Showing papers in "IEEE Transactions on Terahertz Science and Technology in 2019"

Terahertz Imaging and Sensing Applications With Silicon-Based Technologies

Abstract: Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.

A High-Sensitivity AlGaN/GaN HEMT Terahertz Detector With Integrated Broadband Bow-Tie Antenna

Abstract: Many emerging applications in the terahertz (THz) frequency range demand highly sensitive, broadband detectors for room-temperature operation. Field-effect transistors with integrated antennas for THz detection (TeraFETs) have proven to meet these requirements, at the same time offering great potential for scalability, high-speed operation, and functional integrability. In this contribution, we report on an optimized field-effect transistor with integrated broadband bow-tie antenna for THz detection (bow-tie TeraFET) and compare the detector's performance to other state-of-the-art broadband THz detector technologies. Implemented in a recently developed AlGaN/GaN MMIC process, the presented TeraFET shows a more than twice performance improvement compared to previously fabricated AlGaN/GaN-HEMT-based TeraFETs. The detector design is the result of detailed modeling of the plasma-wave-based detection principle embedded in a full-device detector model to account for power coupling of the THz radiation to the intrinsic gated FET channel. The model considers parasitic circuit elements as well as the high-frequency impedance of the integrated broadband antenna, and also includes optical losses from a silicon substrate lens. Calibrated characterization measurements have been performed at room temperature between 490 and 645 GHz, where we find values of the optical (total beam power referenced) noise-equivalent power of 25 and ${\text{31 pW}}/\surd{\text{Hz}}$ at 504 and 600 GHz, respectively, in good agreement with simulation results. We then show the broadband detection capability of our AlGaN/GaN detectors in the range from 0.2 to 1.2 THz and compare the TeraFETs’ signal-to-noise ratio to that of a Golay cell and a photomixer. Finally, we demonstrate an imaging application in reflection geometry at 504 GHz and determine a dynamic range of >40 dB.

Measurement, Simulation, and Characterization of Train-to-Infrastructure Inside-Station Channel at the Terahertz Band

Abstract: In this paper, we measure, simulate, and characterize the train-to-infrastructure (T2I) inside-station channel at the terahertz (THz) band for the first time. To begin with, a series of channel measurements is performed in a train test center at 304.2 GHz with 8 GHz bandwidth. Rician $K$ -factor and root-mean-square (RMS) delay spread are extracted from the measured power-delay profile. With the aid of an in-house-developed ray-tracing (RT) simulator, the multipath constitution is physically interpreted. This provides the first hand information of how the communicating train itself and the other train on site influence the channel. Using this measurement-validated RT simulator, we extend the measurement campaign to more realistic T2I inside-station channel through extensive simulations with various combinations of transmitter deployments and train conditions. Based on RT results, all cases of the target channel are characterized in terms of path loss, shadow fading, RMS delay spread, Rician $K$ -factor, azimuth/elevation angular spread of arrival/departure, cross-polarization ratio, and their cross correlations. All parameters are fed into and verified by the 3GPP-like quasi-deterministic radio channel generator. This can provide the foundation for future work that aims to add the T2I inside-station scenario into the standard channel model families, and furthermore, provides a baseline for system design and evaluation of THz communications.

A 400-GHz High-Gain Quartz-Based Single Layered Folded Reflectarray Antenna for Terahertz Applications

Abstract: Compact high-gain antennas are highly desired in the high-speed terahertz (THz) wireless system, especially for the space limited application, such as the high-speed inter link inside the high density wireless communication base station. To this end, a 400-GHz folded reflectarray (FRA) antenna with high gain, high aperture efficiency, and compact profile is proposed in this paper. It is composed of a feed source, a single-layered reflectarray using a lithography process on quartz, and a wire-grid polarizer implemented by the printed-circuit-board technology. A 3-D printed fixture is used to assemble all parts together. In order to design accurately the proposed antenna, the THz electromagnetic properties of the supporting dielectric materials are extracted by using a THz time-domain spectrometer system. Then, a single-layered phasing element, made up of a pair of orthogonally I-shaped structures with an open square ring, is proposed and designed based on the extracted material characteristics. Both phase compensation and polarization conversion can be realized by the proposed unit cell. A reflectarray is designed by using the proposed phasing element with the conventional array synthesizing theory, and a THz grid polarizer is designed with strips on a 0.127 mm Taconic TLY-5 substrate. The THz grid is placed in front of the THz feed and the reflectarray, which is fully reflective to the feed and transparent to the reflectarray. All components of the FRA antenna have been fabricated and assembled. Experiments show that the FRA prototype has a peak gain of 33.66 dBi at 400 GHz with an aperture efficiency of 33.65%, and a 3-dB gain bandwidth of 16% (357–421 GHz).

