Search

This paper presents a high-power 240–290 GHz waveguide enclosed two-dimensional (2-D) grid heterostructure barrier varactor (HBV) frequency multiplier. A 35 mW of output power is produced at 247 GHz with an input power of 900 mW. The operational bandwidth is tunable within a 50 GHz span by the use of an input tuner able to adjust the input matching of the 2-D grid HBV frequency multiplier. Tuning is achieved by moving a suspended dielectric slab in the input waveguide.

An empirical model for field-effect transistor (FET) based power detectors is presented. The electrical model constitutes a Volterra analysis based on a Taylor series expansion of the drain current together with a linear embedding small-signal circuit. It is fully extracted from S-parameters and IV curves. The final result are closed-form expressions for the frequency dependence of the noise equivalent power (NEP) in terms of the FET intrinsic capacitances and parasitic resistances. Excellent model agreement to measured NEP of coplanar access graphene FETs with varying channel dimensions up to 67 GHz is obtained. The influence of gate length on responsivity and NEP is theoretically and experimentally studied.

This paper presents a compact varactor grid frequency doubler encapsulated in a waveguide environment, thus providing single mode (H10) waveguide connection at both input and output. Schottky diodes are used as varactors in this 128-element grid frequency doubler. By packaging the grid and its embedding network together with a stepped waveguide taper on the output, a module measuring 9 mm $\times $ 19 mm by 19 mm is created. A peak output power of 0.25 W is produced at 183 GHz with 1.32 W of input power and a corresponding conversion efficiency of 19%. The peak conversion efficiency is 23% at 183 GHz with 666 mW of input power.

This letter presents a graphene field effect transistor (GFET) detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz 1/2. This letter shows how the detector performance degrades as a function of the residual carrier concentration in the graphene channel, which is an important material parameter that depends on the quality of the graphene sheet and contaminants introduced during the fabrication process. In this work, the exposure of the graphene channel to liquid processes is minimized resulting in a low residual carrier concentration. This is in part, an important contributing factor to achieve the record high GFET detector performance. Thus, our results show the importance to use graphene with high quality and the importance to minimize contamination during the fabrication process.

Low-frequency vibrations are crucial for protein structure and function, but only a few experimental techniques can shine light on them. The main challenge when addressing protein dynamics in the terahertz domain is the ubiquitous water that exhibit strong absorption. In this paper, we observe the protein atoms directly using X-ray crystallography in bovine trypsin at 100 K while irradiating the crystals with 0.5 THz radiation alternating on and off states. We observed that the anisotropy of atomic displacements increases upon terahertz irradiation. Atomic displacement similarities develop between chemically related atoms and between atoms of the catalytic machinery. This pattern likely arise from delocalized polar vibrational modes rather than delocalized elastic deformations or rigid-body displacements. This method can ultimately reveal how the alignment of chemically related atoms and the underlying polar vibrational dynamics make a protein structure stable.

In this study, simulations and measurements were used to investigate stray light properties of the three-mirror off-axis telescope of a new satellite for atmospheric research called Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS). A 700 mm breadboard baffle for stray light rejection has been designed and tested. Good performance was achieved by coating the baffle’s inside with Vantablack S-VIS R , which has a hemispherical reflectance of 0.2-0.6% across the instrument’s detection band (270-776 nm). A point source transmittance (PST) down to 10−6 was measured for the full-size baffle breadboard. This is in excellent agreement with simulations performed in OpticStudio/LightTools, where scattering was modeled using empirical BRDF data. From the breadboard results, a simulation model of a flight-representative prototype model of the entire instrument was set up in OpticStudio. Strong signals just outside the field of view constitute the biggest challenge, where a PST in the order of 10-6 − 10-4 is required. Simulations suggest that the PST of the prototype limb instrument will be lower than this. Adding to these simulations, an instrument model was developed, which will be utilized by the end-users to remove unwanted features in the data stemming from the instrument itself. Besides stray light, the model also takes into account the most relevant aspects of the instrument, such as image resolution (from measured/simulated point spread functions), image sensor characteristics as well as temperature and wavelength dependencies.

In many cases, metallic planar waveguides are required in the design of integrated circuits. However, at terahertz frequencies, metallic planar waveguides present high losses, which make necessary the use more efficient waveguides to avoid power limitations. In this work, the attenuation constant of two popular planar waveguides for terahertz frequencies, Coplanar Waveguide (CPW) and Planar Goubau Line (PGL), are compared between 0.75 THz and 1.1 THz. To measure the PGL, its transition is deembeded using a multiline Thru-Reflect-Line calibration standard. Measurement results show a lower attenuation constant across the band for a PGL $(0.13\ \mathrm{mm}^{-1} than for a CPW $(0.68\ \mathrm{mm}^{-1} when an ultra-thin substrate is used suspended in air, which greatly reduces the substrate mode coupling from the PGL. These results put the PGL as a less lossy metallic planar waveguide for terahertz applications.

