Search

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.

We investigate the methods of fabrication effective antenna structures for broadband detection by antenna-coupled fin-shaped field-effect transistors (FinFETs) in the terahertz radiation range, using AlGaN/GaN technological process. We demonstrate the efficiency of a split bow-tie antenna solution for a single metal layer design and the way to improve antenna design by FinFET shifting to the middle antenna’s part. Modified bow-tie antennas for FinFETs improve the THz response in the frequency range up to 800 GHz with a maximum of response in the range between 200 GHz and 300 GHz.

A free space 252 GHz quasi-optical emitter-detector pair (optopair) is implemented in standard Si CMOS technology. The THz source consists of a voltage-controlled differential field-effect-transistor (FET) based Colpitts oscillator emitting at the third harmonic with total radiated power -11.1 dBm and 35 mW DC power consumption. The detector is a resonant-antenna-coupled FET-based power detector for quasi-optic coupling through the substrate lens. The detector exhibits a minimum optical noise equivalent power as low as $22{\text{pW}}/\sqrt {{\text{Hz}}} $ at 252 GHz. The system reveals 61.7 dB power signal to noise for 1 Hz equivalent noise bandwidth in the direct detection regime. The practical application of the pair for two-dimensional imaging is also demonstrated.

In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.

Over the last two decades, photomixer-based continuous wave systems developed into versatile and practical tools for terahertz (THz) spectroscopy. The high responsivity to the THz field amplitude of photomixer-based systems is predetermined by the homodyne detection principle that allows the system to have high sensitivity. Here, we show that the advantages of homodyne detection can be exploited with broadband power detectors combined with two photomixer sources. For this, we employ a THz detector based on a complementary metal-oxide-semiconductor field-effect transistor and a broadband bow-tie antenna (TeraFET). At 500 GHz and an effective noise bandwidth of 1 Hz, the response from one photomixer-based THz source resulted in an about 43 dB signal-to-noise ratio (SNR). We demonstrate that by employing a homodyne detection system by overlaying the radiation from two photomixers, the SNR can reach up to 70 dB at the same frequency with an integration time 100 ms. The improvement in SNR and the spectroscopic evidence for water vapor lines demonstrated up to 2.2 THz allow us to conclude that these detectors can be successfully used in practical continuous wave THz spectrometry systems.

We report on the improvement of the optical performance of THz detectors based on antenna-integrated Si CMOS field-effect transistors (TeraFETs) when cooled with liquid nitrogen (temperature range 77-297 K). The minimum optical noise-equivalent power (NEP) at 77 K is as low as 11.7 pW/√Hz at 0.6 THz, which corresponds to a decrease by a factor of 3.8 compared to room temperature, and approaching the optical sensitivity levels of commercial helium-cooled bolometers. The channel's static resistance, optical responsivity and NEP measurements reveal a specific feature for this family of detectors. There are gate biasing regimes for which the selected performance parameter can be maintained almost independent of the temperature. This feature is desirable in applications requiring stability from the variations in the environmental conditions.

In this paper, we show experimental evidence of applicability of a field-effect transistor-based THz detector (TeraFET) for the simultaneous hyperspectral raster scanning imaging. For this work, we have used a dual frequency comb setup extended to THz frequencies, as well as a TeraFET detector adopted for a high frequency readout circuit. Such a spectroscopic imaging system can acquire simultaneously many frequency lines with outstanding frequency accuracy and adjustable frequency resolution even at very low THz source power. The dual frequency comb THz system is also capable of narrow linewidth, which makes spectroscopy measurements with high resolution more feasible. The TeraFET detector has optical NEP below 19 pW/Hz^0.5 at 296 GHz. Additionally, the detector was supplemented with a low-noise amplifier of 40.8 dB gain and 200 kHz 3-dB bandwidth.

