Double-Sided Rhombic Shaped-Nanoantenna for Broadband IR Detection

Wireless Personal Communications - A double-sided rhombic nanoantenna for infrared (IR) detection in frequency band from 10 to 60 THz is proposed. The nanoantenna consists of square...     

Selective Growth of Type-II Weyl-Semimetal and Van der Waals Stacking for Sensitive Terahertz Photodetection

The emergence of novel topological semimetal materials, accompanied by exotic non-equilibrium properties, not only provides a fertile playground for a fundamental level of interest but also opens exciting opportunities for inventing new applications by making use of different light-induced effects such as nonlinear optics, optoelectronics, especially for the highly pursued terahertz (THz) technology due to the gapless electronic structures. Exploring type-II Weyl semimetal endowed with the richness of quantum wavefunction and peculiar band structure, underlie strong nonlinear coupling with THz waves. Here, the selective growth of type-II Weyl semimetal NbIrTe₄ by means of a self-flux approach is reported, which hosts strongly tilted Weyl cones and exotic Fermi arcs. The oscillating THz field induced by the antenna is engineered in terms of planar metal-topological semimetal-metal structure, along with van der Waals stacking, which allows for self-powered photodetection at room temperature. The results elucidate the superior performance of NbIrTe₄-graphene heterostructure-based photodetectors with responsivity up to 264.6 V W⁻¹ at 0.30 THz, fast response of 1 µs as well as low noise equivalent power ˂0.28 nW Hz^−0.5 is achieved, already exhibiting high-quality imaging at THz frequency. The results promise superb impacts in exploring topological Weyl semimetals for efficient low-energy photon harvesting.     

Complex permittivity characterization of benzocyclobutene for terahertz applications

High-quality polymers such as the benzocyclobutene polymer (BCB) provide interesting dielectric feature for terahertz applications. Already used in silicon integrated circuit technologies, this material could become one of the most promising candidates for the realization of future THz waveguides and interconnections on a silicon substrate but also after active devices process on the top of any other technology (GaAs, InP, GaN…). A frequency-dependent complex permittivity of spin-coated thick layers of this low-k dielectric is obtained from transmittance spectra measured with Fourier transform spectroscopy in the frequency range of 0.5-5.4 THz. The dielectric constant and the loss tangent are discussed according to curing conditions of the photosensitive resin used. A low loss tangent value of 7 − 9 × 10^-3 at 1 THz is obtained with polymerisation in oxygen-free atmosphere. An incomplete curing and a high dose UV exposure have a weak impact on losses. These results associated with the high compatibility of this polymer with silicon and metals make BCB layers well suited for the design of microelectronic THz devices and circuits.     

Gyrotron Development for High Power THz Technologies at IAP RAS

The review summarizes recent experimental results in the field of THz gyrotrons developed for various applications. A CW gyrotron with the operation frequency of 0.26 THz has been successfully used for DNP spectroscopy A pulsed high-harmonic Large Orbit Gyrotron (LOG) with the frequency of 0.55 THz and kW level of output power has been used for THz breakdown and obtaining dense plasma in gases. A powerful pulsed 0.67 THz/200 kW gyrotron is under development for remote detection of ionization sources.     

运用太赫兹时域光谱技术定量分析柴油的含硫量

测量柴油样品的太赫兹时域光谱并对其含硫量进行了定量计算.在0.2 ～1.5 THz范围内,柴油的吸收系数随着太赫兹频率和硫含量呈规律性递增.基于这一变化关系建立起了硫含量与太赫兹吸收系数和频率的多元非线性模型,通过测量柴油的太赫兹光谱代入此模型即可计算出柴油的硫含量.这一结果为太赫兹时域光谱技术应用于柴油含硫量的无损快速检测奠定了理论基础,显示出巨大的应用前景.     

Enhanced Continuous-Wave Terahertz Imaging with a Horn Antenna for Food Inspection

Continuous-wave (CW) terahertz (THz) imaging with a horn antenna is proposed to enhance the spatial resolution of a THz imaging system. The attached waveguide that is smaller than the wavelength can easily increase the spatial resolution, and the optimized horn flare can significantly increase the transmission power. Consequentially, transmission THz images of a phantom obtained by the amplitude signal using a 0.2 THz wave reveal that the spatial resolution is achieved up to 500 μm. Also, the transmitted power is increased up to 6 times higher compared to the pinhole aperture. The feasibility of CW THz imaging with a horn antenna is demonstrated by the inspection of the organic samples inside food resulting in a relatively high sensitivity for soft organic samples compared with the sensitivity of X-ray imaging to this kind of samples.     

