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This contribution presents the results of investigations performed on monolayer graphene field effect transis-tor-based (GFET-based) terahertz detectors. We have implemented three different types of planar antennae: a bow-tie, a bow-tie with transmission lines and a slot-disc, allowing us to realize different conditions for high-frequency impedance matching. We present a semi-empirical model which uses physical parameters derived from electrical characterization results of devices and electrodynamic characteristics of antennae, allowing us to predict THz re-sponsivity. Model predictions have been compared with the responsivity measurements performed at room temperature in a frequency range from 50 to 1250 GHz. Good agreement between the model predictions and experimental results implies the eligibility of a distributed resistive mixing approximation for GFET. In addition, the device stabil-ity, the temperature dependence and the origin of noise in the transistor channel have been investigated. Finally, to the best of our knowledge, we demonstrate the record performance values for room temperature graphene-based terahertz detectors: 80V/W optical responsivity without the normalization to the antenna effective area and a noise equivalent power of 111 pW/√—Hz at 336 GHz.

This paper presents a fully-electronic wireless link operating at the 250 GHz frequency. The key elements of the developed system are the voltage-controlled harmonic oscillator implemented in 65 nm complementary metal-oxide-semiconductor technology (CMOS) and a quasi-optical detector with a resonantantenna- coupled field-effect transistor completed in 90 nm CMOS. The source is optimized for the third harmonic emission at 252 GHz with radiated power reaching up to –11 dBm (decibels with reference to one milliwatt) level. The detector has a resonance maximum of 254 GHz with a bandwidth of 25% and a minimal optical noise equivalent power of 22 pW/√Hz . We employ an on-off keying technique for data coding and demonstrate digital signal transmission from 0.4 to 18 m distances. At 0.4 m distance and modulation frequency of 32 MHz, we achieve a 15.9 dB signal-to-noise ratio. The channel capacity of assembled communication link reaches 266 Mbit/s. However, it is limited by external electronic components – the amplifier and the modulator bandwidths. Implementing state-of-the-art high-frequency circuits should allow directly scaling the throughput to 10 Gbit/s.

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.

Fourier imaging allows to record full scenes with a variable spatial resolu-tion down to the diffraction limit. Here, a 600-GHz Fourier holography system, based on subharmonic detection with CMOS transistors, is demonstrated.

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices.

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.

This contribution presents an overview of main methods what are currently used for the sensitivity, i.e., responsivity and noise equivalent power determination of state-of-the-art antenna-coupled field-effect-transistor-based THz detectors/sensors. We show that, the reported results strongly depend on the sensor effective area determination method, especially if it is not unambiguously determined leading to up to orders of magnitude discrepancy. Furthermore we illustrate our statements by presenting results of characterisation of detectors which are coupled with different antennas and presenting state-of-the-art sensitivity values.

An electrically tunable detector for sub-THz-frequency radiation based on Si CMOS technology is proposed. The tunability range of the implemented detector covers frequencies from 150 GHz up to 270 GHz with optical responsivity close to 1 kV/W and noise equivalent power below 20 ${\text{pW}}/\sqrt {{\text{Hz}}} $. Potential applications include microwave bio and medical sensing, spectroscopy, amateur radio use and nondestructive imaging.

The validation of simulations tools for microelectronic devices and circuits at frequencies reaching high into the THz regime is a challenging task. For the case of Si MOSFETs used as detectors of cw THz radiation, we show that both the TSMC RF foundry model and our physics-based in-house tool ADS-HDM, which is designed to capture plasmonic channel effects at all frequencies correctly, reproduce the performance of detectors from 0.4 to 1.2 THz remarkably well. The good agreement between the two models has its roots in the short momentum scattering time of charge carriers in the channels of MOSFETs.

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.

The spread of practical terahertz (THz) systems dedicated to the telecommunication, pharmacy, civil security, or medical markets requires the use of mainstream semiconductor technologies, such as complementary metal-oxide-semiconductor (CMOS) lines. In this paper, we discuss the operation of a CMOS-based free space all-electronic system operating near 250 GHz, exhibiting signal-to-noise ratio (SNR) with 62 dB in the direct detection regime for one Hz equivalent noise bandwidth. It combines the state-of-the-art detector based on CMOS field-effect-transistors (FET) and a harmonic voltage-controlled oscillator (VCO). Three generations of the oscillator circuit are presented, and the performance characterization techniques and their improvement are explained in detail. The manuscript presents different emitter–detector pair operation modalities, including spectroscopy and imaging.

We present the study of the influence of different thin silver nanowire layers on electrical and optical properties in the Terahertz (THz) frequency range. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2~THz to 1.2~THz is non-monotonic and depends on the nanowire dimensions and density. We present and validate also a theoretical approach describing well the experimental results and allowing to model the THz response as function the nanowire layer structure. Our results pave the way toward the application of silver nanowires as a perspective material for transparent and conductive coatings, and printable antennas operating in the terahertz range – significant for future wireless communication devices.

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.

We report the successful implementation of antenna-coupled terahertz field-effect transistors (TeraFETs) as homodyne detectors in a scattering-type scanning near-field optical microscope (s-SNOM) operating with radiation at 246.5 GHz. The devices were fabricated in Si CMOS foundry technology with two different technologies, a 90 nm process, which provides a better device performance, and a less expensive 180 nm one. The high sensitivity enables s-SNOM demodulation at up to the 10th harmonic of the cantilever's oscillation frequency. While we demonstrate application of TeraFETs at a fixed radiation frequency, this type of detector device is able to cover the entire THz frequency range.

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.

We report on the design procedure and characterization of a set of antenna-coupled terahertz detectors based on a single-transistor layout and fabricated on commercial CMOS technology. Preliminary detector measurements yield noise-equivalent-power below 100 pW/Hz for all the detectors, achieving performance-to-bandwidth tradeoffs close to reported state-of-the-art silicon detectors in the mid-terahertz range.

This work presents to our knowledge the first completely passive imaging of human body radiation below 1 THz at the room temperature without additional illumination or cooling. The successful image shows an impressive sensitivity limit of a terahertz FET detector (TeraFET). We made a 2.7 cm × 4 cm size image of a hand with the spatial resolution of 1 mm and scanning speed equal to 1 cm2/100 s. The SNR is only 2.5 dB and the NETD of the hand radiation at room temperature is 4.8 K.

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.