Abstract: In this article, we propose a hybrid electromechanical scanning lens antenna array architecture suitable for the steering of highly directive beams at submillimeter wavelengths with field-of-views (FoV) of ±25°. The concept relies on combining electronic phase shifting of a sparse array with a mechanical translation of a lens array. The use of a sparse-phased array significantly simplifies the RF front-end (number of active components, routing, thermal problems), while the translation of a lens array steers the element patterns to angles off-broadside, reducing the impact of grating lobes over a wide FoV. The mechanical translation required for the lens array is also significantly reduced compared to a single large lens, leading to faster and low-power mechanical implementation. In order to achieve wide bandwidth and large steering angles, a novel leaky wave lens feed concept is also implemented. A 550-GHz prototype was fabricated and measured demonstrating the scanning capabilities of the embedded element pattern and the radiation performance of the leaky wave fed antenna.

Abstract: For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are attractive in that regard since they do not require complex feeding control circuits and beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus, it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP)-based photonic or electronic THz sources and receivers. Therefore, in this article, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50-μm thin InP THz antenna chip has been established. By changing the operating frequency from 230 to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from −46° to 42°, i.e., the total scanning angle is 88°. The measured average realized gain and 3-dB beam width of a 1.5-mm wide InP LWA are ∼11 dBi and 10°. This article furthermore discusses the use of the fabricated LWA for THz interconnects.

Abstract: In this article, a hollow core antiresonant photonic crystal fiber is analyzed for terahertz applications. A numerical analysis of the proposed fiber is first carried out to minimize coupling between the core and cladding modes. The modeling of the scaled-up and inhibited coupling fiber is carried out by means of a finite element method, which is then demonstrated using a Zeonex filament fiber, fabricated by fused deposition modeling of 3-D printing technology. The simulation is carried out to analyze both the transmission and possibility of refractometric sensing, whereas the experimental analysis is carried out using terahertz time-domain spectroscopy, and supports our numerical findings, illustrating how the proposed fibers can be used for low-loss transmission of terahertz waves. The simplicity of the proposed fiber structures facilitates fabrication for a number of different transmission and sensing applications in the terahertz range.

Abstract: This article presents the experimental demonstration of a fully photonics-based heterodyne subterahertz (sub-THz) system for wireless communications. A p-i-n photodiode is used as a broadband transmitter to upconvert the signal to the sub-THz domain and a photoconductive antenna downconverts the received wave to an intermediate frequency around 3.7 GHz. The optical signals used for photomixing are extracted from two independent optical frequency combs with different repetition rates. The optical phase locking reduces the phase noise of the sub-THz signal, greatly improving the performance of the system when phase modulation formats are transmitted. The sub-THz carrier is tuned between 80 and 320 GHz in 40-GHz steps, showing a power variation of 21.8 dB. The phase noise at both ends of the communication link is analyzed and compared with the phase noise of the received signal with different wireless carriers. As a proof-of-concept, a 100-Mbit/s binary-phase-shift-keying signal is successfully transmitted over 80-, 120-, and 160-GHz carriers, achieving a bit error rate below 10−5 in the first two cases. These results show the great potential of THz communications driven by photonics to cover an extensive portion of the THz range without relying on electronic components that limit the operating range of the system to a concrete frequency band.

Abstract: In this article, three frequency independent optical paths are designed and analyzed. A two-parabolic-mirror system and a four-parabolic-mirror system are studied and developed over 140–220 GHz to achieve precision complex permittivity measurements of a dielectric substrate. To achieve a wide plane wave zone for the center of the four-parabolic-mirror system, two 80-mm-length corrugated horns are designed and fabricated for the measurement systems. The Gaussicity of the corrugated horn is larger than 97.4%. For the multiple reflection model and direct wave model, two closed-form expressions of loss tangent are derived from the transmission parameters (insertion losses) of the measurement systems. Meanwhile, the resolution and uncertainty of loss tangent can be calculated according to the working frequency, the thickness of the wafer, the real part of the relative permittivity, and the $| {{S_{21}}} |$ measurement uncertainty. The complex permittivity of the Rogers/Duroid series PCB substrates, which are commonly used at microwave frequencies, and silicon wafers are measured in G -band.

