Sensors and Communication Technologies in the 1 GHz to 10 THz Band

In this paper, we propose a method to simultaneously improve spatial resolution in a low-cost configuration and achieve high-precision imaging for higher moving speed targets. Simulation results showed that the proposed method outperformed conventional methods in direction finding evaluations for a faster moving target.

This paper proposes a novel approach to 3-D microwave imaging using dynamic metasurface antennas in a multistatic configuration. By introducing a panel-to-panel model and a preprocessing technique, raw measurements are converted into the space-frequency domain for efficient data acquisition and reconstruction. The adaptation of the range migration algorithm enables fast Fourier-based image reconstructions. Simulation results showcase the effectiveness of the proposed method, highlighting its potential for real-world applications.

This paper introduces an approach for 3-D near-field microwave imaging, combining a special 2-D multiple-input multiple-output (MIMO) structure with orthogonal coding and Fourier domain processing. The proposed MIMO coded generalized reduced dimension Fourier algorithm effectively reduces data dimensionality while preserving valuable information, streamlining image reconstruction. Through mathematical derivations, we show how the proposed approach includes phase and amplitude compensators and reduces the computational complexity. The algorithm includes both phase and amplitude compensators, reduces the computational complexity, and mitigates propagation loss effects. Numerical simulations confirm the approach’s satisfactory performance in terms of information retrieval and processing speed.

In this talk, I will give an overview of the unique applications of terahertz waves for communication, chemical identification, material characterization, biomedical sensing and diagnostics and describe the state of the existing terahertz imaging and sensing technologies and their limitations. I will introduce a game changing technology that enables high performance, low cost, and compact terahertz spectroscopy and imaging systems for various applications. More specifically, I will introduce plasmonic terahertz imaging and spectroscopy systems, which offer several orders of magnitude higher signal-to-noise ratio levels compared to the state of the art.

The detection of buried objects with GPR poses a significant challenge in many sectors, including utilities, military operations and humanitarian efforts. It is a difficult task partly due to the presence of clutter and the strong signal attenuation presented by many soil types. This paper seeks to improve the detection of buried objects using the combination of Synthetic Aperture Radar (SAR) and Polarimetry (PolSAR). In this study a Stepped Frequency Continuous Wave (SFCW) air-coupled radar is used to acquire polarimetric measurements of buried metallic and dielectric objects between the frequency range of 1 - 6.5 GHz. A 3D Synthetic Aperture Radar (SAR) algorithm is developed and following a polarimetric calibration procedure the SAR algorithm is used to create sub-surface images of each polarization channel. Using polarimetric decompositions, the dominant scattering mechanisms are identified and used to synthesize polarization signatures of the buried objects. Analysis is conducted to determine the optimal polarization state for sub-surface detection, enhancing target identification and discrimination capabilities.

This work covers the development of a low-cost volumetric moisture content sensor based on commercial off the shelf hardware, a TI IWR1642 76 GHz automotive radar module, coupled with a sample specific effective medium model. The resulting sensor is orders of magnitude less expensive than competing sensors while providing measurements of volumetric moisture content accurate to within 5% dry-basis moisture content.

A novel cGAN is leveraged to achieve image restoration, where both the condition and the input are the CI-based microwave back-scattered measurements. The proposed cGAN consists of two parts. One is defined as the generator, which is leveraged to achieve image restoration. The other one is known as the discriminator, which serves the purpose of optimizing the generator by judging the similarity of the estimation and the ground truth. The training samples and testing samples are randomly selected from the open-source dataset MNIST. During the optimization process, the generator is adversarial with the discriminator. The optimized generator can retrieve high-fidelity image reconstructions directly from the CI backscattered measurements, eliminating the need for the computationally expensive image reconstruction step required by conventional imaging techniques. This contributes to the successful reconstruction of the scene images by deep learning methods. The performance of the proposed approach and its efficacy are confirmed by numerical simulations.

Recent developments in the design of the epitaxial structure of the asymmetrical spacer layer tunnel (ASPAT) diode have seen the inclusion of a quantum well, leading to a substantially improved curvature coefficient due to a reduction in the leakage current, introducing yet further advantages over the standard ASPAT diode which has temperature independence and zero bias operations as well as a high dynamic range. In this work these diodes have been developed into a fully integrated miniature rectenna solution, integrating a meandered loop antenna, rectifier, and RC filter into a 2x0.8 mm² die. Targeting an operating frequency of 26 GHz, a solution such as this could see applications in wireless power transfer, energy harvesting, or signal detection.

A novel deep learning technique is leveraged to achieve a sensing matrix estimation, where inputs are two unique aperture distribution fields from the transmitter and the receiver of a CI-based system, respectively. The proposed network utilizes the residual skip connection to ensure the delivery of information. By learning the features of the aperture distribution fields, the sensing matrix is accurately predicted. This contributes to the successful reconstruction of the sensing matrix by using deep learning methods. The performance of the proposed approach and its efficacy are confirmed by numerical simulations.

