Longitudinal Modulation Research Articles

Evaluation of Elastic Properties and Conductivity of Chitosan Acetate Films in Ammonia and Water Vapors Using Acoustic Resonators.

Novel bio-materials, like chitosan and its derivatives, appeal to finding a new niche in room temperature gas sensors, demonstrating not only a chemoresistive response, but also changes in mechanical impedance due to vapor adsorption. We determined the coefficients of elasticity and viscosity of chitosan acetate films in air, ammonia, and water vapors by acoustic spectroscopy. The measurements were carried out while using a resonator with a longitudinal electric field at the different concentrations of ammonia (100–1600 ppm) and air humidity (20–60%). It was established that, in the presence of ammonia, the longitudinal and shear elastic modules significantly decreased, whereas, in water vapor, they changed slightly. At that, the viscosity of the films increased greatly upon exposure to both vapors. We found that the film’s conductivity increased by two and one orders of magnitude, respectively, in ammonia and water vapors. The effect of analyzed vapors on the resonance properties of a piezoelectric resonator with a lateral electric field that was loaded by a chitosan film on its free side was also experimentally studied. In these vapors, the parallel resonance frequency and maximum value of the real part of the electrical impedance decreased, especially in ammonia. The results of a theoretical analysis of the resonance properties of such a sensor in the presence of vapors turned out to be in a good agreement with the experimental data. It has been also found that with a growth in the concentration of the studied vapors, a decrease in the elastic constants, and an increase in the viscosity factor and conductivity lead to reducing the parallel resonance frequency and the maximum value of the real part of the electric impedance of the piezoelectric resonator with a lateral electric field that was loaded with a chitosan film. This leads to an increase in the sensitivity of such a sensor during exposure to these gas vapors.

Bragg solitons in topological Floquet insulators.

We consider a topological Floquet insulator consisting of two honeycomb arrays of identical waveguides having opposite helicities. The interface between the arrays supports two distinct topological edge states, which can be resonantly coupled by additional weak longitudinal refractive index modulation with a period larger than the helix period. In the presence of Kerr nonlinearity, such coupled edge states enable topological Bragg solitons. Theory and examples of such solitons are presented.

Phase-field collagen fibrils: Coupling chirality and density modulations

To describe the interaction between longitudinal density modulations along collagen fibrils (the D-band) with the radial twist-field of molecular orientation (double-twist), we couple phase-field-crystal (PFC) with liquid-crystalline free-energies to obtain a hybrid model of equilibrium collagen fibril structure. We numerically compute the resulting axial and radial structure. We find two distinct fibrillar phases, `L' and `C', with a coexistence line that ends in an Ising-like critical point. We propose that coexistence between these phases can explain the bimodal distribution of fibril radii that has been widely reported within tendon tissues. Tensile strain applied to our model fibrils straightens the average fibrillar twist and flattens the D-band modulation. Our PFC approach should apply directly to other longitudinally-modulated chiral filaments, such as fibrin and intermediate filaments.

Trajectory engineering via a space-fractional Schrödinger equation with dynamic linear index potential**Project supported by the Natural Science Research Project of Anhui Provincal Education Department of China (Grant Nos. KJHS2018B01 and KJ2018A0407), the National Natural Science Foundation of China (Grant No. 11804112), the Natural Science Foundation of Anhui Province of China (Grant No. 1808085QA22), and Start-up Fund of Huangshan University, China

We theoretically and numerically study the propagation dynamics of a Gaussian beam modeled by the fractional Schrödinger equation with different dynamic linear potentials. For the limited case α = 1 (α is the Lévy index) in the momentum space, the beam suffers a frequency shift which depends on the applied longitudinal modulation and the involved chirp. While in the real space, by precisely controlling the linear chirp, the beam will exhibit two different evolution characteristics: one is the zigzag trajectory propagation induced by multi-reflection occurring at the zeros of spatial spectrum, the other is diffraction-free propagation. Numerical simulations are in full accordance with the theoretical results. Increase of the Lévy index not only results in the drift of those turning points along the transverse direction, but also leads to the delocalization of the Gaussian beam.