Abstract: Efficient and compact frequency converters are essential for frequency stabilization of terahertz sources. In this article, we present a ${3.5}$ -THz, ${\times 6}$ -harmonic, integrated Schottky diode mixer operating at room temperature. The designed frequency converter is based on a single-ended, planar Schottky diode with a submicron anode contact area defined on a suspended 2- ${\mu }$ m ultra-thin gallium arsenide substrate. The dc-grounded anode pad was combined with the radio frequency E-plane probe, which resulted in an electrically compact circuit. At $\text{200}$ -MHz intermediate frequency, a mixer conversion loss of about $\text{59}$ dB is measured resulting in a ${40}$ -dB signal-to-noise ratio for phase locking a ${3.5}$ -THz quantum-cascade laser. Using a quasi-static diode model combined with electromagnetic simulations, good agreement with the measured results was obtained. Harmonic frequency converters without the need of cryogenic cooling will help in the realization of highly sensitive space and air-borne heterodyne receivers.

Abstract: We report on the design and characterization of a novel backside-radiating antenna-coupled direct terahertz detector fabricated in 65 nm CMOS technology. The novelty of the design lies in the low-metal coverage of the biquad antenna geometry, which adapts well to the particular challenging conditions of on-chip antenna integration in silicon and allows optimization for a versatility of operation conditions. The biquad antenna was modified here to achieve wideband radiation and matching to a gate-coupled single-finger field-effect transistor with ac open condition at the drain terminal. The successful detector performance was the result of a careful treatment of transistor, antenna, and optics from a codesign perspective, since the beginning of the design. This included the frequency-dependent complex impedance for optimum matching, the technology restrictions to ensure proper chip fabrication, and the overall detection efficiency after backing the device with a silicon lens. Calibrated detector measurements for 7777 Hz modulation frequency yielded minimum optical noise-equivalent-power (NEP) of 25 pW/ $\sqrt{\,}$ Hz at 1 THz, with NEP values below 50 pW/ $\sqrt{\,}$ Hz in the 0.84–1.29 THz frequency range. These figures achieve state-of-the-art of wideband CMOS-based detectors and are only a factor of ${{\sim}2}$ inferior to the best reported narrowband devices close to 1 THz.

Abstract: In this work, a novel 670-GHz integrated direct-detection receiver using 25-nm InP HEMT technology is presented. This is the first demonstration of an integrated direct-detection radiometer architecture at these frequencies. The receiver exhibits a noise figure of 11.4 dB with a total dc power consumption of 0.25 W. The integrated receiver measures only 0.8 cm × 1.3 cm × 4.8 cm (0.3” × 0.5” × 1.9”). These results show that transistor-based direct-detection receivers are a viable technology for submillimeter-wave applications, with low SWaP with no compromise in performance.

Abstract: In this article, a high-performance wideband double-ridged waveguide orthomode transducer (OMT) for the 275–500 GHz band has been designed, fabricated, and tested. Such wideband performance would allow to cover two full RF bands in radio astronomy, 275–373 GHz and 385–500 GHz, with a single receiver, allowing for additional science cases and simplified operations. Successful fabrication of prototypes by direct machining in two split blocks and successful testing prove the readiness for complex fabrication of OMT components in this frequency range. The reported performance of our prototype OMT is to our knowledge the best at sub-mm wavelengths over such as wide bandwidth.