Terahertz Microfluidic Metamaterial Biosensor for Sensitive Detection of Small-Volume Liquid Samples

Abstract: Metamaterial (MM) assisted terahertz (THz) label-free biosensing has promising applications. However, the sensitive THz detection of highly absorptive liquid samples remains challenging. Here, we present a novel multi-microfluidic-channel MM biosensor for highly sensitive THz sensing of small-volume liquid samples. The multichannels are set mostly in the strong electric field enhancement area of the MM, which significantly decreases the liquid's amount and enhances the interaction between the sensing targets and the THz wave (thus increasing the sensitivity). The sensing results of isopropyl-alcohol–water mixtures and bovine serum albumin solutions based on the bow-tie array MM with multichannels demonstrate the effectiveness of the proposed design and the great potential in THz biosensing. This design has the advantages of being highly sensitive, label-free, cost-effective, easy to operate, and only needing a tiny liquid volume. Thus, our device provides a robust route for MM-assisted THz label-free biosensing of liquid-based substances.

Terahertz Wireless Links Using Diffuse Scattering From Rough Surfaces

Abstract: We describe measurements of the diffuse bistatic scattering of a modulated terahertz beam incident on five metallic rough surfaces, to investigate the implications of surface roughness for non-line-of-sight (NLOS) wireless data links at frequencies at and above 100 GHz. The measurements were performed using transmitter and receiver modules, operating at several frequencies from 100 to 400 GHz. We investigate the dependence of the scattering patterns on surface roughness parameters, including rms height and correlation length. The results are consistent with numerical models for scattering from a rough surface. They support the design of bistatic methods for future multiple antenna systems, remote sensing, imaging, and localization in the terahertz range. We demonstrate for the first time that data links which incorporate an NLOS reflection in a nonspecular direction can be established at frequencies above 100 GHz, with low bit error rates.

THz-TDS Time-Trace Analysis for the Extraction of Material and Metamaterial Parameters

Abstract: We report on a method to fit time-trace data from a terahertz time-domain-spectroscopy system enabling the extraction of physical parameters from a material or metamaterial. To accomplish this, we developed a Python-based open-source software called Fit@TDS that functions on a personal computer. This software includes commonly used methods where the refractive index is extracted from frequency-domain data. This method has limitations when the signal is too noisy or when an absorption peak saturates the spectrum. Thus, the software also includes a new method where the refractive indices are directly fitted from the time trace. The idea is to model a material or a metamaterial through parametric physical models (Drude–Lorentz model and time-domain coupled mode theory) and implement this in the propagation model to simulate the time trace. Then, an optimization algorithm is used to retrieve the parameters of the model corresponding to the studied material/metamaterial. In this paper, we explain the method and test it on fictitious samples to probe its feasibility and reliability. Finally, we used Fit@TDS on real samples of high-resistivity silicon, lactose, and gold metasurface on quartz to show the capacity of the method.

Novel Air-Filled Waveguide Transmission Line Based on Multilayer Thin Metal Plates

Abstract: This paper presents a novel way of constructing waveguiding structures by stacking several thin metal plates for millimeter-wave and terahertz applications. The metallic layers do not require any electrical contacts among them. An air-filled multilayer waveguide (MLW) transmission line is successfully designed and manufactured at $D$ -band. Five vertically stacked thin metal layers are used to form an air-filled rectangular waveguide line. The layers are simply assembled by allowing a small air gap among them, without the need of using advanced manufacturing methods such as adhesive bonding techniques. The possible field leakage due to the air gaps is prevented by using an electromagnetic bandgap structure, consisting of glide-symmetric holes. A straight MLW line and a line with double 90 $^\circ$ bends are fabricated by using chemical metal etching, as a proof of concept. The measurement results of the straight line show that the reflection coefficient is better than $-$ 18 dB with an average insertion loss of 0.02 dB/mm over the frequency band 110–170 GHz. For the double 90 $^\circ$ bend line, the reflection coefficient is better than $-$ 18 dB with a similar average insertion loss of 0.02 dB/mm over the frequency band 110–150 GHz. The proposed concept could be an excellent candidate for designing compact and thin passive waveguide components and active components packaging, with a great potential for low-cost, light weight, and mass producible at millimeter-wave frequencies.

