Transverse Intensity Profile Research Articles

Performance optimization of a SERF atomic magnetometer based on flat-top light beam

We explore the impact of pumping beams with different transverse intensity profiles on the performance of the spin-exchange relaxation-free (SERF) atomic magnetometers (AMs). We conduct experiments comparing the traditional Gaussian optically-pumped AM with that utilizing the flat-top optically-pumped (FTOP) method. Our findings reveal that the FTOP-based approach outperforms the conventional method, exhibiting a larger response, a narrower magnetic resonance linewidth, and a superior low-frequency noise performance. Specifically, the use of FTOP method leads to a 16% enhancement in average sensitivity within 1 Hz–30 Hz frequency range. Our research emphasizes the significance of achieving transverse polarization uniformity in AMs, providing insights for future optimization efforts and sensitivity improvements in miniaturized magnetometers.

Variation of structured laser beam pattern and optimization for an alignment reference line creation.

The alignment of particle accelerators demands a dedicated measurement system based on a straight-line reference. This straight line can be provided by a laser beam. The alignment then involves accurately measuring the offset of accelerator components with respect to this light path. In order to be efficient, the laser beam needs to serve as a stable and straight reference for distances of several hundreds of meters. The attainable accuracy depends, among other parameters, on the laser spot size, which should ideally change very little over the distances at which the alignment system needs to operate. Due to the significant divergence of Gaussian laser beams, we propose using a structured laser beam (SLB) for alignment. Its transversal intensity profile is similar to a Bessel beam and consists of an intense inner core (IC) and concentric rings. The divergence of the IC, i.e., the growth of its size with distance, can be limited to 10μrad using a favorable generator configuration. Thus an SLB may be suitable as a straight-line reference for long-distance alignment applications. However, the SLB is distorted if obstructions cover parts of the outermost ring (OR) of the beam within, which should therefore also be small. In this paper, we investigate the relationship between the size of the IC and OR depending on the design parameters of the SLB generator. We use numerical simulations and experiments with different generators over distances up to 50 m to analyze the transversal intensity profile and wavefronts of different SLBs. The results indicate the general suitability of an SLB as a reference line for long-distance alignment but also expose tradeoffs between small IC and small OR. The findings outlined in the paper help to describe the optimal SLB parameters for given conditions.

Non-Diffraction Self-Acceleration Beams With Customized Transverse Intensity Profiles Based on 3-D-Printed Meta-Structure

In this article, two 3-D-printed all-dielectric meta-structures that can produce diffraction-free self-acceleration beams with different transverse intensity profiles are proposed in the microwave frequency band. The first single-layer meta-structure, combining the Fourier lens phase and cubic phase distributions, is engineered to generate Airy beams with highly asymmetric transverse intensity profiles. In order to flexibly customize the propagation trajectory and transverse spot shape, Bessel-like beams with symmetric transverse profiles along arbitrary trajectories are realized by the second compact meta-structure based on phase synthesis. Calculating the phase mask of the metasurface is the most critical part. In this work, phase-modulated meta-structures can map the phase information to height distribution of the cube elements that can be precisely controlled. To verify the validity, two prototypes are printed and measured. From the beam intensities at different cross sections along the propagation trajectory, the oscillating multilobe intensity pattern of the 2-D Airy beam and the symmetric non-spreading central main lobe of the Bessel-like beam can be observed in simulation and measurement. Besides that, two kinds of beams have transverse acceleration behavior and self-reconstruction property. Therefore, the proposed meta-structures have great potential in numerous challenging fields, like microwave power transmission, secure communication, imaging, and detection.

Theory of space-time supermodes in planar multimode waveguides.

When an optical pulse is focused into a multimode waveguide or fiber, the energy is divided among the available guided modes. Consequently, the initially localized intensity spreads transversely, the spatial profile undergoes rapid variations with axial propagation, and the pulse disperses temporally. Space-time (ST) supermodes are pulsed guided field configurations that propagate invariantly in multimode waveguides by assigning each mode to a prescribed wavelength. ST supermodes can be thus viewed as spectrally discrete, guided-wave counterparts of the recently demonstrated propagation-invariant ST wave packets in free space. The group velocity of an ST supermode is tunable independently-in principle-of the waveguide structure, group-velocity dispersion is eliminated or dramatically curtailed, and the time-averaged intensity profile is axially invariant along the waveguide in absence of mode-coupling. We establish here a theoretical framework for studying ST supermodes in planar waveguides. Modal engineering allows sculpting this axially invariant transverse intensity profile from an on-axis peak or dip (dark beam) to a multi-peak or flat distribution. Moreover, ST supermodes can be synthesized using spectrally incoherent light, thus paving the way to potential applications in optical beam delivery for lighting applications.