Abstract: To undertake THz spectroscopy and imaging, and accurately design and predict the performance of quasi-optical components, knowledge of the parameters of the beam (ideally Gaussian) emitted from a THz source is paramount. Despite its proliferation, relatively little work has been done on this in the frame of broadband THz photoconductive antennas. Using primarily pinhole scanning methods, along with stepwise angular spectrum simulations, we investigate the profile and polarization characteristics of the beam emitted by a commercial silicon-lens-integrated THz photoconductive antenna and collimated by a TPX (polymethylpentene) lens. Our study flags the limitations of the different beam profiling methods and their impact on the beam Gaussianity estimation. A non-Gaussian asymmetric beam is observed, with main lobe beam waists along x and y varying from 8.4 $\,\pm\,$ 0.7 mm and 7.7 $\,\pm\,$ 0.7 mm at 0.25 THz, to 1.4 $\,\pm\,$ 0.7 mm and 1.4 $\,\pm\,$ 0.7 mm at 1 THz, respectively. Additionally, we report a maximum cross-polar component relative to the on -axis co-polar component of $-$ 11.6 dB and $-$ 21.2 dB, at 0.25 THz and 1 THz, respectively.

Abstract: In this article, we present an ultrahigh- Q cavity at terahertz (THz) frequencies. The designed cavity is built on a low-loss suspended silicon (Si) waveguide. The substrate removal under the waveguide and the use of optimized deep reactive ion etching processing are the main reasons for observing very low losses of this design α Q > 18000. Different cavity layouts are adjusted in order to maximize the transmittance while maintaining high Q . A design with reduced number of etched crystal holes achieve Q > 1500 and high transmittance T > 70%. These structures are presented at sub-mm waves (around 600 GHz) for the design of a gas sensor in this frequency region, but the principles can be scaled and redesigned for other frequencies in the THz band.

Abstract: The modeling, design, and experimental evaluation of both a 400-GHz transmitter and receiver submillimeter-wave monolithic integrated circuit (S-MMIC) is presented in this article. These S-MMICs are intended for a radar-based system in the aforementioned operating frequency. The transmitter occupies a total chip area of $750\times 2750\,{\mu {\rm m}^2}$ . It consists of a multiplier-by-four, generating the fourth-harmonic of the WR-10 input signal, which drives the integrated WR-2.2 power amplifier. The latter has an output-gate width of 128 $\mu$ m. The receiver S-MMIC, $750\times 2750\,{\mu {\rm m}^2}$ , consists of a multiplier-by-two, providing the second harmonic of the WR-10 input signal for the local-oscillator port of the subsequent integrated subharmonic mixer. The radio-frequency port of the latter, connects via a Lange coupler to a WR-2.2 low-noise amplifier (LNA). All the components included, are processed on a 35-nm ${\text{InAlAs}}/{\text{InGaAs}}$ metamorphic high-electron-mobility transistor integrated-circuit technology, utilizing two-finger transistors and thin-film microstrip lines (TFMSLs). The modeling approach of the amplifier cores and the respective design decisions taken are listed and elaborated-on in this work. Accompanying measurements and simulations of the transmitter and receiver are presented. The individual components of the aforementioned S-MMICs, are characterized and the results are included in this article. The state-of-the-art, for S-MMIC based circuits operating in the WR-2.2 band, is set by the LNA, on one side, spanning an operational 3-dB bandwidth (BW) of 310 to 475 GHz, with a peak gain of 23 dB and, on the other side, by the final transmitter design, which covers an operating range of 335 to 425 GHz with a peak-output power of 9.0 dBm and accompanying transducer gain of 11 dB. The included transmitter- and receiver-designs represent a first-time implementation in the mentioned technological process, utilizing solely TFMSLs, boasting the integration level, operating in the WR-2.2 frequency band, and setting the state of the art—to the authors’ best knowledge—for all S-MMIC based solutions in the respective frequency band, in terms of output power and gain over the operating 3-dB BW.

