sub-THz Range Research Articles

Continuum absorption in pure N[formula omitted] gas and in its mixture with Ar

The N2−N2and N2−Arcontinuum absorption spectra are calculated using the classical trajectory-based simulation (CTS). The spectra obtained are validated by new measurements in the subTHz spectral range along with the previously reported data in the far infrared. A novelty of our approach consists in the use of the CTS method to simulate both the fundamental nitrogen absorption band and the rototranslational band in N2−Ar, i.e., we succeeded to step beyond the conventionally used approximation of the only rigid monomers. This extension of the theory made it possible, in particular, to demonstrate the validity of the rigid monomer assumption for the CTS simulation of the rototranslational N2−Arband. The broadband spectra within 77–354 GHz were measured using the resonator spectrometer at temperatures of 278–333 K and pressures of 900–1600 Torr. A minor underestimation of the calculated absorption by 3.7% and 5% is shown for the N2−N2and N2−Arsystem, respectively. On the basis of the obtained data, a new analytical model is developed for the N2−N2absorption in the subTHz range, which can be used in radiation propagation codes for the Earth, Titan, or other nitrogen-rich atmospheres. The advantage of the model proposed here over those previously published is discussed.

Review of Luneburg Lens Antenna Designs Manufactured Using 3D Printing

Introduction. The interest in multibeam dielectric lens antenna arrays has been growing in recent years due to the development of millimeter-wave telecommunication and radar systems. Progress in the development of mobile communication systems based on adaptive beamforming technology is increasingly associated with multibeam systems based on lens antenna structures, providing an alternative to hard-to-implement and energy-consuming phased antenna arrays. In recent years, spherical and cylindrical Luneburg lens antennas implemented using additive manufacturing technology have attracted research attention. Despite their complexity of execution, these design exhibit excellent electromagnetic characteristics. This paper provides a review of Luneburg lens antennas manufactured using 3D printing, which can find application in 5G and 6G communication systems.Aim. To review achievements in the design of lens antenna structures manufactured using additive manufacturing.Materials and methods. Materials for analysis, comparison, and systematization were derived from various sources, including research articles, publications in proceedings of Russian and international conferences, and websites of manufacturers of lens antennas over the past 20 years. The material selection mechanism was based on the originality of the presented designs of printed Luneburg lens antennas.Results. A review of Luneburg lens antennas manufactured using 3D printing, which differ from each other in terms of mechanical strength, complexity of execution, and electrodynamic characteristics, was carried out. The results of a comparative analysis of the key characteristics of these antennas are presented, along with examples of their practical implementation.Conclusion. The disadvantage of Luneburg lens antennas has always been the complexity of their manufacture; however, additive manufacturing technologies open up new opportunities for their fast, high-quality, and automated production. Various 3D printing technologies can be used to create dielectric lens antennas, which differ in the resolution of printers, printing speed, and cost. Additive manufacturing methods are constantly developing, having reached the technological possibility of printing Luneburg lens for the sub-THz range with a high level of resolution and accuracy. In addition, 3D printers capable of printing multiple lenses simultaneously have also appeared.

Characterization of Nematic Liquid Crystal Dielectric Properties Using Complementary FSSs Featuring Electrically Small Cell Gaps Across a Wide Sub-THz Range

Characterization of Nematic Liquid Crystal Dielectric Properties Using Complementary FSSs Featuring Electrically Small Cell Gaps Across a Wide Sub-THz Range

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene.

Graphene is a nonlinear material in the terahertz (THz) frequency range, with χ(3) ∼ 10-9 m2/V2 ∼ 15 orders of magnitude higher than that of other materials used in the THz range, such as GaAs or lithium niobate. This nonlinear behavior, combined with ultrafast dynamic for excited carriers, proved to be essential for third harmonic generation in the sub-THz and low (<2.5 THz) THz range, using moderate (60 kV/cm) fields and at room temperature. Here, we show that, for monochromatic high peak power (1.8 W) input THz signals, emitted by a quantum cascade laser, the nonlinearity can be controlled using an ionic liquid gate that tunes the graphene Fermi energy up to >1.2 eV. Pump and probe experiments reveal an intense absorption nonlinearity at 3.2 THz, with a dominant 3rd-order contribution at EF > 0.7 eV, hence opening intriguing perspectives per engineering novel architectures for light generation at frequencies > 9 THz.

