Millimeter-wave Region Research Articles

Efficient linear analysis of coupled-cavity traveling-wave tubes using minimum number of dispersion frequency points

Coupled-cavity traveling-wave tubes (CCTWTs) are high-power amplifiers operating in the microwave and millimeter-wave regions of the spectrum, utilizing beam-wave coupling within an electromagnetic slow-wave structure (SWS). The linear (small-signal) analysis of these amplifiers is a crucial initial step in their electromagnetic design. This paper presents and evaluates a method for analyzing these structures in the linear regime using only three frequency points from the SWS. The results obtained from this method are compared with full-wave particle-in-cell simulations, demonstrating satisfactory agreement. This approach offers a rapid analysis method for examining CCTWTs in the linear state, eliminating the need for lumped circuit modeling and detailed knowledge of the SWS dispersion characteristics.

Advancing spectroscopic understanding of HOCS+: Laboratory investigations and astronomical implications

Sulphur-bearing species play crucial roles in interstellar chemistry, yet their precise characterisation remains challenging. Here, we present laboratory experiments aimed at extending the high-resolution spectroscopy of protonated carbonyl sulphide (HOCS+), a recently detected molecular ion in space. Using a frequency-modulated free-space absorption spectrometer, we detected rotational transitions of HOCS+ in an extended negative glow discharge with a mixture of H2 and OCS, extending the high-resolution rotational characterisation of the cation well into the millimetre wave region (200–370 GHz). Comparisons with prior measurements and quantum chemical calculations revealed an overall agreement in the spectroscopic parameters. With the new spectroscopic dataset in hand, we re-investigated the observations of HOCS+ towards G+0.693−0.027, which were initially based solely on Ka = 0 lines contaminated by HNC34S. This re-investigation enabled the detection of weak Ka ≠ 0 transitions, free from HNC34S contamination. Our high-resolution spectroscopic characterisation also provides valuable insights for future millimetre and submillimetre astronomical observations of these species in different interstellar environments. In particular, the new high-resolution catalogue will facilitate the search for this cation in cold dark clouds, where very narrow line widths are typically observed.

Synchrotron radiation far infrared spectrum of the astrophysically significant Ethanol (CH3CH2OH) molecule in the gauche states in the vibrational ground state and other infrared observations

In this communication, the analyses of synchrotron radiation far infrared (FIR) spectrum corresponding to the gauche- (e1ando1) states of Ethyl Alcohol are reported. Detailed assignments have been performed for b-type transitions for K values ranging from 5 to 32 and up to a maximum J value of 50. The assignments confirmed the earlier microwave (MW) and millimeter wave (MMW) spectroscopic results. The transition wavenumbers have been used to determine the term values for the gauche-states involved for all the K and J values for which observation has been made. About 2000 spectral lines have been assigned, including some accurately calculated lines that fall in the MW and MMW regions. Although transitions between the trans-species and gauche species are not allowed in Ethanol, many resonances and level crossings of energy levels cause forbidden transitions. One such system of level crossings between K=10o1 and 12ee00 has been analyzed in detail, and the interaction coefficient determined. The state mixing seems quite strong and causes forbidden transitions (ΔK=0,2,3), including transitions for trans↔gauche, have been found. In addition, many new strong transitions have been assigned, which belong to the trans-species. To extend our work to higher wave numbers to facilitate observations made by the infrared detector on board the James Webb Space Telescope (JWST), the analysis of the torsional fundamental band transitions (around 220cm-1) for o2←e0 and o2←e1 has been taken up. Some of the assignments are discussed here. Lastly, the lower-lying vibrational bands centered around 425cm-1 (CCO– bending mode) and the band at around 800cm-1 (CH3- rocking mode) have been recorded. The CCO bending band shows an unmistakable parallel character for the trans-species. The assignment work on the CCO-bending band is in progress, and the assignments for K=0andK=1(+and-) are included in this report. Accurate term values for the CCO-bending mode could be obtained. The results allowed the determination of the leading rotational constants for the trans-species in the excited vibrational state of the CCO-bending state. The complete known assigned lines of about 16,000 lines for the parent isotopomer and about 650 lines for 13 -C1 and 13−C2 (see graphical abstract) substituted Ethanol isotopomers are gathered in Appendix I and II, respectively. These atlases should be a valuable tool for further energy level analysis and astronomical detection of Ethanol in interstellar space.

