Microwave Spectroscopy Techniques Research Articles

Temporal shaping of high‐voltage picosecond electric pulses for electronic spectroscopy and bioelectric applications

AbstractIn this paper, we present the temporal and spectral shaping of kilovolt picosecond electrical pulses obtained by using the linear switching process with femtosecond laser source. The frozen wave generator integrating up to four photoconductive semiconductor switches is activated by varying the optical pulse amplitude and time delay. Electric pulses with rise time and duration as short as 60 and 100 ps, respectively are used to implement a microwave spectroscopy experiment. This optoelectronic device is synchronous with the optical pulses which can be used for optical spectroscopy, after nonlinear frequency conversion. This single‐shot microwave spectroscopy technique was used to characterize the resonances of a passive dielectric resonator. Pulse shaping integrating coherent wave combining is also demonstrated.

Observation of Collective Resonance Modes in a Chiral Spin Soliton Lattice with Tunable Magnon Dispersion.

A chiral spin soliton lattice (CSL), one of the representative systems of a magnetic superstructure, exhibits reconfigurability in periodicity over a macroscopic length scale. Such coherent and tunable characteristics of the CSL lead to an emergence of elementary excitation of the CSL as phononlike modes due to translational symmetry breaking and bring a controllability of the dispersion relation of the CSL phonon. Using a broadband microwave spectroscopy technique, we directly found that higher-order magnetic resonance modes appear in the CSL phase of a chiral helimagnet CrNb_{3}S_{6}, which is ascribed to the CSL phonon response. The resonance frequency of the CSL phonon can be tuned between 16 and 40GHz in the vicinity of the critical field, where the CSL period alters rapidly. The frequency range of the CSL phonon is expected to extend over 100GHz as extrapolated on the basis of the theoretical model. The present results indicate that chiral helimagnets could work as materials useful for broadband signal processing in the millimeter-wave band.

Surface Spin-Wave Propagation in the Orthogonal Transverse Junction of YIG-Based Magnonic Stripes

In this article, we experimentally demonstrate the propagation of magnetostatic surface spin wave (MSSW) in the orthogonally joined yttrium iron garnet stripes in the form of T-shaped connection. By the means of microwave spectroscopy technique, we show how the transmission is affected by the distance between orthogonal yttrium iron garnet stripes. Micromagnetic modeling demonstrates the presence of nonreciprocal propagation of spin waves (SWs) between all three ports of the T-junction. A mechanism for controlling the spatial-frequency selection of a propagating signal by variation of the air gap in the area of connected orthogonal sections is shown. The proposed structure can act as a magnonic splitter for 3-D topology of multilevel magnonic networks (MNs). The transition to a multilayer topology of MNs opens the possibility of increase of the packing density of computational units based on the principles of magnonics.

Anisotropic microwave propagation in a reconfigurable chiral spin soliton lattice

We investigated microwave propagation in the chiral spin soliton lattice (CSL) phase of micrometer-sized crystals of the monoaxial chiral helimagnet ${\mathrm{CrNb}}_{3}{\mathrm{S}}_{6}$. An advantage of the CSL is that its periodicity can be reconfigured over a macroscopic length scale by means of an external magnetic field. Using a two-antenna microwave spectroscopy technique, we measured the anisotropic response of the transmitted microwaves via the spin dynamics of the CSL. When propagating along the direction parallel to the helical axis, the microwave amplitude increased up to a factor of twenty with decreasing the number of chiral soliton kinks. When the propagation direction was rotated by ${90}^{\ensuremath{\circ}}$ with regards to the helical axis, the microwave amplitude increased by one order of magnitude upon formation of the chiral helimagnetic order in the vicinity of zero magnetic field, exceeding that of the ferromagnetic phase above the critical field. Our findings open a novel route for controlling the characteristics of microwave propagation using noncollinear spin textures.

Reflection-less width-modulated magnonic crystal

The interest in artificial magnetic media such as magnonic crystals increased substantially in recent years due to their potential applications in information processing at microwave frequencies. The main features of these crystals are the presence of band gaps in the spin-wave spectra, usually formed due to Bragg reflections of spin-waves on the artificially created periodic structures. Here, we study spin-wave propagation in longitudinally magnetized width- and thickness-modulated yttrium iron garnet waveguides by means of Brillouin light scattering and microwave spectroscopy techniques. It is found that the width modulated crystal does not manifest noticeable Bragg reflections, but still demonstrates a pronounced band gap in its transmission characteristic. The phenomenon can be explained by the destructive interference between different frequency-degenerated spin-wave modes excited by the crystal. Such a reflection-less crystal is promising for future design of multi-element magnonic devices.