Abstract: We present a novel compact wideband waveguide T-junction power divider, especially suited for millimeter-wave and THz frequencies. It incorporates substrate-based elements into a waveguide structure to provide the output port's isolation and matching. The internal port is introduced at the apex of the T-junction formed as an E-probe on a substrate. This facilitates efficient coupling of the reflected energy from the output port to a novel thin-film-based resistive termination integrated with the E-probe onto the same substrate and fabricated by means of thin-film technology. A power divider was designed, simulated, and fabricated for the frequency band 150–220 GHz to experimentally verify the theoretical and simulated performance. The results showed excellent agreement between the simulations and measurements with the devices demonstrating a remarkable return loss of 20 dB for both the input and output ports for a three-port device with equal split and isolation better than 17 dB between the output ports. Furthermore, the measured insertion loss is less than 0.3 dB and the amplitude and phase imbalance are 0.15 dB and 0°, respectively. Moreover, the divider's remarkable tolerance to the dimensions and sheet resistance of the resistive material of the built-in absorbing load makes the device a very practical component for millimeter-wave and THz systems, in particular radio-astronomy receivers.

Abstract: We report on a terahertz (THz) detector based on a photoconductive antenna (PCA) utilizing an artificially strained undoped InGaAs/InAlAs superlattice (SL). Using our laboratory pulsed THz time-domain spectrometer, we demonstrate the advancement of the strain-induced SL-based PCA detector (SID) when operating with an optical probe power of $P_{\text{opt}}>$ 6 mW over the PCA detector based on a lattice-matched InGaAs/InAlAs SL (LMD). Both detectors demonstrate a broad frequency bandwidth of 3.5 THz at the excitation wavelength of 780 nm with a signal-to-noise ratio (SNR) of $\sim$ 70 dB. The experimental results demonstrate a change in the behavior of two detectors: At $P_{\text{opt}}$ = 1 mW, the LMD shows a bit increased SNR compared to that for SID, while vice versa at $P_{\text{opt}}$ = 10 mW. Also, SID shows a quadratic dependence of its SNR on $P_{\text{opt}}$ while the SNR for LMD starts saturating at $P_{\text{opt}}\;\sim$ 5 mW. Moreover, the noise floor in SID is almost independent on probe power, while the noise floor for LMD demonstrates a rapid growth with an increase of $P_{\text{opt}}$ . We believe that SID coupled to a fiber telecommunication wavelength laser could open a pathway toward the development and fabrication of portable and cost-effective THz photoconductive devices.

Abstract: Quasi-optical polarization beam splitters are important components of terahertz instrumentation widely used in interferometric and polarimetric measurements. Recently metasurfaces, i.e., two-dimensional periodic or quasi-periodic optically dense structures composed of unit cells with subwavelength dimensions, have been shown to operate as compact and efficient beam splitters. Typically, their design was based on careful optimization of anisotropic metal or dielectric resonant scatterers confined in each unit cell. In this work, we propose and experimentally demonstrate a simple and useful approach to designing circular-polarization beam splitters taking advantage of intrinsically frequency-independent properties of single-layer self-complementary metasurfaces (SCMSs). Theoretically, when illuminated with a circularly polarized beam, any SCMS at any frequency transmits a copolarized beam with a complex transmission coefficient of 1/2. At the same time, a cross-polarized beam of the same magnitude is produced, with a transmission phase that can be controlled at every point of a metasurface aperture. In this work, to split the copolarized and cross-polarized transmitted beams, we spatially modulate this phase by constructing a spatially nonuniform metasurface of self-complementary unit cells. With this approach, we experimentally demonstrate a focusing circular-polarization beam splitter operating near 0.345 THz.

