For decades, the terahertz (THz) frequency band had been primarily explored in the context of radar, imaging, and spectroscopy, where multi-gigahertz (GHz) and even THz-wide channels and the properties of THz photons offered attractive target accuracy, resolution, and classification capabilities. Meanwhile, the exploitation of the THz band for wireless communication had originally been limited due to several reasons:1) no immediate need for such high data rates available via THz bands and 2) challenges in designing sufficiently high-power THz systems at reasonable cost and efficiency, leading to what was often referred to as “the THz gap.” Over the recent decade, advances on many fronts have drastically changed the THz landscape. First, the evolution from 5G-to 6G-grade wireless systems dictates the need to support novel bandwidth-hungry applications and services for both data transfer i.e., eXtended Reality (XR), the Metaverse, and vast modeling needs of artificial intelligence (AI) and machine learning (ML), as well as centimeter-precision sensing and classification (i.e., for standalone position location, vehicle-to-everything (V2X), or unmanned aerial vehicle (UAV) tracking). Second, substantial progress in THz hardware has been achieved, offering promise that the THz technology gap will be closed. Hence, THz-band wireless communication seems inevitably an essential part of the future networking technology landscape in the coming decades. To design efficient THz systems, the peculiarities of THz hardware and THz channels need to be understood and accounted for. This roadmap paper first reviews the evolution of the hardware design approaches for THz systems, including electronic, photonic, and plasmonic approaches, and the understanding of the THz channel itself, in diverse scenarios, ranging from common indoors and outdoors scenarios to intrabody and outer space environments. This article then summarizes the lessons learned during this multidecade process and the cutting-edge state-of-the-art findings, including novel methods to quantify power efficiency, which will become more important in making design choices. Finally, this article presents the authors’ perspective and insights on how the evolution of THz systems design will continue toward enabling efficient THz communications and sensing solutions as an integral part of next-generation wireless systems.

Accurate timing is critical in many industries, from time-stamping grocery receipts to facilitating stock-market transactions and supporting precise navigation, localization, and guidance. High levels of precision are currently achieved using physically large, power-hungry atomic clocks that rely on energy-level transitions in a chosen atomic species. Recently, commercially available chip-scale atomic clocks (CSACs) have emerged, offering atomically precise timing in a compact, low-power package that can be operated on a battery for extended periods. Such clocks have important applications in underwater timing for oil and gas exploration, enhanced GPS receivers, and are being adopted for application to the energy grid, where large clocks are impractical. These devices contain a microwave local oscillator (LO) that generates the RF used to interrogate the atoms and, once locked to the atomic resonance, serves as the clock output. This article reviews the state of the art in microwave CSACs, with a focus on the engineering requirements for compact, low-power microwave LOs used to lock to the atomic transition. Several different LO designs are surveyed and compared in terms of power consumption, phase noise, and suitability for use in miniaturized clocks. The frequency control electronics needed to operate CSACs are also discussed. Finally, potential future improvements to all clock electronics for CSACs are highlighted.

This month’s special issue provides an overview of the state-of-the-art technology and theory for future networks with wireless power transfer and energy harvesting. Wireless communication using radio frequency (RF) radiation has been around for over 100 years and has shaped our society significantly over the past 40 years. However, wireless is not just about communications. For very short ranges, wireless power supply by inductive power transfer has become a reality through several commercial products and standards (Wireless Power Consortium, Power Matters Alliance, Alliance for Wireless Power, and Rezence). Far-field wireless power transfer (WPT) and wireless energy harvesting (WEH) via RF (as in wireless communications) on the other hand could be used for a longer range. Although it has long been seen as a potential for powering low-power devices, it is only recently that wireless (via RF) power has been recognized as a promising technology to cope with the explosion of low-power devices in future networks. Driven by the reduction in the energy needs of electronic devices (remember Koomey’s law according to which, in 20 years, a device will require 10 000 times less energy to compute a given task) and the advent of trillions of internet device objects [Internet of Things (IoT)], there is a need to rethink the design of the network of the future so that wireless can reach its full potential not only to carry information but also to transfer energy.