We developed process technologies dedicated to high frequency graphene field-effect transistors (GFETs). We used graphene from different sources (graphene growth directly on silicon carbide (SiC) substrate by chemical vapor deposition (CVD) technique [1], CVD graphene growth on Cu foil and transferred on Si02/Si substrate [2]) to realize two GFET structures (top gate and back gate transistors). After fabrication, we have explored the high frequency performances of GFETs including noise performances. The performances of GFET achieved are comparable to the state of the art with the gate length. The noise parameters of the transistors are mandatory for circuit design (linear application such as amplifier). These parameters limit the performances of the device when small signals are considered. The effort was made here to measure the noise parameters of GFET $(\mathrm{NF}_{\min}, \mathrm{R}_{\mathrm{n}}, \Gamma_{\mathrm{opt}})$ . It is well known that these parameters depend both on intrinsic properties of the graphene material (noise generated in the channel under the gate), as well as extrinsic parameters related to the technological process (mainly noise generated by access resistances, …). Based on the reliability of the process, RF circuits were designed and fabricated. These circuits demonstrated the potential to achieve high performances in the field of linear circuit such as amplifiers, and non-linear circuits such as mixers.

We report on the development of two 874 GHz receiver channels with orthogonal polarizations for the international submillimetre airborne radiometer. A spline horn antenna and dielectric lens, a Schottky diode mixer circuit, and an intermediate frequency (IF) low noise amplifier circuit were integrated in the same metallic split block housing. This resulted in a receiver mean double sideband (DSB) noise temperature of 3300 K (minimum 2770 K, maximum 3400 K), achieved at an operation temperature of 40 C and across a 10 GHz wide IF band. A minimum DSB noise temperature of 2260 K at 20 C was measured without the lens. Three different dielectric lens materials were tested and compared with respect to the radiation pattern and noise temperature. All three lenses were compliant in terms of radiation pattern, but one of the materials leads to a reduction in a noise temperature of approximately 200 K compared to the others. The loss in this lens was estimated to be 0.42 dB. The local oscillator chains have a power consumption of 24W and consist of custom-designed Schottky diode quadruplers (5% power efficiency in operation, 8%-9% peak), commercial heterostructure barrier varactor (HBV) triplers, and power amplifiers that are pumped by using a common dielectric resonator oscillator at 36.43 GHz. Measurements of the radiation pattern showed a symmetric main beam lobe with full width half maximum

We present a multiline Thru-Reflect-Line (TRL) calibration standard for Planar Goubau Line (PGL) which allows setting the calibration plane along the PGL and thus removing the effect of the embedding structure. This opens the possibility of characterizing PGL-circuits. The presented structures were used for calibrating S-parameters measurements between 0.75 THz and 1.1 THz to characterize a 1 mm long and $10\ \mu \mathrm{m}$ wide PGL. The line shows negligible dispersion with an effective relative permittivity of 2.0 and an attenuation constant lower than 0.35 Np/mm $(0.65\ \mathbf{dB}/\lambda)$ .

We present a 220 GHz imaging radar prototype that has been developed in the European Defense Agency (EDA) project TIPPSI. The purpose of the development was to demonstrate short-range high-resolution 3D imaging for security applications at checkpoints, and to guide the development of stand-off real-time millimeter wave and sub-millimeter wave imaging systems for detection of larger objects at greater distances. An additional goal was to experimentally verify simulation techniques for active (sub)-mmw imaging systems, the verified simulation techniques can then be used to explore different system architectures. The 220 GHz imaging radar prototype consist of a flexible, mechanically scanned optical system that can support linear arrays of transmit/receive (TxRx) units up to 150 mm in length. The optical system is divided into two parts: A compact Dragonian system including the mechanical scanner that can be used as a stand-alone imager at reduced target distance and resolution, and a confocal system that can be added to achieve the full resolution of 1 cm x 1 cm x 1 cm at 4.5 m target distance. The field of view of the full resolution system is 70 cm x 70 cm. The front-end is currently populated by 4 TxRx units that are sparsely distributed along the 150 mm focal plane. The TxRx units operate in frequency modulated continuous wave (FMCW) mode and have a bandwidth of 24 GHz. Each TxRx unit use a single horn antenna and the transmit- and receive signals are generated and received using the same circuits which avoids the need of a duplexer. We will demonstrate high resolution 3D videos taken at 1 Hz frame rate and compare the individual images with simulations using electromagnetic simulators and character/clothes animation.

