Journal of Infrared, Millimeter, and Terahertz Waves

Chúng tôi trình bày các phép đo trực tiếp các meta-atom terahertz, đơn vị cơ bản của vật liệu siêu hạt, bằng cách sử dụng các sóng terahertz được tạo ra tại chỗ trong khu vực gần trường. Khác với phổ quang học hoặc hình ảnh terahertz trong khoảng cách xa thông thường, kỹ thuật của chúng tôi có đặc điểm là phát xạ cục bộ của các xung terahertz đồng bộ trên quy mô nhỏ hơn bước sóng, cho phép hình dung chi tiết chuyển động của từng meta-atom. Dữ liệu thu được cho thấy sự tương tác gần trường giữa các meta-atom và ảnh hưởng của phân bố điện trường từ meta-atom bị kích thích tới các meta-atom hàng xóm. Phản hồi cộng hưởng LC có thể quan sát được được tăng cường với sự gia tăng số lượng các meta-atom. Hơn nữa, phương pháp của chúng tôi cũng có tiềm năng để hình dung các mode riêng lẻ của meta-atom tại các điểm chiếu xạ terahertz khác nhau. Những dữ liệu này có thể giúp chúng tôi hiểu vai trò quan trọng của meta-atom trong vật liệu siêu hạt và phát triển các linh kiện và thiết bị terahertz mới như vật liệu siêu hạt terahertz chủ động và thiết bị cảm biến sinh học compact, nhạy bén.

Terahertz backward wave oscillators based on double corrugated waveguides are enabling devices for modern satellite communication systems. This research focuses on the design of a 0.34 THz double corrugated waveguide-based interaction structure using a sheet beam. This choice allows the use of shorter pillars along with a narrow gap between pillar rows. Shorter pillars are easier to manufacture and a narrow gap is required for better interaction impedance. Circular beams restrict the use of larger pillars and narrow gap between pillars. The performance of this interaction structure is compared with a folded waveguide. Under the same operating conditions involving a 20 kV beam voltage and a 30 mA beam current, the double corrugated waveguide interaction structure exhibits impressive performance in simulations, featuring an interaction impedance of 0.52 $${\varOmega }$$ at 0.34 THz, an output power of 3.2 W, and a bandwidth extending to approximately 20 GHz. In contrast, the folded waveguide, as per simulation results, registers values of 0.43 $${\varOmega }$$ , 2.6 W, and a 12 GHz bandwidth, respectively. The proposed double corrugated waveguide-based interaction structure is fabricated using modern CNC machining. Experimental validation reinforces the effectiveness of this design, with measurements indicating reflection below −20 dB and transmission exceeding −2 dB.

The present work proposes a novel design of a dual-band-printed antenna for operation at the millimeter-wave frequencies 28 and 38 GHz that are utilized for the modern and future generations of mobile communications. The antenna is composed of two radiating elements. The first element is the main patch that is fed through a microstrip line with inset feed, and the second element is a parasitic element that is fed through capacitive coupling with the main patch. The design parameters of the proposed antenna are optimized through a complete parametric study to give excellent impedance matching at 28 GHz over the band 27.7–28.3 GHz and at 38 GHz over the band 37.7–38.3 GHz. The surface current distributions at the two operational frequencies are investigated. The designed antenna is used to construct a four-port efficient multi–input–multi–output (MIMO) system. The MIMO system performance is investigated regarding the envelope correlation coefficient (ECC), diversity gain (DG), and the channel capacity loss (CCL) showing very good performance. The single-element antenna and the MIMO are fabricated and experimentally evaluated showing excellent impedance matching over the lower and higher frequency bands, which come in agreement with the simulation results. It is shown that the antenna produces maximum gain of 7.4 and 8.1 dBi at 28 and 38 GHz, respectively. The average radiation efficiencies of the proposed antenna are 88% and 88.8% over the lower and higher frequency bands, respectively. In addition, the coupling coefficients between the MIMO antenna systems are measured experimentally showing very low coupling values resulting in an efficient MIMO system that is suitable for future millimeter-wave (mm-wave) applications.

