Millimeter-wave Absorption Research Articles

Numerical simulation of electromagnetic-wave interference induced by ionization-front of millimeter-wave discharge at subcritical conditions and application to discharge structure identification

A standing wave induced in front of the ionization-front of a millimeter-wave discharge was numerically investigated to develop an interferometric discharge structure identification method. The time-varying waveform of the standing-wave intensity obtained at a distant observation point was smooth when a continuous comb-shaped structure was formed, whereas it was noisy with high-frequency components when a discrete structure was formed. The peak frequency of the Fourier spectrum of the time-varying waveform was proportional to the ionization-front propagation speed. The rapid time-variation of the waveform was caused by an increase in millimeter-wave absorption in a new plasma spot formation in the discrete structure. The results suggest that discharge structure identification, measurement of ionization-front propagation, and timing of plasma spot formation can be conducted experimentally without using a high-speed camera.

Tunable optically transparent graphene–ITO patterning-based millimeter-wave absorber

In this work, we demonstrated a multilayer millimeter-wave absorber capable of tuning the absorption frequency while simultaneously maintaining the optical transparency. It comprises indium tin oxide (ITO) as the bottom ground plane deposited over polyvinyl chloride (PVC) substrate over which a patterned ITO layer is formed. Above this ITO layer, a PVC spacer is put to isolate the upper graphene sandwich structure (GSS) comprising of two patterned graphene mediums separated by a diaphragm paper. The configuration shows the tunability of reflectance minima (which correspond to high absorptance of ≥ 98 %) in a frequency range of 92.5–97.5 GHz due to the adjustable bias voltage applied to GSS. Additionally, the designed absorber demonstrates adequate shielding effect, generally required for both commercial and military applications in the stated frequency range. This simultaneously configurable and optically transparent device has potential in multispectral and smart stealth technologies, particularly 6G applications.

Enhanced electromagnetic wave absorption of La3+–Zr4+ cosubstituted M−type barium hexaferrite

The development and breakthrough of electronic information technology have led to a profound focus on exploring new millimeter-wave absorbing materials. Herein, La–Zr ions co-substituted barium ferrite BaxLa1−xZrxFe12−xO19 (BLZFO) was prepared using a sol–gel method. BaFe12O19 possesses a dual loss mechanism (electrical/magnetic) and exhibits a natural resonance frequency of ∼ 45 GHz, making it effective for millimeter-wave absorption. The presence of La–Zr ions enhances the electromagnetic parameters of the material, improving its attenuation ability and optimizing impedance matching. Consequently, a wide, effective absorption frequency band and a minimal matching thickness are achieved. Specifically, within the 26.5–40 GHz range (Ka-band), BLZFO (x = 0.4) demonstrates an effective absorption bandwidth of 12.15 GHz, an optimal reflection loss of − 21.17 dB, and a corresponding matching thickness of 0.6 mm. This material not only exhibits strong reflection loss but also meets the requirements of an effective absorption frequency bandwidth and a small matching thickness, underscoring its profound importance for millimeter-wave absorption material research.

On the nature of sub-THz continuum absorption in CO2 gas, its mixture with Ar, and in pure water vapor.

Detailed analysis of the unique broadband millimeter-wave (70-360 GHz) collision-induced absorption spectra in pure CO2 and in its mixture with Ar is presented. The nature of the observed continuum absorption is examined using classical trajectory simulation along with statistical physics consideration. Bimolecular continuum is decomposed in the phase space into separate contributions from the so-called free, quasibound, and true bound molecular pairs, the proportions of which greatly vary with temperature. This partitioning is supported by consideration of the second virial coefficient and excluded volume in pure CO2, Ar, and CO2-Ar. Close similarity between collision-induced absorption in the CO2 containing gases and the water vapor continuum in the subterahertz spectral range is demonstrated. This similarity suggests that the physical principles underlying both continuum absorption phenomena have much in common and, therefore, can be used for continuum modeling.

Microwave Cavity Sensor for Measurements of Air Humidity under Reduced Pressure.