We have developed a test structure for evaluating the quality of Al 2 O 3 gate dielectrics grown on graphene for graphene field effect transistors on flexible substrates. The test structure consists of a metal/dielectric/ graphene stack on a PET substrate and requires only one lithography step for the patterning of the topside metal electrodes. Results from measurements of leakage current, capacitance and loss tangent are presented.

A receiver composed by a graphene FET 200-GHz mixer and a 1-GHz intermediate frequency amplifier integrated on a silicon substrate was modelled, fabricated and characterized. This is the first demonstration of a millimeter wave integrated receiver based on graphene FETs. The receiver conversion loss is measured to be 25 dB across the 185-205-GHz band with 16 dBm of local oscillator pump power, which is in good agreement with the circuit simulations. The simulations show that the receiver conversion loss can be significantly reduced to 16 dB by reducing the contact resistance and by realizing a higher charge carrier mobility in the mixer transistor. As high-speed communication systems are evolving, the need for new materials and technologies arises, which allow for the development of advanced millimeter wave and terahertz circuitry [1], [2]. In this context, a receiver is an important system component. A typical heterodyne receiver consists of an antenna, filters, amplifiers, and a down converting mixer. Usually, the amplifiers and the mixers are based on field-effect transistors (FETs) [3]. To realize high speed amplifier FETs the charge carrier mobility and velocity in the transistor channel should be high. In this context, the 2D material graphene is a promising candidate, since it reveals high room temperature charge carrier mobility and saturation velocity [4]. Graphene FETs (GFETs) with a current and power gain comparable to that of Si n-channel MOSFETs have been demonstrated [5]. Still, the power gain of GFETs lays behind that of InP and GaAs transistors, partly, due to the lack of a bandgap in graphene. However, the application of GFETs in subharmonic resistive mixers utilizes the unique symmetric resistance characteristics of GFETs (attributed to the lack of the bandgap) allowing a simplified receiver circuitry, since no balun is required. The simplification of the circuitry and the potential of graphene being grown in large scale and being transferred to any arbitrary substrate, explains the interest of developing a receiver circuit fully based on GFETs. In our recent works a 10-dB small-signal amplifier designed for 1 GHz and a 200 GHz subharmonic resistive mixer based on graphene FETs (GFETs) have been demonstrated [6], [7]. In this work the GFET -mixer and IF amplifier are integrated together on a silicon substrate chip, as shown in Fig. 1 and the characterization results are presented.

The papers in this special issue were presented at the 30th International Symposium on Space Terahertz Technology (ISSTT 2019) that took place in Gothenburg, Sweden, on April 15–17, 2019.

We present a full wave, large-signal analysis of a waveguide-embedded frequency multiplier array, solving the entire geometry using FEM EM modeling and including every nonlinear element in a Harmonic Balance simulation. The full 3D model employed provides the exact field distribution across the array, thus enabling us to account for substrate resonances, higher order mode excitation, instabilities, and diode yield. The model is compared with measurements on a 247 GHz fixed tuned 72-diode HBV tripler. In this example, higher order mode excitation was found due to the interaction between the filter and the diode array. This can only be observed using a full 3D model, which more accurately predicts the output power versus input power and frequency. Finally, the pros and cons of a full 3D model and a unit cell model for waveguide embedded multiplier arrays are discussed.

We present a THz heterodyne detector based on a single layer graphene field effect transistor (GFET) integrated with a bowtie antenna at 400 GHz. The heterodyne detection is achieved by coupling RF and LO signals quasi-optically to the same GFET. The down converted IF signal is extracted via a coplanar stripline connected to the GFET source and drain terminals. The measured IF bandwidth is 5 GHz.

There is a high need for >2 THz room temperature direct-detectors/harmonic-mixers to characterize THz optical sources, phase lock THz-QCLs as LO sources for multi-pixel receivers, and realize absolute frequency calibration sources for applications in astrophysics, earth science, and remote sensing. Thus, we have developed a WM-52 (WR-0.22) harmonic mixer for the 3–5 THz operation. The design, fabrication and assembly of the THz mixer are discussed.

