THz Detection Research Articles

THz Detection and Quantitative Analysis of Interlayer Air Gap in Power Insulators

THz Detection and Quantitative Analysis of Interlayer Air Gap in Power Insulators

CMOS Platform for Everyday Applications Using Submillimeter Electromagnetic Waves

CMOS (Complementary Metal Oxide Silicon) integrated circuits (ICs) technology is emerging as a means for realization of capable and affordable systems that operate at frequencies near 300GHz and higher. This is lowering a key barrier for utilizing the sub-millimeter electromagnetic waves in everyday applications. Despite the fact that the unity maximum available gain frequency, fmax of NMOS transistors (with connections to the tope metal layer) has peaked at 320 GHz, signal generation up to 1.33 THz, coherent detection up to 1.2 THz and incoherent detection up to 10 THz have been demonstrated using CMOS integrated circuits. Furthermore, highly integrated rotational spectroscopy transceivers operating at frequencies up to near 300 GHz, 300-GHz and 400-GHz concurrent transceiver pixels and arrays for high resolution radar imaging, and 300-GHz and 390-GHz transmitters, and 300-GHz receivers for high data-rate communication have been demonstrated in CMOS. The performances of these CMOS circuits are sufficient or close to being sufficient to support electronic smelling using rotational spectroscopy that can detect and quantify concentrations of a wide variety of gases; imaging that can enable operation in a wide range of visually impaired conditions; and high-bandwidth communication. Lastly, techniques for affordable packaging and testing sub-millimeter wave systems are suggested based on experimental demonstrations.

Broadband THz Detection by YBaCuO Josephson Junctions Having Finite Capacitance

Broadband classical detection of THz radiation by a YBaCuO Josephson junction was studied on the basis of resistively-capacitively shunted junction model. Numerical simulation was based on the parameters of the samples experimentally studied in other works at nitrogen temperatures. It is shown that taking into account the damping of the Josephson junction becomes essential for high frequencies of external signal. The absolute value of responsivity decreases as junction capacitance increases. Damping parameter also influences the choice of optimal IC and RN parameters.

Crosstalk in CMOS Terahertz Detector Array With On-Chip SPR Antenna

Although surface plasmon resonance (SPR) antennas have been integrated into CMOS THz detectors as an alternative to the traditional radio-wave antenna, spatial crosstalk of SPR antenna in detector arrays remains an issue and there is little study in this field. In this paper, a CMOS THz detector array at 0.65 THz with on-chip SPR antennas is designed in standard CMOS technology and the pixel crosstalk related to the SPR antenna parameters are investigated. Simulation results show that the adjacent SPR antenna spacing significantly affect the antenna crosstalk due to the surface wave propagation. To suppress the pixel crosstalk, the adjacent SPR antenna spacing is designed as 1.65 times larger than half of the antenna size. Under the condition, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S₁₂</i> parameter between the adjacent antennas is reduced to -15 dB. High antenna isolation can be further available by the increase of the isolation layer thickness of the pixel up to 1 μm. Based on the simulation results, a 1×16 THz detector array at 0.65THz was fabricated. The experimental results show that the pixels in the array achieve uniform <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NEP</i> , with an average value of about 32.6 pW/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> and a standard deviation coefficient of about 9.6%. The results can be further optimized to meet various crosstalk requirements for CMOS THz detectors.

THz detection and amplification using plasmonic field effect transistors driven by DC drain currents

We report on the numerical and theoretical results of sub-THz and THz detection by a current-driven InGaAs/GaAs plasmonic field-effect transistor (TeraFET). New equations are developed to account for the channel length dependence of the drain voltage and saturation current. Numerical simulation results demonstrate that the effect of drain bias current on the source-to-drain response voltage (dU) varies with the device channel length. In a long-channel TeraFET where plasmonic oscillations cannot reach the drain, dU is always positive and rises rapidly with increasing drain current. For a short device in which plasmonic oscillations reach the drain, the current-induced nonuniform electric field leads to a negative response, agreeing with previous observations. At negative dU, the amplitude of the small-signal voltage at the drain side becomes larger than that at the source side. Thus, the device effectively serves as a THz amplifier in this condition. Under the resonant mode, the negative response can be further amplified near the resonant peaks. A new expression of dU is proposed to account for this resonant effect. Based on those expressions, a current-driven TeraFET spectrometer is proposed. The ease of implementation and simplified calibration procedures make it competitive or superior compared with other TeraFET-based spectrometers.