High-Resolution Parameter Estimation for Wideband Radio Channel Sounding

Multidimensional channel sounding measures the geometrical structure of mobile radio propagation. The parameters of a multipath data model in terms of directions, time-of-flight and Doppler shift are estimated from observations in frequency, time and space. A maximum likelihood estimation framework allows joint high-resolution in all dimensions. The prerequisite for this is an appropriate parametric data model that represents the multipath propagation correctly. At the same time, a device data model is necessary that typically results from calibration measurements. The used model should be as simple as possible since its structure has a considerable effect on the estimation effort. For instance, the inherent effort in parameter search is reduced if the influence of the parameters is kept independent. Therefore, the data model is characterized by several approximations. The most important is the “narrowband assumption” which assumes a low relative bandwidth and also avoids considering any frequency response in magnitude and phase. We extend the well-known multidimensional RIMAX parameter estimation framework by including proper frequency responses. The advantage reveals itself with high bandwidth in the mmWave and sub-THz range. It allows for a more realistic modeling of antenna arrays, it breaks with the usual narrowband model and allows a better modeling of mutual coupling and time delay effects. If the interacting object extends over several delay bins (hence an extended target in radar terminology) we propose a model that assigns a short delay spread, respectively a frequency response to the propagation path that associates it to the respective object. We verify the validity of the device model by numerical experiments on simulated and measured antenna data and compare it to RIMAX. Additionally, we use synthetic data based on ray-tracing results and measurements both ranging from 27.0GHz to 33.0GHz with known ground truth information and show that the proposed estimator delivers better performance for higher relative bandwidths than the conventional RIMAX implementation.

Biothermal Heating on Human Skin by Millimeter and Sub-Terahertz Waves in Outdoor Environment—A Theoretical Study

The frequency band in the millimeter-wave (MMW) and sub-terahertz (sub-THz) range has shown great potential in mobile communication technology due to the advantages of ultra-large bandwidth and ultra-high data rates. Based on the increasing research activities on MMW/sub-THz waves, biological safety at relevant frequencies must be explored, especially when high-power illumination occurs. Here, its non-ionizing nature plays a vital role, which makes it safe for humans at low illumination powers. However, under high power, the biothermal heating on the skin surface is still a main concern, and lots of research has been conducted in a laboratory. In this article, we analyze the thermal heating effect of human skin in outdoor environments, where atmospheric conditions can significantly impact the propagation of MMW/sub-THz waves. Our analysis is based on rat skin, which has a similar structure to human skin. A theoretical model combining Pennes’ bioheat transfer equation (BHTE), the ITU model, and the Mie scattering theory is developed. Good agreement between calculation results and measured data confirms the efficiency of this model. The influence of rainfall rate, humidity, operating frequency, illumination time, power density, and propagation distance is presented and discussed.

Sub-THz Vibrational Dynamics in Ordered Mesoporous Silica Nanoparticles.

The vibrational dynamics in the sub-THz range of mesoporous silica nanoparticles (MSNs) having ordered cylindrical mesopores was investigated. MCM-41 and SBA-15 particles were synthesized, and their structure was determined using scanning electron microscopy (SEM), low-angle X-ray diffraction (XRD), N2 physisorption analyses, and Raman scattering. Brillouin scattering measurements are reported and enabled determining the stiffness of the silica walls (speed of sound) using finite element calculations for the ordered mesoporous structure. The relevance of this approach is discussed based on the comparison between the numerical and experimental results and previous works reported in the literature.

Low-Cost and Low-Profile Sub-Terahertz Luneburg Lens Beamformer on Polymer

A Luneburg lens is used to excite a leaky-wave antenna (array of slots) in the sub-terahertz (sub-THz) frequency range. The antenna is built on a 140-μm thick Cyclic Olefin Copolymer (COC) substrate. The radiating slots (58×24 slots) are etched on the polymer by a photolithographic process, for which the gold (Au) layers are first deposited by evaporation on the COC and, then, patterned by chemical etching. No metalized vias or drilling are applied to the substrate. The full system is fed by a standard WR03 waveguide, and it has been fabricated and tested. Experimental results show an input reflection coefficient better than −7 dB over a 25% fractional bandwidth, spanning from 220 GHz to 290 GHz. The radiated beam can be steered in elevation from −80° to −20° over the same band. The antenna efficiency is estimated to be 20% at the center frequency (260 GHz). This solution provides a simple alternative for low-cost and low-profile beam scanning antennas in the sub-THz range.

