Off-axis Displacement Research Articles

Vortex lattices with transverse-mode-locking states switching in a large-aperture off-axis-pumped solid-state laser

We report on optical vortex lattices (OVL) beams with phase-singularity arrays from a large-aperture diode-pumped Yb:CALGO laser. A distinct transverse-mode-locking (TML) effect is found in the OVL formation via controlling pump aperture and off-axis displacement. Various OVL patterns with vortex arrays can be generated in a TML state. While in a non-TML state, the emission is always a high-order Hermite–Gaussian or Ince–Gaussian mode. Interestingly, a stable TML can be started from a non-TML state by an intentionally induced perturbation. Based on the TML principle, a new closed-form expression is theoretically proposed for interpreting the transverse patterns and the evolution principle of various OVL modes, showing good agreement with the experimental results.

DNA molecules deviate from shortest trajectory when driven through hydrogel.

Dynamic fluorescence-based single-molecule imaging of λ-DNA molecules driven through agarose hydrogels by DC electric fields reveals that passage through the hydrogel (98.5% water content) induces mobility orthogonal to the external field. Tortuous paths followed by the DNA molecules, which are heavily entangled in the hydrogel mesh as their contour length is nearly 100 times the hydrogel mesh size of 200 nm, cause them to appear to diffuse orthogonal to the driving force. The higher the driving field, from 2 to 16 V/cm, the higher the off-axis dispersion is, over the same time interval. We measure the off-axis displacement distribution over 3 orders of magnitude of probability density and find a master curve after normalizing for time (t) elapsed, but the power of time for normalizing increases with the external field, from t0.25 to t0.6 with increasing field. Comparing trajectories over the same distance traveled in the electric field direction, we observe whereas for the highest field strengths DNA molecules come closest to taking the shortest trajectory between two points in space, deviations from the shortest trajectory grow larger and larger (up to 40% larger) as one approaches the case of small yet finite external field strength.

Tightly focused field modulated by spiral phase with an off-axis singularity

We examine the influences of an off-axis singularity in spiral phase on the tightly focused field. Through vectorial diffraction theory, the tightly focused fields modulated by the off-axis singularity displacements and polarization states have been presented and analyzed. The results demonstrate that, the intensity patterns in the focal plane transform along the direction determined by the off-axis singularity displacement and handedness of spiral phase. The off-axis singularity induces the transverse intensity patterns rotate along the propagation, and the direction of rotation is corresponding to the handedness of spiral phase. The profiles and transformation degree of all intensity patterns are dependent on off-axis singularity displacement, polarization state and amplitude distribution of incident light beam. Collaborating with polarization and amplitude modulation, the off-axis singularity in spiral phase is helpful to construct various tightly focused fields potentially applied in these areas, such as optical imaging, manipulation or measurement.

A method of solving tilt illumination for multiple distance phase retrieval

Multiple distance phase retrieval is a technique of using a series of intensity patterns to reconstruct a complex-valued image of object. However, tilt illumination originating from the off-axis displacement of incident light significantly impairs its imaging quality. To eliminate this affection, we use cross-correlation calibration to estimate oblique angle of incident light and a Fourier-based strategy to correct tilted illumination effect. Compared to other methods, binary and biological object are both stably reconstructed in simulation and experiment. This work provides a simple but beneficial method to solve the problem of tilt illumination for lens-free multi-distance system.

High-peak-power large-angular-momentum beams generated from passively Q-switched geometric modes with astigmatic transformation

The criterion of achieving efficient passive Q-switching is analyzed to design an off-axis pumped Nd:YVO4/Cr4+:YAG laser with a degenerate cavity. Experimental results reveal that pure high-order HG0,m or HGm,0 eigenmodes with the order m between 0 and 14 can be generated, depending on the off-axis displacement along the y axis or the x axis. On the other hand, lasing modes naturally turn into planar geometric modes when the off-axis displacement is larger than the value for exciting the HG0,m or HGm,0 eigenmodes with m>14. The overall peak powers for high-order eigenmodes or geometric modes can exceed 140 W. Furthermore, the high-order eigenmodes and geometric modes are employed to generate vortex beams with large orbital angular momentum by using an external cylindrical mode converter. Theoretical analyses are performed to confirm experimental results and to manifest the phase structures of the generated vortex beams.