In this paper we report the terahertz detection by field-effect transistors in 65-nm CMOS technology with on-chip integrated patch antennas. In conjunction with continuous wave photomixer source, detectors demonstrate > 40 dB signal-to-noise ratio and minimal NEP of $12{\text{pW}}/\sqrt {{\text{Hz}}} $ at 1 kHz of chopping at the resonant frequency of 620 GHz. This NEP value is by few ${\text{pW}}/\sqrt {{\text{Hz}}} $ lower than best reported values for this category of detectors which gradually approaches the state-of-the-art characteristics of THz bolometers.

In this paper, a terahertz hyperspectral imaging architecture based on an electro-optic terahertz dual-comb source is presented and demonstrated. In contrast to single frequency sources, this multiheterodyne system allows for the characterization of the whole spectral response of the sample in parallel for all the frequency points along the spectral range of the system. This hence provides rapid, highly consistent results and minimizes measurement artifacts. The terahertz illumination signal can be tailored (in spectral coverage and resolution) with high fexibility to meet the requirements of any particular application or experimental scenario while maximizing the signal-to-noise ratio of the measurement. Besides this, the system provides absolute frequency accuracy and a very high coherence that allows for direct signal detection without inter-comb synchronization mechanisms, adaptive acquisition, or post-processing. Using a feld-efect transistor-based terahertz resonant 300GHz detector and the raster-scanning method we demonstrate the two-dimensional hyperspectral imaging of samples of diferent kinds to illustrate the remarkable capabilities of this innovative architecture. A proof-of-concept demonstration has been performed in which tree leaves and a complex plastic fragment have been analyzed in the 300 GHz range with a frequency resolution of 10 GHz. This project has received funding from the ATTRACT project funded by the EC under Grant Agreement 777222 and from the Spanish Ministry of Economy and Competitiveness under Project TEC2017-86271-R. The work has also been partially funded by the EU H2020 Celta project under Grant Agreement 675683 and the Foundation for Polish Science under grant IRA CENTERA. Viktor Krozer is thankful for partial financial support in the frame of the Chairs of Excellence program of the Universidad Carlos III, Madrid, Spain.

Traditional imaging systems including microscopes depend on the generation of real images to be recorded by the sensor element, which is only sensitive to the intensity and not the phase. The object distance, which is crucial for the spatial resolution of the system, therefore is restricted to be larger than the focal length of the objective lens. This leads to a limitation of the achievable lateral, diffraction-limited spatial resolution. In order to reach a resolution enhancement with the same system components, we explore – in the sub-THz frequency regime – heterodyne detection of the scene’s complex-valued spatial Fourier spectrum in the image-sided focal plane of the optical system. The existence of the Fourier spectrum is independent of the object distance. The measured complex-valued field distribution enables a numerical back-propagation to any place in the object-sided free space from which radiation has reached the detector. Heterodyne Fourier imaging hence enables 3D imaging and – the relevant theme here – it lifts the restriction of the imaging distance. This enables object distances smaller than the focal length of the objective lens and, with it, an enhanced diffraction-limited spatial resolution. In the experiments presented here, the heterodyne data acquisition of the 0.3-THz continuous wave radiation employs Si CMOS TeraFET detectors, i.e., THz sensors based on antenna-coupled field-effect transistors which have been developed in our laboratory. First imaging results show an enhancement of the maximal resolution by a factor of 1.4 for the specific measurement conditions of our experiments, with considerable room for improvement.

Aiming for non-destructive testing and security applications, we investigate transmission-mode imaging and spectroscopic sensing using terahertz (THz) pulses from a commercial optical parametric oscillator (OPO) in combination with THz detectors based on antenna-coupled field-effect transistors (TeraFETs). The Q-switched OPO generates quasi-continuous-wave THz pulses with a peak power of up to 1 W at a repetition rate between 12 and 90 Hz. The pulses are frequency-tunable between 0.7 and 2.6 THz with a typical linewidth of 50 GHz. We explore detection with fast GaN/AlGaN TeraFETs which hold the potential for multi-pixel and homodyne detection.