Rotational spectrum, internal dynamics, and molecular structure of methylphenylsilane.

The rotational spectra of two isotopologues of methylphenylsilane were measured and assigned by using the supersonic-jet Fourier transform microwave spectroscopic technique in the 2.0-20.5 GHz range. The feature of rotational spectra of methylphenylsilane indicates that the doublets of rotational transitions in methylphenylsilane are contributable to the methyl internal rotation with a V3 barrier of 559 (25) cm-1. No splitting has been observed due to the methyl internal rotation in its carbon analog, ethylbenzene, which indicated that the barrier to such motion should be higher than 700 cm-1. Silicon incorporation of the ethyl group in ethylbenzene leads to a much lower barrier to the methyl internal rotation.

The Shape of the Simplest Non-proteinogenic Amino Acid α-Aminoisobutyric Acid (Aib).

The simplest non-proteinogenic amino acid α-aminoisobutyric acid (Aib), an analogue of glycine and alanine, has been vaporized by laser ablation and probed by high-resolution Fourier transform microwave spectroscopic techniques. Comparison of the experimental rotational and 14 N nuclear quadrupole constants with that predicted ab initio has allowed the identification of three conformers of Aib exhibiting three types of hydrogen-bond interactions I (NH⋅⋅⋅O=C, cis-COOH), II (OH⋅⋅⋅N, trans-COOH), and III (N-H⋅⋅⋅O-H, cis-COOH) within the amino acid backbone. The observation of conformer III, not detected previously for related proteinogenic amino acids with a nonpolar side chain in a supersonic expansion, indicates that the presence of the methyl groups should restrict the conformational relaxation from conformer Aib-III to Aib-I. For conformer Aib-II, the rotational spectra of the 13 C isotopomers reveal a tunneling motion arising from the two equivalent methyl groups in the molecule. The observation of a single spectrum at the midpoint between those predicted for the two 13 C of the methyl groups has been explained by considering a double-minimum potential function with a low-energy interconversion barrier for a large amplitude internal motion. This singular fact has been corroborated by the anomalous centrifugal distortion effects determined in conformer Aib-II.

Characterization of Bi2O3/ZnS heterojunctions designed for visible light communications

In the current work, the structural, morphological and optical properties of the Bi2O3/ZnS heterojunctions as visible light communication (VLC) technology element are explored. Bismuth oxide layers of thicknesses of 200 nm are used as substrate to evaporate ZnS films of thicknesses of 500 nm by the thermal evaporation technique under vacuum pressure of 10−5 mbar. The heterojunction devices are studied by the x-ray diffraction, scanning electron microscopy, optical spectrophotometry and microwave spectroscopy techniques. The Bi2O3/ZnS heterojunctions are found to form a highly strained structure with extremely large lattice mismatches. By the strained structure and with the valence and conduction band offsets that exhibit values of 1.04 and 0.41 eV, respectively, it was possible to enhance the light absorbability of ZnS by 459 times at 3.10 eV. In addition, the dielectric constant spectra of the device display a linear and nonlinear optical properties below and above 1.94 eV, respectively. Moreover, the optical conductivity parameters including the drift mobility and plasmon frequency and the cutoff frequency spectra of an area of 0.50 cm2 of Bi2O3/ZnS interfaces have shown the ability of using these heterojunction devices as light signal receivers that attenuate signals at terahertz frequencies in the range of 0.27–1.00 THz. As an additional demonstration, the Bi2O3/ZnS heterojunction devices were subjected to a microwave signal propagation in the frequency domain of 0.01–2.90 GHz. The device performed as band filters at gigahertz frequencies.

Helimagnon Resonances in an Intrinsic Chiral Magnonic Crystal.