On a Modified VCO Nozzle Layout for Diesel Common Rail Injectors under Actual Needle Displacement

The mechanical-hydraulic behavior of the nozzle and the features of the internal flow are two fundamental factors in the spray formation process. In the current multi-injection strategies, short injections with reduced values of the needle lift are usual. In these cases, the nozzle can give rise to abnormalities in spray development, highlighted by irregularities and asymmetries. The main hole to hole differences are encountered in the rate of injection, in the spray shape and in the atomization process. The off-axis of the needle, the characteristics of the volume upstream of the holes and their geometry play a key role in causing such anomalies.In the paper, an unconventional VCO nozzle layout has been defined and analyzed by modeling. The study is oriented at evaluating its capability on reducing the dispersion among the flow features of the holes, when the pure axial needle displacement is perturbed. The investigated nozzle is based on a multihole layout but, referring to the injector axis, nozzle holes have a non-radial arrangement. In comparison with the standard VCO nozzle configuration, 3D-CFD simulations have been devoted to highlight the influence of the alternative nozzle layout on the internal flow and on the resulting spray features in the external ambient.In the modelling process, data concerning rate of injection, spatial distribution of velocity and cavitation behaviour at hole exit have been computed; specific attention has been focused on the fuel flow distribution among the holes; the computations have evidenced the behaviour of the unconventional layout when off-axis needle displacement is considered. Results have been resumed indicating how the internal flow pattern properties are reflected by the spray features.

Propagation dynamics of off-axis symmetrical and asymmetrical vortices embedded in flat-topped beams

In this paper, propagation dynamics of off-axis symmetrical and asymmetrical optical vortices(OVs) embedded in flat-topped beams have been explored numerically based on rigorous scalar diffraction theory. The distribution properties of phase and intensity play an important role in driving the propagation dynamics of OVs. Numerical results show that the single off-axis vortex moves in a straight line. The displacement of the single off-axis vortex becomes smaller, when either the order of flatness N and the beam size ω0are increased or the off-axis displacement d is decreased. In addition, the phase singularities of high order vortex beams can be split after propagating a certain distance. It is also demonstrated that the movement of OVs are closely related with the spatial symmetrical or asymmetrical distribution of vortex singularities field. Multiple symmetrical and asymmetrical optical vortices(OVs) embedded in flat-topped beams can interact and rotate. The investment of the propagation dynamics of OVs may have many applications in optical micro-manipulation and optical tweezers.

Design of axisymmetric multi-mirror grazing incidence system to increase the numerical aperture of neutron and X-ray microscopes

An axisymmetric multi-mirror system for neutron and X-ray microscopes is proposed to increase their numerical aperture and collection efficiency. A Wolter type-I mirror is used as the basis of the multi-mirror system at grazing incidence. The addition of an even number of hyperboloid mirrors to the Wolter type-I mirror can satisfy both an equal optical path length and Abbe’s sine condition. The numerical aperture increases in proportion to the number of mirrors. The optical parameters of the system with four tandem mirrors are calculated for neutrons and X-rays with a wavelength of 0.4 nm by assuming that the average grazing angle of incidence is 5.4 mrad and the magnification is 10. The inner diameters of the mirrors are limited to <10 mm considering the total length of the optical system. Tolerance of off-axis distance was calculated using a ray-tracing computer simulation. Ray tracing shows that a blur size <14 nm will be possible at an off-axis displacement of 10 μm.

Effect of electromagnetic pulse transverse inhomogeneity on ion acceleration by radiation pressure

During ion acceleration by radiation pressure, a transverse inhomogeneity of an electromagnetic pulse leads to an off-axis displacement of the irradiated target, limiting the achievable ion energy. This effect is analytically described within the framework of a thin foil target model and with particle-in-cell simulations showing that the maximum energy of the accelerated ions decreases as the displacement from the axis of the target's initial position increases. The results obtained can be applied to the optimization of ion acceleration by the laser radiation pressure with mass-limited targets.