Abstract: This article presents a low-loss silicon microelectrical mechanical system (MEMS) phase shifter operating in the 500–600 GHz band. The phase shifter consists of a $\text{30-}\mu \text{m}$ thick perforated silicon slab that is moved in and out of a waveguide in the E-plane with a large deflection MEMS actuator. By implementing different hexagonal patterns in the silicon slab, a stepped permittivity is created to impedance match, and thus, reduce return loss. When the silicon slab is inserted into the waveguide, the phase velocity of the incoming wave is decreased, thus resulting in different phase shifts depending on the position of the slab inside the waveguide. The MEMS phase shifter is fully actuated at around $50\,{\text{V}}$ and can move up to $\pm 95\,\mu \text{m}$ , depending on the applied voltage. The insertion loss, when the maximum phase shift is achieved, is measured to be $\text{1.8}\,\text{dB}$ , compared to a $1.6\text{-}\text{dB}$ insertion loss for a waveguide of equivalent length. The return loss is better than $\text{18}\,\text{dB}$ for the desired band. The measured phase shift, with the slab fully inserted into the waveguide at $\text{550}\,\text{GHz}$ was $145^\circ$ . The MEMS phase shifter enables a variety of applications including phased array antenna systems with scanning capability for mapping of planetary surfaces with an electronically steerable antenna.

Abstract: The design, simulation, and characterization of a quasioptical system for submillimeter-wave quantification of corneal thickness and water content are presented. The optics operate in the 220–330 GHz band and are comprised of two, custom aspheric, biconvex lenses in a Gaussian beam telescope configuration. The design produces broadband wavefront curvature matching to 7.5 mm radius of curvature target surfaces thus emulating a plane-wave-on-planar-media condition and enabling application of stratified medium theory to data analysis. Aspheric lens coefficients were optimized with geometric ray tracing subject to optical path length penalties and physical-optics simulations showed aspheric designs achieved wavefront coupling to spherical surfaces, superior to equivalent, canonical hyperbolic lenses. The fabricated lens system was characterized in a planar near-field scanner system and demonstrated good agreement to physical-optics simulations. An average central corneal thickness of 652 μ m and free water content volume of 47% were extracted from ex vivo sheep corneas via complex s -parameters and agree with literature values. Extracted contact lens thickness and water content agreed with independently validated values. Moreover, the quasioptical system enabled observation of dynamic changes in artificial tear-film, thickness, and water content. This work elucidates two major findings related to submillimeter-wave wavefront matching on spherical surfaces, with wavelength order radii of curvature: 1) An asphere whose sag coefficients are optimized via field phase variation on a spherical surface produces coupling superior to a plano-hyperbolic lens. 2) For most feasible apertures, the Gaussian beam waist is located in the aperture near field, suggesting consideration for operating in the beam near field.

Abstract: An improved topology for sub-THz radiation detection realized in 65 nm CMOS, including an on-chip antenna and using zero biasing is presented in this article. The topology is based on a differential Colpitts topology working in reverse-mode and leverages the nonlinearity with respect to vDS in subthreshold operation to rectify. The use of tuned inductors at the gates and sources of the transistor core create degenerative resonance feedback, which further enhances the responsivity while working with zero bias eliminates 1/ f noise to improve the NEP. Measurements demonstrated a voltage responsivity as high as 2 kV/W with a 3 dB RF BW of at least 50 GHz centered at 315 GHz and a record NEP of down to 3.5 pW/√Hz, verified both at zero-IF and using chopping from 0.5 Hz up to 2 kHz. The chip occupies an area of 0.165 mm2 including pads.

Abstract: The performance of a CMOS transmitter designed for planetary science in situ molecular sensing applications having an operational bandwidth of 180–190 GHz and peak output power of 0.6 mW (−2.22 dBm) is evaluated with a series of spectroscopic-based experimental trials. Continuous wave frequency modulated absorption schemes are exploited to probe the Doppler and sub-Doppler lineshape profiles of the water rotational transition at 183.310 GHz. These results demonstrate the tuning finesse and phase-noise characteristics of the integrated circuit embedded phase lock loop used to generate coherent mm-wave radiation are sufficient for high-precision molecular spectroscopy applications. A description of the pulse modulator designed into the CMOS circuitry allowing for implementation of sensitive emission-based Fourier transform detection schemes is provided with performance characterized for spectroscopically relevant pulse durations (40–500 ns). Results are accompanied by a spectral analysis of the transmitter pulse signal leakage, where the total isolation is measured to be 22 dB. The first emission-based molecular detections obtained with this source are presented demonstrating viability for this transmitter to be incorporated into future planned resonant cavity enhanced in situ molecular sensing systems.

