Helical Propagation Research Articles

Helical waves in easy-plane antiferromagnets

Effective spin torques can generate the N\'eel vector oscillations in antiferromagnets (AFMs). Here, it is theoretically shown that these torques applied at one end of a normal AFM strip can excite a helical type of spin wave in the strip whose properties are drastically different from characteristic spin waves. An analysis based on both a N\'eel vector dynamical equation and the micromagnetic simulation identifies the direction of magnetic anisotropy and the damping factor as the two key parameters determining the dynamics. Helical wave propagation requires the hard axis of the easy-plane AFM to be aligned with the traveling direction, while the damping limits its spatial extent. If the damping is neglected, the calculation leads to a uniform periodic domain wall structure. On the other hand, finite damping decelerates the helical wave rotation around the hard axis, ultimately causing stoppage of its propagation along the strip. With the group velocity staying close to spin-wave velocity at the wave front, the wavelength becomes correspondingly longer away from the excitation point. In a sufficiently short strip, a steady-state oscillation can be established whose frequency is controlled by the waveguide length as well as the excitation energy or torque.

Implementation of helical wave propagation motion in snake robot moving on exterior of a pipe

In this study, we developed a snake robot that goes through a branch point on exterior of a pipe in a helical shape. A hyperbolic function is utilised to generate a wave along the helical curve. The joint angles of the snake robot are derived by calculating the curvature and torsion of the curve by using the formula of the continuous curve model. We implemented the new motion in a real snake robot by considering parameters of the curve. In addition, we propose a method to compensate for the effect of gravity to achieve the motion by a real snake robot. Experimental results are shown to validate the effectiveness of the proposed method.

Implementation of helical wave propagation motion in snake robot moving on exterior of a pipe

In this study, we developed a snake robot that goes through a branch point on exterior of a pipe in a helical shape. A hyperbolic function is utilised to generate a wave along the helical curve. The joint angles of the snake robot are derived by calculating the curvature and torsion of the curve by using the formula of the continuous curve model. We implemented the new motion in a real snake robot by considering parameters of the curve. In addition, we propose a method to compensate for the effect of gravity to achieve the motion by a real snake robot. Experimental results are shown to validate the effectiveness of the proposed method.

Wavelength independency of spatially multiplexed communication channels in standard multimode optical fibers

Spatial domain multiplexing (SDM), also known as space division multiplexing, adds a new degree of photon freedom to existing optical fiber multiplexing techniques by allocating separate radial locations to different channels of the same wavelength as a function of the input launch angle. These independent MIMO channels remain confined to their designated locations while traversing the length of the carrier fiber owing to helical propagation of light inside the fiber core. As a result, multiple channels of the same wavelength can be supported inside a single optical fiber core, thereby allowing spatial reuse of optical frequencies and multiplication of fiber bandwidth. It also shows that SDM channels of different operating wavelengths continue to follow an output pattern that is based on the input launch angle. As a result, the SDM technique can be used in tandem with wavelength division multiplexing (WDM), to achieve higher optical fiber bandwidth through increased photon efficiency and added degrees of photon freedom. This endeavor presents the feasibility of a hybrid optical fiber communication architecture in which the spectral efficiency of the combined system increases by a factor of “n” when each channel of an “ n ” channel SDM system carries the entire range of WDM spectra.

Combining spatial domain multiplexing and orbital angular momentum of photon-based multiplexing to increase the bandwidth of optical fiber communication systems

Spatial domain multiplexing/space division multiplexing (SDM) can increase the bandwidth of existing and futuristic optical fibers by an order of magnitude or more. In the SDM technique, we launch multiple single-mode pigtail laser sources of the same wavelength into a carrier multimode fiber at different angles. The launching angles decide the output of the carrier fiber by allocating separate spatial locations for each channel. Each channel follows a helical trajectory while traversing the length of the carrier fiber, thereby allowing spatial reuse of optical frequencies. We launch light from five different single-mode pigtail laser sources (of same wavelength) at different angles (with respect to the axis of the carrier fiber) into the carrier fiber. Owing to helical propagation, five distinct concentric donut-shaped rings with negligible crosstalk at the output end of the fiber were obtained. These SDM channels also exhibit orbital angular momentum (OAM), thereby adding an extradegree of photon freedom. We present the experimental data of five spatially multiplexed channels and compare them with simulated results to show that this technique can potentially improve the data capacity of optical fibers by an order of magnitude: A factor of five using SDM and another factor of two using OAM.

