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Showing papers in "IEEE Transactions on Terahertz Science and Technology in 2021"


Journal ArticleDOI
TL;DR: In this paper, effective medium-clad dielectric waveguides with low loss and low dispersion were investigated for terahertz-integrated platforms, and the results showed an average measured attenuation coefficient of 0.075 dB/cm and a group velocity dispersion ranging from around 10ps/THz/mm across the whole band.
Abstract: Effective-medium-clad dielectric waveguides are purely built into a single high-resistivity float-zone silicon wafer with their claddings defined by deep subwavelength perforations. The waveguides are substrate-free while supporting both $E_{11}^x$ and $E_{11}^y$ modes with low loss and low dispersion. This article extends the investigations of the waveguides by analyzing the dispersion, cross-polarization, and crosstalk together with the characteristics of bends and crossings over an operation frequency range of 220–330 GHz (WR-3 band). Taking the $E_{11}^x$ mode as an example, the experimental results show an average measured attenuation coefficient of 0.075 dB/cm and a group velocity dispersion ranging from around $\pm$ 10 ps/THz/mm across the whole band. A crosstalk level below $-$ 10 dB is measured for parallel waveguides with a separation of 0.52 $\lambda _{0}$ at 300 GHz. The realized waveguides show a bending loss ranging from 0.500 to 0.025 dB per bend and a crosstalk at crossing below $-$ 15 dB from 220 to 330 GHz. Due to the different dispersion characteristics, the $E_{11}^y$ mode has similar performances but with its operation frequency range reduced to 260–330 GHz. Limited by the measurement setup, a cross-coupling between the $E_{11}^x$ and $E_{11}^y$ modes is measured to be below $-$ 20 dB over the whole band. This in-depth investigation of effective-medium-clad waveguides will form a basis for terahertz-integrated platforms.

33 citations


Journal ArticleDOI
TL;DR: In this paper, a hybrid electromechanical scanning lens antenna array architecture is proposed for the steering of highly directive beams at submillimeter wavelengths with field-of-views (FoV) of ±25°.
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.

30 citations


Journal ArticleDOI
TL;DR: In this paper, the indium phosphide (InP)-based leaky-wave antennas (LWAs) are used for beam steering in mobile THz applications. But, the InP-based LWAs are based on polymer or graphene substrates and thus, it is quite impossible to monolithically integrate these antennas with state-of-the-art InPbased photonic or electronic THz sources and receivers.
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.

27 citations


Journal ArticleDOI
TL;DR: In this article, a fully photonics-based heterodyne subterahertz (sub-THz) system for wireless communications is presented, where 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.
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.

21 citations


Journal ArticleDOI
TL;DR: In this paper, a hollow core antiresonant photonic crystal fiber is analyzed for terahertz applications, and a numerical analysis of the proposed fiber is first carried out to minimize coupling between the core and cladding modes.
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.

20 citations


Journal ArticleDOI
TL;DR: In this article, an ultrahigh-Q cavity at terahertz (THz) frequencies is presented, which is built on a low-loss suspended silicon (Si) waveguide.
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.

17 citations


Journal ArticleDOI
TL;DR: In this paper, a two-parabolic-mirror system and a four-paraboloid-based optical path were designed and analyzed to achieve precision complex permittivity measurements of a dielectric substrate.
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.

14 citations


Journal ArticleDOI
TL;DR: In this paper, a transistor-based direct-detection receiver for submillimeter-wave applications is presented, which exhibits a noise figure of 11.4 dB with a total dc power consumption of 0.25 W.
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.

14 citations


Journal ArticleDOI
TL;DR: In this paper, a high-performance wideband double-ridged waveguide orthomode transducer (OMT) for the 275-500 GHz band has been designed, fabricated, and tested.
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.

14 citations


Journal ArticleDOI
TL;DR: In this paper, 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.
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.

14 citations


Journal ArticleDOI
TL;DR: In this paper, 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 are investigated.
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.