Terahertz time-domain-spectroscopy system using a 1 micron wavelength laser and photoconductive components made from low-temperature-grown GaAs

A terahertz time-domain spectroscopy (TDS) system based on a femtosecond Yb:KGW laser, photoconductive emitters and detectors made from as-grown and from annealed at moderate temperatures (~400°C) low-temperature-grown GaAs (LTG GaAs) layers was demonstrated. The measured photoconductivity of these layers increased linearly with the optical power, showing that transitions from the defect band to the conduction band are dominant. The largest amplitude THz pulse with a useful signal bandwidth reaching 3 THz and its signal-to-noise ratio exceeding 50 dB was emitted by the device made from the LTG GaAs layer annealed at 420°C temperature. The detector made from this material was by an order of magnitude less sensitive than conventional GaBiAs detectors.     

A Technology Demonstrator for 1.6–2.0 THz Waveguide HEB Receiver with a Novel Mixer Layout

In this paper, we present our studies on a technology demonstrator for a balanced waveguide hot-electron bolometer (HEB) mixer operating in the 1.6–2.0 THz band. The design employs a novel layout for the HEB mixer combining several key technologies: all-metal THz waveguide micromachining, ultra-thin NbN film deposition and a micromachining of a silicon-on-insulator (SOI) substrate to manufacture the HEB mixer. In this paper, we present a novel mixer layout that greatly facilitates handling and mounting of the mixer chip via self-aligning as well as provides easy electrical interfacing. In our opinion, this opens up a real prospective for building multi-pixel waveguide THz receivers. Such receivers could be of interest for SOFIA, possible follow up of the Herschel HIFI, and even for ground based telescopes yet over limited periods of time with extremely dry weather (PWV less than 0.1 mm).     

A Novel Terahertz Rat-Race Hybrid Coupler Based on PPDW

A novel rat-race hybrid coupler based on parallel-plate dielectric waveguide (PPDW) is proposed. PPDW has been proposed as a basic terahertz transmission line for its remarkable simple structure and comparatively low attenuation at terahertz frequency band. It can be applied to design many terahertz components. In this paper, a (3 N + 3/2) × lambda circumference PPDW rat-race hybrid coupler operating at quasi-TE₁₀ mode is studied. The investigation results show that, at 0.34 THz, the transmission losses of the two split output ports are both equals −3.2 dB; the return loss at its input port is below −20 dB; the insertion loss at each split transmission port is less than 0.2 dB; and the two isolated ports show good isolation of above 20 dB. Therefore, it will have wide potential applications at terahertz frequency band.     

XPS and IPE analysis of HfO2 band alignment with high-mobility semiconductors

A detailed analysis of the band alignment between atomic-layer deposition (ALD)-grown HfO₂ thin films and n-type Ge(1 0 0) substrate is presented. The valence band offset (VBO) is determined by X-ray photoelectron spectroscopy (XPS) after careful evaluation of the experimental data and accurate removal of the artefacts induced by differential charging phenomena occurring during XPS measurement. VBO values are 2.7±0.1 eV for all samples independent of the different growth conditions. A conduction band offset (CBO) of 2.0±0.1 eV and a band gap of 5.6±0.1 eV have been obtained by internal photoemission (IPE) and photoconductivity measurements, respectively. VBO and CBO values obtained by the different techniques are in excellent agreement within the experimental error.     

Terahertz wireless data communication

In recent years Terahertz (THz) Band communications have gained even greater interest and higher expectations to meet an ever increasing demand for the speed of wireless communications. This paper provides the characteristics of electromagnetic waves propagating in the THz Band, which is one of the key technology to satisfy the increasing demand for Terahertz Wireless Data Communication (ThWDC). The performance of future terabit super channels implemented using bipolar phase-shift-keying which gives the best BER (Bit Error Rate) with today’s technology is investigated through the simulations for ThWDC. The objective of this paper is to describe the important issues related to the transmission in of ThWDC in air environment and to determine the best transmission windows in the THz range. In particular, ThWDC channel is modeled considering effects like capacity, channel performance and BER is investigated through simulation. The simulation results and the theoretical analysis show that data communication is possible from 0.01 to 0.5 THz frequency range and the best transmission window in this range have been found ω₁ = [0.01–0.05 THz], ω₂ = [0.06–0.16 THz] and ω₃ = [0.2–0.3 THz] in this paper.