A high-sensitivity sensor for measuring moisture content in the air or air humidity under low pressure was designed on the basis of a half-wave coaxial microwave cavity. The method of measuring small variations in the signal phase at a cavity excitation frequency of 1.63 GHz was applied to detect low densities of water vapor. It allows the detection of variations in dielectric air permittivity in the seventh decimal place. A prototype of the sensor was tested in a vacuum chamber. It was calibrated by comparing the test results with the readings of a commercial pressure gauge and using the independent method of measuring the moisture content in rarefied air on the basis of the absorption of millimeter waves in the local line of water vapor at 183 GHz. The developed sensor can be used in laboratory experiments and full-scale geophysical research in the atmosphere onboard various aircrafts.

Toward the limits of complexity of interstellar chemistry: Rotational spectroscopy and astronomical search for n- and i-butanal

Context. In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium. Aims. In this context, we aim to provide accurate frequencies of the ground vibrational state of two key aliphatic aldehydes, n-butanal and its branched-chain isomer, i-butanal, to enable their eventual detection in the interstellar medium. We also want to test the level of complexity that interstellar chemistry can reach in regions of star formation. Methods. We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features of n- and i-butanal. We analyze the assigned rotational transitions of each rotamer separately using the A-reduced semirigid-rotor Hamiltonian. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search for n- and i-butanal toward the star-forming region Sgr B2(N). We also search for both aldehydes toward the molecular cloud G+0.693−0.027 with IRAM 30 m and Yebes 40 m observations. The observational results are compared with computational results from a recent gas-grain astrochemical model. Results. Several thousand rotational transitions belonging to the lowest-energy conformers of two distinct linear and branched isomers have been assigned in the laboratory spectra up to 325 GHz. A precise set of the relevant rotational spectroscopic constants has been determined for each structure as a first step toward identifying both molecules in the interstellar medium. We report non-detections of n-and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find that n- and i-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and that n-butanal is at least 63 times less abundant than acetaldehyde toward G+0.693−0.027. While propanal is not detected toward Sgr B2(N1S) either, with an abundance at least 5–11 lower than that of acetaldehyde, propanal is found to be 7 times less abundant than acetaldehyde in G+0.693−0.027. Comparison with astrochemical models indicates good agreement between observed and simulated abundances (where available). Grain-surface chemistry appears sufficient to reproduce aldehyde ratios in G+0.693−0.027; gas-phase production may play a more active role in Sgr B2(N1S). Model estimates for the larger aldehydes indicate that the observed upper limits may be close to the underlying values. Conclusions. Our astronomical results indicate that the family of interstellar aldehydes in the Galactic center region is characterized by a drop of one order of magnitude in abundance at each incrementation in the level of molecular complexity.

Resonance Frequency Tuning of a 200 GHz Band Absorber by an External Magnetic Field

Resonance Frequency Tuning In article number 2100319, Shin-ichi Ohkoshi and co-workers investigate the effect of the external magnetic field on the millimeter-wave absorption of rhodium-substituted epsilon iron oxide, a 200-GHz band absorber, and achieve tuning of the resonance frequency.

Micro- and millimeter-wave spectra of five conformers of cysteamine and their interstellar search

Context. Cysteamine (NH2CH2CH2SH), a molecule of potential astrobiological interest, has not yet been detected in the interstellar medium. Furthermore, the sulfur- stituted isomer of ethanolamine (or 2-aminoethanol) has been recently detected in the molecular cloud G+0.693–0.027. Aims. In order to conduct a new interstellar search for cysteamine in the molecular cloud G+0.693–0.027, its pure rotational spectrum needs to be investigated in the laboratory. Methods. A pulsed-jet Fourier transform microwave spectrometer and a Stark-modulated free-jet millimeter-wave absorption spectrometer were used to measure the purely rotational spectrum of cysteamine in the range of 6.5-18 GHz (46.12–16.66 mm) and 59.6–120.0 GHz (5.03–2.72 mm), respectively. We used a deep spectral line survey toward the molecular cloud G+0.693-0.027 obtained with the IRAM 30 m and Yebes 40 m radiotelescopes to search for cysteamine. Results. We assigned 815 rotational transition lines of five conformers (gGt, gGg, g′Gg, g′Gg′, and g'Gt) to fit the rotational constants, quartic centrifugal distortion constants, and the 14N nuclear quadrupole coupling constants. For four conformers (gGt, gGg, g′Gg, and g′Gg′), the 34S isotopologs were observed, and for two of them (gGg and g'Gg), the 13C and 15N isotopolog spectra were also detected; all in natural abundance. The five conformers of cysteamine were not detected toward the G+0.693–0.027 molecular cloud. We derived upper limits for their molecular abundances compared to molecular hydrogen of <(0.2–1.3) × 10–10. The relative abundances with respect to the oxygen counterpart ethanolamine, previously detected toward this cloud, are NH2CH2CH2OH/NH2CH2CH2SH > 0.8–5.3.