A biogenic organic molecule involved in the formation of secondary organic aerosols: Microwave and millimeter-wave spectroscopy of 3-methylcatechol backed by quantum chemistry

3-methylcatechol (3MC) is a molecule of environmental interest due to its presence in the atmosphere during biomass burning events, leading to the formation of secondary organic aerosols. This paper goes one step further than the paper from A.S. Hazrah et al. (2022) [10]. A three-dimensional torsional potential energy surface was determined, confirming the existence of three rotamers for 3MC. One dimensional potential energy curves along the rotation of the methyl group were computed and fitted to estimate the barrier height in each conformer. Experiments were performed at low temperature in the microwave domain (2−20GHz range) and, for the first time, in the millimeter-wave domain (70−110GHz range) at T=323K with a specially designed heated absorption cell. The rotational linelist was fitted to provide updated spectroscopic parameters for the two 3MC conformers of lowest energies. In particular, the A−E splitting is observed in the microwave region for both conformers while in the millimeter-wave region, it is only seen for the conformer of the lowest energy.

Laboratory Measurement of CH2DOH Line Intensities in the Millimeter-wave Region

Deuterium fractionation in molecules is known as one of the most powerful tools to study chemical processes during star and planet formation. Among various interstellar molecules, methanol often shows very high deuterium fractionation. It is the most abundant saturated organic molecule and is known as a parent species to form more complex organic molecules. However, deriving the abundance of deuterated methanol suffers from the uncertainty in the intrinsic line intensities (S μ 2) of CH3OH isotopologues. Due to their floppy nature, theoretical evaluation of the S μ 2 values is not straightforward, particularly for asymmetric-top asymmetric-frame isotopologues such as CH2DOH. In this study, we have measured the line frequencies and their intensities for CH2DOH in the millimeter-wave region from 216 to 264 GHz by using an emission-type millimeter and submillimeter-wave spectrometer. For the a-type J = 5 − 4 transition, the derived S μ 2 values are 13%–27% larger than those theoretically calculated, except for the transitions of K a = 2 for e 0 and K a = 1 for e 1 affected by avoided level crossing. For b-type transitions, significant systematic differences are found between theoretical and experimental S μ 2 values. The results of the present study enable us to accurately derive from observations the CH2DOH abundances, which are essential for understanding deuterium fractionation in various sources.

The missing conformer: A comprehensive rotational spectroscopy study and astronomical search of two conformers of methyl cyanoacetate

We performed a comprehensive investigation of methyl cyanoacetate (MCA) using high-resolution Fourier transform rotational spectroscopy. Two low energy conformers of MCA were observed in the vibrational ground state, in selected frequency regions from 2 to 110 GHz. We report accurately determined line lists, rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants for both conformers, as well as for several singly substituted heavy-atom isotopologues. One of the conformers was previously reported; however, the rotational transitions of the second conformer, which is newly described here, are observed to be generally more intense than the latter. The accurate predictions of rotational transitions into the millimeter-wave region can facilitate the detection of these conformers of MCA in the interstellar medium. Using the rotational spectroscopy data provided here, we searched for the two conformers of MCA in a deep and unbiased spectral survey of the molecular cloud G+0.693-0.027 located in the Galactic Center. None of the MCA conformers were detected. The upper limits for their abundances were derived with respect to the abundance of molecular hydrogen.