Three-dimensionally woven glass fiber composite struts: characterization and mechanical response in tension and compression

Three-dimensionally woven E- and S2-glass fiber textiles have been used in the past to create delamination-resistant corrugated core sandwich panels. During subsequent out-of-plane loading, the E-glass composite core struts and S2-glass composite faces are subjected to either compressive or tension loads. This study has investigated the relationships between the three-dimensional fiber architecture, fiber properties and the mechanical response of representative samples of the core and faces. Using X-ray computed tomography and optical microscopy to characterize the three-dimensional fiber architectures, it is found that the in-plane warp and weft fibers suffer significant off-axis displacement (waviness) due to their interaction with through thickness z-fiber tows. The consequence of this fiber waviness on the relationships of the in-plane tensile and compressive mechanical properties, along with fiber type, fiber volume fraction, and strut aspect ratio are experimentally investigated. The large initial misalignment angle of the warp and weft fiber tows results in a strut compressive strength that is substantially lower than its tensile strength due to compressive failure by either elastic or localized fiber microbuckling. Simple micromechanical models are used to relate the compressive strength of the three-dimensional woven composite struts to strut aspect ratio, fiber volume fractions in the three directions and the three-dimensional fiber architecture.

Nonparaxial propagation of a vectorial apertured off-axis anomalous hollow beam

Based on the vectorial Rayleigh–Sommerfeld integral formula and the complex Gaussian expansion of the hard-edge aperture function, an analytical propagation expression for a nonparaxial vectorial off-axis anomalous hollow beam (AHB) passing through a rectangular aperture is derived. The corresponding closed-forms for unapertured case, the far field expression and the scalar paraxial result are also given as special cases of the general formulae. By using the derived formulae, some numerical examples are given to illustrate the nonparaxial propagation properties of a vectorial off-axis AHB through a rectangular aperture. It is shown that the f parameter, the off-axis displacement and the truncation parameter all play an important role in determining nonparaxial propagation behavior.

Electric and magnetic polarization singularities of first-order Laguerre–Gaussian beams diffracted at a half-plane screen

Based on the vector Fresnel diffraction integrals, analytical expressions for the electric and magnetic components of first-order Laguerre-Gaussian beams diffracted at a half-plane screen are derived and used to study the electric and magnetic polarization singularities in the diffraction field for both two- and three-dimensional (2D and 3D) cases. It is shown that there exist 2D and 3D electric and magnetic polarization singularities in the diffraction field, which do not coincide each other in general. By suitably varying the waist width ratio, off-axis displacement parameter, amplitude ratio, or propagation distance, the motion, pair-creation, and annihilation of circular polarization singularities, and the motion of linear polarization singularities take place in 2D and 3D electric and magnetic fields. The V point, at which two circular polarization singularities with the same topological charge but opposite handedness collide, appears in the 2D electric field under certain conditions in the diffraction field and free-space propagation. A comparison with the free-space propagation is also made.

Off-axial plasma displacement suitable for antihydrogen production in AEgIS experiment

Antihydrogen experiments are currently based on non neutral electron, positron or antiproton plasma manipulation techniques in cylindrical Malmberg-Penning traps. An experimental study of a plasma manipulation technique based on off-axis diocotron displacement is presented. The use of the autoresonant excitation of (1, 0) diocotron mode of pure electron plasma allows a precise positioning of the plasma by moving it across the magnetic field and allows dumping such plasma in a desired angular position. The experimental procedure described here will pave the way to positron loading into an off-axial Penning trap terminated with a positronium converter target as it is proposed for the AEgIS experimental apparatus. The technique was studied over a range of confining magnetic field values and reproduces experimental conditions similar to most of the currently running antihydrogen experiments. The efficiency of the autoresonant excitation – in terms of plasma expansion rate and particle loss – is analyzed, studying the behaviour of electron plasma subjected to large off-axial displacements, showing that this method fulfills the requirements imposed by the AEgIS experiment.

Novel Terahertz Antenna Based on a Silicon Lens Fed by a Leaky Wave Enhanced Waveguide

We introduce a novel antenna concept suitable for future integrated arrays at terahertz frequencies. The antenna consists of an extended hemispherical lens antenna fed by a leaky wave waveguide feed that can be integrated with sensors and detectors such as Schottky diodes. In this antenna architecture, a couple of TE/TM leaky wave modes are excited in a resonant cavity formed by a waveguide opening ground plane and a silicon lens. Due to these modes, the field radiated by the waveguide inside the lens is a very directive pattern that illuminates the upper part of the lens. Having a directive primary field helps to increase the f-number of the lens improving several factors as spill over, off axis distortions and coating layer fabrication. The antenna structure is compatible with modern semiconductor fabrication technology and lends itself nicely for large format imaging arrays. In this contribution, we investigate the important parameters of a single antenna such as reflection coefficient, directivity, Gaussicity, phase center and off-axis displacement tolerances, and we validate our simulations by measuring the far field radiation patterns of a 545 GHz prototype.