Broadband millimeter-wave absorbers: a review

AbstractThis paper reports an exhaustive review of recent research progress in the development of millimeter-wave absorbers. With the advancement in technologies, microwave absorbers have shown their evolution in several applications such as defense, security identification, stealth technology, and several more. The importance of these absorbers is increasing due to electromagnetic interference (EMI) effects. Primarily, an abundant amount of absorbers has been developed in lower frequency bands and with the increasing demand for 5G technology, the usage of millimeter-wave bands has gained attention. This in turn requires the development of millimeter-wave absorbers to provide EMI shielding in several applications. Out of several materials, polymers have grabbed attention in the mitigation of EMI. An absorber that combines carbonaceous elements with polymers offers large design flexibility and tunability per the filler concentration. This paper focuses on the classification of these absorbers based on geometry as well as that by using polymers with their design challenges, merits, and demerits with their industrial applications. Comparative studies of geometrically based absorbers and polymeric-based absorbers are also shown.

Optical Properties of Epsilon Iron Oxide Nanoparticles in the Millimeter- and Terahertz-Wave Regions

Abstract Epsilon iron oxide (ε-Fe2O3) is attracting global attention as a magnetic material with a large magnetic anisotropy. In this article, the optical properties of ε-Fe2O3 nanoparticles and the metal-substituted series of ε-MxFe2−xO3 (M = Ga, In, and Al) are studied over a wide frequency range from the millimeter-wave to terahertz-wave region, 30 GHz–30 THz, using terahertz time-domain, far-infrared, and Raman spectroscopies. To understand the spectroscopic data, first-principles calculations of the electronic structure and phonon modes are performed. First, an ε-Fe2O3 bar magnet is introduced and its atomic movements are calculated by phonon mode calculations. Second, the phonon modes of Ga-substituted ε-Fe2O3 are calculated. Far-IR, mid-IR, and Raman spectroscopies confirm that the calculated and observed spectra show good agreement. Third, the influences of In-substitution on the crystal structure, magnetic properties, and millimeter-wave absorption are described. In high-frequency millimeter-wave absorption due to magnon, the resonance frequency decreased with In-substitution. Finally, the millimeter-wave absorption property of ε-AlxFe2−xO3 is described. An absorption peak due to the natural resonance occurs at 100 GHz. The rotation data of the transmitted millimeter wave are determined by millimeter-wave–polarization-plane measurements.

Particle size effect on millimeter-wave absorption, rotation, and ellipticity of gallium-substituted epsilon iron oxide.

Various applications employ millimeter waves. For example, the carrier frequencies of vehicle radar in advanced driver assistance systems are 76–81 GHz millimeter waves. Here, we investigate the particle size effect on millimeter-wave absorption of gallium-substituted epsilon iron oxide ε-GaxFe2−xO3 with x = 0.44 ± 0.01. Samples were composed of nanoparticles with sizes of 16.9(1) nm, 28.8(2) nm, and 41.4(1) nm. Millimeter wave absorption, Faraday rotation, and Faraday ellipticity were measured by terahertz time-domain spectroscopy. This series exhibits millimeter-wave absorption at 78.7, 78.2, and 77.7 GHz without an external magnetic field. The millimeter-wave absorption increases from 4.6 dB to 9.4 dB as the particle size increases. In the magnetized sample, the Faraday rotation angle increases from 9.1° to 18.4°, while the Faraday ellipticity increases from 0.27 to 0.52. The particle size effect can be explained by the change in the ratio of the surface and core of the nanoparticles. The present study should contribute to the realization of high-performance millimeter-wave absorbers.