We present a novel waveguide-to-suspended stripline loop-probe transition operating over the entire WR-1.0 waveguide band. The loop probe is designed for broadband response with simulated RL > 15 dB, and has an integrated DC return path, which can also be extended for biasing. The measured insertion loss for a back-to-back configuration is 1–2 dB in almost the entire frequency range of 750–1100 GHz.

We present a flexible terahertz (THz) detector based on a graphene field-effect transistor fabricated on a plastic substrate. At room temperature, this detector reveals voltage responsivity above 2 V/W and estimated noise equivalent power (NEP) below 3 nW/ Hz at 487 GHz. We have investigated the effects of bending strain on DC characteristics, voltage responsivity, and NEP of the detector, and the results reveal its robust performance. Our findings have shown that graphene is a promising material for the development of THz flexible technology.

We present terahertz (THz) detectors based on top-gated graphene field effect transistors (GFETs) with integrated split bow-tie antennas. The GFETs were fabricated using graphene grown by chemical vapor deposition (CVD). The THz detectors are capable of room-temperature rectification of a 0.6 THz signal and achieve a maximum optical responsivity better than 14 V/W and minimum optical noise-equivalent power (NEP) of 515 pW/Hz(0.5). Our results are a significant improvement over previous work on graphene direct detectors and are comparable to other established direct detector technologies. This is the first time room-temperature direct detection has been demonstrated using CVD graphene, which introduces the potential for scalable, wafer-level production of graphene detectors.

This paper describes a single flange 2-port measurement setup for S-parameter characterization of waveguide integrated devices. The setup greatly reduces calibration and measurement uncertainty by eliminating vector network analyzer (VNA) extender cable movement and minimizing the effect of waveguide manufacturing tolerances. Change time of standards is also improved, reducing the influence of VNA drift on the uncertainty. A TRL calibration kit has been manufactured and measurements are demonstrated in WR-03 (220–325 GHz).

This paper presents a new method for thermal characterization of THz Schottky diodes. The method is based on the transient current behavior, and it enables the extraction of thermal resistances, thermal time-constants, and peak junction temperatures of THz Schottky diodes. Many typical challenges in thermal characterization of small-area diode devices, particularly those related to self-heating and electrical transients, are either avoided or mitigated. The method is validated with measurements of commercially available single-anode Schottky varactor diodes. A verification routine is performed to ensure the accuracy of the measurement setup, and the characterization results are compared against an in-house measurement-based method and against simulation results of two commercial 3-D thermal simulators. For example, characterization result for the total thermal resistance of a Schottky diode with an anode area of 9 μm2 is within 10% of the average value of 4020 K/W when using all four approaches. The new method can be used to measure small diode devices with thermal time constants down to about 300 ns with the measurement setup described in the paper.

We present a silicon integrated Heterostructure Barrier Varactor (HBV) frequency quintupler ( $\times$ 5) operating between 440 GHz and 490 GHz. By epitaxial transfer of InP-based HBV material structure onto silicon-on-insulator (SOI), a uniform and accurate thickness (20 $\mu{\hbox{m}}$ ) of the frequency quintupler chip is achieved. In a single stage this device delivers 2.8 mW of output power at 474 GHz, when pumped with 400 mW at 94.75 GHz, corresponding to conversion efficiency of 0.75%. The present device exhibits a 3-dB bandwidth of 4%.

We present a 220-GHz homodyne transceiver module intended for frequency modulated continuous wave radar applications. The RF transceiver circuits are fabricated on 3-μm-thick GaAs membranes, and consist of a Schottky diode based transmitter frequency doubler that simultaneously operates as a sub-harmonic down-converting mixer. Two circuits are used in a balanced configuration to improve the noise performance. The output power is > 3 dBm over a 40-GHz bandwidth (BW) centered at 220 GHz, and the receiver function is characterized by a typical mixer conversion loss of 16 dB. We present radar images at 4-m target distance with up to 60-dB dynamic range using a 30-μs chirp time, and near-BW-limited range resolution. The module is intended for applications in high-resolution real-time 3-D radar imaging, and the unit is therefore designed so that it can be assembled into 1-D or 2-D arrays.

We present a sensitivity analysis on TRL calibrated S-parameter measurements of membrane circuits in the WR-03 waveguide band (220-325 GHz). The impact of waveguide and membrane circuit misalignment, as well as waveguide dimension mismatch is investigated. The analysis is performed for the thru-reflect-line (TRL) calibration applied to E-plane split waveguide blocks carrying membrane circuits. The analysis shows a large influence of the waveguide width tolerance on transmission and reflection phase after the TRL calibration. For a 20 mm long rectangular waveguide with a ± 5 μm width tolerance a phase uncertainty as large as ± 45° for reflection and ± 30° for transmission measurements is observed.