The effects of the stellar wind and orbital motion on the jets of high-mass microquasars

High-mass microquasar jets propagate under the effect of the wind from the companion star, and the orbital motion of the binary system. The stellar wind and the orbit may be dominant factors determining the jet properties beyond the binary scales.} An analytical study is performed to characterize the effects of the stellar wind and the orbital motion on the jet properties. Accounting for the wind thrust transferred to the jet, we derive analytical estimates to characterize the jet evolution under the impact of the stellar wind. We include the Coriolis force effect, induced by orbital motion and enhanced by the wind presence. Large-scale evolution of the jet is sketched accounting for wind-to-jet thrust transfer, total energy conservation, and wind-jet flow mixing. If the angle of the wind-induced jet bending is larger than its half-opening angle, the following is expected: (i) a strong recollimation shock; (ii) bending against orbital motion, caused by Coriolis forces and enhanced by the wind presence; and (iii) non-ballistic helical propagation further away. Even if disrupted, the jet can reaccelerate due to ambient pressure gradients, but wind entrainment can weaken this acceleration. On large scales, the opening angle of the helical structure is determined by the wind-jet thrust relation, and the wind-loaded jet flow can be rather slow. The impact of stellar winds on high-mass microquasar jets can yield non-ballistic helical jet trajectories, jet partial disruption and wind mixing, shocks, and possibly non-thermal emission. Among several observational diagnostics at different bands, the radio morphology on milliarcsecond scales can be particularly insightful regarding the wind-jet interaction.

Current-induced three-dimensional domain wall propagation in cylindrical NiFe nanowires

We report on the magnetization configurations in single NiFe cylindrical nanowires grown by template-assisted electrodeposition. Angular anisotropic magnetoresistance measurements reveal that a three-dimensional helical domain wall is formed naturally upon relaxation from a saturated state. Micromagnetic simulations support the helical domain wall properties and its reversal process, which involves a splitting of the clockwise and anticlockwise vortices. When a pulsed current is applied to the nanowire, the helical domain wall propagation is observed with a minimum current density needed to overcome its intrinsic pinning.

One-way helical electromagnetic wave propagation supported by magnetized plasma

In this paper we reveal the presence of photonic one-way helical surface states in a simple natural system- magnetized plasma. The application of an external magnetic field to a bulk plasma body not only breaks time-reversal-symmetry but also leads to separation of Equi-Frequency Contour surfaces (EFCs) to form topologically nontrivial gaps in k space. Interestingly, these EFCs support topologically protected surface states. We numerically investigate an interface between magnetized plasma, using a realistic model for parameter dispersion, and vacuum, to confirm the existence of one-way scatter-immune helical surface states. Unlike previous proposals for achieving photonic one-way propagation, our scheme does not require the use of artificial structures and should therefore be simple to implement experimentally.

Chiral streamers

The interaction of time-varying electromagnetic fields and solid, liquid, and gaseous matter may lead to electrical breakdown phenomena through the excitation of ionization waves or streamers that control the dynamics of localized plasma propagation through the media. The streamers usually propagate along straight lines, either between random points in space or along a certain direction in a guided mode. Here, we report on a new type of plasma discharges with the regular helical propagation pattern driven by a pulsed dc voltage in nitrogen at sub-atmospheric-pressure conditions. The helical guided streamers, named chiral streamers or chi-streamers, are excited without any external magnetic fields, which commonly cause helical plasma motions. We also demonstrate a hybrid propagation mode involving the interchangeable chiral streamers and the straight-line propagating plasmas. High-speed, time-resolved optical imaging reveals that the chiral streamers and the hybrid patterns are made of spatially localized discrete plasma bullets, similar to the straight-line guided streamers. These results may enable effective control of propagation of confined plasmas and electromagnetic energy along pre-determined, potentially deterministic paths, which have important implications for the development of next-generation plasma-based radiation sources, communication devices, and medical treatments.

The first polymorph in the family of nucleobases: a second form of cytosine.

A new polymorph of cytosine, C4H5N3O, is reported half a century after the report of its first known crystal structure [Barker & Marsh (1964). Acta Cryst. 17, 1581-1587]. Cytosine thus provides the first polymorphic example in the category of parent nucleobases. The new form, denoted (Ib), was observed unexpectedly during an attempt to cocrystallize cytosine with catechol. Form (Ib) crystallizes in the orthorhombic centrosymmetric space group Pccn with two molecules in the asymmetric unit. The previously known form, denoted (Ia), crystallizes in the orthorhombic noncentrosymmetric space group P212121. The cytosine molecule is planar in both forms. Hydrogen-bonding interactions are also similar for both forms. Infinite one-dimensional ribbons composed of cytosine base-pair dimers in R2(2)(8) arrangements are observed in both (Ia) and (Ib). However, the way that the ribbons are packed differs in (Ia) and (Ib). This appears to guide the centrosymmetric versus noncentrosymmetric space-group selection through the formation of an inversion-related motif in polymorph (Ib) and a helical propagation in polymorph (Ia). A few selected polymorphic systems have been gathered from the Cambridge Structural Database to understand possible structural features responsible for achiral molecules adopting centro- and noncentrosymmetric space groups.