     

A Low-Noise Terahertz SIS Mixer Incorporating a Waveguide Directional Coupler for LO Injection

We have developed a low-noise heterodyne waveguide Superconductor-Insulator-Superconductor (SIS) mixer with a novel local oscillator (LO) injection scheme for the Atacama Large Millimeter/submillimeter Array (ALMA) band 10, over the frequency range 0.78–0.95 THz. The SIS mixer uses radio frequency (RF) and LO receiving horns separately and a waveguide 10 dB LO coupler integrated in the mixer block. The insertion loss of the waveguide and coupling factor of the coupler were evaluated at terahertz frequencies at both room and cryogenic temperatures. The double-sideband (DSB) receiver noise temperatures were below 330 K (7.5hf/k _B) at LO frequencies in the range 0.801–0.945 THz. The minimum temperature was 221 K at 0.873 THz over the intermediate frequency range of 4–12 GHz at an operating temperature of 4 K. This waveguide heterodyne SIS mixer exhibits great potential for practical applications, such as high-frequency receivers of the ALMA.     

Scalable Microstructured Photoconductive Terahertz Emitters

The development of scalable emitters for pulsed broadband terahertz (THz) radiation is reviewed. Their large active area in the 1 – 100 mm² range allows for using the full power of state-of-the-art femtosecond lasers for excitation of charge carriers. Large fields for acceleration of the photogenerated carriers are achieved at moderate voltages by interdigitated electrodes. This results in efficient emission of single-cycle THz waves. THz field amplitudes in the range of 300 V/cm and 17 kV/cm are reached for excitation with 10 nJ pulses from Ti:sapphire oscillators and for excitation with 5 μJ pulses from amplified lasers, respectively. The corresponding efficiencies for conversion of near-infrared to THz radiation are 2.5 × 10^-4 (oscillator excitation) and 2 × 10^-3 (amplifier excitation). In this article the principle of operation of scalable emitters is explained and different technical realizations are described. We demonstrate that the scalable concept provides freedom for designing optimized antenna patterns for different polarization modes. In particular emitters for linearly, radially and azimuthally polarized radiation are discussed. The success story of photoconductive THz emitters is closely linked to the development of mode-locked Ti:sapphire lasers. GaAs is an ideal photoconductive material for THz emitters excited with Ti:sapphire lasers, which are widely used in research laboratories. For many applications, especially in industrial environments, however, fiber-based lasers are strongly preferred due to their lower cost, compactness and extremely stable operation. Designing photoconductive emitters on InGaAs materials, which have a low enough energy gap for excitation with fiber lasers, is challenging due to the electrical properties of the materials. We discuss why the challenges are even larger for microstructured THz emitters as compared to conventional photoconductive antennas and present first results of emitters suitable for excitation with ytterbium-based fiber lasers. Furthermore an alternative concept, namely the lateral photo-Dember emitter, is presented. Due to the strong THz output scalable emitters are well suited for THz systems with fast data acquisition. Here the application of scalable emitters in THz spectrometers without mechanical delay stages, providing THz spectra with 1 GHz spectral resolution and a signal-to-noise ratio of 37 dB within 1 s, is presented. Finally a few highlight experiments with radiation from scalable THz emitters are reviewed. This includes a brief discussion of near-field microscopy experiments as well as an overview over gain studies of quantum-cascade lasers.     

Fast Design and Cold-Circuit Properties Simulation for the Slow Wave Structure of a 0.14 THz Broadband Folded Waveguide Traveling Wave Tube

This article applies an explicit method to the fast structural design for the slow wave circuit of a 0.14 THz broadband folded waveguide traveling wave tube. To valid the primary design, cold-circuit properties, such as dispersion and interaction impedance, were calculated by using both theoretical method and electromagnetic software (CST MWS) simulation. The S parameter S ₁₁ was also calculated by CST MWS with lossy metal being considered. Simulation results show that, the cold pass band is about 70 GHz, and the interaction impedance is about 1 ohm, and indicate that the design method is reasonable for obtaining a 0.14 THz broadband slow wave structure.     

Study of atmospheric attenuation characteristics of terahertz wave based on line‐by‐line integration

Terahertz band communication promises new solution for satisfying the increasing demand for ultrahigh‐speed wireless communication. Channel models capturing the unique peculiarities of the terahertz (THz) band are required for communication systems designing. Extreme high molecular absorption is a distinctive phenomenon that has to be involved in terahertz communication models. Research in this field has mainly focused on the characteristics along the horizontal propagation path. In this paper, we developed a unified molecular absorption model along the slant propagation path of the THz wave, based on the line‐by‐line integration method developed by Van‐Vleck and Weisskopf and combining the molecular spectral line in the HITRAN database. Then, an in‐depth analysis on the THz channel characteristics is carried out by the developed propagation models. The attenuation characteristics of terahertz waves with frequencies in the range of 0.1 to 1 THz are analyzed by theoretical and mathematical modeling. The results show that the terahertz communication channel has a strong dependence on both the molecular composition of the medium and the transmission distance. The experimental results also indicate the strong absorption frequency points, weak absorption frequency points, and spectral windows.     