In this paper, we report on efficient detection of the radiation emitted by a THz quantum cascade laser (QCL) using an antenna-coupled field effect transistor (TeraFET). In the limiting case when all radiated power would be collected, the investigated TeraFET can show up to 230 V/W responsivity with the noise equivalent power being as low as 85 pW/ $\sqrt {\text {Hz}}$ at 3.1 THz, which is several times lower than that of the typical Golay cell. A combination of the QCL and a set of off-axis parabolic mirrors with 3-inch and 2-inch focal lengths was used to measure the signal-to-noise ratio (SNR) of the TeraFET. The practically achieved SNR was five times lower than that of the Golay cell and two orders of magnitude lower than a bolometer’s. However, TeraFETs are much faster and do not need a signal modulation, thus can be used both in a continuous mode for power monitoring or for investigation of transient processes on a sub-microsecond time scale.

We use a THz dual-comb optical source to perform high-speed hyperspectral imaging in the 300 GHz range with high sensitivity field-effect transistor-based THz detectors. First hyperspectral imaging results of different samples are shown using a scanning system, demonstrating the localization of interesting spectral features in the specimens.

This paper presents an overview of the different methods used for sensitivity (i.e., responsivity and noise equivalent power) determination of state-of-the-art field-effect transistor-based THz detectors/sensors. We point out that the reported result may depend very much on the method used to determine the effective area of the sensor, often leading to discrepancies of up to orders of magnitude. The challenges that arise when selecting a proper method for characterisation are demonstrated using the example of a 2×7 detector array. This array utilises field-effect transistors and monolithically integrated patch antennas at 620 GHz. The directivities of the individual antennas were simulated and determined from the measured angle dependence of the rectified voltage, as a function of tilting in the E- and H-planes. Furthermore, this study shows that the experimentally determined directivity and simulations imply that the part of radiation might still propagate in the substrate, resulting in modification of the sensor effective area. Our work summarises the methods for determining sensitivity which are paving the way towards the unified scientific metrology of FET-based THz sensors, which is important for both researchers competing for records, potential users, and system designers.

A novel continuous-wave terahertz holography imaging system based on heterodyne detection is presented. It employs a field-effect transistor as a fast coherent detector for digital data recording.

In the above paper [1] , the footnote acknowledges financial support received from the Research Council of Lithuania with wrong project number. The statement “This work was supported by the Research Council of Lithuania under contract TAP LZ-06/2015, …” should be read as “This work was supported by the Research Council of Lithuania under contract LAT-04/2016, …”.

THz in-line holography, is a simple and effective way of three-dimensional (3D) imaging. In this work, a THz in-line holography system is presented to investigate the imaging performance. An electrical multiplier-chain emitter working at 300 GHz was utilized as illuminating source. A highly sensitive broad-band CMOS TeraFET detector mounted on a 2D mechanical translation stage for raster-scanning acted as a virtual camera to record the hologram. The data capture area was 60x60 mm2 with a pixel size of 0.25 0.25 mm2. The test objects were polyvinyl chloride (PVC) boards, one taped with 2-mm-wide straight aluminum stripes separated by 2-mm intervals, another one taped with 1.5-mm-wide aluminum stripes forming the word ’GUF’ (size of each letter: 6x8 mm2), and the other two with the same patterns, but now realized as grooves in the material. The imaging results show that a resolution of at least 2 mm is achieved, with a large dynamic range of 60 dB due to the high sensitivity of the TeraFET detector. A property of in-line holography is its self-homodyning capability with the consequence that, in addition to the intensity, also the phase information is encoded in the hologram. The phase information of the transparent part of the object, and of the scattered and diffracted waves are retrieved with the same spatial resolution as the intensity, revealing the 3D imaging capability of the system. The quality and fidelity of the image results can be substantially improved in the future by enlarging the recording area and using an iterative phase recovery method. Furthermore, with a camera substituting the single-pixel scanning technique, the system will have the capability of real-time imaging. Combined with the 3D imaging ability, it will then have a wide application range for various topical areas.