Synchrotron intensity plots from a relativistic stratified jet

ABSTRACT We examine the effect of a jet transversal structure from magnetohydrodynamic semi-analytical modelling on the total intensity profiles of relativistic jets from active galactic nuclei. In order to determine the conditions for forming double- and triple-peaked transverse intensity profiles, we calculate the radiative transfer for synchrotron emission with self-absorption from the jets described by the models with a constant angular velocity and with a total electric current closed inside a jet. We show that double-peaked profiles appear either in the models with high maximal Lorentz factors or in optically thick conditions. We show that triple-peaked profiles in radio galaxies constrain the fraction of the emitting particles in a jet. We introduce the possible conditions for triple-peaked profiles under the assumptions that non-thermal electrons are preferably located at the jet edges or are distributed according to Ohmic heating.

Impact of focusing and polarization inhomogeneity on SHG in type-II ppKTP

We study the effects of pump beam focusing and polarization inhomogeneity on second-harmonic generation (SHG) in a 3 cm long type-II ppKTP crystal. Focusing leads to an asymmetry in the SHG output power around the phase-matching temperature (PMT), along with a shift of the PMT to a lower temperature. Theoretical predictions are experimentally verified for different focusing parameters at various pump wavelengths, showing maximum SHG of W−1 (close to optimal focusing). Further, an inhomogeneous polarization across the pump beam cross-section is theoretically predicted to break up the SHG transverse (intensity) profile into several lobes, resulting in an overall decrease in the total output power. We experimentally show that a radially/azimuthally polarized pump beam (generated via a Q-plate) results in a four-lobed SHG similar to the Hermite–Gaussian HG 11 mode with an optimal focusing parameter that is four times higher than that for the homogeneously polarized pump.

Optical transport over millimeter distances of a microscopic particle using a novel all-fiber Bessel-like beam generator

Whilst free space Bessel modes can show particle guidance over extended distances, this has been limited for fiber-based Bessel-like beams which have importance for microfluidic applications. We propose and experimentally demonstrate a novel all-fiber Bessel-like beam generator (BBG) that is shown transport a dielectric particle distance in excess of 2mm. This was achieved by optimizing the multimode interference (MMI) in the BBG structure to create a Bessel-like beam of appropriate propagation invariant length (PIL) and judicious choice of laser wavelength suppress thermal effects. By varying the diameter of the region where the MMI occurred we analyzed its impact on PIL and the transverse intensity profile of the Bessel-like beam. Our study paves the way for the fiber optic applications such as novel beam shaping, optical transport, and optical imaging.

Metasurface-stabilized optical microcavities

Cavities concentrate light and enhance its interaction with matter. Confining to microscopic volumes is necessary for many applications but space constraints in such cavities limit the design freedom. Here we demonstrate stable optical microcavities by counteracting the phase evolution of the cavity modes using an amorphous Silicon metasurface as cavity end mirror. Careful design allows us to limit the metasurface scattering losses at telecom wavelengths to less than 2% and using a distributed Bragg reflector as metasurface substrate ensures high reflectivity. Our demonstration experimentally achieves telecom-wavelength microcavities with quality factors of up to 4600, spectral resonance linewidths below 0.4 nm, and mode volumes below 2.7{lambda }^{3}. The method introduces freedom to stabilize modes with arbitrary transverse intensity profiles and to design cavity-enhanced hologram modes. Our approach introduces the nanoscopic light control capabilities of dielectric metasurfaces to cavity electrodynamics and is industrially scalable using semiconductor manufacturing processes.

Generation of Perfect Electron Vortex Beam with a Customized Beam Size Independent of Orbital Angular Momentum.