Facile synthesis of epsilon iron oxides via spray drying for millimeter-wave absorption

A simple, scalable spray drying method was developed for high-yield epsilon iron oxide (ε-Fe2O3) synthesis. The ε-Fe2O3 particle size can be tailored by varying the annealing temperature and molar ratio of Fe/Si, producing a high-purity ε-phase. This strategy also enables ferromagnetic resonance tuning, making it potentially usable in millimeter-wave absorbers.

A solid solution-based millimeter-wave absorber exhibiting highly efficient absorbing capability and ultrabroad bandwidth simultaneously via a multi-elemental co-doping strategy

A multi-elemental co-doped solid solution, i.e. BaZr0.2Ti0.2Ni0.2W0.2Fe11.2O19, was developed, featuring an optimal reflection loss of −61.8 dB and a −20 dB bandwidth of 9.15 GHz at a matching thickness of 0.97 mm in the frequency range of 18–40 GHz.

Structure and dynamics of methacrylamide, a computational and free-jet rotational spectroscopic study

The conformational space of methacrylamide was explored by quantum mechanical modeling and surveyed in the 59.6–104.0 GHz frequency range using a millimeter-wave Stark-modulated free-jet absorption spectrometer. According to the relative orientation of the two unsaturated bonds, two conformers were observed, namely s-trans (A=5234.360(1), B=3364.9717(8) and C=2173.099(1) MHz) and s-cis (A=5207.292(1), B=3470.930(1) and C=2113.496(1) MHz). The s-trans conformation is the global minimum, with relative energy 4(2) kJ mol−1 and calculated isomerization barrier 15 kJ mol−1. Except for the methyl hydrogen atoms, s-cis-methacrylamide is planar and its methyl internal rotation barrier is 10.2(1) kJ mol−1. In s-trans-methacrylamide the allyl and amino frames form a dihedral angle of about 30° and the methyl internal rotation barrier is 7.4 kJ mol−1. This different behaviour is explained in terms of attractive and repulsive intramolecular interactions between groups: CH2/CO and CH3/NH2 for s-cis, CH2/NH2 and CH3/CO for s-trans. The tunneling splitting related to the double-well potential describing the interconversion between the two equivalent s-trans forms is 837.97(2) MHz and was reproduced by a one-dimensional flexible model using a 3.6 kJ mol−1 interconversion barrier.

A 1-D Model for the Millimeter-Wave Absorption and Heating of Dielectric Materials in Power Beaming Applications

A heat exchanger, based on a millimeter-wave absorbing ceramic composite, is under development. This article describes a 1-D finite-difference model that is used in the design of the heat exchanger. The purpose of this model is to offer a design tool that can rapidly estimate the overall performance of the heat exchanger. This fast model allows for absorber performance to be evaluated over a wide parameter space as opposed to 3-D finite-difference time-domain methods, which provide accurate results but require substantially more computational resources. The model enables quick calculations by approximating the electric field such that the simulation runs on the slower thermal dynamics time scale without having to resolve the faster electromagnetic time scale. Example calculations were performed to illustrate the performance of a realistic absorber. These calculations used experimentally measured material properties for a molybdenum-loaded aluminum nitride (AlN:Mo) ceramic composite. Simulation results show dielectric tiles reaching equilibrium temperature in around 20 s and the samples absorbing up to 70% of the power from the millimeter-wave beam. Parameter studies over Mo loading percentage and boundary condition temperatures highlight the complexity of this coupled system. An AlN:Mo composite with 3% Mo (by volume) exhibits uniform power absorption across multiple boundary condition temperatures and suggests robustness to variety of possible experimental testing conditions.