First-principles high pressure structure searching, longitudinal-transverse mode coupling and absence of simple cubic phase in sulfur

We use first-principles density functional theory to conduct an extensive structure search using the AIRSS package for elemental sulfur in the range 50–550 GPa. We then obtain the low-temperature phase diagram of sulfur in the same pressure range, including vibrational effects through the harmonic approximation. We do not find any structures lower in energy than those already reported in experiment, although the phase diagram below 100 GPa is found to be crowded with structures separated by only a few meV. We report the transition sequence ICM and obtain accurate pressures for each transition, although we find the second-order transition particularly difficult to define. Contrary to previous first-principles works (Pavel et al; Rudin and Liu 1999 Phys. Rev. Lett. 83 3049–52), we do not reproduce a trigonal simple cubic transition at either the static lattice or harmonic level. We also undertake a detailed analysis of the incommensurately modulated (ICM) phase of sulfur phase using a commensurate approximant found in the structure search. We find that the modulation amplitude is zero above 96 GPa; some 40 GPa below the experimentally reported transition to the unmodulated phase. We find that the body-centred atoms in the relaxed ICM approximant are, in addition to the dominant transverse modulation (which is a frozen-in optical phonon mode), slightly displaced longitudinally in the b-direction. We subsequently discover that this (small) longitudinal modulation is coupled to the transverse mode, and hence report previously unnoticed weak-mode coupling between transverse and longitudinal optical phonons in the ICM phase.

Parity-time symmetry in a single-loop photonic system

Non-Hermitian photonics based on parity-time (PT) symmetry has been considered an effective solution to achieve mode selection for optical or microwave single-mode oscillation. A PT-symmetric system is usually implemented using two mutually coupled elements with identical geometry or a single element based on precise longitudinal refractive index modulation. In this study, we propose a novel scheme to achieve PT symmetry based on polarization mode manipulation in a single optical loop. By controlling the polarization states of two light waves that are propagating along the clockwise (CW) and counter-clockwise (CCW) directions in the single optical loop, two equivalent mutually-coupled loops with identical geometry and a balanced gain and loss are formed. Based on this concept, a wavelength-tunable single-mode fiber laser with a sub-kHz linewidth is realized. The tunability is achieved by thermally tuning the wavelength of an integrated microdisk resonator (MDR) incorporated in the optical loop. Experimental results show that a continuously tunable single-mode laser with a sub-kHz linewidth of 640 Hz and a wavelength tuning range from 1552.953 to 1554.147 nm is realized. The key advantages of using a single optical loop to implement PT symmetry include greatly simplified implementation and highly improved stability. The demonstration opens new avenues to implement high-performance PT-symmetric systems for applications in photonic and microwave photonic systems.

Feedback Challenges in the Formative Assessment of the Interinstitutional Program of Teaching-Service-Community Interaction: the Students’ Perspective

Abstract: Introduction: This present study assesses the use of the feedback mechanism in the formative assessment during medical training, considering this instrument is of great importance for this training stimulation, which is based on active teaching methodologies. The study aims to characterize the application of this tool and to identify possible failures in its application in the daily routine of the PINESC module in the Anhanguera University (Uniderp) medical course. Method: This is a quantitative, sectional research, including students from the first to the eighth semesters of the medical course, attending the PINESC longitudinal module. In the data collection, a questionnaire was used with questions related to the formative assessment performed by the preceptors, which is reported to the students according to the sample calculation. The feedback is the core activity of the formative assessment and allows the development of reflective and self-evaluation capacities and the development of skills. However, the use of feedback still faces barriers, mainly due to the difficulty of the evaluators in listing negative points and capabilities of the evaluated individuals. Result: In this study, some shortcomings and positive points were reported regarding the application process of this tool, listing and suggesting improvements for a better use of feedback and thus a more favorable academic achievement. Conclusion: In conclusion, as the basis of formative evaluation, the feedback still shows many weaknesses regarding its form and application.