We report on circuit simulation, modeling, and characterization of field-effect transistor based terahertz (THz) detectors (TeraFETs) with integrated patch antennas for discrete frequencies from 1.3 to 5.7 THz. The devices have been fabricated using a standard 90-nm CMOS technology. Here, we focus in particular on a device showing the highest sensitivity to 4.75-THz radiation and its prospect to be employed for power monitoring of a THz quantum cascade laser used in a heterodyne spectrometer GREAT (German REceiver for Astronomy at Terahertz frequencies). We show that a distributed transmission line based detector model can predict the detector's performance better than a device model provided by the manufacturer. The integrated patch antenna of the TeraFET designed for 4.75 THz has an area of $13\times 13$ $\mathrm{\mu }$ m $^2$ and a distance of 2.2 $\mu$ m to the ground plane. The modeled radiation efficiency at the target frequency is 76% with a maximum directivity of 5.5, resulting in an effective area of 1750 $\mu$ m $^2$ . The detector exhibits an area-normalized minimal noise-equivalent power of 404 pW/ $\sqrt{\mathrm{Hz}}$ and a maximum responsivity of 75 V/W. These values represent the state of the art for electronic detectors operating at room-temperature and in this frequency range.

Field-effect transistors owe their usefulness as detectors of THz radiation (TeraFETs) to their rectification capability. At low frequencies, the dominant mechanism is resistive mixing, supported at THz frequencies by the Dyakonov-Shur mechanism involving charge density waves in the transistor's channel. Additionally, a hot-carrier photo-thermoelectric effect can significantly contribute. We have developed a hydrodynamic transport model and implemented it in a commercial device simulator which handles these contributions on an equal footing. We show that thermo-electrics can significantly contribute not only in graphene TeraFETs but also in devices made from conventional semiconductors such as AlGaN/GaN TeraFETs. The simulations also allow us to address the question of why it has proven to be difficult to experimentally verify the signal enhancement by Fabry-Perot-like plasmon resonances in the channel predicted by Dyakonov and Shur to occur at high frequencies.

We report on the modeling, implementation and characterization of optimized, ultrabroadband antenna-coupled field-effect transistors for the detection of terahertz (THz) radiation. The detectors were fabricated in 90-nm silicon CMOS technology with integrated bow-tie and log-spiral antennas. We also investigated a third design with a narrowband dipole antenna. In the frequency range from 0.5 to 1.5 THz, the detector with the bow-tie antenna design exhibited a nearly flat frequency response at an optical responsivity of ~100 V/W. We found minimum optical noise equivalent powers (NEPs) as low as 48 pW/√Hz at 0.6 THz and 70 pW/√Hz at 1.5 THz. The dipole-design detector achieved an optical NEP of 17 pW/√Hz at resonance frequency of 490 GHz. Our detectors were optimized with the help of a self-developed circuit model based on device parameters such as transistor geometry, antenna impedance matching and the peculiarities of the employed fabrication technology.

We report on design and implementation of narrowband and broadband field-effect transistor-based detectors for THz frequency range (TeraFETs) using commercial CMOS fabrication process. Exploiting the plasma wave mixing principle, which becomes important above the transistor's cut-off frequency, we achieve highly sensitive detection at room temperature operation conditions. At 297 GHz, the resonant detector demonstrates optical noise-equivalent power (NEP) including all losses of 31 pW per square root of Hz. For the same conditions, broadband device exhibits NEP below 100 pW per square root of Hz in a frequency range from 500 GHz to 750 GHz. The optical, antenna effective area-limited NEP of resonant device at 620 GHz is below 10 pW per square root of Hz. These performance values are competitive with the best commercially available room temperature THz detectors.

We present a 24 × 24 pixel camera capable of high-speed THz imaging in power-detection mode. Each pixel of the sensor array consists of a pair of 150-nm NMOS transistors coupled to a patch antenna with resonance at 600 GHz. The camera can operate with a speed of up to 450 frames per second where it exhibits a minimum resolvable power of 10.5 nW per pixel. For a 30-Hz frame rate, the minimum resolvable power is 1.4 nW.