Abstract: Current state-of-the-art security video cameras operating in the THz regime employ up to a few hundred detectors together with optomechanical scanning to cover an adequate field-of-view for practical concealed object detection. As a downside, the scanning reduces the integration time per pixel compromising sensitivity, increases the complexity, and reduces the reliability of the system. In contrast to this, we demonstrate a video camera, for the first time, basing its operation on the concept of a fully staring 2-D detector array with a single detector element responsible for a single imaged pixel. The imaging system is built around the detector technology of kinetic inductance bolometers, allowing the operation in the intermediate temperature range $>$ 5 K and the scale-up of the detector count into multikilo-pixel arrays and beyond. The system is designed for a field-of-view of 2 × 1 m $^2$ and an imaging distance of 2.5 m. We describe the main components of the system and show images from concealed object experiments performed at a near-video rate of 9 Hz.

Abstract: We present evidence that in the sub-THz frequency band, human skin can be considered as an electromagnetic bio-metamaterial in which its natural emission is a product of skin tissue geometry and embedded structures. Radiometry was performed on 32 human subjects from 480 to 700 GHz. Concurrently, the subjects were exposed to stress, while heart pulse rate (PS) and galvanic skin response (GSR) were also measured. The results are substantially different from the expected blackbody radiation signal of the skin surface. PS and GSR correlate to the emissivity. Using a simulation model for the skin, we find that the sweat duct is a critical element. The simulated frequency spectra qualitatively match the measured emission spectra and show that our sub-THz emission is modulated by our level of mental stress. This opens avenues for the remote monitoring of the human state.

Abstract: Free-space transmission of terahertz waves open great opportunities for wireless communication and sensing in the Beyond 5G/6G paradigm. Nevertheless, terahertz transmission suffers from severe diffraction losses due to the shorter wavelengths than the microwaves. To compensate for the diffraction losses, point-to-point transmission by directional beams is indispensable. However, implementing terahertz beam steering is still challenging due mainly to the lack of practical phase shifters. To circumvent this challenge, we demonstrate a novel strategy of two-dimensional (2-D) terahertz beam steering based on trajectory deflection of leaky-mode at around 300 GHz. We use a pair of metal plates with a mesh surface, in which the phase velocity exceeds the light speed in free-space. The leaky-mode can be steered vertically by a phase matching condition controllable with frequency sweep and horizontally by graded refractive index controllable with small tilt of the plate. The result confirms 2-D beam steering without relying on phase shifters.

Abstract: Sheet beam vacuum electron tubes are an attractive solution for high-power sources or amplifiers at millimeter-wave and terahertz range. In this article, a high-order coalesced TM11-like mode operation for 220 GHz sheet beam traveling-wave tube is proposed. The feasibility of coalesced TM11-like mode operation is investigated. The utilization of high-order mode expands the RF circuit power capacity, increases the beam current and reduces the cathode emission density. The self-consistent nonlinear code and three-dimensional particle-in-cell simulations are used to verify the 6.7% beam-wave interaction at 220 GHz without phase matching optimization. The maximum output power reaches 370 W. The results demonstrate this methodology is a good solution to build a high-power device for a wide region of terahertz.

Abstract: In this article, a full aperture phase error reconstruction method upon the single range bin data is proposed based on the sparsity-promoting parameter estimation, for the effective motion compensation of synthetic aperture radar (SAR) in terahertz (THz) band. To improve the potential of the method for different forms of motion errors, the motion errors of the SAR platform are generally modeled as variant phase errors in each azimuth sampling points within the full aperture. A sensing matrix including the contribution of the phase errors is successfully derived to sparsely represent the SAR echo. After range compression, an iterative algorithm based on the single range bin data is developed to efficiently estimate the phase errors by updating the reflectivities of the dominant scatterers and the elements of the sensing matrix. Based on the reconstructed phase errors, the motion compensation can be successfully performed for the THz SAR to obtain the highly focused images. For the proof-of-principle experiments, a vehicle-borne SAR at 0.3-THz band with frequency modulation continuous wave transmitter and dechirp heterodyne receiver is developed. The simulation and the experimental results verify the effectiveness of the proposed method.