Beam Scanning of Silicon Lens Antennas Using Integrated Piezomotors at Submillimeter Wavelengths

Abstract: This paper presents a lens antenna that scans the beam using an integrated piezomotor at submillimeter-wave frequencies. The lens antenna is based on the concept presented by Llombart et al. in 2011, a leaky-wave waveguide feed in order to achieve wide angle scanning and seamless integration with the receiver. The lens is translated from the origin of the waveguide producing the scanning of the beam over a 50 $^\circ$ field of view (FoV) (or about 6.25 beamwidths) with a maximum scanning loss of 1 dB. The lens movement is achieved with a piezoelectric motor that is integrated within the antenna and receiver block. A prototype was built and measured at 550 GHz achieving scanning beam angles close to 20 $^\circ$ with only 0.6 dB of loss. The scanning of the 50 $^\circ$ FoV, which corresponds to a lens displacement of approximately 2 mm, takes about 0.9 s achieving a scanning rate of 0.75 Hz of the FoV. The accuracy in continuous mode of the piezoactuator has been measured to be less than $28\,\mu$ m in the worse of cases for displacements of 2 mm, which corresponds to a beam steering of 0.76 $^\circ$ , much smaller than the antenna half-power beamwidth of 8 $^\circ$ .

Study of Automatic Detection of Concealed Targets in Passive Terahertz Images for Intelligent Security Screening

Abstract: The automatic extraction of the targets in which we are interested from a given image is the fundamental of the automatic detection and identification for security screening systems based on imaging technologies. Suffering from the comparatively low signal-to-noise ratio (SNR), the automatic detection of targets in a passive terahertz (THz) imaging system facing great challenges, but in urgent necessary. In this paper, a comprehensive method for automatic detection of concealed targets in passive THz image by making the best use of the “block statistics uniformity” properties of the passive images is first studied. A theoretical model for the “featured regions” decomposition based on the minimization of a “fit energy” functional with respect to a “surface function” is established, to overcome the drawbacks of conventional methods with gradient-based edge operators for their unsuccessful application in low SNR passive images with blurred boundaries. Based on earlier theoretical basis and taking advantages of the distinguished contrasts of the convergent “surface function” in different “featured regions,” an automatic detection algorithm with three steps was further developed to automatically extract the number, the locations and the shapes of all the concealed targets, with the shape of each target derived as the contour point series arranged in clockwise direction. With plenty of experimental results in 0.2 THz band, it is found that, the proposed method has high detection accuracy about 95% with quite good realtime performance, even for the single channel proof-of-state system with low SNR. The theorem, algorithm, and results, in this paper, may have important applications in unmanned and intelligent security screening systems without any artificial interventions.

Micromachined Filters at 450 GHz With 1% Fractional Bandwidth and Unloaded Q Beyond 700

Abstract: This letter presents two silicon-micromachined narrowband fourth-order waveguide filter concepts with center frequency of 450 GHz, which are the first narrowband submillimeter-wave filters implemented in any technology with a fractional bandwidth as low as 1%. Both filters designs are highly compact and have axial port arrangements, so that they can be mounted directly between two standard waveguide flanges without needing any split-block interposers. The first filter concept contains two TM110 dual-mode cavities of circular shape with coupling slots and perturbations arranged in two vertically stacked layers, while the second filter concept is composed of four TE101 series resonators arranged in a folded, two-level topology without crosscouplings. Prototype devices are fabricated in a multilayer chip platform by high-precision, low-surface roughness deep-silicon etching on silicon-on-insulator wafers. The measured passband insertion loss of two prototype devices of the dual-mode circular-cavity filters is 2.3 dB, and 2.6 dB for three prototypes of the folded filter design. The corresponding extracted unloaded quality factors of the resonators are 786 ± 7 and 703 ± 13, respectively, which are the best so far reported for submillimeter-wave filters in any technology. The presented filters are extremely compact in terms of size; their footprints have areas of only 0.53 and 0.55 mm2, respectively, and the thickness between the waveguide flanges is 0.9 mm.