Quantitative analysis of aqueous biomolecular mixtures by THz spectroscopy based on high-power LiNbO3 radiation source

The rapid and accurate identification of the types and contents of early pathological markers by THz technology are of particular importance for the prevention and treatment of major diseases. Nevertheless, these markers usually contain interference from water and other non-target molecules, resulting in low signal-to-noise ratio (SNR) and making identification and quantitative analysis difficult. Here, based on THz spectroscopy from a high-power THz source radiated by LiNbO3, we perform quantitative and real-time THz detection of mixtures (α-lactose monohydrate and 4-aminobenzoic acid) in liquids. The results demonstrate that the absorption spectra of the aqueous biomolecular mixtures exhibit an accumulation of THz features of each pure product, i.e., the amplitude of the absorption peaks is proportional to the mixing ratio, while the corresponding absorption baseline increases with decreasing concentration. Furthermore, the content of the target substance can be calculated from the linear relationship between the absorption spectra of pure and mixed samples. This technology will support the future application of THz-TDS in early disease diagnosis under complex states and environments.

Graphene plasmons-enhanced terahertz response assisted by metallic gratings

Abstract Terahertz detectors based on two-dimensional Dirac materials offer a new approach for room-temperature terahertz detection with high response and low noise. However, these devices can hardly show high response over a broad frequency range, mainly due to the poor absorption caused by their ultrathin nature. Here we apply metallic gratings to enhance the excitation efficiency of graphene plasmons. When nonzero source-drain bias is applied, graphene plasmons can generate terahertz response orthogonal to the polarization of the incidence. The response is attributed to the orthogonal overdamped plasmon rectification effect, and graphene plasmons-enhanced photo-thermoelectric effect. By comparing the normalized on/off ratio, the metallic gratings are found to effectively enhance the coupling efficiency between graphene plasmons and THz incidence, and thus the absorption and responsivity. The results are beneficial for improving the response of room temperature THz detectors.

Determination of a High-Power THz Detector for EA-FEL Radiation Using Optical Sampling of GaAs and ZnTe Crystals

This paper presents a diagnostic method for THz pulses produced by EA-FEL. Experimental results present a comparison between ZnTe and GaAs crystals for the detection of a single terahertz pulse using electro-optic sampling. In order to match an electro-optic detector for the EA-FEL radiation, the THz pulse from a source was simultaneously detected by ZnTe and GaAs electro-optic detectors. The GaAs detection system was found to have a shorter response time but low signal-to-noise ratio (SNR) compared to the ZnTe system. The GaAs crystal is suitable for the detection of short THz pulses due to fiber coupling, and the SNR of the GaAs system can be improved using a faster and more sensitive photodetector.

Thermoemission-Based Model of THz Detection and Its Validation—JLFET Case Studies

Silicon junctionless field-effect transistors (JLFETs) detect THz radiation even at frequencies above a few THz. This effect cannot be explained by classic laws of electron transport. The same behavior is observed for other types of FETs. However, the JLFET architecture makes this device an effective tool for testing the THz detection mechanism. In particular, a deeper insight into this effect is provided by a case study in which, the low concentration of electrons in the gate-controlled region contradicts potential plasmonic effects. Considering the experimental results, the authors critically discuss the plasmon-based theory of THz detection by JLFETs. Then, taking into account the revealed inconsistencies and based on numerical simulation results, they propose a simple, one-dimensional JLFET photoresponse model based on local electron heating at the channel–source contact which is located in the path of the THz signal between the gate and the source. The model is verified in a simulation-assisted experiment showing that the energy of hot electrons generates a sufficient photoelectric voltage, typical of silicon FETs integrated with antennas illuminated by THz radiation. The authors suggest that the model is universal and in the three-dimensional version it can successfully explain the THz detection by various FETs, especially those operating in the subthreshold range in which the electron concentration under the gate is very low.