Review: Waveform Design for Improve Perform Analysis of 5G

We need to access very large bandwidths in order to accomplish the goal of ubiquitous mobile broadband, where radio access performance should not be a limitation for customer experience. As a result, higher frequency bands up to the millimetre wave region should be taken into consideration. In terms of mobility, energy efficiency, and cost efficiency, the design of the air interface, which involves waveforms, is a crucial element. Four candidate multicarrier waveforms are evaluated by comparing in this study based on a variety of parameter bit error rates (BER), which include orthogonal frequency division multiplexing, filtered orthogonal frequency division multiplexing, universal filtered multicarrier, and generalized frequency division multiplexing to decide which of them is a better option for 5G mobile. The waveforms significantly enhanced spectrum localization and produced suitable spectrum fragmentation, based on the results of the MATLAB simulation. These waveforms successfully solve the problem of time-frequency synchronization by being able to merge different traffic parameters. As a result, these waveforms have a significant amount of promise for orthogonally and synchronization and can manage multiple users without signal degradation. Additionally, they handle all multiple-input and multiple-output applications and scenarios. The simulation's results show the range of these waveforms and propose the optimal ones for use in 5G systems.

A W-Band SPDT Switch With 15-dBm P1dB in 55-nm Bulk CMOS

Power-handling capability of bulk CMOS-based single-pole double-throw (SPDT) switch operating in millimeter-wave (mm-wave) and subterahertz region is significantly limited by the reduced threshold voltage of deeply scaled transistors. A unique design technique based on impedance transformation network (ITN) is presented in this work, which improves 1-dB compression point, namely P1dB, without deteriorating other performance. To prove the presented solution is valid, a 70–100-GHz switch is designed and implemented in a 55-nm bulk CMOS technology. At 90 GHz, it achieves a measured P1dB of 15 dBm, an insertion loss (IL) of 3.5 dB, and an isolation (ISO) of 18 dB. The total area of the chip is only 0.14 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Laboratory Measurement of Millimeter-wave Transitions of 13CH2DOH for Astronomical Use

Methanol (CH3OH) is an abundant interstellar species and is known to be an important precursor of various interstellar complex organic molecules. Among the methanol isotopologues, CH2DOH is one of the most abundant isotopologues and it is often used to study the deuterium fractionation of CH3OH in interstellar medium. However, the emission lines of CH2DOH can sometimes be optically thick, making the derivation of its abundance unreliable. Therefore, observations of its presumably optically thin 13C substituted species, 13CH2DOH, are essential to overcome this issue. In this study, the rotational transitions of 13CH2DOH have been measured in the millimeter-wave region from 216 GHz to 264 GHz with an emission-type millimeter- and submillimeter-wave spectrometer by using a deuterium and 13C enriched sample. The frequency accuracy of measured 13CH2DOH is less than a few kHz, and the relative line intensity error is less than 10% in most of the frequency range by taking advantage of the wide simultaneous frequency-coverage of the emission-type spectrometer. These results offer a good opportunity to detect 13CH2DOH in space, which will allow us to study the deuterium fractionation of CH3OH in various sources through accurate determination of the CH2DOH abundance.

Excitation of tunable plasmons in silicon using microwave transmission through a metallic aperture

Plasmon resonances in semiconductors at microwave frequencies offer the possibility for many functionalities and integration schemes. Semiconductor materials, such as germanium, gallium arsenide, and silicon, have the further advantage of being able to be integrated with standard electronics technology. Here, we probe the bulk plasmon modes in silicon in the vicinity of a copper plate perforated by a single aperture at frequencies between 10 and 60 GHz. Sharp transmission minima are observed at discrete frequencies. The observed frequencies depend on the size of the aperture and the carrier concentration in the silicon; they are well reproduced by the dispersion relation for bulk plasmons. Our results show that one can excite plasmons in silicon in the millimeter-wave region, opening a route to microwave plasmonics for large-scale applications, using low-cost technology.

Balanced-ternary-inspired reconfigurable vortex beams using cascaded metasurfaces

Abstract Electromagnetic vortex carries the orbital angular momentum, one of the most fundamental properties of waves. The order of such vortex can be unbounded in principle, thus facilitating high-capability wave technologies for optical communications, photonic integrated circuits and others. However, it remains a key challenge to generate the high-order vortex beams in a reconfigurable, broadband and cost-effective manner. Here, inspired by the balanced-ternary concept, we demonstrate the reconfigurable generation of order-controllable vortices via cascaded N-layer metasurfaces. We theoretically showed that 3 N − 1 ${3}^{N}-1$ different vortex modes can be generated by cascading N metasurfaces, each one serving as an individual vortex beam generator for the order of 3 k ${3}^{k}$ (k = 0,1,2 …, N − 1 $N-1$ ). As a proof-of-concept demonstration, a reconfigurable generation of 26 different vortex beams, with orders from 1 to 13 and from −1 to −13, is showcased in a broad millimeter-wave region by a cascade of 3 metasurfaces. Our method can be easily extended to vortex beam generator of arbitrary orders in a reconfigurable and easily implementable manner, paving a new avenue towards tremendous practical applications.