Feedback Challenges in the Formative Assessment of the Interinstitutional Program of Teaching-Service-Community Interaction: the Students’ Perspective

Abstract: Introduction: This present study assesses the use of the feedback mechanism in the formative assessment during medical training, considering this instrument is of great importance for this training stimulation, which is based on active teaching methodologies. The study aims to characterize the application of this tool and to identify possible failures in its application in the daily routine of the PINESC module in the Anhanguera University (Uniderp) medical course. Method: This is a quantitative, sectional research, including students from the first to the eighth semesters of the medical course, attending the PINESC longitudinal module. In the data collection, a questionnaire was used with questions related to the formative assessment performed by the preceptors, which is reported to the students according to the sample calculation. The feedback is the core activity of the formative assessment and allows the development of reflective and self-evaluation capacities and the development of skills. However, the use of feedback still faces barriers, mainly due to the difficulty of the evaluators in listing negative points and capabilities of the evaluated individuals. Result: In this study, some shortcomings and positive points were reported regarding the application process of this tool, listing and suggesting improvements for a better use of feedback and thus a more favorable academic achievement. Conclusion: In conclusion, as the basis of formative evaluation, the feedback still shows many weaknesses regarding its form and application.

All-Optical Scheme for Generation of Isolated Attosecond Electron Pulses.

We theoretically propose an all-optical scheme for generation of isolated electron pulses with subfemtosecond durations for ultrafast electron microscopy and diffraction. The scheme is based on simultaneous longitudinal and transverse momentum modulation of freely propagating electrons at two distinct frequencies via ponderomotive interaction with tailored light fields of three phase-controlled laser pulses in vacuum. After a drift distance, an attosecond electron pulse train is formed with the individual pulses displaced in the transverse direction. Spatial filtering of the electron beam by an aperture leads to isolation of an individual attosecond pulse with a duration of <100 as. Subfemtosecond isolated electron pulses will enable direct space-time imaging of attosecond electronic dynamics in atoms, molecules, and solids with atomic spatial resolution, allowing us to directly observe phenomena such as electron tunneling or electron-electron interaction in strong laser fields of few-cycle laser pulses.

Coherent terahertz emission from a plasma layer due to linear conversion of laser wakefields on pre-modulated ion density

In this paper, electromagnetic emission at the plasma frequency produced by a short laser pulse in a finite-size plasma layer with a longitudinal density modulation is studied using both analytical theory and particle-in-cell simulations. The radiation mechanism suggests that a laser pulse excites a long-lived plasma wake which, in the presence of ion density modulation with the appropriate period, generates a superluminal satellite capable of matching in phase with vacuum electromagnetic waves. It is found that such a mechanism can be used for generating tunable narrow-band (5%) multi-mJ terahertz pulses with high efficiency (>0.3%) due to ability of superluminal plasma oscillations at the cut-off frequency to diffuse through a plasma that is several times wider than the radiation wavelength.

Control of spatial four-wave-mixing efficiency in Bessel beams using longitudinal intensity shaping

Diffraction-free Bessel beams have attracted major interest because of their stability even in regimes of nonlinear propagation and filamentation. However, Kerr nonlinear couplings are known to induce significant longitudinal intensity modulation, detrimental to the generation of uniform plasma or for applications in the processing of transparent materials. These nonlinear instabilities arise from the generation of new spatio-spectral components through an initial stage of continuous spectral broadening followed by four wave mixing. In this paper, we investigate analytically and numerically these processes and show that nonlinear instabilities can be controlled through shaping the spatial spectral phase of the input beam. This opens new routes for suppressing the nonlinear growth of new frequencies and controlling ultrashort pulse propagation in dielectrics.

Reducing the memory usage of computer-generated hologram calculation using accurate high-compressed look-up-table method in color 3D holographic display.