The electron vortex beam (EVB)-carrying quantized orbital angular momentum (OAM) plays an essential role in a series of fundamental research. However, the radius of the transverse intensity profile of a doughnut-shaped EVB strongly depends on the topological charge of the OAM, impeding its wide applications in electron microscopy. Inspired by the perfect vortex in optics, herein, we demonstrate a perfect electron vortex beam (PEVB), which completely unlocks the constraint between the beam size and the beam's OAM. We design nanoscale holograms to generate PEVBs carrying different quanta of OAM but exhibiting almost the same beam size. Furthermore, we show that the beam size of the PEVB can be readily controlled by only modifying the design parameters of the hologram. The generation of PEVB with a customized beam size independent of the OAM can promote various in situ applications of free electrons carrying OAM in electron microscopy.

Generation of multiple-vortex beams in the process of diffraction of cosine-Laguerre–Gaussian laser beam by the fork-shaped grating

We present the theoretical study about Fraunhofer diffraction of cosine-Laguerre–Gaussian laser beam of mode l,n=0 by the fork-shaped grating with a phase singularity of integer order p. The analytical expressions for the diffracted wave field amplitude and intensity distributions are derived. The transverse intensity and phase profiles of the diffracted beam, in a given diffraction order and for different values of l and p, are calculated. They show that the higher diffraction orders are multiple-vortex beams. In addition, the interference patterns of the diffracted wave field with a slightly inclined plane wave are presented, for confirming the presence of the optical vortices and determining the values and signs of their topological charges. We have shown how an azimuthally shaped, non-vortex beam, consisting of 2l bright lattices, can be transformed into a fan of multiple-vortex beams in the higher diffraction orders.

Generation of arbitrary vector Bessel beams on higher-order Poincaré spheres with an all-dielectric metasurface

Vector Bessel beams may have a spatially varying polarization state combined with a Bessel-type transversal intensity profile. Such beams can carry both spin angular momentum and orbital angular momentum (OAM) and be characterized by points on higher-order Poincar\'e (HOP) spheres. We report a method to produce vector Bessel beams on any point of a HOP sphere of any topological charge by utilizing only a single all-dielectric planar metasurface. Specifically, by a proper choice of the input polarization and distance from the metasurface, the entire HOP sphere surface can be covered. The proposed method is simple and efficient and applies to any $n\mathrm{th}$-order HOP sphere. Furthermore, it yields an extremely high OAM mode purity. Our paper demonstrates a robust approach for generating arbitrary high-quality vector Bessel beams and will likely inspire future applications ranging from optical manipulation to laser beam engineering.

The morphology of the HD 163296 jet as a window on its planetary system

Context. HD 163296 is a Herbig Ae star which drives a bipolar knotty jet with a total length of ~6000 au. Strong evidence exists that the disk of HD 163296 harbors planets. Studies have shown that the presence of companions around jet-driving stars could affect the morphology of the jets. This includes a ‘wiggling’ of the jet axis and a periodicity in the positions of the jet knots. Aims. In this study we investigate the morphology (including the jet width and axis position) and proper motions of the HD 163296 jets, and use our results to better understand the whole system. Methods. This study is based on optical integral-field spectroscopy observations obtained with VLT/MUSE in 2017. Using spectro-images and position velocity diagrams extracted from the MUSE data cube, we investigated the number and positions of the jet knots. A comparison was made to X-shooter data collected in 2012 and the knot proper motions were estimated. The jet width and jet axis position with distance from the star were studied from the extracted spectro-images. This was done using Gaussian fitting to spatial profiles of the jet emission extracted perpendicular to the position angle of the jet. The centroid of the fit is taken as the position of the jet axis. Results. We observe the merging of knots and identify two previously undetected knots. We find proper motions that are broadly in agreement with previous studies. The jet width increases with distance from the source and we measure an opening angle of ~5° and 2.5° for the red and blue lobes, respectively. Measurements of the jet axis position, derived from Gaussian centroids of transverse intensity profiles, reveal a similar pattern of deviation in all forbidden emission lines along the first 20 arcsec of the jets. This result is interpreted as being due to asymmetric shocks and not due to a wiggling of the jet axis. Conclusions. The number of new knots detected and their positions challenge the 16-yr knot ejection periodicity proposed in prior studies, arguing for a more complicated jet system than was previously assumed. We use the non-detection of a jet axis wiggling to rule out companions with a mass >0.1 M⊙ and orbits between 1 au and 35 au. Any object inferred at these distances using other methods must be a brown dwarf or planet, otherwise it would have impacted the jet axis position. Both the precession and orbital motion scenarios are considered. Overall it is concluded that it is difficult to detect planets with orbits >1 au through a study of the jet axis.