Rotational spectrum of anti- and gauche-4-cyano-1-butyne (C5H5N) – An open-chain isomer of pyridine

The analysis of the rotational spectrum of 4-cyano-1-butyne, an open-chain isomer of pyridine, is presented herein for the first time. Transitions for the ground vibrational states of both conformational isomers, anti and gauche, were observed in the 8–38 GHz and 130–360 GHz frequency ranges and fit to effective sextic distorted-rotor Hamiltonians with low uncertainty (σfit < 50 kHz), allowing for their potential detection by radioastronomy or in harsh reaction environments. The microwave region includes hyperfine-split transitions for both conformers, allowing for the experimental determination of the χaa, χbb, and χcc quadrupole coupling constants. In the millimeter-wave region, not every observable ground-state transition could be satisfactorily addressed by a single-state Hamiltonian due to Coriolis coupling with the low-energy vibrationally excited states. Thus, effective single-state fits of the ground states were obtained by including only Ka-series transitions without obviously perturbed energy levels. Importantly, the effective fit does include the most intense transitions throughout the observed frequency ranges. The predicted values (MP2/cc-pVTZ) are in close agreement with the experimental spectroscopic constants. These data provide a sufficient foundation for the identification of anti- and gauche-4-cyano-1-butyne in the interstellar medium by radioastronomy. The gauche conformer is estimated to lie 1.00 (5) kcal/mol higher in energy than the anti conformer, as determined using experimental transition intensities.

Millimetre-wave laboratory study of glycinamide and a search for it with ALMA towards Sagittarius B2(N)

Context. Glycinamide (NH2CH2C(O)NH2) is considered to be one of the possible precursors of the simplest amino acid, glycine. Its only rotational spectrum reported so far has been in the centimetre-wave region on a laser-ablation generated supersonic expansion sample. Aims. The aim of this work is to extend the laboratory spectrum of glycinamide to the millimetre (mm) wave region to support searches for this molecule in the interstellar medium and to perform the first check for its presence in the high-mass star forming region Sagittarius B2(N). Methods. Glycinamide was synthesised chemically and was studied with broadband rotational spectroscopy in the 90–329 GHz region with the sample in slow flow at 50°C. Tunnelling across a low-energy barrier between two symmetry equivalent configurations of the molecule resulted in splitting of each vibrational state and many perturbations in associated rotational energy levels, requiring careful coupled state fits for each vibrational doublet. We searched for emission of glycinamide in the imaging spectral line survey ReMoCA performed with the Atacama Large Millimetre/submillimetre Array towards Sgr B2(N). The astronomical spectra were analysed under the assumption of local thermodynamic equilibrium. Results. We report the first analysis of the mm-wave rotational spectrum of glycinamide, resulting in fitting – to experimental measurement accuracy – of over 1200 assigned and measured transition frequencies for the ground-state tunnelling doublet and of many lines for tunnelling doublets for two singly excited vibrational states. We also determine the precise vibrational separation in each doublet. We did not detect emission from glycinamide in the hot molecular core Sgr B2(N1S). We derived a column density upper limit of 1.5 × 1016 cm−2, which implies that glycinamide is at least seven times less abundant than aminoacetonitrile and 1.8 times less abundant than urea in this source. A comparison with results of astrochemical kinetics models for species related to glycinamide suggests that its abundance may be at least one order of magnitude below the upper limit obtained towards Sgr B2(N1S). This means that glycinamide emission in this source likely lies well below the spectral confusion limit in the frequency range covered by the ReMoCA survey. Conclusions. Thanks to the spectroscopic data provided by this study, the search for glycinamide in the interstellar medium can continue on a firm basis. Targetting sources with a lower level of spectral confusion, such as the Galactic Center shocked region G+0.693-0.027, may be a promising avenue.