A noise bandwidth (NBW) of 6-7 GHz was obtained for hot-electron bolometer (HEB) mixers made of 10 nm MgB2 films. A systematic investigation of the (IF) gain bandwidth as a function of the MgB2 film thickness (30, 15, and 10 nm) is also presented. The gain bandwidth (GBW) of 3.4 GHz was measured for a 10 nm film, corresponding to a mixer time constant of 47 ps. For 10 nm films a reduction of the GBW was observed with the reduction of the critical temperature (Tc). Experimental data were analyzed using the two-temperature model. From the theoretical analysis, the electron-phonon time (τe - ph), the phonon escape time (τesc) and the electron and phonon specific heats (ce, cph) were extrapolated giving the first model for HEB mixers of MgB2 films.

This paper reports on photodetection properties of the graphene-Si schottky junction by measuring current–voltage characteristics under 1.55- $\mu{\rm m}$ excitation laser. The measurements have been done on a junction fabricated by depositing mechanically exfoliated natural graphite on top of the pre-patterned silicon substrate. The electrical Schottky barrier height is estimated to be (0.44–0.47) eV with a minimum responsivity of 2.8 mA/W corresponding to an internal quantum efficiency of 10%, which is almost an order of magnitude larger than regular Schottky junctions. A possible explanation for the large quantum efficiency related to the 2-D nature of graphene is discussed. Large quantum efficiency, room temperature IR detection, ease of fabrication along with compatibility with Si devices can open a doorway for novel graphene-based photodetectors.

We present an analytic method to extract Schottky diode parasitic model parameters. All the ten unknown model parameters are extracted via a straightforward step-by-step procedure. The challenges for a proper finger inductance and series resistance extraction are discussed and solutions are recommended. The proposed method is evaluated using three sets of $S$ -parameter data for GaAs-based planar Schottky diodes, i.e., data from measurement up to 110 GHz and 3-D electromagnetic full-wave simulations up to 600 GHz. The extracted models agree well with the measured and simulated data.

A 30-GHz integrated subharmonic mixer based on a single graphene field-effect transistor (G-FET) has been designed, fabricated, and characterized. The mixer is realized in microstrip technology on a 250- μm-high-resistivity silicon substrate. In order to enhance the current on-off ratio, the G-FET utilizes a channel consisting of an array of bow-tie structured graphene, yielding a current on-off ratio of 7. A conversion loss (CL) of 19 ± 1 dB over the frequency range of 24-31 GHz is obtained with a local oscillator (LO) to RF isolation better than 20 dB at an LO power of 10 dBm. The overall minimum CL is 18 dB at 27 GHz. The mixer has a 3 GHz ± 1-dB IF bandwidth, which is achieved with a fixed LO signal of 15 GHz. The mixer linearity is characterized and the highest third-order intercept point is measured to be 12.8 dBm.

We report on the first complete RF characterization of graphene field-effect transistor subharmonic resistive mixers in the frequency interval fRF=2-5 GHz . The analysis includes conversion loss (CL), noise figure (NF), and intermodulation distortion. Due to an 8-nm thin Al2O3 gate dielectric, the devices operate at only ~ 0 dBm of local oscillator (LO) power with an optimum measured CL in the range of 20-22 dB. The NF closely mimics the CL, thus determining the noise to be essentially thermal in origin, which is promising for cryogenic applications. The highest input third-order intercept point is measured to be 4.9 dBm at an LO power of 2 dBm.

A room-temperature Schottky diode-based WM-86 (WR-0.34) harmonic mixer was developed to build high-resolution spectrometers, and multipixel receivers in the terahertz (THz) region for applications such as radio astronomy, plasma diagnostics, and remote sensing. The mixer consists of a quartz-based local oscillator (LO), intermediate-frequency (IF) circuits, and a GaAs-based beam-lead THz circuit with an integrated diode. Measurements of the harmonic mixer were performed using a 2 THz solid-state source and 2.6906 THz QCL. A conversion loss of 27 dB for the third harmonic mixing and a conversion loss of 30 dB for the fourth harmonic mixing were achieved. This is the first development of a wideband WM-86 (WR-0.34) harmonic mixer with a planar Schottky diode integrated on a beam-lead THz circuit that uses a lower LO harmonic factor for 1.8–3.2 THz RF frequency. Furthermore, this result represents the best Schottky-based mixer in this frequency range.