Towards wireless data transmission with compact all-electronic THz source and detector system

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.

A Novel Terahertz Detector Based on Asymmetrical FET Array in 55-nm Standard CMOS Process.

This paper reports a novel, one-dimensional dense array of asymmetrical metal-oxide-semiconductor field-effect-transistor (MOSFET) THz detector, which has been fabricated in GlobalFoundries 55-nm CMOS technology. Compared with other technologies, the Si-based complementary metal-oxide-semiconductor (CMOS) dominates in industrial applications, owing to its easier integration and lower cost. However, as the frequency increases, the return loss between the antenna and detector will increase. The proposed THz detector has a short-period grating structure formed by MOSFET fingers in the array, which can serve as an effective antenna to couple incident THz radiation into the FET channels. It not only solved the problem of return loss effectively, but also greatly reduced the detector area. Meanwhile, since the THz signal is rectified at both the source and drain electrodes to generate two current signals with equal amplitude but opposite directions, the source drain voltage is not provided to reduce the power consumption. This leads to a poor performance of the THz detector. Therefore, by using an asymmetric structure for the gate fingers position to replace the source drain voltage, the performance of the detector in the case of zero power consumption can be effectively improved. Compared with the symmetrical MOSFET THz detector, Rv is increased by 183.3% and NEP is decreased by 67.7%.

Reflective Terahertz Metasurfaces Based on Non-Volatile Phase Change Material for Switchable Manipulation

Recently, metasurfaces have been investigated and exploited for various applications in the THz regime, including modulators and detectors. However, the responsive properties of the metasurface in THz stay fixed once the fabrication process is complete. This limitation can be modified when integrating the phase change material (PCM), whose states are switchable between crystalline and amorphous, into the metasurface structure. This characteristic of the PCM is appealing in achieving dynamic and customizable functionality. In this work, the reflective metasurface structure is designed as a hexagonal unit deposited on a polyimide substrate. The non-volatile PCM chosen for the numerical study is germanium antimony tellurium (GST). Our proposed phase change metasurface provides two resonant frequencies located at 1.72 and 2.70 THz, respectively; one of them shows a high contrast of reflectivity at almost 80%. The effects of geometrical parameters, incident angles, and polarization modes on the properties of the proposed structure are explored. Finally, the performances of the structure are evaluated in terms of the insertion loss and extinction ratio.

Absorption Enhancement in a Quantum Dot Thz Detector with a Metal-Semiconductor-Metal Structure

The low absorptivity of quantum dot nano-structures cannot meet the requirements for high-performance next-generation Thz detectors which can be used for environmental pollution detection. In this study, a novel metal-semiconductor-metal (MSM) cavity structure with a square hole array instead of a traditional planar metal electrode was developed to improve and enhance the absorptivity of a quantum dot Thz detector. The possible modes and loss problems in the metal resonant cavity were analyzed using the finite-element transmission matrix, the eigenvector method, and Kirchhoff diffraction theory. The results demonstrate that the MSM cavity structure introduced in the detector can enhance absorption up to 8.666 times higher than that of the conventional counterpart.