Internal rotation arena: Program performances on the low barrier problem of 4-methylacetophenone.

In the rotational spectroscopy community, several popular codes are available to treat multiple internal rotors in a molecule. In terms of the pros and cons of each code, it is often a difficult task to decide which program to apply to a specific internal rotation problem. We faced this issue when dealing with the spectroscopic fingerprintof 4-methylacetophenone (4MAP), recently investigated in the microwave region, which we here extended into the millimeterwave region. The methyl group attached to the phenyl ring in 4MAP undergoes internal rotation with a very low barrier of only 22cm-1. The acetyl methyl group features a much higher barrier of about 580cm-1. The performances of a program using the so-called "local" approach in terms of Herschbach's perturbative treatment, SPFIT, as well as three programs XIAM, ERHAM, and ntop, representing "global" fits, were tested. The results aim at helping spectroscopists in the decision on how to tackle their own internal rotation problems.

Mechanical Regulation of the Magnetic Properties of Uniaxial Anisotropic Hexaferrite Thin Films

Flexible oxide thin films with uniaxial magnetocrystalline anisotropy $({K}_{u})$ not only have many applications in flexible electronics, but also provide an ideal low-crystal-symmetry material for a fundamental understanding of the competitive interplay among magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy. However, an understanding of the mechanical tuning of magnetic parameters in flexible oxide films with ${K}_{u}$ has not been realized up to now. In this work, epitaxial flexible hexaferrite ${\mathrm{Ba}\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ thin films with a room temperature out-of-plane ${K}_{u}$ of 1.1 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ J/m${}^{3}$ are fabricated. The continuous and controllable tuning of magnetic parameters is found to be realizable by different bending radii of the film. The changes in magnetic parameters are symmetrical under tensile and compressive bending due to the dominance of film geometry and bending-stress directions, which are perpendicular to the easy axis of ${K}_{u}$. The micromagnetic simulations further show that the magnitude of ${K}_{u}$ plays a critical role in the mechanical tuning performance of the film, where high ${K}_{u}$ drastically reduces the role of stress anisotropy and increases the magnetic bending repeatability of the film by suppressing bending-defect-related anisotropy degradation. This work not only provides an understanding of the role of the direction and magnitude of ${K}_{u}$ in the flexible thin films, but also demonstrates that the flexible hexaferrite film is one of the best materials for mechanical tunable devices up to the millimeter-wave region.

Precise equilibrium structure of thiazole (c-C3H3NS) from twenty-four isotopologues.

The pure rotational spectrum of thiazole (c-C3H3NS, Cs) has been studied in the millimeter-wave region from 130 to 375GHz. Nearly 4800 newly measured rotational transitions for the ground vibrational state of the main isotopologue were combined with previously reported measurements and least-squares fit to a complete sextic Hamiltonian. Transitions for six singly substituted heavy-atom isotopologues (13C, 15N, 33S, 34S) were observed at natural abundance and likewise fit. Several deuterium-enriched samples were prepared, which gave access to the rotational spectra of 16 additional isotopologues, 14 of which had not been previously studied. The rotational spectra of each isotopologue were fit to A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The experimental values of the ground-state rotational constants (A0, B0, and C0) from each isotopologue were converted to determinable constants (A0″, B0″, and C0″), which were corrected for effects of vibration-rotation interactions and electron-mass distributions using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The moments of inertia from the resulting constants (Ae, Be, and Ce) of 24 isotopologues were used to determine the precise semi-experimental equilibrium structure (re SE) of thiazole. As a basis for comparison, a purely theoretical equilibrium structure was estimated by an electronic structure calculation [CCSD(T)/cc-pCV5Z] that was subsequently corrected for extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise re SE structure is compared to the resulting "best theoretical estimate" structure. Some, but not all, of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the re SE results. The possible origin of the discrepancies between the best theoretical estimate re and semi-empirical re SE structures is discussed.