Toward Industrial Exploitation of THz Frequencies: Integration of SiGe MMICs in Silicon-Micromachined Waveguide Systems

Abstract: A new integration concept for terahertz (THz) systems is presented in this article, wherein patterned silicon-on-insulator wafers form all DC, IF, and RF networks in a homogeneous medium, in contrast to existing solutions. Using this concept, silicon-micromachined waveguides are combined with silicon germanium (SiGe) monolithic microwave integrated circuits (MMICs) for the first time. All features of the integration platform lie in the waveguide's H-plane. Heterogeneous integration of SiGe chips is achieved using a novel in-line H-plane transition. As an initial step toward complete systems, we outline the design, fabrication, and assembly of back-to-back transition structures, for use at D -band frequencies (110–170 GHz). Special focus is given to the industrial compatibility of all components, fabrication, and assembly processes, with an eye on the future commercialization of THz systems. Prototype devices are assembled via two distinct processes, one of which utilizes semiautomated die-bonding tools. Positional and orientation tolerances for each process are quantified. An accuracy of $\pm \text{3.5}\; \mu \text{m}$ , $\pm \text{1.5} ^\circ$ is achieved. Measured $S$ -parameters for each device are presented. The insertion loss of a single-ended transition, largely due to MMIC substrate losses, is 4.2–5.5 dB, with a bandwidth of 25 GHz (135–160 GHz). Return loss is in excess of 5 dB. Measurements confirm the excellent repeatability of the fabrication and assembly processes and, thus, their suitability for use in high-volume applications. The proposed integration concept is highly scalable, permitting its usage far into the THz frequency spectrum. This article represents the first stage in the shift to highly compact, low-cost, volume-manufacturable THz waveguide systems.

Terahertz Dual-Polarization Beam Splitter Via an Anisotropic Matrix Metasurface

Abstract: A metasurface composed of multiple resonator arrangements with a certain phase distribution is capable of efficient wavefront manipulation. An anisotropic matrix metasurface consisting of asymmetric metal cross particles with a near-field coupling-induced achieves simultaneous distinctive dual-polarization anomalous reflections. Using polarization splitting, due to the anisotropic phase gradients of the matrix subunit, the x- and y- polarization components of the incident terahertz waves can be efficiently deflected with different deflection angles in a single plane or in orthogonal planes. Both simulated and experimental results show that the anisotropic matrix metasurface efficiently reflects the x- and y- polarization components of normal incident waves in different directions at 0.32–0.42 THz with a relative bandwidth of 27%, and the maximum deflection coefficient is measured to exceed 85%. The proposed anisotropic matrix metasurface is promising for applications in high-performance terahertz polarization beam splitters, spectral splitting, directional emitters, polarization-sensitive imaging, and polarization multiplexers.

High- T c Superconducting Fourth-Harmonic Mixer Using a Dual-Band Terahertz On-Chip Antenna of High Coupling Efficiency

Abstract: This paper presents a dual-band on-chip antenna-coupled high- Tc superconducting (HTS) Josephson-junction subterahertz (THz) fourth-harmonic mixer. The antenna utilizes a couple of different structured twin slots to enable the resonant radiations at two frequencies, and integrates a well-designed coplanar waveguide network for achieving good radiation coupling and signal isolation characteristics. The electromagnetic simulations show that coupling efficiencies as high as −4 and −3.5 dB are achieved for the 160- and 640-GHz operating frequency bands, respectively. Based on this dual-band antenna, a 640-GHz HTS fourth-harmonic mixer is developed and characterized in a range of operating temperatures. The mixer exhibits a measured conversion gain of around −18 dB at 20 K and −22 dB at 40 K, respectively. The achieved intermediate frequency bandwidth is larger than 23 GHz. These are the best results reported for HTS harmonic mixers at comparable sub-THz frequency bands to date.

Comparison of On-Wafer TRL Calibration to ISS SOLT Calibration With Open-Short De-Embedding up to 500 GHz

Abstract: Sub-mm circuit design requires accurate on-wafer characterization of passive and active devices. In industry, characterization of these devices is often performed with off-wafer short-open-load-thru (SOLT) calibration. In this paper, the validity of this characterization procedure above 110 GHz is investigated by an exhaustive study of on-wafer and alumina off-wafer calibration using measurement and electromagnetic (EM) simulation up to 500 GHz. The EM simulation is performed at two different levels, first at the intrinsic level of the devices under test for reference and afterward up to the probe level to simulate different standards used in the off-wafer calibration or in the on-wafer calibration in the presence of the probe. Furthermore, EM simulation data are calibrated with the same procedures and tools that are used in the measurement; therefore, it includes the probe-to-substrate coupling. In addition, precise EM model of a commercial impedance standard substrate is developed and used to perform the SOLT calibration. A good agreement is observed between measurement and EM modeling for the off-wafer calibration as well as for the on-wafer calibration. Results clearly highlight a limitation of alumina off-wafer methodology above 200 GHz for characterization of silicon-based technologies. Finally, a discussion is given on the pros and cons of the off-wafer and on-wafer methodologies.