Helical Plasma Propagation of Microsecond Plasma Gun Discharges

Atmospheric pressure helium plasma inside a dielectric capillary and in the plume may exhibit stable and fascinating helix patterns. Such plasma propagation mode is achieved over an extended range of pulse repetition and gas flow rates. A key role of electric field is inferred from voltage, and electromagnetic configuration perturbation impacts on the plasma pattern. Helical plasma propagation is suggested to represent a specific case of previously reported wall-hugging plasma propagation.

Computationally-validated surrogate models for optimal geometric design of bio-inspired swimming robots: Helical swimmers

Research on micro-swimming robots without tether is growing fast owing to their potential impact on minimally invasive medical procedures. Candidate propulsion mechanisms of robots are vastly based on micro-organisms with rotating helical tails. For design of swimming robots, accurate models are necessary to compute velocities with corresponding hydrodynamic forces. Resistive force theory (RFT) provides an excellent framework for six degrees-of-freedom (dof) surrogate models in order to carry out effective design studies. However, resistance coefficients reported in literature are based on approximate analytical solutions for asymptotical cases, and do not address the effect of hydrodynamic interactions between the body and the tail, even in unbounded fluid media. Here, we use hydrodynamic interaction coefficients that multiply the body resistance coefficients along with no further modification to local resistance coefficients of the tail. Interaction coefficients are obtained from the solution of the inverse problem once for a fixed representative design with a computational fluid dynamics (CFD) simulation or an experiment. Results of the RFT-based hydrodynamic model are compared against further CFD simulations, and indicate that the model with hydrodynamic interaction coefficients obtained from a representative design provides a viable surrogate for computationally intensive three-dimensional time-dependent CFD models for a range of design variables. Finally, the validated hydrodynamic model is employed to investigate efficient geometric designs with helical wave propagation method within a wider range of design parameters.

On vortex motion in chromospheric network boundaries

A large percent of spicules shows a surge-like behavior on the solar limb, supporting a multi-component model with twisted threads. The counterpart of limb spicules foot-points is investigated on the disk, and this re-examination indicates that the interpretation of transverse motion of off-limb spicules could directly be related to rotational motion at the feet of disk spicules. Related bright elements move and vary in brightness on the timescales of chromospheric oscillations. The motions are similar to random displacements of the bright elements along the network boundaries with amplitudes of about 200 to 400 km, with evidence of “spinning” or vortex motion. We find a clear evidence and in several cases for splitting process and suggesting a formation mechanism for doublet (or multi-component) spicules. A general inter chromospheric network velocity pattern with twists existing before the emergence and eruption of spicules seems to be required. In this paper a helical- kink mode propagation consistent with the new evidence of spicule multiple structure is presented and provides an explanation for the origin of the Alfvenic wave propagation along the spicules. The evidence of spinning spicules remains unclear from disk Dopplergram observations, we use a 3D time slice “column” diagrams (2D in x and y and time in z being the 3d dimension) by consecutive partly transparent slices put in perspective to show the rotational behavior at the chromospheric rosettes, and provide a wealth of information on spinning motion, helical wave propagation and splitting.

Examination on Plasma Production and Ion Heating for Electric Propulsion Using a Single Helical Antenna

A new structure of RF control of plasma for electric propulsion has been proposed. With excitation characteristics of a helical antenna and propagation characteristics of waves with rotating field, both plasma production and ion heating will be expected simultaneously by employing a single helical antenna. Basic experiments to examine the proposed structure have been performed, showing consistent results with expected scenario.