The millimeter-wave spectrum of doubly deuterated propylene oxide [formula omitted

Spectra of doubly deuterated propylene oxide, CH3CHCD2O, were recorded in the millimeter-wave spectral region up to 330GHz utilizing a 2f frequency modulated Terahertz spectrometer. Rotational and centrifugal distortion constants and tunneling parameters for the description of the internal rotation of the methyl group were retrieved. The software XIAM was used to describe the A–E tunneling splitting with a frequency uncertainty of 82kHz. Molecular parameters derived from our measurements are complemented by quantum chemical anharmonic frequency calculations on the B3LYP/aug-cc-pVTZ level of theory. In addition, the barrier height to internal rotation of the methyl group, V3, was derived experimentally and found to slightly differ from the value reported for the main isotopologue. In view of astrophysical observations we provide accurate line lists to search for doubly deuterated propylene oxide in space. Furthermore, we deliver improved molecular parameters to further investigate the properties of a simple chiral molecule possessing internal large amplitude motions.

MIMO Communication Measurements in Small Cell Scenarios at 28 GHz

Massive multiple-input-multiple-output (MIMO) systems operating in the centimeter-wave (cmWave) and millimeter-wave (mmWave) region offer huge spectral efficiencies, which enables to satisfy the urgent need for higher data rates in mobile communication networks. However, the proper design of those massive MIMO systems first requires a deep understanding of the underlying wireless propagation channel. Therefore, we present a fully digital MIMO measurement system operating around 28 GHz. The system enables to take fast subsequent snapshots of the complex MIMO channel matrix. Based on this method, we statistically analyze the time-dependent channel behavior, the achievable signal quality and spectral efficiency, as well as the channel eigenvalue profile. Furthermore, the presented calibration approach for the receiver enables an estimation of the dominant absolute angle of arrival (AoA) and allows us to draw conclusions about the line-of-sight (LOS) dominance of the scenario. In total, 159 communication measurements over 20 s are conducted in three different small cell site scenarios to investigate the wireless propagation behavior. The measurements reveal the existence of several spatial propagation paths between the mobile transmitter and the base station. Furthermore, an insight into their likelihood in different propagation scenarios is also given.

Digitized Reconfigurable Metal Reflectarray Surfaces for Millimeter-Wave Beam-Engineering

Digitized beam-forming metal reflectarray antennas are designed for the millimeter-wave region. The phase control of antennas has been implemented by the reconfiguration of rectangular grooves on a metal plate. The antenna has 1147 elements arranged in an aluminum metal plate. The depths of all metal grooves are manipulated for designed phase control of high-gain beam-aimed reflectors. We have demonstrated a digitized reconfigurable metal reflectarray to steer a re-radiated millimeter-wave field from the reflector in a two-dimensional scanning plane from −20∘ to 20∘. The far-field patterns show that the measured gain of the 2-bit reflectarray is only 1 dB lower than that of a non-digitized reflectarray antenna. The measured peak gain is higher than 31.7 dB, and the measurements show that the gain of the full 40∘ scanned beam is 31.7 dB and well-defined scanned beams are obtained with a maximum scan gain loss of 0.2 dB. The proposed reconfigurable antennas can be a useful candidate for high-gain beam-aimed antennas for practical reflecting surfaces and a variety of wireless and satellite communication systems.

Characterization of Hydrogen-Bonded Liquid Crystal Materials in the Millimeter-Wave Region

A novel liquid crystal (LC) materials consisting of hydrogen bonding are investigated in the millimeter-wave (MMW) frequency region of electromagnetic wave spectra. Relative permittivity (or refractive index) and loss tangent (or absorption coefficient) are evaluated using a rectangular waveguide test cell and are compared with those values of conventional LC materials. The most unusual result is that the refractive index anisotropy increases significantly in the MMW region comparing with that for visible light, which is contrary to what is observed in the conventional LC materials. Although the anisotropy, which determines the tunability of LC devices, is not so large in this stage, the loss tangent is as small as, or better than, those of the excellent conventional-type LC materials synthesized for the microwave applications. We find out a potential of the hydrogen-bonded LCs as one of the best classes of LC materials for MMW application.