Progress in ITER ECE diagnostic design and integration

The ITER electron cyclotron emission (ECE) diagnostic system has primary roles in providing measurements of the core electron temperature profile and the electron temperature fluctuation associated with the neoclassical tearing modes. The ITER ECE system includes a radial and oblique line-of-sight. Four 43-meter long low-loss transmission lines (TLs) are designed to transmit millimeter wave power in the frequency range of 70–1000 GHz in both X- and O-mode polarization from the port plug to the ECE instrumentation room in the diagnostic building. The measurement instrumentation includes two Fourier transform spectrometer (FTS) systems and two radiometer systems. The Indian Domestic Agency (IN-DA) and United States Domestic Agency share the responsibility. The IN-DA scope excluding instrumentation and control has passed its preliminary design review and is progressing towards the final design review (FDR). In parallel, the diagnostic integration in different areas is ongoing. Several captive components for the TLs have passed FDR and will be manufactured for installation in the tokamak building soon. A peer review meeting has been held on the prototype hot calibration source, and its integration and new thermal analysis in the diagnostic shield module are continuing. A prototype TL is being tested. A prototype polarizing Martin-Puplett type FTS, operating in the frequency range 70–1000 GHz, features an in-vacuo fast scanning mechanism and a cryo-cooled dual-channel THz detector system. Its performance has been assessed in detail against ITER requirements.

The Role of the Directivity of Various THz Detectors in Multiplexing Systems

Many modern and future systems, based on the wireless communication at the THz frequencies, could benefit from multichannel transmission. One of the possible approaches is to (de)multiplex several separate signals to and from a single transmission channel using dedicated diffractive optical elements. Proper selection of receivers for such systems is crucial and strongly depends not only on the frequencies used but also on the geometry of the setup. In this article, we present a complex analysis of the applicability of various detectors for the characterization of highly convergent and off-axis beams. Three three-focal-spot diffractive lenses have been designed, optimized and manufactured to verify the influence of parameters such as focal length, focal position shift, deflection angle or radiation frequency on the proper detection and separation of focal spots using different receivers. The reliable characterization of multi-focal-point structures can be performed only with high-acceptance-angle detectors, such as, for example, field-effect transistors equipped with a patch antenna. On the other hand, for the detection of a single demultiplexed signal, a much more directive receiver can be applied, as long as it is placed at a proper angle.

Multiple Modes Response of Co‐Aperture 2D/1D Phototransistors

AbstractStrict requirements for target detection and identification of the complex background have increased because a single detection method is susceptible to environmental factors. The abilities to use terahertz imaging systems with high photoresponsivity and polarization sensitivity are of central importance to practical photodetectors. The present technology for high‐sensitivity, polarization THz detectors integrated into a compact, common aperture (co‐aperture) system remains largely unexplored challenges. By exploiting the inherent features of graphene/Bi2Te3 Dirac material and InAs nanowire, the plasma‐wave and thermoelectric co‐aperture phototransistors with a selective, synchronous, and controllable operating modes are devised in the same field of view, which provide remarkable efficiencies for THz‐light harvesting on‐chip. The achieved selective detection, better than that of single nanowire phototransistor, exhibits excellent sensitivity of 18.5 mA W−1 (12 V W−1) in graphene/Bi2Te3 channel and 3.5 mA W−1 (400 V W−1) in InAs nanowire channel, corresponding to the noise equivalent power of less than 0.1 nW/Hz0.5. The results are exploited to highlight a novel strategy for the realization of efficient co‐aperture phototransistors toward tunable, multiple modes, polarization THz detection by combining with in situ integration advantages of various materials, compatibility mechanisms, integration structures with metamaterials.

Terahertz control of photoluminescence emission in few-layer InSe

A promising route for the development of opto-electronic technology is to use terahertz radiation to modulate the optical properties of semiconductors. Here, we demonstrate the dynamical control of photoluminescence (PL) emission in few-layer InSe using picosecond terahertz pulses. We observe a strong PL quenching (up to 50%) after the arrival of the terahertz pulse followed by a reversible recovery of the emission on the timescale of 50 ps at T=10 K. Microscopic calculations reveal that the origin of the photoluminescence quenching is the terahertz absorption by photo-excited carriers: this leads to a heating of the carriers and a broadening of their distribution, which reduces the probability of bimolecular electron-hole recombination and, therefore, the luminescence. By numerically evaluating the Boltzmann equation, we are able to clarify the individual roles of optical and acoustic phonons in the subsequent cooling process. The same PL quenching mechanism is expected in other van der Waals semiconductors, and the effect will be particularly strong for materials with low carrier masses and long carrier relaxation time, which is the case for InSe. This work gives a solid background for the development of opto-electronic applications based on InSe, such as THz detectors and optical modulators.