Guiding of Laguerre–Gaussian pulses in high-order plasma channels

In laser wakefield accelerators, guiding of drive laser pulses in preformed plasma channels plays a key role to overcome laser diffraction for effective acceleration. Different from guiding schemes studied previously, where a Gaussian laser pulse and a parabolic plasma channel were investigated, here we investigate the guiding of Laguerre–Gaussian (LG) pulses in plasma channels. Analytical studies and three-dimensional particle-in-cell simulations show that the matched conditions still exist for high order laser pulses and high order plasma channels. For usual Gaussian and high order LG pulses, the second order parabolic channel gives the best guiding. Although the laser pulse can also be guided in even higher order channels, its envelope deforms during propagation. For laser pulses with combined multi-LG modes, determined by their azimuthal orbit angular momenta, there is axisymmetric or non-axisymmetric evolution for the transverse laser intensity profile. The preformed plasma channel can guide the combined pulses but the transverse intensity profile of the laser pulses always evolves.

GPU accelerated Monte Carlo simulation of high-intensity pulsed laser-electron interaction

We present a new Monte Carlo code for simulating strong field QED effects in high intensity Laser-Electron collisions, accelerated by massively parallel computations on GPUs. The numerical simulation tool is paired with a separate code which solves the Stratton-Chu vector diffraction integrals given the incoming beam on the focusing mirror, to more accurately describe the electromagnetic field at the focus of the mirror. Program summaryProgram title: SFQED codeCPC Library link to program files:https://doi.org/10.17632/bd5m7tf5yr.1Licensing provisions: MIT licenseProgramming language: C++ and CUDANature of problem: Evaluating the electromagnetic field, in the focus of a laser pulse with an arbitrary wavefront and transverse intensity profile focused by a parabolic mirror, is analytically not possible without performing several approximations along the way. It may prove vital in upcoming strong field QED experiments to know the field of an imperfect focused laser pulse exactly when laser intensities become large, as the interaction between high energy electrons and the focused laser pulse is numerically modeled when comparing to experiments.Solution method: We propose to numerically evaluate the Stratton-Chu vector diffraction integrals, given the wavefront and spatial intensity profile of the laser on the focusing mirror, through CUDA, and evaluate the electromagnetic field on a spatial grid around the focus of the laser pulse. A Monte-Carlo code that simulates the interaction between high energy particles and an electromagnetic field, will then use the evaluated focused laser field through trilinear interpolation to find the resulting electric field at the location of the particle in each time step.

Probing the rotational spin-Hall effect in a structured Gaussian beam

Spin-to-orbit conversion of light is a dynamical optical phenomenon in nonparaxial fields leading to various manifestations of the spin and orbital Hall effect. However, the effects of the spin-orbit interaction (SOI) have not been explored extensively for structured Gaussian beams carrying no intrinsic orbital angular momentum. Indeed, the SOI effects on such structured beams can be directly visualized due to azimuthal rotation of their transverse intensity profiles, a phenomenon we call the rotational spin-Hall effect. In this paper we show that for an input circularly polarized (right or left) Hermite-Gaussian $({\text{HG}}_{10})$ mode, the SOI leads to a significant azimuthal rotation of the transverse intensity distribution of both the orthogonal circularly polarized (left or right) component and the longitudinal field intensity with respect to the input intensity profile. We validate our theoretical and numerically simulated results experimentally by tightly focusing a circularly polarized ${\text{HG}}_{10}$ beam in an optical tweezers configuration and projecting out the opposite circular polarization component and the transverse distribution of the longitudinal field intensity at the output of the tweezers. We also measure the rotational shift as a function of the refractive index contrast in the path of the tightly focused light and in general observe a proportional increase. The enhanced spin-orbit conversion in these cases may lead to interesting applications in inducing complex dynamics in optically trapped birefringent particles using structured Gaussian beams with no intrinsic orbital angular momentum.

Polarization-independent optical spatial differentiation with a doubly-resonant one-dimensional guided-mode grating.