Evolution of an optical vortex dipole diffracted by a half screen

The analytical expression for the propagation of an optical vortex dipole (OVD) nested in a Gaussian beam diffracted by a half screen is derived and used to study the evolution of the diffracted OVD. It is shown that Compared with the case of the OVD evolution in free space, multi-pairs of the OVD may take place in the diffracted field, and the annihilation fashion depends on the off-axis displacement. A variety of the evolution behaviors of OVDs appear by varying the off-axis parameter, but the topological charge is conserved in the evolution process.

Polarization singularities in near-field of Gaussian vortex beam diffracted by a circular aperture

Polarization singularities in the near-field of Gaussian vortex beams diffracted by a circular aperture are studied by a rigorous electromagnetic theory. It is shown that there exist C-points and L-lines, which depend on off-axis displacement parameters along the x and y directions, waist width, wavelength, and topological charge of the diffracted Gaussian vortex beam, as well as on propagation distance. The results are illustrated by numerical calculations.

Study correlation vortices in the near-field of partially coherent vortex beams diffracted by an aperture

We have derived the analytical expression of the electric cross-spectral density in the near- field of partially coherent vortex beams diffracted by an aperture. Taking the Gaussian Schell-model vortex beam as a typical example of partially coherent vortex beams, the spatial correlation properties and correlation vortices in the near-field of partially coherent vortex beams diffracted by a rectangle aperture are studied. It is shown that the off-axis displacement, spatial degree of coherence parameter, propagation distance, and the opening factor of the aperture affect the spectral degree of coherence and positions of correlation vortices. With the optimization algorithm, we obtain the symmetric distributing coherent vortex.

Influence of pointing fluctuation on intense laser beams propagation in plasma channels

An off-axis incident model is presented to analyze the influence of beam pointing fluctuation on the propagation properties of intense laser beams in plasma channels. The equations for the beam spot size and centroid are obtained by applying the variational method. The beam pointing fluctuation contributes additional focusing effect by amplifying relativistic self-focusing, leading to periodically modified oscillations of the spot size. The beam centroid oscillates along the channel axis with the amplitude close to its initial off-axis displacement, while the oscillation frequency is scaled as the square of the dimensionless channel strength parameter.

Photonic crystal lens for coupling two waveguides

We report the design, fabrication, and characterization of a new nanophotonic device comprising a two-dimensional photonic crystal (PhC) lens of size 3x4 microm fabricated in silicon-on-insulator. The PhC lens is put at the output of a planar waveguide of width 4.5 microm to couple light into a planar waveguide of width 1 microm, with two waveguides being of length 5 mm. A 1 microm off-axis displacement of the smaller waveguide leads to an 8-fold reduction of output light intensity, which means that the focal spot size at output of the PhC lens in silicon is less than 1 microm. The simulation has shown that the PhC lens has maximal transmittance at 1.55 microm, with the coupling efficiency being 73%. The focal spot size of the lens in air calculated at the FWHM is 0.32lambda (where lambda is the wavelength).

Binding of O2, NO, and CO to model active sites in ferrous heme proteins: Ligand geometry, electronic structure, and quadrupole splittings

The electronic structures of model oxy-, nitrosyl-, and carbon monoxy ferroporphyrin compounds with N-methylimidazole as an axial ligand are calculated for a number of different conformations of the O2, NO, and CO ligands using the iterative extended Huckel method. For the dioxygen ligand a bent end-on (Pauling) geometry with a low-energy off-axis displacement is favored. The “anomalously large” electric field gradient at the iron nucleus can be accounted for by a paired iron(II)-dioxygen configuration. The carbon monoxide ligand is linear in model compounds but bent or tilted in protein. The reduced affinity observed for the intact protein is explained by these results. The nitrosyl ligand is bent both in model compounds and in the protein. The calculated electronic structure is consistent with the spin density observed in hyperfine splitting in electron spin resonance spectra and gives a quadrupole splitting in excellent agreement with experiment.