In this paper, measurements with a THz imaging system at 346 GHz together with electromagnetic simulations have demonstrated the capability of THz sensing for the non-destructive density determination of intermediate products in the pharmaceutical industry. The results confirm the ability of THz imaging to detect density changes in pharmaceutical samples by measuring the phase variation of the wave transmitted through the sample.

We present a solution where one single LO chain is used to feed a homodyne FMCW radar transceiver. An InGaAs pHEMT active frequency multiplier MMIC (x8) and a Schottky diode frequency doubler make up the LO chain. The novel Schottky diode based transceiver operates both as a frequency multiplier (x2) and as a sub-harmonic mixer. The modules operate at a center frequency of 340 GHz with a 30 GHz modulation bandwidth. An output power of 0 dBm, an IF noise level of −168 dBm/Hz and a receiver conversion loss of 18 dB is achieved in the band. The form factor of the modules is adapted to build one- or two-dimensional FMCW radar arrays. State of the art system performance is achieved while system complexity, size and cost is significantly reduced.

We report on the design and characterization of a 200 GHz resistive subharmonic mixer based on a single, multi-channel CVD graphene field effect transistor (G-FET). The device has gate length 0.5 µm and width 2×40 µm. The integrated mixer circuit is implemented in coplanar waveguide (CPW) technology and realized on a 100 µm thick high resistive silicon substrate. The measured mixer conversion loss (CL) is 34 ± 3 dB across 190–210 GHz band with 10 dBm local oscillator (LO) pumping power and the overall minimum CL gives 31.5 dB at 190 GHz.

This paper discusses the integration of the split bowtie antenna with a graphene FET THz detector to maximize the detector efficiency at 1 THz. The detector utilizes the principle of distributed resistive self-mixing in GFET, and the split bowtie antenna provides an asymmetric feed to the GFET. The antenna is placed on a substrate lens to improve the directivity and can be used to create an imaging array. The dimensions of the split bowtie antenna are optimized for the best impedance matching with the GFET and to improve the pixel density of the array. The off-axis pixel performance is improved by modifying the edge-pixel antennas. The improvement in directivity of corresponding pixels is up to 1.3 dB.

To ensure the high performance of graphene-based field effect transistors (GFETs) on flexible substrates, an uniform and manufacturable dielectric film with good electrical properties is needed. Thus, electrical characterization of the dielectric film on graphene on flexible substrates is very important for the development of flexible electronics based on GFETs. Here, we have fabricated and characterized parallel-plate capacitor test structures consisting of 35 nm thick Al2O3 dielectric film and with graphene as bottom electrode on polyethylene terephthalate (PET). It was found that the leakage current density in the Al2O3 film is less than 100 μA/cm2 at 5 V, which allows for applying it as a gate dielectric in GFETs on flexible substrates. The extracted dielectric constant of the Al2O3 film is approx. 7.6, which is close to the bulk value and confirms the good quality of the Al2O3 film. Analysis indicates that the measured loss tangent, which is up to 0.2, is governed mainly by the dielectric loss in the Al2O3 and can be associated with defects from the Al2O3 film and the Al2O3/graphene interface. Our results will be used in further development of GFETs on flexible substrates.

Studies of biological and artificial membrane systems, such as niosomes, currently rely on the use of fluorescent tags, which can influence the system under investigation. For this reason, the development of label-free, non-invasive detection techniques is of great interest. We demonstrate an open-volume label-free millimeter-wave sensing platform based on a coplanar waveguide, developed for identification and characterization of niosome constituents. A design based on a λ/2-line resonator was used and on-wafer measurements of transmission and reflection parameters were performed up to 110 GHz. Our sensor was able to clearly distinguish between common niosome constituents, non-ionic surfactants Tween 20 and Span 80, measuring a resonance shift of 3 GHz between them. The complex permittivities of the molecular compounds have been extracted. Our results indicate insignificant frequency dependence in the investigated frequency range (3 GHz – 110 GHz). Values of permittivity around 3.0 + 0.7i and 2.2 + 0.4i were obtained for Tween 20 and Span 80, respectively.