We experimentally study magnetic resonances in the helical and conical magnetic phases of the chiral magnetic insulator Cu_{2}OSeO_{3} at the temperature T=5 K. Using a broadband microwave spectroscopy technique based on vector network analysis, we identify three distinct sets of helimagnon resonances in the frequency range 2 GHz≤f≤20 GHz with low magnetic damping α≤0.003. The extracted resonance frequencies are in accordance with calculations of the helimagnon band structure found in an intrinsic chiral magnonic crystal. The periodic modulation of the equilibrium spin direction that leads to the formation of the magnonic crystal is a direct consequence of the chiral magnetic ordering caused by the Dzyaloshinskii-Moriya interaction. The mode coupling in the magnonic crystal allows excitation of helimagnons with wave vectors that are multiples of the spiral wave vector.

Experimental study on the optimisation of chemical treatment to reduce waste rubber aggregates absorption properties

Utilisation of abundant waste rubber generated from scrap tyres in pavement engineering applications could be a useful route to consume large extent of the waste from the scrap tyres. It may contribute towards maintaining a greener environment by reducing the associated environmental and social impacts as well as improve the properties of bituminous mixture used for pavement engineering applications. This paper focuses on the upgrading of waste rubber aggregates surface properties by identifying and applying optimised chemical treatment (oxidation and cross-linking). The focus was to increase the life span of bituminous mixtures containing waste rubber as an aggregate by reducing a phenomenon of migration of light fractions (maltenes) from bitumen to rubber which occur at elevated temperatures ranging between 160 and 180°C. The results of various tests conducted showed the potential of the treatment to reduce the absorption of maltenes into rubber. The existence of the surface treatment was verified through the Scanning Electron Microscope (SEM) analysis of the rubber aggregate. The reduction in the absorption properties of rubber due to the existence of the treatment was verified using the microwave spectroscopy technique. The results of both the tests were promising in terms of indicating the durability of the treatment and the reduction in the absorption properties of rubber. Mechanical tests such as Indirect Tensile Stiffness Modulus (ITSM) and Repeated load axial (RLA) tests were conducted on the newly devised bituminous mix design with 10% of added rubber (by mass of the sample). The results were promising in the case of the aged samples with both the stiffness and load bearing capacity being higher for the aged samples. However, the results need improvement in terms of its applications on the light traffic areas through the replacement of the ordinary filler with the mineral filler. Also, a higher percentage of waste rubber should be added to study the suitability of its use in the flexible surface applications such as children’s playgrounds, sports pitches and surfaces, etc.

Spin Vortex Resonance in Non-planar Ferromagnetic Dots.

In planar structures, the vortex resonance frequency changes little as a function of an in-plane magnetic field as long as the vortex state persists. Altering the topography of the element leads to a vastly different dynamic response that arises due to the local vortex core confinement effect. In this work, we studied the magnetic excitations in non-planar ferromagnetic dots using a broadband microwave spectroscopy technique. Two distinct regimes of vortex gyration were detected depending on the vortex core position. The experimental results are in qualitative agreement with micromagnetic simulations.

Breaking inversion symmetry in a state-dependent honeycomb lattice: artificial graphene with tunable band gap

Here, we present the application of a novel method for controlling the geometry of a state-dependent honeycomb lattice: the energy offset between the two sublattices of the honeycomb structure can be adjusted by rotating the atomic quantization axis. This enables us to continuously tune between a homogeneous graphene-like honeycomb lattice and a triangular lattice and to open an energy gap at the characteristic Dirac points. We probe the symmetry of the lattice with microwave spectroscopy techniques and investigate the behavior of atoms excited to the second energy band. We find a striking influence of the energy gap at the Dirac cones onto the lifetimes of bosonic atoms in the excited band.

Recent Advances on Rotational Spectroscopy and Microwave Spectroscopic Techniques

Rotational spectroscopy is a branch of fundamental science to study the rotational spectra of molecules, free radicals and ions based on the quantum mechanics. It has great applications on many fields such as radio astronomy and atmospheric remote sensing. In this paper we reviewed the basic theory of rotational spectroscopy, two different types of Fourier transform microwave spectrometers, spectroscopic analysis of certain typical cases, and future prospect of the microwave spectroscopic techniques as well.

Pinning the most stable HxCyOz isomers in space by means of high-level theoretical procedures

It has been recently demonstrated that there is a high statistical correlation between the relative energies of isomers and their relative abundances in the interstellar medium (ISM). In the present work we use the high-level W2–F12 thermochemical protocol to obtain accurate isomerization energies for a set of 109 HxCyOz isomers, 18 of which have been observed in the ISM so far. We use our benchmark isomerization energies to (i) rationalize the presence of the isomers that have been detected, and (ii) predict which new isomers are likely to be detected in the future. We find that the energetically most stable isomers of H2C3O (1,2-propadien-1-one), H8C3O (2-propanol), and H6C3O2 (propanoic acid) have not been observed, despite the fact that higher-energy isomers of these chemical formulas have been detected in the ISM. The dipole moments of these isomers are sufficiently large that these species should be observed using microwave spectroscopy techniques.