Magnetron sputtering deposited large-scale Weyl semimetal THz detector

THz detectors are considered as a building block for a wide range of applications including THz imaging system as well as THz communication system. However, beset by low sensitivity, small effective detection area, high preparation cost and high noise equivalent power, further research still needs especially in high-performance and large-scale THz detectors. In order to fulfill this requirement, Weyl semimetal MoTe2 was chosen as the active layer due to its lower dark current and Weyl-point enhancement. Moreover, high-quality and large-scale THz MoTe2-based detectors were fabricated with an effective detection area of 1 mm × 6.3 mm by means of magnetron sputtering deposition method. With an external voltage applied, our device exhibits excellent surface plasmon polaritons effect for THz detection and demonstrates a photoresponse of 4 A/W and a noise equivalent power of 9.74 × 10-12 W Hz−1/2 under the illumination of 0.1 THz wave at room temperature. Our research paves the way to the fabrication of large-scale THz detectors, also proves that the Weyl semimetal can be an excellent candidate of high-performance material in long-wave detection.

Enhanced terahertz detection of multigate graphene nanostructures

Abstract Terahertz (THz) waves have revealed a great potential for use in various fields and for a wide range of challenging applications. High-performance detectors are, however, vital for exploitation of THz technology. Graphene plasmonic THz detectors have proven to be promising optoelectronic devices, but improving their performance is still necessary. In this work, an asymmetric-dual-grating-gate graphene-terahertz-field-effect-transistor with a graphite back-gate was fabricated and characterized under illumination of 0.3 THz radiation in the temperature range from 4.5 K up to the room temperature. The device was fabricated as a sub-THz detector using a heterostructure of h-BN/Graphene/h-BN/Graphite to make a transistor with a double asymmetric-grating-top-gate and a continuous graphite back-gate. By biasing the metallic top-gates and the graphite back-gate, abrupt n+n (or p+p) or np (or pn) junctions with different potential barriers are formed along the graphene layer leading to enhancement of the THz rectified signal by about an order of magnitude. The plasmonic rectification for graphene containing np junctions is interpreted as due to the plasmonic electron-hole ratchet mechanism, whereas, for graphene with n+n junctions, rectification is attributed to the differential plasmonic drag effect. This work shows a new way of responsivity enhancement and paves the way towards new record performances of graphene THz nano-photodetectors.

Enhanced room-temperature terahertz detection and imaging derived from anti-reflection 2D perovskite layer on MAPbI3 single crystals.

Terahertz (THz) detection technology is getting increasing attention from scientists and industries alike due to its superiority in imaging, communication, and defense. Unfortunately, the detection of THz electromagnetic waves under room temperature requires a complicated device architecture design or additional cryogenic cooling units, which increase the cost and complexity of devices, subsequently imposing an impediment in its universal application. In this work, THz detectors operated under room temperature are designed based on the thermoelectric effect with MAPbI3 single crystals (SCs) as active layers. With solution-processed molecular growth engineering, the anti-reflection 2D perovskite layers were constructed on SCs' surfaces to suppress THz reflection loss. Simultaneously, by finely regulating the main carrier types and the direction of the applied bias across the inclined energy level, the thermoelectric effect is further promoted. As a result, THz-induced ΔT in MAPbI3 SCs reaches 4.6 °C, while the enhancement in the bolometric and photothermoelectric effects reach ∼4.8 times and ∼16.9 times, respectively. Finally, the devices achieve responsivity of 88.8 μA W-1 at 0.1 THz under 60 V cm-1, noise equivalent power (NEP) less than 2.16 × 10-9 W Hz-1/2, and specific detectivity (D*) of 1.5 × 108 Jones, which even surpasses the performance of state-of-the-art graphene-based room-temperature THz thermoelectric devices. More importantly, proof-of-concept imaging gives direct evidence of perovskite-based THz sensing in practical applications.