2016 41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz) : [Proceedings] - IEEE, 2016. - ISBN 978-1-4673-8485-8 - doi:10.1109/IRMMW-THz.2016.7758457 41st International Conference on Infrared, Millimeter, and Terahertz waves, IRMMW-THz 2016, Copenhagen, Denmark, 25 Sep 2016 - 30 Sep 2016; IEEE, 2 pp. (2016). doi:10.1109/IRMMW-THz.2016.7758457, We have periodically (25ms on - 25ms off) illuminated lysozyme crystals with 0.4 THz radiation and simultaneously monitored their X-ray diffraction intensity in order to study non-thermal structural changes in the protein. In this work we analyze the X-ray scaled and unmerged diffraction intensity observations using a multivariate Bayesian model in order to improve the accuracy of the intensity estimates. The diffraction intensity pairs of the illuminated and non-illuminated state show a predominantly positive correlation. The correlation decreases with increasing resolution suggesting that finer slicing and faster sampling of the rocking curve may further improve the accuracy and effect size of structure factor amplitude differences, making the interpretation of structural changes more straightforward. The improved analysis retains the most important structural features described previously (in helix 3) and provide addition details about the B-factor changes close to the substrate binding site., Published by IEEE

We present a self-consistent electro-thermal model for multi-anode Schottky diode multiplier circuits. The thermal model is developed for an n-anode multiplier via a thermal resistance matrix approach. The nonlinear temperature responses of the material are taken into consideration by using a linear temperature-dependent approximation for the thermal resistance. The electro-thermal model is capable of predicting the hot spot temperature, providing useful information for circuit reliability study as well as high power circuit design and optimization. Examples of the circuit analysis incorporating the electro-thermal model for a substrateless- and a membrane-based multiplier circuits, operating up to 200 GHz, are demonstrated. Compared to simulations without thermal model, the simulations with electro-thermal model agree better with the measurement results. For the substrateless multiplier, the error between the simulated and measured peak output power is reduced from ~ 13% to ~ 4% by including the thermal effect.

A THz imaging system, operating at 346 GHz and tailored for implementation of an imaging algorithm based on the Born approximation, is presented. The imaging algorithm provides focusing by compensating for the antenna footprint. This allows for using a more simple antenna system without optical focusing. Several aspects of implementing an imaging algorithm based on the Born approximation in THz imaging are discussed and key system properties are highlighted. The performance of the imaging algorithm is verified by imaging two simple dielectric targets. The results indicate that this approach provides a qualitative indication of the distribution of contrast in the samples complex permittivity and is a potential complement to existing imaging techniques.

We propose a semi-empirical graphene field-effect-transistor (G-FET) model for analysis and design of G-FET-based circuits. The model describes the current-voltage characteristic for a G-FET over a wide range of operating conditions. The gate bias dependence of the output power spectrum is studied and compared with the simulated values. Good agreement between the simulated and the experimental power spectrums up to the third harmonic is demonstrated, which confirms the model validity. Moreover, S-parameter measurements essentially coincide with the results obtained from the simulation. The model contains a small set of fitting parameters, which can be straightforwardly extracted from S-parameters and dc measurements. The developed extraction method gives a more accurate estimation of the drain and source contact resistances compared with other approaches. As a design example, we use a harmonic balance load-pull approach to extract optimum embedding impedance values for a subharmonic G-FET mixer.

We present an overview of research on integrated diode circuits for terahertz applications carried out at Chalmers University of Technology. This includes progress on heterogeneous integration of heterostructure barrier varactor multipliers on silicon substrates. The described technology uses silicon-on-insulator (SOI) substrates, for which accurate and well-defined substrate thickness for the microstrip circuitry is obtained.

We present a novel room temperature bolometer with nanosecond response that can be used both for coherent and incoherent detection through the entire terahertz frequency range. A responsivity of up to 15 V/W, and a noise equivalent power (NEP) ~ 450 pW/Hz0.5 were measured at modulation frequencies from 0.5 kHz to 100 kHz. A conversion gain of -28 dB was demonstrated at an intermediate frequency of 20 MHz with a Local Oscillator power of 0.74 mW. Possible improvements of the bolometer characteristics are discussed.

A novel high performance waveguide integrated sideband separating (2SB) Schottky receiver operating in the 320-360 GHz band is presented. The unique receiver design is based on a core of two subharmonic Schottky diode mixers with embedded LNA's with a minimum noise figure of 1.8 dB, fed by LO and RF quadrature hybrids. At room temperature, a typical receiver SSB noise temperature of 3000 K is measured over most of the band reaching a minimum of 2700 K, with only 4 mW of LO power. The sideband ratio (SBR) is typically below 15 dB over the whole band and the measured LO input return loss is typically below 15 dB broadband. High performance sideband separating Schottky receivers can now for the first time be considered for submillimeter wave systems enabling new types of instrument concepts.