A new class of spatial harmonic magnetrons with potentials for CW and sub-THz operation

AbstractA strictly physically based design theory of a new class of Spatial Harmonic Magnetrons (SHM) is thoroughly derived which leads to analytically evaluable expressions. Thus two advantages are obtained: (1) The design parameters appear grouped into two separate categories – one just containing geometrical and material parameters, the other one exclusively containing beam current-related ones, which are in product-form determining performance. The influence of each parameter can thus easily be discovered and investigated. (2) Numerical efforts for any new design will be reduced by at least one order of magnitude. Subsequently and based on feature (1) it is derived that loading the anode structure by a suitably selected meta-material will increase output power and efficiency, will pave the way to CW operation, and can extend oscillation frequency well into the sub-THz range. Finally, it is shown that the Rising Sun Magnetron is a first step toward a meta-material loaded SHM offering the same but quantitatively reduced features and less stringent requirements to fabrication technology.

Sub-terahertz silicon-based on-chip absorption spectroscopy using thin-film model for biological applications

Spectroscopy in the sub-terahertz (sub-THz) range of frequencies has been utilized to study the picosecond dynamics and interaction of biomolecules. However, widely used free-space THz spectrometers are typically limited in their functionality due to low signal-to-noise ratio and complex setup. On-chip spectrometers can revolutionize THz spectroscopy allowing integration, compactness, and low-cost fabrication. In this paper, a low-loss silicon-based platform is proposed for on-chip sub-THz spectroscopy. Through functionalization of silicon chip and immobilization of bio-particles, we demonstrate the ability to characterize low-loss nano-scale biomolecules across the G-band (0.14–0.22 THz). We also introduce an electromagnetic thin-film model to account for the loading effect of the immobilized biomolecules, i.e. dehydrated streptavidin and immunoglobulin antibody, as two key molecules in the biosensing discipline. The proposed platform was fabricated using a single mask micro-fabrication process, and then measured by a vector network analyzer (VNA), which offers high dynamic range and high spectral resolution measurements. The proposed planar platform is general and paves the way towards low-loss, cost-effective and integrated sub-THz biosensors for the detection and characterization of biomolecules.

Terahertz magnetic resonance in MnCr2O4 under high magnetic field

Terahertz (THz) time-domain spectroscopy (THz-TDS) of polycrystalline MnCr2O4 was performed at <9 T and low temperatures. A resonance absorption in the sub-THz range with linear blueshifts was observed as the magnetic field was increased from 4 T to 9 T. These magnetism-driven absorptions originated from a ferromagnetic resonance, which agrees with low-field electron spin resonance measurements and ferromagnetic resonance theory. The low-temperature g-factors of MnCr2O4 were also obtained using THz-TDS. This work provides new insights into the spin dynamics of chromite spinel compounds in the THz region.

Development of Nd (III)-Based Terahertz Absorbers Revealing Temperature Dependent Near-Infrared Luminescence.

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [NdIII (phen)3 (NCX)3] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN− by SeCN−. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.

Джозефсоновские туннельные переходы с интегральным СИН-шунтированием

This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured.

Josephson tunnel junctions with integral SIN shunting.

This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured. Keywords: Superconducting devices, superconductor-insulator-superconductor tunnel junction, Josephson effect, shunting of Josephson junctions.

Chip-Scalable, Room-Temperature, Zero-Bias, Graphene-Based Terahertz Detectors with Nanosecond Response Time.