Journal ArticleDOI
TL;DR: In this paper, the authors extracted absolute thickness and free-water-content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature and aqueous backing via coherent submillimeter-wave reflectometry at 220-330 GHz.
Abstract: Absolute thickness and free-water-content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature and aqueous backing were extracted via coherent submillimeter-wave reflectometry at 220–330 GHz. Fourier-domain-based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10-3 and 0.1°, respectively, for signal-to-noise ratios (SNRs) between 40 and 50 dB. Total 12 phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical-coherence tomography-based sphericity metrics via image segmentation. The extracted free-water volume fraction varied less than 5% in the five phantoms whose fabrication yielded the most spherical geometry. Submillimeter-wave-based thickness accuracy was better than 111 μ m (∼λ/9) with an average of 65 μ m (∼λ/17) and standard deviation of 44 μ m (∼λ/25) for phantoms with physiologically relevant geometry. Monte-Carlo simulations of measurement noise and uncertainty limits agree with the experimental data analysis and indicate a lower thickness accuracy limit of 33 μ m, and water-content sensitivities of 0.5% and 11.8% for the anterior and posterior segments, respectively. Numerical analysis suggests that the measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produces statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique, which is able to extract both the thickness and water-content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.

Journal ArticleDOI
TL;DR: In this paper, a quasi-optical system for submillimeter-wave quantification of corneal thickness and water content is presented, which is comprised of two custom aspheric, biconvex lenses in a Gaussian beam telescope configuration.
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.

Journal ArticleDOI
TL;DR: In this article, the concept of a fully-staring 2D detector array with a single detector element responsible for a single imaged pixel is introduced. But the system is designed for a field-of-view of 2 × 1/m $^2$ and an imaging distance of 2.5m.
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.

Posted ContentDOI
TL;DR: In this paper, a planar Schottky diode with a submicron anode contact area defined on a suspended 2-m ultra-thin gallium arsenide substrate was used for frequency stabilization of terahertz sources.
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.

Journal ArticleDOI
TL;DR: In this article, a thin-film-based power divider for millimeter-wave and THz frequency bands was proposed. But the performance of the power dividers is limited to the frequency band 150-220 GHz.
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.

Journal ArticleDOI
TL;DR: In this article, a novel 1/f noise mitigation technique is presented to improve the receiver 1/F noise performance of a 670 GHz receiver, which can be applied to any direct-detection receiver in the terahertz frequency range.
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.

Journal ArticleDOI
TL;DR: In this paper, a high-order coalesced TM11-like mode operation for 220 GHz sheet beam traveling-wave tube is proposed, and the feasibility of coalescedTM11-based mode operation is investigated.
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.

Journal ArticleDOI
TL;DR: 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, intended for a radar-based system in the aforementioned operating frequency.
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.

Journal ArticleDOI
TL;DR: In this paper, a long-path terahertz (THz) communication channel is characterized in the frequency range of 0.32-1.1 GHz using a custom, silicon-based THz pulse radiator chip.
Abstract: The terahertz (THz) band has opened up a new frontier for high-speed, wireless communication, high-resolution radars, and highly precise remote sensing. Identifying the low-loss atmospheric windows is vital for these applications. In this study, a long-path THz communication channel is characterized in the frequency range of 0.32–1.1 THz using a custom, silicon-based THz pulse radiator chip. The chip radiates 1.7-ps pulses via an on-chip antenna at a repetition rate of 8 GHz, resulting in a broadband 0.1–1.1 THz frequency comb. It is fabricated in 130-nm SiGe BiCMOS process and consumes 45 mW of dc power. A specular link was created using the impulse radiator, parabolic reflector antennas, a plane mirror, and a downconverter mixer. The THz channel was characterized up to a distance of 110 m. The measurement results demonstrate channel path loss, atmospheric absorption, and low-loss frequency windows suitable for wireless links in the THz range. The results correspond well with the HITRAN database [1].

Journal ArticleDOI
Shuyun Shi1, Chao Li1, Jianmin Hu1, Xiaojuan Zhang1, Guangyou Fang1 
TL;DR: 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 and can be successfully performed for the THz SAR to obtain the highly focused images.
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.

Journal ArticleDOI
TL;DR: In this article, a submillimeter-wave reflectometry was used to estimate the permittivity of gelatin hydrogels between 220 and 330 GHz, and a constant nonfreezing bound-water mass of 0.6 g/g was observed and confirmed with differential-scanning calorimetry.
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.