Terahertz Frequency-Selective Surface With Polarization Selection and Conversion Characteristics

Abstract: In this paper, a novel frequency-selective surface (FSS) with polarization selection and dual-band polarization conversion characteristics is proposed for terahertz applications. The FSS is composed of two layers of metallic periodic arrays separated by a polymer dielectric spacer. The top and bottom arrays provide bandpass and bandstop characteristics for TE- and TM-polarized incidences, respectively. Geometrical parameters of the top and bottom arrays are tuned such that the phase differences of the TE and TM transmissions at the lower ( f 1) and upper ( f 2) crossover frequency points are −90° and +90°, respectively. Therefore, for normal incidence, a 45° linearly polarized incident wave is converted into a right-handed circularly polarized and left-handed circularly polarized transmitted wave at f 1 and f 2, respectively. Equivalent circuit models are established to explain the operation principle of the proposed FSS. A prototype is fabricated using micromachining with its transmission characteristics, namely, polarization selectivity and dual-band polarization conversion, validated by time-domain and frequency-domain measurements.

Compact Silicon-Micromachined Wideband 220–330-GHz Turnstile Orthomode Transducer

Abstract: This paper reports on a turnstile-junction orthomode transducer (OMT) implemented by silicon micromachining in the 220–330-GHz band. Turnstile OMTs are very wideband and allow for co-planar ports but require accurate and complex geometries, which makes their fabrication challenging at higher frequencies. The compact 10 mm × 10 mm × 0.9 mm OMT-chip presented in this paper is the first micromachined full-band OMT in any frequency range and only the second turnstile OMT implemented above 110 GHz. The measured insertion loss (0.3 dB average, 0.6 dB worst case) and the cross polarization (60 dB average, 30 dB worst case) over the whole waveguide band represent the best performance of any wideband OMT, regardless of design or fabrication technology, in the 220–330-GHz band. The return loss with 22 dB average (16 dB worst case) is comparable with or better than previous works. This paper discusses design considerations and compromises of this complex 9-layer silicon micromachined device, including the influence of side-wall slopes, underetching, and postbonding misalignment between the chips. It is shown that for a device that is very sensitive to geometrical variations, such as a turnstile OMT, it is necessary to anticipate and compensate for any fabrication imperfections in the design to achieve high RF performance.

A 183-GHz InP/CMOS-Hybrid Heterodyne-Spectrometer for Spaceborne Atmospheric Remote Sensing

Abstract: A complete 183-GHz CMOS/Indium Phosphide (InP) hybrid heterodyne-spectrometer is realized for spaceborne atmospheric remote sensing applications. It captures spontaneous emission from thermally populated pure rotational states for investigating Earth/planetary atmosphere and interstellar media. Recent missions such as the heterodyne instrument for far-infrared, the microwave limb sounder, and the microwave instrument for Rosetta orbiter highlighted the wealth of scientific knowledge that can be attained by utilizing heterodyne-spectrometers. However, major challenges still remain in mission cost, instrument size, weight, and power (SWaP). Highly integrated CMOS circuits are designed to prototype a low-cost/SWaP spectrometer system and conduct challenging science missions. Within the prototype system, a 183-GHz receiver with a frequency synthesizer is designed in the 28-nm CMOS, and a back-end 6-GS/s 4096-point spectrometer processor is designed in the 65-nm CMOS. The receiver's center frequency is tunable from 180 to 200 GHz. An external InP low-noise amplifier is added at the front-end to provide the required receiver sensitivity. The demonstrated 28-nm CMOS receiver and 65-nm CMOS spectrometer consume 515 mW under 1.5/1.15-V and 1500 mW under 1 V supply, respectively. The complete system achieves 700 to 1000 K noise temperature within the interested bandwidth and RMS uncertainty improvement up to 10 s integration according to the Allan deviation measurements. The spectrometer's capability in detecting gas phase molecular compounds is verified with laboratory trials employing water (H2O) and methyl-cyanide (CH3CN).