A high sensitivity broadband terahertz direct detector based on YBa2Cu3O7 high-Tc superconductor microbolometers is presented. At 77 K, the responsivity of the spiral antenna-integrated microbolometers (1.5 μm ×1.5 μm) is 190 V/W, referenced to the input of the silicon substrate lens, across the frequency range of 330 GHz-1.63 THz in a single device. The response time is approximately 300 ps. Using a room temperature readout, we measure an optical noise equivalent power (NEP) of 20 pW/Hz0.5 (readout noise limited) for modulation frequencies ranging from 500 Hz to 100 kHz.

In this paper, we present the influence of eddy currents, skin and proximity effects on high-frequency losses in planar terahertz Schottky diodes. The high-frequency losses, particularly losses due to the spreading resistance, are analyzed as a function of the ohmic-contact mesa geometry for frequencies up to 600 GHz. A combination of 3-D electromagnetic (EM) simulations and parameter extraction based on lumped equivalent circuit is used for the analysis. The extracted low-frequency spreading resistance shows a good agreement with the results from electrostatic simulations and experimental data. By taking into consideration the EM field couplings, the analysis shows that the optimum ohmic-contact mesa thickness is approximately one-skin depth at the operating frequency. It is also shown that, for a typical diode, the onset of eddy current loss starts at ~ 200 GHz, and the onset of a mixture of skin and proximity effects occurs around ~ 400 GHz.

A typical uni-traveling carrier photodiode (UTC-PD) and a commercial PIN-PD are compared in the context of the analog performance at 10 GHz in a realistic noise environment.The behavior of the carrier-to-noise ratio (CNR), the linearity, represented by the third-order output intercept point (OIP3), and the spurious free dynamic range are studied as a function of photocurrent. We have found that the increment of the CNR is only minor because of excess noise of the amplifier, as an optical amplifier is used to reach high optical power. However, the OIP3 of the UTC-PD is substantially higher and increases with photocurrent, in contrast to the PIN-PD. Thus, even though the responsivity is lower, it is beneficial to use a UTC-PD to have superior dynamic range.

To extend the frequency range of transistors into the terahertz domain, new transistor technologies, materials, and device concepts must be continuously developed. The quality of the interface between the involved materials is a highly critical factor. The presence of impurities can degrade device performance and reliability. In this paper, we present a method that allows the study of the charge carrier velocity in a field-effect transistor vs impurity levels. The charge carrier velocity is found using high-frequency scattering parameter measurements followed by delay time analysis. The limiting factors of the saturation velocity and the effect of impurities are then analysed by applying analytical models of the field-dependent and phonon-limited carrier velocity. As an example, this method is applied to a top-gated graphene field-effect transistor (GFET). We find that the extracted saturation velocity is ca. 1.4×107 cm/s and is mainly limited by silicon oxide substrate phonons. Within the considered range of residual charge carrier concentrations, charged impurities do not limit the saturation velocity directly by the phonon mechanism. Instead, the impurities act as traps that emit charge carriers at high fields, preventing the current from saturation and thus limiting power gain of the GFETs. The method described in this work helps to better understand the influence of impurities and clarifies methods of further transistor development. High quality interfaces are required to achieve current saturation via velocity saturation in GFETs.To extend the frequency range of transistors into the terahertz domain, new transistor technologies, materials, and device concepts must be continuously developed. The quality of the interface between the involved materials is a highly critical factor. The presence of impurities can degrade device performance and reliability. In this paper, we present a method that allows the study of the charge carrier velocity in a field-effect transistor vs impurity levels. The charge carrier velocity is found using high-frequency scattering parameter measurements followed by delay time analysis. The limiting factors of the saturation velocity and the effect of impurities are then analysed by applying analytical models of the field-dependent and phonon-limited carrier velocity. As an example, this method is applied to a top-gated graphene field-effect transistor (GFET). We find that the extracted saturation velocity is ca. 1.4×107 cm/s and is mainly limited by silicon oxide substrate phonons. Within the considered ran...

We present a method to optimize the epitaxial layer structure of an InGaAs/InP uni-traveling-carrier photo-diode (UTC-PD) for continuous THz-wave generation. The design approach used is general in that it can be applied for any target frequency while this study focuses on 340 GHz. The photodiode epitaxy is modeled and optimized using a TCAD software implementing the hydrodynamic semiconductor equations. This physical device model was found to be in good agreement with reported experimental results. It is shown that the UTC-PD can generate ~1 mW at 340 GHz by choosing the optimum absorption layer and collection layer thicknesses.