Journal ArticleDOI
TL;DR: In this paper, the characteristic absorption spectra of Vanillin and ortho-Vanillin were investigated by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4-2.0 THz.
Abstract: In this article, the characteristic absorption spectra of Vanillin and ortho-Vanillin were investigated by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4–2.0 THz. Using quantum chemistry tools, density functional theory calculations were performed to obtain the theoretical THz spectra. To better understand the THz absorption spectra on a molecular level and to systematically and comprehensively interpret the origin of the characteristic absorption peaks, the vibrational characteristics and the intermolecular weak interactions of these two compounds were analyzed using a cluster model. We used the vibrational mode automatic relevance determination method to assign the vibrational modes of all absorption peaks and provide the percentage contribution of these modes. The vibrational modes of Vanillin and ortho-Vanillin were mainly dihedral angle torsion. The intermolecular weak interactions were analyzed using the independent gradient model based on Hirshfeld partition of molecular density method. The results indicated that the weak interaction types of Vanillin and ortho-Vanillin were dominated by van der Waals interaction. This work has demonstrated that using THz-TDS combined with quantum chemical calculations is an effective way to identify and investigate isomers. This method has significant potential applications for exploring the relationship between the biomolecular structure and the material performance.

Journal ArticleDOI
TL;DR: A terahertz fundamental oscillator IC with high efficiency and compact chip size based on an indium phosphide (InP)-based resonant tunneling diode (RTD) is proposed in this article.
Abstract: A terahertz (THz) fundamental oscillator IC with high efficiency and compact chip size based on an indium phosphide (InP)-based resonant tunneling diode (RTD) is proposed. The oscillator is designed by utilizing a low-frequency bias stabilizer, along with utilizing a negative differential resistance oscillator topology. The oscillator is fabricated by using an InP monolithic THz integrated circuit multilayer process. The fabricated oscillator shows a high dc-to-RF power efficiency ( η ) of 0.274 % with a low dc power consumption of 3.4 mW and an RF output power of 9.3 μ W at an oscillation frequency of 692 GHz. The IC exhibits a chip size of 300 × 380 μ m2 excluding the measurement pads. The achieved η and chip size are good values among THz oscillators above 500 GHz reported to date.

Journal ArticleDOI
TL;DR: In this paper, a focusing circular-polarization beam splitter based on a spatially nonuniform metasurface of self-complementary unit cells is proposed.
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.

Journal ArticleDOI
TL;DR: In this paper, the authors show that 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.
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.

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TL;DR: In this article, 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.
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.

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TL;DR: In this paper, a 2D terahertz beam steering based on trajectory deflection of leaky-mode at around 300 GHz is proposed, in which the phase velocity exceeds the light speed in free space.
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.

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TL;DR: In this paper, a superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy.
Abstract: A superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy. The proposed filter geometry has a free spectral range of an octave of bandwidth without introducing spurious resonances, reaches a high coupling efficiency in the pass-band and shows very high rejection in the stop-band to minimize reflections and cross-talk with other filters. A spectrally sparse prototype filter-bank in the band 300–400 GHz has been developed employing these filters as well as an equivalent circuit model to anticipate systematic errors. The fabricated chip has been characterized in terms of frequency response, reporting an average peak coupling efficiency of 27% with an average spectral resolution of 940.

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TL;DR: In this article, the authors achieved high identification accuracy of reagents hidden by thick shielding materials, by combining injection-seeded terahertz (THz) wave parametric generator measurements and machine learning analysis.
Abstract: We achieved high identification accuracy of reagents hidden by thick shielding materials, by combining injection-seeded terahertz (THz) wave parametric generator measurements and machine learning analysis. The analysis performance of three methods, support vector machine (SVM), k-nearest neighbor, and random forest, was compared in an attempt to identify the optimal approach. SVM proved to be the best model. Conventional systems could only identify reagents through premeasured shields; however, incorporation of machine learning allowed us to identify the reagents through shielding materials that had not been premeasured. Moreover, spectroscopic imaging of the reagents revealed the distribution pattern of the reagents, even through thick shielding materials that attenuated THz frequencies such that they were close to the noise level.