A Polarization-Resolved Study of Nanopatterned Photoconductive Antenna for Enhanced Terahertz Emission

Abstract: Terahertz (THz) frequencies, despite having the potential for several important applications, have been relatively underexplored in the past owing to the unavailability of proper sources and detectors. The scenario has been changing over the past few decades due to the advent of convenient THz sources and detectors. THz photoconductive antennas (PCA), due to their attractive features, such as cost effectiveness and room temperature operation, are playing a key role in current and future research prospect in the field of THz spectroscopy, both as sources and detectors. Complex PCA designs have been proposed and studied to boost the THz emission efficiencies. Elucidating the underlying physics in such devices requires a thorough investigation of a few physical parameters. This requires the integration of several experimental techniques under identical conditions. In this paper, we show such a study, including a parametric variation of pump polarization, conducted on a PCA with a nanopatterned active region, which boosts the emitted THz radiation. Through the set of measurements, we unravel the subtle interplay of the various physical processes responsible for the emission of THz radiation from the device.

A Connected Array of Coherent Photoconductive Pulsed Sources to Generate mW Average Power in the Submillimeter Wavelength Band

Abstract: A pulsed photoconductive terahertz (THz) source is presented that is able to radiate milliwatt (mW) level average power over a large bandwidth, by exploiting both the optical and electrical properties of photoconductive sources and the ultrawideband properties of connected antenna arrays. An optical system composed of a microlenses array splits the laser beam into $\boldsymbol{N \times N}$ spots that host the active excitation of the antenna arrays. An “ad hoc” network is introduced to bias the array active spots in order to implement a connected antenna array configuration. The array feeds a silicon lens to increase the directivity of the radiated THz beam. A dipole and a slot array are designed. Prototypes have been fabricated and measured. Power and spectrum measurements of the prototypes are in excellent agreement with the expected results. The proposed solutions achieve excellent power radiation levels by exploiting accurate electromagnetic design. Thus, they can offer enhancements to any active system relying on pulsed photoconductive antennas.

G-band Rectangular Beam Extended Interaction Klystron Based on Bi-Periodic Structure

Abstract: A rectangular beam G-band extended interaction klystron (EIK) based on planar bi-periodic slow wave structure (SWS) is proposed. Such structure has the advantages of uniform and strong electric field, as well as the large frequency spacing, which implies that EIK adopting such structure will have strong beam–wave interaction and will be easy to avoid the mode competition. Furthermore, the planar structure is compatible to the microfabrication technology, thus reducing the fabrication difficulty of THz vacuum electron devices with small sizes. Compared with the sheet beam and large beam current, the rectangular beam with small aspect ratio and the decreased current is easy to realize the beam focusing and high power in engineering. Under the voltage of 16.5 kV, current of 0.3 A, and beam size of 0.30 mm × 0.13 mm, particle in cell simulations show a considerable output power of 390 W, with gain of 38.9 dB, efficiency of 7.9%, and relatively wide bandwidth of 800 MHz. The high-frequency structure has been fabricated by computer numerical control method, and the measurement results are close to the simulations. The proposed planar bi-periodic SWS shows certain superiorities in the performance improvement of millimeter-wave and terahertz EIK.

Extending Spatial and Temporal Characterization of Indoor Wireless Channels From 350 to 650 GHz

Abstract: Communication at terahertz carrier frequencies is a promising way to satisfy the ever-growing demands for high-speed wireless networks. The studies of terahertz wireless channels have so far been limited to the atmospheric transmission bands below 350 GHz. Availability of high-power transmitters and high-sensitivity receivers at higher frequencies necessitates extending the wireless channel studies to enable higher data-rate communication systems. With a view to assessing communication system design requirements at higher frequencies, we present the channel measurement results for 650-GHz carrier frequencies in comparison with 350 GHz carrier frequencies in a typical indoor environment. To obtain the spatial and temporal characteristics of the channel, the power angle profile and the power delay profile are measured based on new measurement methods. Multiple spatially resolvable paths are observed at both 350- and 650-GHz carrier frequencies. Signal-to-noise ratio of the received signal through the non-line-of-sight (NLoS) paths is sufficiently high to enable robust communication when the direct line-of-sight (LoS) path is blocked due to a moving object. The measurement results are used to calculate the reduction in the 650-GHz channel capacity in comparison with that of the 350-GHz channel for both LoS and NLoS paths. Channel dispersion characterization over a 10-GHz bandwidth shows that the delay spreads for the resolved LoS and NLoS paths are less than 80 ps for both 350- and 650-GHz bands. Therefore, no complicated equalizer is required to compensate for channel dispersion at both 350 and 650-GHz, which greatly simplifies the terahertz transceiver design.