This letter reports on the influence of illumination conditions on the detector response of three-terminal devices. Antenna-coupled field-effect transistors for the plasmonic detection of THz radiation (TeraFETs) were realized using a 0.25- $\mu\hbox{m}$ AlGaN/GaN process. Integrated bow-tie antennas are connected to the source, drain and gate terminals of electrically identical transistors in various ways. If radiation power is coupled symmetrically to the source and drain sides of the transistors' channels, TeraFETs become sensitive to gradient-intensity of the illumination power. It is found that fully asymmetric coupling, nontrivial to ensure at THz-frequencies, is required for a pure response to incident power.

We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular, we show that the build-up time of the intrinsic rectification signal of a patch-antenna-coupled CMOS detector changes from 20 ps in the deep sub-threshold voltage regime to below 12 ps in the vicinity of the threshold voltage.

Many THz applications require detection of sub-picosecond THz pulses. Electronic detectors, in particular, can address this challenge. We report on the detection of sub-picosecond THz pulses generated by a large-area interdigitated photoconductive antenna using a AlGaN / GaN high electron mobility transistor with integrated bow-tie antenna. We demonstrate that the detectors photoresponse is linear in a wide range of gate bias voltages regarding the available THz radiation power with peak power levels of a few hundreds of milliwatts. We apply an autocorrelation technique to investigate the spectral response of our detector within a bandwidth exceeding 1 THz. We observe an unexpected frequency roll-off of responsivity, which can not be predicted using a framework of standard distributed transmission line theory. However, we show that the data can be understood if one accounts for only partial plasmon screening by the gate electrode, so that the results adhere simply to the distributed resistive mixing approximation, whereby the device suffers from the observed roll-off. This indicates, that for novel detectors and radiation sources, which intend to utilize plasma waves, it is important to ensure efficient screening by the gate electrode.

We present broadband high sensitivity terahertz (THz) detectors based on 90 nm CMOS technology with the state-of-the-art performance. The devices are based on bow-tie and log-spiral antenna-coupled field-effect transistors (FETs) for the detection of free-space THz radiation (TeraFETs). We report on optimized performance, which was achieved by employing an in-house developed physics-based model during detector design and thorough device characterization under THz illumination. The implemented detector with bow-tie antenna design exhibits a nearly flat frequency response characteristic up to 2.2 THz with an optical responsivity of 45 mA/W (or 220 V/W). We have determined a minimum optical noise-equivalent power as low as 48 pW/ $\sqrt {\textsf {Hz}}$ at 0.6 THz and 70 pW/ $\sqrt {\textsf {Hz}}$ at 1.5 THz. The results obtained at 1.5 THz are better than the best narrowband TeraFETs reported in the literature at this frequency and only up to a factor of four inferior to the best narrowband devices at 0.6 THz.

We present the field-effect-transistor-based THz (TeraFET) detector as a tool for monitoring power and its low-frequency fluctuations of 4.75-THz quantum cascade laser (QCL). The resonant patch antenna coupled TeraFET was implemented in 90 nm CMOS technology. At 4.75 THz, detector exhibits area-normalized minimal noise-equivalent power (NEP) of 370 pW/√Hz and the maximum responsivity of 82 V/W.

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.

We summarize three lines of development and investigation of foundry-processed patch-antenna-coupled Si MOSFETs as detectors of THz radiation: (i) Exploiting the pinciple of plasma-waved-based mixing in the two-dimensional electron gas of the transistors’ channels, we demonstrate efficient detection at frequencies as high as 9 THz, much above the transit-time-limited cut-off frequencies of the devices (tens of GHz). Real-time imaging at 600 GHz with a 12 × 12 detector array is explored. (ii) Given the limited THz power usually available for applications, we explore imaging with enhanced sensitivity in heterodyne mode. We show that real-time operation of a 100 × 100-pixel heterodyne camera should be possible at 600 GHz with a better dynamic range (30 dB) than for direct power detection (20 dB), even if only a quarter-milliwatt of local-oscillator power, distributed radiatively over all detector pixels, is available. (iii) Finally, we present an all-electronic raster-scan imaging system for 220 GHz entirely based on CMOS devices, combining the CMOS detectors with an emitter circuit implemented in a 90-nm CMOS process and delivering radiation with a power on the 100- μW scale. Considering progress in the field, we anticipate that the emitter concept of oscillator-based power generation with on-chip frequency multiplication will carry well into the sub-millimeter-wave regime.