Pulse-level beam-switching for terahertz networks

Communication in Terahertz (THz) band is envisioned as a promising technology to meet the ever-growing data rate demand, and to enable new applications in both nano-scale and macro-scale wireless paradigms. In this study, we propose the first system-level design that is suitable for THz communication in macro-scale range with 100+ Gbps data rate. The design is based on the proposed terahertz pulse-level beam-switching with energy control (TRPLE), and motivated by the rise in Graphene-based electronics, which include not only compact generator and detector for pulse communication, but also the capability of beam scanning aided with nano-antenna-arrays. The very high path loss seen in THz wireless channel requires the use of narrow beam to reach longer transmission ranges. On the other hand, impulse radio that emits femtosecond-long pulses allows the beam direction to steer at pulse-level, rather than at packet-level. For TRPLE, we mathematically analyze the data rate for an arbitrary wireless link under the THz channel characteristics and the energy modulation scheme. Then, a novel optimization model is formulated to solve the parameters of the inter-pulse separation and the inter-symbol separation, in order to maximize the data rate while meeting the interference requirement. With the optimization, the data rate of 167 Gbps is shown achievable for most users in 20-m range. A MAC protocol framework is then presented to harness the benefits of the pulse separation optimization.

     

Terahertz Photometer to Observe Solar Flares in Continuum

Solar observations at sub-THz frequencies detected a new flare spectral component peaking in the THz range, simultaneously with the well known microwaves component, bringing challenging constraints for interpretation. Higher THz frequencies observations are needed to understand the nature of the mechanisms occurring in flares. A THz photometer system was developed to observe outside the terrestrial atmosphere on stratospheric balloons or satellites, or at exceptionally transparent ground stations. The telescope was designed to observe the whole solar disk detecting small relative changes in input temperature caused by flares at localized positions. A Golay cell detector is preceded by low-pass filters to suppress visible and near IR radiation, a band-pass filter, and a chopper. A prototype was assembled to demonstrate the new concept and the system performance. It can detect temperature variations smaller than 1 K for data sampled at a rate of 10/s, smoothed for intervals larger than 4 s. For a 76 mm aperture, this corresponds to small solar burst intensities at THz frequencies. A system with 3 and 7 THz photometers is being built for solar flare observations on board of stratospheric balloon missions.     

碳纳米器件光电特性的电磁散射分析

用经典的宏观电磁理论对由碳纳米材料制成的光敏器件的光电特性──主要为功率吸收特性[1],作了适当的建模分析.在该模型中,将光处理为频率较高的电磁波,纳米材料处理为有损介质材料,应用经典的电磁散射理论中的并矢格林函数建立了相应的散射模型,由此求得器件中碳纳米管束处的电场强度,并分析其吸收功率特性,从而对该光敏器件的设计给出一些指导性的意见.最后用商业全波仿真软件HFSS对该结果进行了验证.     

Comparison of CW THz Wave Spectrometer with Laser Diode Excitation and Pulsed THz Wave Spectrometer with Cr:forsterite Sources Based on Difference Frequency Generation of Near-infrared Lasers in GaP

We constructed two types of terahertz (THz) spectrometers with automatic scanning control based on the difference frequency generation method by the excitation of the phonon-polariton mode in GaP. The pulsed THz wave spectroscopic systems were based on an optical parametric oscillator and Nd: YAG laser sources having a frequency resolution of 1.5 GHz, and on Cr:fosterite sources with a resolution of 20 GHz. Following these, we recently constructed a continuous wave (CW) THz wave spectroscopic system with laser diode excitation. One of the advantages of the CW THz wave spectrometer is its wide frequency tuning range with fine frequency resolution of < 8 MHz. In this study, we compare both types of spectrometers (pulsed versus CW) to show the characteristics of each system in terms of frequency resolution. The absorption spectra of a non-deformed white polyethylene crystal and ultra-high molecular weight polyethylene with/without deformation are measured by using the CW THz wave spectrometer and pulsed THz wave spectrometer. The effect of the high-resolution CW THz wave spectrometer is shown based on the THz spectroscopic results.     

Rectangular Microstirp Patch Antenna Design at THz Frequency for Short Distance Wireless Communication Systems

In this paper, we have presented the simulation results of a rectangular microstrip patch antenna at terahertz (THz) frequency ranging from 0.7 to 0.85 THz. THz electromagnetic wave can permit more densely packed communication links with increased security of communication transmission. The simulated results such as gain, radiation efficiency and 10 dB impedance bandwidth of rectangular microstrip patch antenna at THz frequencies without shorting post configuration are 3.497 dB, 55.71% and 17.76%, respectively, whereas with shorting post configuration, corresponding parameters are 3.502 dB, 55.88% and 17.27%. The simulation has been performed by using CST Microwave Studio, which is a commercially available electromagnetic simulator based on the method of finite difference time domain technique.