A Transmitter System-in-Package at 300 GHz With an Off-Chip Antenna and GaAs-Based MMICs

Abstract: This paper presents a novel transmitter system for applications operating in the frequency region around 300 GHz. A relatively simple packaging approach is taken, allowing for a variety of integrated or embedded applications. This paper presents a multilayered stacked patch antenna on quartz with a dielectric lens, functioning in tandem with a GaAs-based multiplier chain. When operating, an input signal between 7.8 and 8.6 GHz is multiplied 36 times to a frequency range between 280 and 310 GHz, via two in-house fabricated GaAs-based monolithic microwave integrated circuits (MMICs). The antenna is fabricated on quartz wafers via an in-house process. Both, the MMICs and the antenna, are placed on a printed circuit board. A high-density polyethylene lens encloses the system-in-package (SiP), with a footprint of 1 cm $^{2}$ . The SiP has a center frequency of 300 GHz and an absolute pattern bandwidth of 21 GHz. The measured antenna gain along the broadside is 23 dBi, which corresponds to an equivalent isotropic radiated power of 20 dBm. This is achieved with 0.3 W power consumption. The design, simulation, and the analysis are performed via an electromagnetic simulation and modeling tool (Computer Simulation Technology Microwave Studio). Both, separate measurement data for the individual parts of the SiP and complete system experimental characterization are included in this paper.

Terahertz Transmittance of Cobalt-Doped VO 2 Thin Film: Investigated by Terahertz Spectroscopy and Effective Medium Theory

Abstract: Vanadium dioxide (VO2), due to its characteristic semiconductor–metal transition (SMT) at 341 K, is a promising material for optical modulators in the terahertz (THz) regime. In this study, doping Co ions into epitaxial VO2 films have been demonstrated to acquire SMT with a reduced critical temperature, a large modulation depth, and a narrow transition region in the THz regime, which would benefit the applications of VO2 film in THz switches and memory devices. Utilizing Co ions, a 77% THz modulation ratio within a 3 °C transition region appeared when the doping content was 4.0 at. %. Moreover, more doping content induced two different phases in VO2 films, identified by X-ray diffraction (XRD), which would decrease the THz modulation ratio. Theoretical analysis based on the Drude-Smith model and the Bruggeman effective medium theory predict that the phase containing more Co ions might be transparent to THz waves and void of switching ability in the temperature range from 30 °C to 80 °C. This phase separation may be responsible for the complex correlation between the Co doping concentration and the SMT properties.

A 270-GHz CMOS Triple-Push Ring Oscillator With a Coupled-Line Matching Network

Abstract: A design procedure is proposed for triple-push ring oscillators, and an oscillator employing a coupled line-matching network is developed following the procedure. This stepwise procedure, which uses power-dependent Z-parameters of transistors, is applied to the design of each amplifier stage constituting a ring oscillator based on its steady-state oscillation condition. It is verified with both L-section and T-section topologies assumed for the load of the amplifier stages of a given triple-push ring oscillator, and the differences between the two topologies are compared. Based on the procedure, a 270 GHz triple-push ring oscillator that employs coupled lines for matching networks has been developed in a 65-nm complementary metal-oxide-semiconductor (CMOS) process. The circuit benefits from the advantages of coupled lines such as compact area and simplified layout. The fabricated oscillator exhibits a measured oscillation frequency of around 270 GHz and output power of –10.9 dBm, with phase noise of –96 dBc/Hz at 10 MHz offset.