We present a study of the emission of terahertz radiation from multi-finger AlGaN/GaN field-effect transistors.

This paper reports on field-effect-transistor-based terahertz detectors for the operation at discrete frequencies spanning from 0.2 to 4.3 THz. They are implemented using a 150-nm CMOS process technology, employ self-mixing in the n-channels of the transistors and operate well above the transistors' cutoff frequency. The theoretical description of device operation by Dyakonov and Shur is extended in order to describe the device impedance, responsivity, and noise-equivalent power for a novel detection concept, which couples the signal to the drain. This approach enables quasi-static (QS) detection and calibration of the detectors. The different transport regimes (i.e., QS, distributed resistive, and plasmonic mixing) and their transitions are theoretically discussed and experimentally accessed. Responsivity values of 350 V/W at 595 GHz, 30 V/W at 2.9 THz, and 5 V/W at 4.1 THz are reported. At 0.595 THz, we determine the optical noise equivalent power (NEP) to be 42 pW/√Hz ; at 2.9 THz, the value is 487 pW/√Hz. All values are reported for optimum gate bias with respect to NEP at 295 K. For 0.595 THz, theory predicts a NEP value at threshold as low as 2 pW/√Hz for ideal coupling of the radiation.

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

We demonstrate that the rectifying field effect transistor, biased to the subthreshold regime, in a large signal regime exhibits a super-linear response to the incident terahertz (THz) power. This phenomenon can be exploited in a variety of experiments which exploit a nonlinear response, such as nonlinear autocorrelation measurements, for direct assessment of intrinsic response time using a pump-probe configuration or for indirect calibration of the oscillating voltage amplitude, which is delivered to the device. For these purposes, we employ a broadband bow-tie antenna coupled Si CMOS field-effect-transistor-based THz detector (TeraFET) in a nonlinear autocorrelation experiment performed with picoseconds-scale pulsed THz radiation. We have found that, in a wide range of gate bias (above the threshold voltage V th = 445 mV), the detected signal follows linearly to the emitted THz power. For gate bias below the threshold voltage (at 350 mV and below), the detected signal increases in a super-linear manner. A combination of these response regimes allows for performing nonlinear autocorrelation measurements with a single device and avoiding cryogenic cooling.

This paper reports on antenna-coupled field-effect transistors for the detection of terahertz radiation (Ter-aFETs), based on AlGaN/GaN high electron mobility transistors (HEMTs). The HEMTs are integrated on-chip with bow-tie antennas. The broadband sensitivity of the fabricated detectors was characterized in the frequency range from 0.4 up to 1.18 THz. We reach record values of optical noise equivalent power (NEP) down to 57 pW/√Hz at 0.9 THz. These values are the result of antenna and fabrication process optimization as well as modelling including plasmonic effects.

We report on the numerical modelling of rectification in a gated two-dimensional electron gas. We demonstrate that drift-diffusion-based and energy-relaxation-based models predict different features of rectified terahertz radiation as a function of gate bias. Whereas the widely accepted mechanism for rectification is considered to be plasmonic-based, there are conditions when diffusion currents originating by non-local carrier heating can dominate the response. Moreover, diffusive contributions can substantially enhance the response becoming an important phenomenon, which has to be considered in future designs of efficient transistor-based terahertz rectifiers.