The recently unveiled in-plane photoelectric effect is a quantum mechanism that opens the doors to a new type of photonic terahertz detectors which utilize quantum transitions within a well-conducting, degenerate 2D electron system (2DES). In this effect, electrons gain energy quanta through the absorption of THz photons and jump on an artificially created, gate-voltage-tunable potential step within the plane of a 2DES. This leads to an electron flow from the high to the low density region of a much higher magnitude than expected from previously known, classical mechanisms. Detectors exploiting this effect were called photoelectric tunable step (PETS) THz detectors. We give an overview of the in-plane photoelectric effect, highlight current developments in the area of PETS THz detectors, and discuss the prerequisites necessary to make photonic terahertz detectors practically useful in terahertz technology to realise adaptive and scalable device architectures for next-generation THz detectors and focal plane arrays.

A GaAs/AlGaAs-based photoelectric tunable-step (PETS) terahertz (THz) detector with a symmetric dipole antenna is demonstrated in this work as a model system to carry out a systematic study of the in-plane photoelectric effect. We derive the optimal values for the antenna gap, depth of the 2DEG, and other geometrical parameters from numerical simulations, and fabricate a detector with optimized dimensions. It shows a high responsivity (~2.5 kV/W) to 1.9-THz radiation with a short response time. The temperature dependence of the photoresponse of the PETS detector shows a capability of operating up to 75 K. The results of our work deepen the understanding of the in-plane photoelectric effect and provide a universal reference for the design of future high responsivity, fast PETS THz detectors operating at high temperatures.

This study introduces new method for cost-effective millimetr wave (MMW) imaging utilizing glow discharge detectors (GDDs) and up-conversion processes. Up-conversion method converts directly MMW radiation to Visual light. The proposed MMW system integrates GDDs with charge-coupled device (CCD) cameras, offering an inexpensive alternative for focal plane arrays (FPAs) compared to existing options. Key to this approach is the up-conversion detection process, wherein incident MMW radiation increases the intensity of the visible light emitted by GDDs, enabling optical detection. FPAs constructed using GDDs exhibit responsiveness to MMW radiations, particularly in the near-infrared (NIR) zone of the electromagnetic spectrum. This new method promises significant advancements in cost-effective and rapid MMW imaging applications.

Traditional THz spectroscopic methods are expensive, time-consuming and expert-operated. In this work, we present a new method for ultrafast selective multispectral terahertz (THz) spectroscopy. The scheme uses a combination of broad THz pulses detected with a Schottky energy sensor coupled to eight frequency-selective surfaces (FSS) inserted on a mechanical chopper and positioned just before the detector. The discrete spectra of the samples can be obtained instantaneously. Samples are identified by measuring their transmittance by combining their unique spectral signatures normalized to each filter transmission. This technique has also proven efficient in identifying samples which lack distinctive spectral features in the THz region, such as paper. A method employing k-fold cross-validation on a neural network model for multi-class classification, incorporating a tailored evaluation metric, is used to improve our system performance. This simple scheme holds promise for advancing THz detection solutions in industrial applications.

Terahertz radiation, which lies between the microwave and infrared regions of the electromagnetic spectrum, is being explored as a possible solution to meet the ever-growing demand for high data transfer rates. However, at these frequencies, strong absorption peaks due to water vapour in the air impose strict limitations on wireless communication. Here, we use a detector relying on a nonlinear optical upconversion technique to characterize spectral transmission of specific bands between 0.5 THz and 3 THz under normal atmospheric conditions. We classify these bands into two categories aiming at different applications: short-range secured communication and long-range high data transfer rates.

In this communication, we consider principles of design and assembling of nonparaxial THz imaging systems based on silicon diffractive optics components. The investigation is dedicated to lensless photonic setups comprising high-resistivity silicon-based DOEs such as Fresnel zone plates, Fibonacci lenses, Bessel axicons, and Airy zone plates, all fabricated from a high-resistance 500 μm thick silicon substrate by femtosecond laser ablation. The exploration underlines the significance of structuring both the illumination and light-collection schemes as well as assembly principles of silicon diffractive optical elements in compact THz imaging.

The paper presents a metalens for a 60 GHz antenna-in-package-patch antenna using a compact dielectric, inverse-designed topology with incorporated metallic boundaries for directivity enhancement. The proposed methodology utilizes the gradient-based optimization algorithm to find the optimized topology. We employ the solid isotropic material penalization (SIMP) and the filter-and-threshold method to achieve a manufacturable design. For a robust design, we included fabrication constraints resulting from the additive manufacturing of the metalens.

Next-generation communication systems necessitate rapid and efficient control of terahertz (THz) signals to encode data streams. Graphene-based metamaterials have emerged as promising candidates for effective THz modulation owing to graphene’s large electrically controllable conductivity. However, a significant challenge arises from graphene’s inability to achieve full depletion at the Dirac point, limiting the transmission modulation depth in most LC-resonant metasurface modulators. To overcome this limitation, we exploit the interference of Fresnel reflection components from the metasurface and the substrate and propose tuneable capacitors as active elements. Our study presents single-layer, all-solid-state, graphene-metal metasurface modulators operating in the THz range, characterized using terahertz time-domain spectroscopy. Our approach enables us to achieve intensity modulation of more than four orders of magnitude. These findings underscore the potential of graphene-based metamaterials in advancing THz communication technologies.