The scalable synthesis and transfer of large-area graphene underpins the development of nanoscale photonic devices ideal for new applications in a variety of fields, ranging from biotechnology, to wearable sensors for healthcare and motion detection, to quantum transport, communications, and metrology. We report room-temperature zero-bias thermoelectric photodetectors, based on single- and polycrystal graphene grown by chemical vapor deposition (CVD), tunable over the whole terahertz range (0.1–10 THz) by selecting the resonance of an on-chip patterned nanoantenna. Efficient light detection with noise equivalent powers <1 nWHz–1/2 and response time ∼5 ns at room temperature are demonstrated. This combination of specifications is orders of magnitude better than any previous CVD graphene photoreceiver operating in the sub-THz and THz range. These state-of-the-art performances and the possibility of upscaling to multipixel architectures on complementary metal-oxide-semiconductor platforms are the starting points for the realization of cost-effective THz cameras in a frequency range still not covered by commercially available microbolometer arrays.

Superconducting Sub-THz Receivers for Space and Ground-Based Radio Astronomy

The developments in the field of low-noise receiving systems in the sub-THz range, performed at the Kotelnikov Institute of Radio Engineering of the Russian Academy of Sciences in recent years and aimed at the creation of receivers with quantum sensitivity for space and ground-based radio telescopes are presented. Superconductor–insulator–superconductor (SIS) mixers based on high-quality tunnel junctions are key elements of the most sensitive sub-THz heterodyne receivers. The article presents the results of the development of SIS receivers in the 211–275 GHz and 800–950 GHz ranges with a noise temperature in the double-sideband (DSB) mode of about 20 K and 220 K, respectively. These achievements will be used to develop receiving systems for APEX and LLMA ground-based telescopes, as well as for the Millimetron space mission.

Resonator Method for Studying Dielectric Characteristics of Сaustobiolithes

For the sub-THz range, a method for studying the electrodynamic characteristics of organic materials – caustobiolithes-is developed on the basis of a resonator spectrometer. In the frequency range 110 ÷ 260 GHz, the dielectric parameters (refractive index n and tanδ) of peat powders after its microwave processing in the process of «soft» pyrolysis, as well as core sections of oil-containing rock, were studied. The influence of natural humidity on the dielectric parameters of the samples is considered. The reasons for the spread of measurement results, which is observed in these materials and is their specific feature, are discussed

Field-effect silicon-plasmonic photodetector for coherent T-wave reception

Plasmonic internal photoemission detectors (PIPED) have recently been shown to combine compact footprint and high bandwidth with monolithic co-integration into silicon photonic circuits, thereby opening an attractive route towards optoelectronic generation and detection of waveforms in the sub-THz and THz frequency range, so-called T-waves. In this paper, we further expand the PIPED concept by introducing a metal-oxide-semiconductor (MOS) interface with an additional gate electrode that allows to control the carrier dynamics in the device and the degree of internal photoemission at the metal-semiconductor interfaces. We experimentally study the behavior of dedicated field-effect (FE-)PIPED test structures and develop a physical understanding of the underlying principles. We find that the THz down-conversion efficiency of FE-PIPED can be significantly increased when applying a gate potential. Building upon the improved understanding of the device physics, we further perform simulations and show that the gate field increases the carrier density in the conductive channel below the gate oxide to the extent that the device dynamics are determined by ultra-fast dielectric relaxation rather than by the carrier transit time. In this regime, the bandwidth can be increased to more than 1 THz. We believe that our experiments open a new path towards understanding the principles of internal photoemission in plasmonic structures, leading to PIPED-based optoelectronic signal processing systems with unprecedented bandwidth and efficiency.

Relativistic Sub-THz Surface-Wave Oscillators With Transverse Gaussian-Like Radiation Output

Surface-wave oscillators driven by intense relativistic sheet electron beams can produce sub-terahertz waves at sub-gigawatt level of output power; however, evanescent nature of the operating wave hampers further radiation transmission and application. For these devices, we propose a method of efficient conversion of the surface operating wave into the output Gaussian-like wavebeam radiated in transverse direction with respect to the grating by applying an additional corrugation with a period twice larger than the main period. The analysis based both on the quasi-optical model and on the PIC simulations shows feasibility of 150-GHz surface-wave oscillator with 35 % efficiency and 100 MW output power. The transverse energy extraction significantly reduces the Ohmic losses down to 10% of the radiated power, which is essential for sub-THz range.