Abstract: 4GREAT is an extension of the German receiver for astronomy at terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are coaligned on the sky. The frequency bands of 4GREAT cover 491–635, 890–1090, 1240—1525, and 2490–2590 GHz, respectively. This article presents the design and characterization of the instrument, and its in-flight performance. The first light of 4GREAT was on June 2018. It has been offered to the interested SOFIA communities starting with observing cycle 6.

Abstract: Thanks to the large bandwidth availability, millimeter and submillimeter wave systems are getting more attractive to be used in a wide range of applications, such as high-resolution radar or high-speed communications. In this contribution, a new lens antenna in-package solution is presented for the H-band (220–320 GHz), including a wideband quartz-cavity leaky-wave feed combined with an air-bridge chip interconnect technology, based on spray coating and laser lithography. This interconnection acts as a wideband, low-loss transition between the GaAs front-end and the quartz antenna, avoiding the use of expensive waveguide split-blocks. An antenna prototype including the interconnect has been manufactured and characterized, validating the full-wave simulated results for the integrated H-band leaky-wave with aperture efficiency higher than 74% over 34% bandwidth, and radiation efficiency higher than 70% over 37% of bandwidth.

Abstract: In this letter, a novel 1/f noise mitigation technique is presented to improve the receiver 1/f noise performance of a 670 GHz receiver. The time-domain 1/f noise corrected samples are compared with the samples obtained without the correction. The spectral-domain analysis shows that the 1/f noise mitigation method improves the receiver noise performance by 19 dB in the receiver under test. The presented 1/f noise mitigation technique can be applied to any direct-detection receiver in the terahertz frequency range.

Abstract: Normally, the conventional sheet beam extended interaction oscillator (SB-EIO) operates in a single mode and has a narrow tunable bandwidth. To broaden the tunable bandwidth, the concept of multiple-mode operation is proposed and a 0.3-THz ladder-shaped interaction circuit is designed as described in this article. The interaction circuit was manufactured by using the computer numerical control milling technique and cold tested by using a vector network analyzer. Measured and simulated results were in good agreement with each other. Beam-wave interaction simulations with the ohmic loss being taken into account verified the feasibility of the multiple-mode operation and demonstrated that the SB-EIO produced an output power of >91 W in the frequency range of 293.7–294.9 GHz. Compared with the single-mode SB-EIO, the tunable bandwidth of the multiple-mode SB-EIO has increased from 0.3 to 1.2 GHz.

Abstract: This article presents measurements of the permittivity of gelatin hydrogels between 220 and 330 GHz. Hydrated gelatin was treated as a binary mixture of free water and a compound consisting of water bound to collagen. Submillimeter-wave reflectometry was used to estimate the hydrated gelatin permittivity, hydrated gelatin density, and free-water volume fraction in phantoms composed of 62, 67, 72, and 77% water by weight. A hydrated dry/wet density ratio of 0.335 was validated with optical-coherence tomography. A constant nonfreezing bound-water mass of 0.6 g/g was observed and confirmed with differential-scanning calorimetry. Good agreement between results from different modalities supports the dielectric spectroscopy methods and data analysis. Depending on the hydrodynamics at the sample/air interface, measurements indicate a bound-water compound permittivity of 3.77−j2.52 to 3.95−j2.49—contrasting the pure-water average permittivity of 5.16–j5.63. The loss related to bound water was much higher than anticipated and characterization will help reduce uncertainty in measurements of gelatin hydrogel-based tissue phantoms; particularly corneal phantoms where adjacent free water creates complex hydration gradients. This is the first known, submillimeter-wave, frequency domain measurement of complex permittivity of the bound-water component in solid, proteinaceous matter.