We demonstrate for the first time the applicability of antenna-coupled field-effect transistors for the detection of terahertz radiation (TeraFETs) for multi-spectral imaging from 0.76 to 4.25 THz. TeraFETs were fabricated in a commercial 90-nm CMOS process and noise-equivalent powers of 59, 20, 63, 85 and 110 pW/√(Hz) at 0.216, 0.59, 2,52, 3.11 and 4.25 THz, respectively, have been achieved. A set of TeraFETs has been applied in raster-scan transmission and reflection imaging of pellets of sucrose and tartaric acid simulating common plastic explosives. Transmittance values are in good agreement with Fourier-transform infrared spectroscopy data. The spatial distribution of the components in the samples has been determined from the transmission data using principal component analysis.

We have fabricated 20 μm gate width graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition (CVD). These GFETs are integrated with split bow-tie antennae for room temperature, direct detection of a 0.6 THz signal. Our detectors reach a maximum optical responsivity of 3.0 V/W and a minimum noise-equivalent power (NEP) of 700 pW/Hz^0.5. The successful demonstration of THz detection using CVD graphene introduces the possibility for scalable detector production.

We discuss the potential for creating a tunable continuous-wave source for the 1–5 THz frequency range based on nonlinear mixing in the active medium of a dual-color semiconductor laser or laser amplifier. Our analysis for the 1.55 μ m wavelength regime shows that intracavity mixing, after minimization of THz waveguide losses and optimization of the second-order nonlinearity of the active medium, should provide up to 100 μ W of THz power at an optical power of 100 mW per color.

We discuss two different implementations of field-effect-transistor based detectors (TeraFETs) optimised for imaging above 1 terahertz (THz). (i) Resonant-antenna-coupled MOSFET rectifiers have been fabricated using CMOS technology and (ii) broad-band devices by exploting GaN/AlGaN high-electron-mobilitiy transistors. Both technologies are scalable to large detector arrays and allow realisation of sensitive detectors deep in the THz frequency range.

We have fabricated AlGaAs/InGaAs/AlGaAs heterostructure field effect transistors (HFET) with integrated on-chip antennas and we have measured their optical responsivity when mounted in a in-house developed quasi-optical package with a silicon substrate lens.

This chapter deals with plasma-wave-based detection of THz radiation with metal–oxide–semiconductor field-effect transistors (MOSFETs). Building on the general presentation of the subject in Chapter 5 , the focus is now on room-temperature detectors and focal-plane arrays in Si complementary metal oxide semiconductor (CMOS) technology. A review of the detection principle emphasizes the close relationship with resistive mixing at low radiation/signal frequencies, which develops into distributed resistive mixing above the transistors’ cut-off frequency, and then into full plasmonic mixing. Detector design, implementation and characterization are discussed for frequencies from 200 GHz to beyond 4 THz. A 10-kilopixel array is emulated experimentally, and imaging in power-detection and heterodyne mode are presented.

Compact wireless Radio-over-Fiber transmitter and receiver modules for 1.025 Gb/s wireless communication in the frequency range of 71-76 GHz.

Wireless links using carrier waves in the millimetre-wave spectrum above 70 GHz provide a cost-effective solution to increase the data rate. Reported is the development of a photonically enabled link, based on two compact modules, both operated uncooled, transmitting a 1 Gbit/s data signal over several metres. The first module includes an optimised high-speed photodiode, delivering an output RF power exceeding 0 dBm (1 mW). Also reported is the development of a direct envelope receiver based on a quasi-optical Schottky barrier diode (SBD) module. Over the carrier frequency range, the broadband log-periodic antenna impedance has been optimised to match the SBD, while at the baseband frequency, the receiver was optimised to a 50 Ω output through an impedance transition. These have been key issues for the excellent performance of the compact receiver module over the entire E-band.

We demonstrate sub-harmonic and heterodyne mixing beyond cutoff in AlGaAs/InGaAs with integrated planar antenna connected to the channel ends.