A Fast 220-GHz Real Aperture 3-D Personnel Screening System With a Novel-Shaped Mirror Design

Abstract: Terahertz systems have attracted much attention recently in the field of public security due to the high penetration of terahertz waves through clothes and the high resolution of terahertz images. A novel real aperture terahertz personnel screening system is demonstrated in this paper. This system uses a single transmitter and receiver pair together with a quasi-optical system to irradiate and collect the terahertz waves. In the quasi-optical system, a novel design of a shaped reflector, used as the main mirror, helps reach a satisfying focus and meanwhile keep the beam direction invariant. By using this main mirror and a hexagonal cylindrical scanning mirror, a fast horizontal scan of 0.6-m field of view can be accomplished within 10 ms. The vertical scan is realized by a customized translation stage. The whole system is able to finish a full-body scan of a person within 2.9 s with a cross-section resolution of better than 8 mm.

Spectral Characterization of a Microbolometer Focal Plane Array at Terahertz Frequencies

Abstract: We have developed a method to characterize the spectral response of an uncooled microbolometer focal plane array at a broad range of terahertz (THz) frequencies (4–50 THz). This is achieved by using a spectrum-shaped blackbody radiator as a broadband THz source and measuring its spectral power with a Fourier transform infrared interferometer. With an additional measurement with a pyroelectric detector as a reference, the spectral response of the microbolometer relative to the pyroelectric reference is obtained with a signal-to-noise ratio of 100 over a $>$ 50-THz bandwidth.

High Gas Void Fraction Flow Measurement and Imaging Using a THz-Based Device

Abstract: Measuring in real-time two-phase flow composition of a mixed fluid having high gas void fraction (GVF) remains a challenging task in oil–gas fields. Such fluid is abundant in gas pipelines where pressure and temperature fluctuations lead to condensate gas. This may also be the case of crude oil produced from CO2 or steam-based enhanced oil recovery, where the injected gas is mixed with the produced oil. This article presents a new concept of high GVF measurement and flow regime determination using a terahertz-based imaging system. It explores the fact that the gas phase has very low absorption of THz waves, while it yields an absorption factor that is proportional to the amount of liquid. The recent availability of low-cost THz imaging systems that can generate two-dimensional images at more than 100 frames/s makes them well suitable for flow metering applications. Two different artificial intelligence algorithms, namely support vector machine (SVM) and artificial neural network (ANN), were assessed using an in-house multiphase flow loop. The corresponding results reveal that while ANN and SVM yield very accurate results, the SVM technique performed slightly better where a maximal error of 0.46% for GVF in the GVF range from 80% to 100% could be achieved. In addition, it could accurately determine all three type of flow regimes (i.e., annular, stratified, or slug flow). This suggests that the technique can be considered as a good candidate for next-generation flow metering and imaging of multiphase flows.

THz Time-Domain Spectroscopic Ellipsometry With Simultaneous Measurements of Orthogonal Polarizations

Abstract: New instrumentation and calibration procedures for terahertz time-domain spectroscopic ellipsometry (THz-TDSE) are demonstrated. The THz-TDSE is capable of simultaneous measurement of two orthogonal components of reflected THz electric fields with no need to rotate a polarizer. In the calibration, the TDSE response function was obtained via the simultaneous polarization measurements reflected by a flat metal mirror, adapted in conventional THz time-domain reflection spectroscopy, and used here for THz-TDSE without the problems of position accuracy. The calibration could be used to determine accurate ellipsometric parameters with high tolerance of imperfect polarizer extinction ratios and of nonideality in the THz reflection components. Results are presented for an opaque Si wafer with heavy doping. The simultaneous measurements rejected significant common-mode noise from the laser, and it extended reliable THz spectra into the frequency range with a low dynamic range of a photoconductive-antenna THz source, which is a fundamental breakthrough for reflection-based measurements and overcomes the hurdle of phase uncertainty.

Stress Test of Nanosecond Semiconductor Cavity Switches With Subterahertz Gyrotrons

Abstract: Nanosecond semiconductor switches based on resonant cavity with reversibly variable Q -factor are the most prospective devices for generating unlimited series of coherent pulses in subterahertz frequency bands. The pulse duration stretches from nanoseconds to tens of seconds for the same device. Although the subterahertz switches have already been tested with various continuous-wave generators, e.g., semiconductor transistors and backward wave oscillators, they are intended to be used with gyrotrons, the most powerful generators of coherent microwave radiation for subterahertz bands. Here, we present results of several experiments with continuous wave and pulsed gyrotrons with frequencies around 300 GHz, pulse durations up to 10 s, and stressful microwave power up to 180 kW. A simple generalized theoretical estimation for the microwave power limit of the switches is provided and found to be in a good agreement with the performed experiments, avoiding blind zones in subterahertz measurements.