This work presents a resonant metamaterial for millimeter-waves that enables telemetric position sensing. The concept is based on a resonant unit cell that can be tuned to enable position encoding. The resonance frequency shift encodes the absolute position via the geometry parameter of the metamaterial. Two types of arrangements were tested: a linear position encoder and a rotational disk for angular position determination. This telemetric position sensing sensor concept offers a compact and contactless readout without mechanical interference with the moving object. The metamaterial is completely passive, resulting in low maintenance and failure issues.

Next-generation communication systems necessitate rapid and efficient readout of terahertz (THz) signals to decode data streams. The in-plane photoelectric (IPPE) effect is a quantum, photonic THz detection mechanism recognized for its remarkable sensitivity and fast detection capabilities. It was initially demonstrated in a dual-gated FET-type device based on a two-dimensional electron gas (2DEG), termed a photoelectric tunable-step (PETS) detector. In this study, we experimentally demonstrate the coupling of the IPPE detection mechanism with a metamaterial antenna array, which results in a substantial performance enhancement for THz detectors. Operating at 1.9 THz using a THz quantum cascade laser setup, our detector exhibits a photocurrent that significantly surpasses the previously recorded maximum for PETS detectors under identical experimental conditions, while concurrently achieving significantly lower output impedance compared to any previously reported detector utilizing the IPPE mechanism. This highly efficient metasurface-based detector with low output impedance holds the potential for developing high-throughput THz communication systems.

A novel THz wave detection scheme is proposed in which the THz radiation is detected by an audible microphone based on the photothermo-acoustic (PTA) effect in graphene foam. Thanks to the room-temperature broadband electromagnetic absorption characteristics of graphene foam and the fast heat transfer between graphene foam and ambient air, this detection method not only inherits the advantages of the photo-thermal THz detector such as room-temperature and full bandwidth, but also has a response time 3 orders of magnitude faster than the photo-thermal detector. Besides, no micro-antenna/electrode is required to fabricate in the graphene foam THz detector which greatly simplifies the detector design and decreases the fabrication cost. It concludes that the room-temperature, full-bandwidth, fastspeed (≥10 kHz), and easy-to-fabricate THz detector developed in this work has superior comprehensive performances among both the commercial THz detectors and the detectors recently developed in laboratory.

Traditional non-destructive test methods utilise acoustic techniques such as ultrasound, while electromagnetic techniques include eddy current and microwave techniques. Radiography and ultrasound are used to perform most volumetric inspections on both metallic and non-metallic materials, even though they may not be best suited for them in some cases. Microwave inspection, by comparison, is a relatively new method although the concepts have been around since the 1950s, microwave, millimetre-wave and THz NDT has had little industrial use mainly being confined to academic labs to date. In this paper we will describe recent advances in mNDT and describe inspection applications which are hopefully gaining traction for commercial use. The presentation will include recent inspections of wind turbine blades and GFRP composites.

This paper proposes an alternative measurement setup and satellite Doppler Profile (DP) estimation technique to address these issues. By replacing the parabolic antenna with a horn antenna, the increased field of view allows exposure to signals from more satellites. Next, using spectral data DPs are accurately estimated. The DP is then matched with the computer predictions based on the orbital motion of the satellites to identify the matching signals. This allows for both satellite identification as well as (self-)localization.

The ultra wideband (UWB) antenna is one of the essential part of vital sign detection radar systems. The antenna characteristics of high gain and reduced backlobe level have a contribution to improve the detection capability of the UWB radar. In this paper, Antipodal Vivaldi antenna backed with frequency-selective surface (FSS) is developed for UWB pulsed radar system in the frequency range of 3.1 to 4. 8GHz. The frequency-selective surface (FSS) reflector is used to improve the gain of the antenna and reduces the back lobe level of the antenna. Substrate used for Vivaldi antenna and FSS is low cost material, FR-4 with a relative permittivity (εr) of 4.3 and a thickness of 1.6 mm. The air gap between the antenna and the FSS and the unit cell number of the FSS were optimized for high gain and low backlobe level. The proposed antenna provides an almost uniform high gain of 9 dB over the entire bandwidth of the antenna. while maintaining the radiation efficiency of over 80%. The prototype the antipodal Vivaldi antenna is fabricated and the simulation results are verified using experimental measurements.

The field of THz imaging continues to rapidly develop with ever more variety and sensitivity in both methods of sensing as well as detectors in array imaging formats. Nevertheless, comparatively the techniques developed in the shorter wavelength regions, such as in the IR, are better developed and far exceed in performance compared to the state of the art in THz imaging components. To that end using passive or active devices that can upconvert the THz radiation into the IR band can be advantageous for development of remote sensing applications such as low-IR visibility target detection as well as naturally radiant THz sources. To achieve such a feat the fundamental approach is to design and THz absorber that can emit in the IR so that in turn can be detected. Using a novel metasurface absorber the THz to IR radiation conversion can be optimized to detect incoherent radiation. Here we show how effective such a method is towards detection of THz radiation.