In this paper, we propose an accurate high-compressed look-up-table method that uses less memory to generate the hologram. In precomputation, we separate the longitudinal modulation factors and only calculate the basic horizontal and vertical factors. Therefore, we obtain other horizontal and vertical modulation factors of object points by simply shifting the basic horizontal and vertical modulation factors while computing holograms. We perform numerical simulations and optical experiments to verify the proposed method. Numerical simulation results show that the proposed method has the least memory usage, the fastest computation time and no distortion. The optical experimental results are in accord with the numerical simulation results. The proposed method is simple and effective to calculate computer-generated holograms for color dynamic holographic display with high speed, less memory usage and high accuracy that could be applied in the holographic field in the future.

MIEZE Neutron Spin-Echo Spectroscopy of Strongly Correlated Electron Systems

Recent progress in neutron spin-echo spectroscopy by means of longitudinal Modulation of IntEnsity with Zero Effort (MIEZE) is reviewed. Key technical characteristics are summarized which highlight that the parameter range accessible in momentum and energy, as well as its limitations, are extremely well understood and controlled. Typical experimental data comprising quasi-elastic and inelastic scattering are presented, featuring magneto-elastic coupling and crystal field excitations in Ho2Ti2O7, the skyrmion lattice to paramagnetic transition under applied magnetic field in MnSi, ferromagnetic criticality and spin waves in Fe. In addition bench marking studies of the molecular dynamics in H2O are reported. Taken together, the advantages of MIEZE spectroscopy in studies at small and intermediate momentum transfers comprise an exceptionally wide dynamic range of over seven orders of magnitude, the capability to perform straight forward studies on depolarizing samples or under depolarizing sample environments, as well as on incoherently scattering materials.

Concatenated analyses of phase space microbunching in high brightness electron beam transport

Microbunching instability (MBI) has been a challenging issue in high-brightness electron beam transport for modern accelerators. The existing theoretical formulation of MBI for recirculating accelerators is based on the single-pass analysis. For multi-pass recirculation or a long beam transport line, the intuitive way of quantifying MBI by successive multiplication of individual scalar amplification gains from subsections of a beamline was found to underestimate the effect. More thorough analyses based on concatenation of gain matrices, aimed to combine the density, energy, and transverse–longitudinal modulations for a general linear transport line, will be developed in this paper. Inclusion of such cross-plane microbunching structures in semi-analytical Vlasov analysis is a crucial step to systematically characterize phase space modulations for a beamline complex in terms of concatenating individual beamline segments. In this paper we outline the concept of gain matrix and develop a semi-analytical formulation to include the microbunching structures in the aforementioned phase space modulations. Then we investigate the phase space microbunching dynamics in an example lattice of the recirculating beamline, compare the semi-analytical calculation with particle tracking simulations, and find that the microbunching amplification calculated from multiplication of concatenated gain matrices can be considered as upper limit to the start-to-end gains.

Highly efficient electromagnetic emission during 100 keV electron beam relaxation in a thin magnetized plasma

In this paper, electromagnetic emissions produced by a beam-plasma system are investigated using particle-in-cell simulations for the particular case when the typical transverse size of both the 100 keV electron beam and the produced plasma channel is comparable to the radiation wavelength. The interest in this regime of beam-plasma interaction is associated with highly efficient generation of electromagnetic waves near the plasma frequency harmonics that has been recently observed in laboratory experiments on the GOL-3 mirror trap. It has been found that the radiation power only from the vicinity of the doubled plasma frequency in these experiments can reach 1% of the total beam power. Subsequent theoretical and simulation studies have shown that the most likely candidate for explaining such efficient generation of electromagnetic radiation is the mechanism of a beam-driven plasma antenna based on the conversion of the most unstable plasma oscillations on a longitudinal density modulation of plasma ions. In this paper, we investigate how effectively this mechanism can work in a real experiment at the GOL-3 facility, when a thin subrelativistic electron beam gets a large angular spread due to compression by a magnetic field, and the gas into which it is injected has macroscopic density gradients.