TeraFETs are THz power detectors based on field-effect transistors (FETs) integrated with antennas. The first part of this paper discusses the design of Si CMOS TeraFETs leading to an optimized noise-equivalent power close to the room-temperature limit. The impact of the choice of the gate width and gate length, the role of the parasitic effects associated with the technology node, and the conjugate matching of antenna and FET impedance – which is possible over narrow THz frequency bands because of the frequency dependence of the channel impedance resulting from plasmonic effects – are highlighted. Taking these aspects into account, we implement narrow-band detectors of two different designs. Using a 90-nm and a 65-nm CMOS technology, we reach a room-temperature cross-sectional NEP of 10 pW/√Hz at 0.63 THz. We then explore the optimization of AlGaN/GaN TeraFETs equipped with broadband antennae. A room-temperature optical NEP of 26 pW/√Hz is achieved around 0.5 THz despite the fact that the existence of pronounced ungated regions leads to a significant hot-electron thermoelectric DC voltage reducing the rectified signal. AlGaN/GaN TeraFETs become competitive and they have the added advantage that they are extraordinarily robust against electrostatic shock even without inclusion of protection diodes into the design.

We present recent developments of two kinds of compact room-temperature detectors for terahertz radiation. These are asymmetrically-shaped bow-tie diodes, and field-effect transistors with sub-micrometer channel lengths. Both kinds of detectors exhibit fast response times which allow operating them as mixers, moreover they are suitable for the fabrication of large multi-pixel arrays. Here, we provide data on the experimental performance of detector arrays and show their applicability in coherent terahertz imaging systems. In addition, we demonstrate real-time operation of a 12×12-pixel CMOS camera in power-detection mode at 590 GHz.

We present an all-electronic raster-scan imaging system for 220 GHz which is fully based on planar CMOS integrated circuit components. The emitter has been implemented in 90-nm CMOS process and delivers up to 50 µW at 220 GHz. The detector, based on a patch-antenna-coupled field-effect transistor pair has been implemented in a 150-nm CMOS process and exhibits a maximum responsivity of 180 V/W at 213 GHz and a bandwidth of 5% at FWHM. Our preliminary imaging results achieve an SNR of 20 dB at the equivalent noise bandwidth of 5 Hz.

We present experimental and theoretical results on sub-harmonic mixing of THz radiation at 639 GHz with radiation at 213 GHz (both being high above f T ) in an antenna-coupled silicon CMOS detector with a fundamental antenna resonance at 213 GHz. The measurements are analyzed in the context of distributed resistive mixing in the channels of the field-effect transistors.

Distributed phenomena permit the use of field-effect transistors for the detection of frequencies far beyond transistor cut-off. This contribution gives details on an improved design for patch antenna-coupled field-effect-transistors and shows sensitive detection from 0.2 to 4.3 THz for monolithically integrated devices relying on commercial 0.15-μm CMOS process technology. Room-temperature responsivity values of 1344 V/W at 585 GHz, 90 V/W at 3.1 THz and 11 V/W at 4.3 THz are reported (values acquired at optimum operational point). A minimum optical noise-equivalent-power (NEP) of 163 pW/√Hz at 3.1 THz is reported (All values are normalized to the physical antenna area).

The detection of THz radiation, until recently the domain of specialized technologies, is now becoming linked to main-stream silicon CMOS technology, which entails significant advantages concerning costs, reliability, function integration, etc. Following the prediction that field-effect transistors (FETs) can detect THz radiation far above their cut-off frequency by a mixing process involving plasma waves in the FET's channel, we and others have developed CMOS FET detectors and arrays of them which are entirely fabricated in commercial foundries. We report an excellent noise-equivalent power of 43 pW/√Hz at room temperature and of a few pW/√Hz at cryogenic temperatures of detectors for 0.6-THz radiation. An added advantage of FETs is their high cut-off frequency which permits their use in heterodyne mode. The sensitivity is enhanced, and the detection of amplitude and phase of the radiation not only allows the determination of the complex dielectric constant of materials but also opens the way to three-dimensional imaging. We finally specify parameters for real-time active imaging at 0.6 THz.