and measurements are made by field-strength probes at the center of the plasma flow field. A difference between S_H and S_E is found in both the experiment and theoretical simulation, with |S_H| being less than |S_E| by 20–30 dB when the frequency of low-frequency electromagnetic waves is 10 MHz. The verification of this theory thus shows that the magnetic field component of the low-frequency electromagnetic waves may potentially be used to transmit a small amount of critical data or vocal communication, allowing continuous communication during a period of radio blackout.

Exhaust-plume-induced plasma from solid rocket engines absorbs electromagnetic (EM) waves and may cause communication interruptions. Previous papers have proposed both physical phenomena and propagation models for this problem, but few papers have analyzed random fluctuations of EM waves propagating through plumes from a communications perspective. This paper attempts to attach these fluctuations, which are caused by the plume-induced turbulent plasma spread medium, to the communications channel characteristics. A multi-parameter stochastic model is established to characterize the exhaust plume channel in which the first-order and the second-order statistical properties are presented. Accordingly, a ground firing test was conducted to validate the EM wave propagation characteristics and the channel characteristics. The experimental results are in good consistence with the theoretical results. The results indicated that the time-varying envelope of the transmitted EM waves obeys a log-normal distribution, whereas the time-varying phase shift obeys a normal distribution. Moreover, the time-frequency analysis method was applied to evaluate the EM-plume interactions in the time-frequency domain. These results are critical for designing the rocket communication systems and evaluating the systems performance.

We propose a method of applying a static magnetic field to reduce the attenuation of the magnetic field component (S _H) of low-frequency electromagnetic (LF EM) waves in dense plasma. The principle of this method is to apply a static magnetic field to limit electron movement, thereby increasing the equivalent resistance and thus reducing the induced current and S _H. We consider the static magnetic field acting on the plasma of the entire induced current loop rather than on the local plasma, where the induced current is excited by the magnetic field component of LF EM waves. Analytical expressions of S _H suitable for magnetized cylindrical enveloping plasma are derived by adopting an equivalent circuit approach, by which S _H is calculated with respect to various plasma parameter settings. The results show that S _H can be reduced under a static magnetic field and the maximum magnetic field strength that mitigates blackout is less than 0.1 T. Experiments in which LF EM waves propagate in a shock-tube-generated magnetized cylindrical enveloping plasma are also conducted. S _H measured under the magnetic field (the magnetic field strength B ₀ acting on the magnetic field probe was about 0.06 T) reduces at f = 10 MHz and f = 30 MHz when n _e ≈ 1.9 × 10¹³ cm⁻³, which is consistent with theoretical results. The verification of the theory thus suggests that applying a static magnetic field with a weak magnetic field has the potential to improve the transmission capacity of LF EM waves in dense plasma.