Ka-band compact dielectric waveguide antenna array for active imaging system is given. Antenna array with WR28 metal waveguide direct feeding is specially designed with small size, high gain, good radiation pattern, easy realization, low insertion loss and low mutual coupling. One practical antenna array for 3-D active imaging system is shown with theoretic analysis and experimental results. The mutual coupling of transmitting and receiving units is less than -30dB, the gain from 26.5GHz to 40GHz is (12-16) dB. The results in this paper provide guidelines for the designing of millimeter wave dielectric waveguide antenna array.

In this paper, millimeter-wave imaging of foreign object debris (FOD)-type objects on the ground is studied with the help of ground-based synthetic aperture radar (GB-SAR) technique. To test the feasibility of detecting runway FODs with this technique, some preliminary experiments are conducted within short antenna-to-target ranges of small imaging patches. An automated stripmap GB-SAR system with stepped-frequency transmission is constructed together with a quasi-monostatic data collection operation. The imaging experiments for various braces and screws are then carried out by using 32–36 GHz and 90–95 GHz frequency bands of the millimeter-wave. Images reconstructed by a matched-filter based algorithm are analyzed to determine the proper system parameters for an efficient imaging and to comprehend the factors against a successful detection. Results demonstrate the capability of GB-SAR imaging in accurately locating these FOD-like targets under near-range operating conditions.

The dual-frequency behavior of stacked high T c superconducting rectangular microstrip patches fabricated on a two-layered substrate is investigated using a full-wave spectral analysis in conjunction with the complex resistive boundary condition. Using a matrix representation of each layer, the dyadic Green’s functions of the problem are efficiently determined in the vector Fourier transform domain. The stationary phase method is used for computing the radiation electric field of the antenna. The proposed approach is validated by comparison of the computed results with previously published data. Variations of the lower and upper resonant frequencies, bandwidth and quality factor with the operating temperature are given. Results showing the effects of the bottom patch thickness as well as the top patch thickness on the dual-frequency behavior of the stacked configuration are also presented and discussed. Finally, for a better comprehension of the dual-frequency operation, a comparison between the characteristics of the lower and upper resonances is given.

Noise power spectral density (NPSD) in time domain terahertz (THz) wave detection systems utilizing GaAs-based photoconductive antennas (PCAs) was investigated quantitatively. The contributions of the PCA noise and the amplifier noises at the amplifier output depend strongly on the resistance of the PCA, the circuit parameters, and the frequency. The PCA has two types of noise: one can be modeled by the Johnson-Nyquist (thermal) noise for the PCA resistance, while the other has an NPSD inversely proportional to the frequency with its intensity dependent on the properties of the GaAs and the metallization. At a high frequency range ~ 100 kHz, voltage-type amplifier noise could appear if the cable capacitance between the PCA and the amplifier is large. As a result, a low-noise range tends to appear in the intermediate frequency range. In comparison with the PCAs with Ti/Au metallization, the PCAs with Pd/Ge/Ti/Au having lower contact resistance lead to lager influence of the Johnson-Nyquist noise at the output.

3D printing is evolving into a standard tool for prototyping various optical components suitable for application in the terahertz range of the electromagnetic spectrum. This work takes the next step in this evolution by demonstrating the fabrication and subsequent evaluation of Fabry–Pérot interferometers (FPIs). Large optical area (centimetre scale) Fabry–Pérot transmission filters have been 3D printed with polylactic acid (PLA) using a commonly used low-cost 3D printer. The advantages of the proposed approach include low cost, rapid prototyping and repeatability. Terahertz transmission measurements for two demonstrated filter designs realised to target optimisation of either signal transmission or spectral filter performance have been performed using terahertz time-domain spectroscopy (THz-TDS) and demonstrate good agreement with the simulated response in the operating spectral band of 0.30–0.75 THz (wavelengths from 1000 down to 400 μm). The critical spectral characteristics assessed were the filter peak transmission magnitude, central wavelength and full width at half-maximum (FWHM) of the transmission peak, as well as the free spectral range (FSR). The signal transmission levels were observed to reach beyond 90% for the first series of filters that targeted optimisation of this aspect; however, this was accompanied by diminished out-of-band rejection and broader transmission peaks in comparison to the second series of filters which targeted the overall performance. For the latter filter series, the resolution in terms of FWHM values of the transmission peaks was reduced to 40–50 GHz, with the out-of-band rejection approaching a ratio of 10:1. This level of spectral performance, along with the achieved signal peak transmission characteristics of 65–75%, provides adequate performance for many applications harnessing the terahertz spectral range.