Microwave spectroscopy of biomolecular building blocks.

Microwave spectroscopy, considered as the most definitive gas phase structural probe, is able to distinguish between different conformational structures of a molecule, because they have unique spectroscopic constants and give rise to distinct individual rotational spectra.Previously, application of this technique was limited to molecular specimens possessing appreciable vapor pressures, thus discarding the possibility of studying many other molecules of biological importance, in particular those with high melting points, which had a tendency to undergo thermal reactions, and ultimately degradation, upon heating.Nowadays, the combination of laser ablation with Fourier transform microwave spectroscopy techniques, in supersonic jets, has enabled the gas-phase study of such systems. In this chapter, these techniques, including broadband spectroscopy, as well as results of their application into the study of the conformational panorama and structure of biomolecular building blocks, such as amino acids, nucleic bases, and monosaccharides, are briefly discussed, and with them, the tools for conformational assignation - rotational constants, nuclear quadrupole coupling interaction, and dipole moment.

Microwave spectroscopy: a potential technique to analyse bitumen dielectric and physical properties

Bitumen is a complex mixture of high molecular weight organic compounds, predominantly hydrocarbons, with carbon numbers greater than C24. It has high carbon to hydrogen ratios, and also contains small amount of metals such as nickel, iron and vanadium. The application of bitumen in the pavement industry is mainly as a binding agent for road construction/surfacing. Hence, this paper discusses the use of microwave spectroscopy (MS) as a potential technique to analyse bitumen's dielectric properties to indicate change in some of its physical properties after its interaction with both treated and untreated coarse rubber in the field of pavement engineering. A comparison of the existing techniques for the quantitative and qualitative analysis of bitumen is given. The drawbacks of the existing techniques are also discussed and the paper addresses how the MS technique can overcome these drawbacks. The MS technique is investigated by analysing different bitumen samples including pure bitumen and samples taken from rubber–bitumen blends. Each of the blends used rubber with different surface properties which have different effects on the properties of the resulting bitumen. The paper highlights the novelty and advantages for the MS technique in reducing analysis time, solvent usage and potential take-up by the industry.

Spectroscopy of a fractional Josephson vortex molecule

In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux $|\Phi|<\Phi_0$, $\Phi_0\approx 2.07\times 10^{-15}$ Wb being the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on $\Phi/\Phi_0$ and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge $\wp=2\pi\Phi/\Phi_0$ and on the bias current $\gamma$ applied to the Josephson junction. We find that with decreasing distance between vortices, a splitting of the eigenfrequencies occurs, that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.

New spin on isomerization

A broadband microwave spectroscopy technique sheds new light on conformational isomerization reactions

Microwave spectrum of 2,4-difluorophenol: cis conformer

The ground state microwave rotational spectra of 2,4-difluorophenol have been investigated and analyzed in the frequency region: 18.0–37.0 GHz using Radiofrequency-Microwave Double Resonance (RFMWDR) and conventional microwave spectroscopic technique. A least-squares analysis of a-type R-branch and b-type Q-branch lines yields a set of rotational and centrifugal distortion constants. The derived rotational constants correspond to the cis conformer of the molecule. A tentative r 0-structure of the cis conformer is proposed.

Microwave spectrum and quadrupole coupling constants of 13CF 3Br

Ground state microwave rotational spectra of 13CF 3Br have been investigated and analyzed in the frequency region: 20.0–37.0 GHz using radiofrequency–microwave double resonance (RFMWDR) and conventional microwave spectroscopic technique. Analysis of rotational transitions includes both isotopic species of Br, viz. 13CF 3 79Br and 13CF 3 81Br. Rotational, centrifugal distortion and quadrupole coupling constants for both isotopes are derived. Finally, an r z structure with precise C–Br, C–F bond lengths and ∠FCF bond angle using the observed rotational constants and the reported values of the parent species for both isotopes has been proposed after correction of the constants for six vibrational modes.