Handedness Inversions in a Simple Model of Fibrillar Superstructure Formation

Aggregation of misfolded proteins into amyloid fibrils has been linked to several disorders such as Alzheimer's disease, or diabetes mellitus type II. Factors such as low pH, high temperature, and agitation have been recognized to accelerate this process in vitro. Amyloid fibrils from unrelated proteins share similar features on different levels of structural hierarchy: e.g. cross-beta structure, and tendency of individual fibrils to form intertwined chiral superstructures with defined morphological handedness and optical activity.In this work we investigate this phenomenon numerically with the use of coarse-grained numerical model of chiral fibrils. The aggregate morphologies, chirality and stability are then analyzed as a function of temperature and initial pitch of the fibrils. The simulations show the existence of a critical pitch below which the handedness of the aggregates is opposite to that of the constituting fibrils. We also observe and analyze the process of spontaneous chirality inversion of individual fibrils in the aggregate. This inversion is accompanied by a helical wave propagation along the fibril axis, with a kink separating left-haded and right-handed regions moving along the chain. The frequency of this process is strongly dependent on the initial pitch of the fibrils with a local maximum near the critical pitch.

Sagnac effect observation in flat and curved spacetime by interference of the opposite topological charge optical vortices

A new interference approach for the Sagnac effect observation is proposed. An interference of the opposite topological charge optical vortices (OVs) in flat and curved spacetime is discussed. We calculate the Sagnac time delay and phase shift for the helical metric in Minkowski spacetime. It is shown that the effective area for interference of the opposite topological charge OVs is proportional to the square of the helical beams propagation distance which leads to an unprecedented interferometer sensitivity. We modified corresponding solutions for the helical metric in the curved spacetime as well.

ChemInform Abstract: Chiral Crystallization and the Origin of Chiral Life on Earth

AbstractReview: 40 refs.

Chiral crystallization and the origin of chiral life on earth

AbstractThe creation of chirality on Earth and the development of chiral life have been discussed in this highlight. Convincing evidence for the introduction of chirality on Earth is still fragmentary. We believe that by a combination of chiral crystallization and formation of helical polymers with preferred chiral conformational structure is the key to this question. This concept of macromolecular asymmetry has inspired ideas and resulted in possible rules for how chiral life as we know it, could have been introduced. These investigations needed the understanding of the requirements for chiral crystallization, for the stereochemistry of the initial formation of helical polymers, the measurements of optical activity of solids and their coordination with the fundamentals of chirality. Spacial modeling of the “oligo‐crystallization” of sodium chlorate led to the conception of “isotactic” linear crystallization, which involves helical propagation. It seems to require unequal sizes of the cations and anions, which, by branching propagation leads to three‐dimensional chiral crystal formation. Linear “isotactic” propagation of crystallization seems to be equivalent to stereo and conformational specific polymerization. One and a half turns of the helix seems to be required for stereo‐ and conformational specificity, that is, between the pentamer and hexamer in chloral polymerization (11/3 or nearly 4/1 helix) and between trimer and tetramer for the sodium chlorate crystal (2/1 helix). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Attenuation and modal dispersion models for spatially multiplexed co-propagating helical optical channels in step index fibers

Spatial reuse of optical frequencies has been shown to be possible through a novel spatial domain multiplexing (SDM) technique that uses spatial multiplexer at the input end to launch multiple channels of the same wavelength inside a single strand of carrier fiber and then employs spatial filtering methods to de-multiplex the different optical channels at the output end. The individual SDM channels are confined to dedicated spatial locations inside the fiber while traversing through it owing to helical propagation of light. This presents attenuation and dispersion models of such a system. Experimentally obtained beam profile and resultant crosstalk of two such spatially multiplexed co-propagating SDM channels of the same wavelength over standard step index multimode optical fibers are also presented.

In Vitro Angioplasty of Atherosclerotic Human Femoral Arteries: Analysis of the Geometrical Changes in the Individual Tissues Using MRI and Image Processing

Existing atherosclerotic plaque imaging techniques such as intravascular ultrasound, multidetector computed tomography, optical coherence tomography, and high-resolution magnetic resonance imaging (hrMRI) require computerized methods to separate and analyze the plaque morphology. In this work, we perform in vitro balloon angioplasty experiments with 10 human femoral arteries using hrMRI and image processing. The vessel segments contain low-grade to high-grade lesions with very different plaque compositions. The experiments are designed to mimic the in vivo situation. We use a semi-automatic image processing tool to extract the three-dimensional (3D) geometries of the tissue components at four characteristic stages of the angioplasty procedure. The obtained geometries are then used to determine geometrical and mechanical indices in order to characterize, classify, and analyze the atherosclerotic plaques by their specific geometrical changes. During inflation, three vessels ruptured via helical crack propagation. The adventitia, media, and intima did not preserve their area/volume during inflation; the area changes of the lipid pool during inflation were significant. The characterization of changes in individual 3D tissue geometries, together with tissue-specific mechanical properties, may serve as a basis for refined finite element (FE) modeling, which is key to better understand stress evolution in various atherosclerotic plaque configurations.