For RF energy harvesting, where the frequencies, power density, polarization, and direction of incident waves are unknown and variable, broadband rectenna arrays offer a simple scalable approach. In this article, we first compare the design methodology and scalability of narrowband and broadband arrays. An example 10-GHz narrowband rectifier array is presented to showcase the challenges involved in ultralow-power (down to 1 mW/m2=$0.1 \mu \text{W}$/cm2) ground-based harvesting of narrowband satellite signals. The circularly polarized array consists of subarrays with RF power combining before rectification. This approach is then contrasted to harvesting over a wide RF bandwidth, through a comparison of several broadband arrays with rectifiers in each element. This includes a 2–5-GHz bow-tie screen-printed wearable rectenna array with an efficiency exceeding 30% above 50-$\mu \text{W}$/cm2incident power density and a 2–18-GHz spiral array tested with powers down to$0.1 \mu \text{W}$/cm2. The two types of arrays are analyzed in the context of applications in a ground-based satellite transmitter with narrowband directional power receiver and in a broadband case with multiple widely spaced simultaneous signals.

This paper discusses far-field wireless powering for low-power wireless sensors, with applications to sensing in environments where it is difficult or impossible to change batteries and where the exact position of the sensors might not be known. With expected radio-frequency (RF) power densities in the 20-200- μW/cm2range, and desired small sensor overall size, low-power nondirective wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the gigahertz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g., thin-film battery) through an efficient power management circuit, and the entire platform, including sensors and a low-power wireless transmitter, and is controlled through a low-power microcontroller. For low incident power density levels, codesign of the RF powering and the power management circuits is required for optimal performance. Results for hybrid and monolithic implementations of the power management circuitry are presented with integrated antenna rectifiers operating in the 1.96-GHz cellular and in 2.4-GHz industrial-scientific-medical (ISM) bands.

This paper presents the design of a 449-MHz radar for wind profiling, with a focus on modularity and solid-state transmitter design. It is one of the first wind profiler radars to use low-cost Laterally Diffused Metal Oxide Semiconductor (LDMOS) power amplifiers (PAs) combined with spaced antennas. The system is portable and designed for 2–3 month deployments. The transmitter PA consists of three 1-kW peak power modules which feed 54 antenna elements arranged in a hexagonal array, scalable directly to 126 elements. The PA is operated in pulsed mode with a 10% duty cycle at 54% power added efficiency. The antenna array is designed to have low sidelobes, confirmed by measurements. The radar was operated in Boulder, Colorado and Salt Lake City, Utah. Atmospheric wind vertical and horizontal components at altitudes between 200 m and 4 km were calculated from the collected atmospheric return signals.

Terahertz imaging makes it possible to acquire images of objects concealed underneath clothing by measuring the radiometric temperatures of different objects on a human subject. The goal of this work is to automatically detect and segment concealed objects in broadband 0.1-1 THz images. Due to the inherent physical properties of passive terahertz imaging and associated hardware, images have poor contrast and low signal to noise ratio. Standard segmentation algorithms are unable to segment or detect concealed objects. Our approach relies on two stages. First, we remove the noise from the image using the anisotropic diffusion algorithm. We then detect the boundaries of the concealed objects. We use a mixture of Gaussian densities to model the distribution of the temperature inside the image. We then evolve curves along the isocontours of the image to identify the concealed objects. We have compared our approach with two state-of-the-art segmentation methods. Both methods fail to identify the concealed objects, while our method accurately detected the objects. In addition, our approach was more accurate than a state-of-the-art supervised image segmentation algorithm that required that the concealed objects be already identified. Our approach is completely unsupervised and could work in real-time on dedicated hardware.

A low-profile microwave antenna suitable for high data-rate radio communications is presented. Although the size, cost, and efficiency are comparable to the microstrip patch, the 2:1 VSWR bandwidth of the antenna exceeds 20%. The radiation pattern of the antenna does not change appreciably within the bandwidth, and the efficiency remains above 70% within the bandwidth. Measurements on antennas around 2 and 4 GHz are presented as well as theoretical results using a full-wave analysis.

Although quasi-optical techniques are applicable to a large variety of solid-state devices, special attention is given to transistors, which are attractive because they can be used as either amplifiers or oscillators. Experimental results for MESFET bar-grid and planar grid oscillators are presented. A MESFET grid amplifier that receives only vertically polarized waves at the input and radiates horizontally polarized waves at the output is discussed. These planar grids can be scaled for operation at millimeter- and submillimeter-wave frequencies. By using modern IC fabrication technology, planar grid oscillators and amplifiers containing thousands of devices can be built, thereby realizing an efficient means for large-scale power combining.>