A 3D printed modular phantom for quality assurance of image-guided small animal irradiators: Design, imaging experiments, and Monte Carlo simulations.

The goal of this work was to develop and test a cylindrical tissue-equivalent quality assurance (QA) phantom for micro computed tomography (microCT) image-guided small animal irradiators that overcomes deficiencies of existing phantoms due to its mouse-like dimensions and composition. The 8.6-cm-long and 2.4-cm-diameter phantom was three-dimensionally (3D) printed out of Somos NeXt plastic on a stereolithography (SLA) printer. The modular phantom consisted of four sections: (a) CT number evaluation section, (b) spatial resolution with slanted edge (for the assessment of longitudinal resolution) and targeting section, (c) spatial resolution with hole pattern (for the assessment of radial direction) section, and (d) uniformity and geometry section. A Python-based graphical user interface (GUI) was developed for automated analysis of microCT images and evaluated CT number consistency, longitudinal and radial modulation transfer function (MTF), image uniformity, noise, and geometric accuracy. The phantom was placed at the imaging isocenter and scanned with the small animal radiation research platform (SARRP) in the pancake geometry (long axis of the phantom perpendicular to the axis of rotation) with a variety of imaging protocols. Tube voltage was set to 60 and 70kV, tube current was set to 0.5 and 1.2mA, voxel size was set to 200 and 275μm, imaging times of 1, 2, and 4min were used, and frame rates of 6 and 12 frames per second (fps) were used. The phantom was also scanned in the standard (long axis of the phantom parallel to the axis of rotation) orientation. The quality of microCT images was analyzed and compared to recommendations presented in our previous work that was derived from a multi-institutional study. Additionally, a targeting accuracy test with a film placed in the phantom was performed. MicroCT imaging of the phantom was also simulated in a modified version of the EGSnrc/DOSXYZnrc code. Images of the resolution section with the hole pattern were acquired experimentally as well as simulated in both the pancake and the standard imaging geometries. The radial spatial resolution of the experimental and simulated images was evaluated and compared to experimental data. For the centered phantom images acquired in the pancake geometry, all imaging protocols passed the spatial resolution criterion in the radial direction (>1.5lp/mm @ 0.2MTF), the geometric accuracy criterion (<200μm), and the noise criterion (<55HU). Only the imaging protocol with 200-μm voxel size passed the criterion for spatial resolution in the longitudinal direction (>1.5lp/mm @ 0.2MTF). The 70-kV tube voltage dataset failed the bone CT number consistency test (<55HU). Due to cupping artifacts, none of the imaging protocols passed the uniformity test of <55HU. When the phantom was scanned in the standard imaging geometry, image uniformity and longitudinal MTF were satisfactory; however, the CT number consistency failed the recommended limit. A targeting accuracy of 282 and 251μm along the x- and z-direction was observed. Monte Carlo simulations confirmed that the radial spatial resolution for images acquired in the pancake geometry was higher than the one acquired in the standard geometry. The new 3D-printed phantom presents a useful tool for microCT image analysis as it closely mimics a mouse. In order to image mouse-sized animals with acceptable image quality, the standard protocol with a 200-μm voxel size should be chosen and cupping artifacts need to be resolved.

Exclusive generation of orbital angular momentum modes in parity-time symmetry fiber gratings

Orbital angular momentum (OAM) modes in fibers process great potentials in providing, from a higher dimension, extra communication channels to meet the ever-growing bandwidth requirement for modern information infrastructures. Long period fiber gratings are commonly used to generate these modes in fibers by coupling fundamental modes to degenerate higher order modes. In order for the generation process to be selective for a certain OAM mode, careful control of the state of polarization (SOP) is required for input fundamental modes. Here, we propose a non-conventional long period fiber grating with not only longitudinal index modulation but also azimuthal parity-time symmetry index modulation, and by placing this grating system at the parity-time exceptional point, an OAM mode with its topological charge matched to the parity-time modulation could be exclusively excited by using any fundamental modes with an arbitrary SOP except for the particular SOP with exactly opposite helicity. Our concept demonstrated here to generate OAM modes in fibers may open up promising applications in parity-time OAM-fiber optics.