Millimeter wave free-jet spectrum of acrolein and several isotopologues

The millimeter-wave absorption spectrum of acrolein has been investigated in the 59.6–120 GHz region with a Free-Jet Absorption MilliMeter Wave spectrometer. New measured rotational transitions belonging to the s-cis conformer and its three 13C isotopologues and those of 13C and 18O isotopologues of the s-trans conformer, observed in natural abundance, are reported.

Structure of the model grignard-type reagent ClZnCH3 ([formula omitted]1A1) by millimeter-wave spectroscopy

Pure rotational spectra of the 37ClZnCH3, ClZnCD3, and ClZn13CH3 isotopologues of monomeric ClZnCH3 (X̃1A1) have been recorded using millimeter-wave direct absorption techniques in the frequency range 263–303 GHz. These species were synthesized in the gas phase in a DC discharge by the reaction of zinc vapor, produced in a Broida-type oven, with 37ClCH3 (in natural chlorine abundance), ClCD3, or Cl13CH3. Five to eight rotational transitions J + 1 ← J consisting of K ladder structure were measured for each isotopologue, identifying all three species as prolate symmetric tops. The data for each isotopologue were analyzed with a symmetric top Hamiltonian and rotational and centrifugal constants determined. In combination with previous measurements of Cl64ZnCH3, Cl66ZnCH3, and Cl68ZnCH3, an rm(2) structure was determined for this organozinc compound. The bond lengths in ClZnCH3 were calculated to be rCl-Zn = 2.0831(1) Å, rZn-C = 1.9085(1) Å, and rC-H = 1.1806(5) Å, considerably different from those established from crystal structures of related species ClZnCH2CH3 and ClZnEtTMEDA. The HCH bond angle was found to be 110.5° – slightly larger than that in methane. These data serve to benchmark future structure calculations of organozinc compounds, which are widely used in organic synthesis.

Nano-Size Hexagonal Ferrites for Microwave and Millimeter-Wave Devices

Complex dielectric and magnetic properties of micro- and nano-size hexagonal ferrites have been studied. Two different measurement techniques have been applied to characterize the samples in the broad frequency range from 1.7 to 120 GHz. It was observed that the constitutive material properties, namely permittivity and permeability, as well as the ferromagnetic resonance frequency of the samples vary with the change in particle dimensions. Based on the results of these measurements, a model for calculating the ferromagnetic resonance frequency of ferrite powders has been derived, which takes into account the size and shape of the particles in the sample. It can be concluded from the size-dependent absorption properties observed in this study that these materials are considered to be tunable millimeter-wave absorbers.

Production method of millimeter-wave absorber with 3D-printed mold.

We established a production method of a millimeter-wave absorber by using a 3D-printed mold. The mold has a periodic pyramid shape, and an absorptive material is filled into the mold. This shape reduces the surface reflection. The 3D-printed mold is made from a transparent material in the millimeter-wave range. Therefore, unmolding is not necessary. A significant benefit of this production method is easy prototyping with various shapes and various absorptive materials. We produced a test model and used a two-component epoxy encapsulant as the absorptive material. The test model achieved a low reflectance: ∼1% at 100 GHz. The absorber is sometimes maintained at a low temperature condition for cases in which superconducting detectors are used. Therefore, cryogenic performance is required in terms of a mechanical strength for the thermal cycles, an adhesive strength, and a sufficient thermal conductivity. We confirmed the test-model strength by immersing the model into a liquid-nitrogen bath.

Revision of the 60-GHz atmospheric oxygen absorption band models for practical use

Two models for millimeter-wave absorption by molecular oxygen in the air are updated. To derive the updated parameter set, earlier resonator spectrometer data are refined and recent data on the collisional parameters’ temperature behavior are taken into account. The updated models provide significantly better agreement between laboratory measurements and calculated absorption band profiles than their earlier versions. The predictive ability of the models is confirmed by new experimental data. The covariance matrix of empirical parameter uncertainties, together with absorption-calculation uncertainty, are evaluated for both updated models. Contributions of the various sources to the total calculation uncertainty are discussed.