Second harmonic electromagnetic emission in a beam-driven plasma antenna

Generation of electromagnetic (EM) radiation near the second harmonic of the plasma frequency during the injection of an electron beam into a rippled-density plasma channel is investigated using both analytical theory and particle-in-cell simulations. The generating scheme is based on nonlinear interaction of the most unstable beam-driven potential plasma wave with its satellite arising due to scattering on the longitudinal modulation of plasma density. Resulting superluminal oscillations of electric current in a finite-size plasma channel radiate EM waves via the same mechanism which has been recently studied for the fundamental harmonic emission and reffered as a beam-driven plasma antenna. It is shown that theoretical predictions for the optimal plasma width and modulation period are confirmed by simulation results and the power conversion efficiency of the second harmonic emission reaches several percent. Such an efficient mechanism opens the path to explanation of laboratory experiments with a thin electron beam at the GOL-3 mirror trap as well as to developing the scheme of terahertz generation at the gigawatt power level.

Large-eddy simulation of transcritical and supercritical jets immersed in a quiescent environment

The study and understanding of transcritical and supercritical jet flow are critical in liquid rockets, gas turbines, and diesel engines, as high-pressure atmospheres in these devices’ mixing chambers drastically modify the morphology of their jets. In the transcritical jet, new elongated entities called finger-like structures appear and characterize the fluid flow phenomenon. This study examines these entities by simulating two classical cases based on Mayer’s experiments. The turbulence is described by the Large-eddy simulation technique with a sub-grid scale model known as the selective structures-function model. Real-gas behavior is evaluated by the Soave-Redlich-Kwong equation, and the transport properties are estimated by Chung’s methods. From the results, a longitudinal modulation which triggers an azimuthal modulation is observed. Then, bulges are formed on the jet surface. Cross-sectional views reveal pairs of streamwise vortices with inverted rotational directions, which are located on either side of the bulges (on the outer edge of the dense core). These transversal, turbulent movements seem to be engaged with the elongation of the bulges and the subsequent formation of finger-like structures. The existence of the counter-rotating vortices is related with the baroclinic vorticity. Then, since one may refer to the Rayleigh-Taylor instability when it is baroclinically generated, this instability could give an explanation of the origin of the finger-like structures. Transcritical parcels emitted at the end of the dense core are bounded by a thermal-shield. Finger-like structures are not observed in the supercritical case. The thermal-shield is absent from the supercritical parcels.

Multi-channel spin exchange relaxation free magnetometer towards two-dimensional vector magnetoencephalography.

Optically pumped atomic magnetometers based on spin exchange relaxation free regime have recently become a powerful tool in the field of magnetoencephalography measurements. For this application of magnetometers, simultaneous multilocation magnetic field measurements are desired. To fulfill the requirement, we develop a multi-channel sensor module based on a single large vapor cell. The probe beam passes through the vapor cell twice by reflection and then records the two-dimensional spatial magnetic field distribution with two 2 × 2 photodiode matrixes. Comparing with the previous multi-channel tangential magnetic field measuring sensors, our magnetometer is sensitive to the normal magnetic field by operating in the longitudinal parametric modulation mode. Measuring the normal component is considered more suitable for magnetoencephalography, because the normal component provides more information. The sensitivities of the channels are approximately 10 fT/Hz1/2 in the normal direction. The auditory evoked magnetic fields of the four adjacent locations perpendicular to the scalp are detected simultaneously. Our magnetometer can measure the normal and tangential magnetic fields simultaneously. The dual-axis vector measurement of magnetic field is very important for magnetoencephalography.