We report on the design and experimental characterization of a suspended silicon nitride subwavelength grating possessing a polarization-independent guided-mode resonance at oblique incidence. At this resonant wavelength we observe that the transverse intensity profile of the transmitted beam is consistent with a first-order spatial differentiation of the incident beam profile in the direction of the grating periodicity, regardless of the incident light polarization. These observations are corroborated by full numerical simulations. The simple one-dimensional and symmetric design, combined with the thinness and excellent mechanical properties of these essentially loss-free dieletric films, is attractive for applications in optical processing, sensing and optomechanics.

Optical shock-enhanced self-photon acceleration

Photon accelerators can spectrally broaden laser pulses with high efficiency in moving electron density gradients. When driven by a conventional laser pulse, the group velocity walk-off experienced by the accelerated photons and deterioration of the gradient from diffraction and refraction limit the extent of spectral broadening. Here we show that a laser pulse with a shaped space-time and transverse intensity profile overcomes these limitations by creating a guiding density profile at a tunable velocity. Self-photon acceleration in this profile leads to dramatic spectral broadening and intensity steepening, forming an optical shock that further enhances the rate of spectral broadening. In this new regime, multi-octave spectra extending from $400 nm - 60 nm$ wavelengths, which support near-transform limited $< 400 as$ pulses, are generated over $<100 \mu$m of interaction length.

Three-dimensional spiral motion of microparticles by a binary-phase logarithmic-spiral zone plate.

Acoustic vortex beams, which have both linear and angular momentum, can be used to make precise acoustic tweezers. Limited by the symmetry of a normal vortex beam, these tweezers are usually used for trapping or rotating particles in two dimensions. Here, the three-dimensional spiral motion of two soft particles of different sizes was realized using a vortex beam with a twisted focus, which was synthesized by a silicone binary-phase logarithmic-spiral zone plate. Numerical simulations and experimental measurements demonstrated that the beam had anisotropic focuses of crescent transverse intensity profiles and a screw phase dislocation with a singularity at the center. Experiments showed that a small particle (k0r ≈ 1.3) can follow the twisted intensity of the beam, but a large particle (k0r ≈ 4.7) spirals up away from the twisted field pattern. This is attributed to the dominant gradient force for the small particle, whereas the scattering effect induced a scattering force combined with a gradient force for the large particle. This focused twisted beam, which was generated with a structured silicone plate, and the three-dimensional spiral motion of microparticles, advance the development of simple, compact, and disposable acoustic devices for the precise and diverse manipulation of microparticles.

Dynamics of femtosecond synthesized coronary profile laser beam filamentation in air

Multiple filamentation in air of high-power ultrashort laser radiation with a transverse intensity profile resembling a corona composed of several annularly distributed independent top-hat sub-beams is theoretically studied. Through the numerical solution of the time-averaged nonlinear Schrödinger equation, we study the spatio-angular dynamics of a near-infrared annular combined beam (CB) along the optical path by varying the number and power of the beamlets (corona-spikes). The evident advances in the use of CB in the multiple-filamentation region manipulation and for achieving long-range filamentation are theoretically revealed. In particular, by adjusting the number and aperture of the constituting sub-beams, it makes it possible to significantly delay the CB filamentation onset distance and increase the filamentation length in air. In addition, at the post-filamentation stage of femtosecond pulse propagation under certain conditions, the CBs exhibit significantly lower angular divergence of their most intense part (post-filamentation light channel) compared to the beams with regular unimodal intensity profiles (Gaussian or plateau-like) that provide enhancement of laser power delivered to the receiver over the atmospheric links.

Radiation-Balanced Lasers: History, Status, Potential

The review of history and progress on radiation-balanced (athermal) lasers is presented with a special focus on rare earth (RE)-doped lasers. In the majority of lasers, heat generated inside the laser medium is an unavoidable product of the lasing process. Radiation-balanced lasers can provide lasing without detrimental heating of laser medium. This new approach to the design of optically pumped RE-doped solid-state lasers is provided by balancing the spontaneous and stimulated emission within the laser medium. It is based on the principle of anti-Stokes fluorescence cooling of RE-doped low-phonon solids. The theoretical description of the operation of radiation-balanced lasers based on the set of coupled rate equations is presented and discussed. It is shown that, for athermal operation, the value of the pump wavelength of the laser must exceed the value of the mean fluorescence wavelength of the RE laser active ions doped in the laser medium. The improved purity of host crystals and better control of the transverse intensity profile will result in improved performance of the radiation-balanced laser. Recent experimental achievements in the development of radiation-balanced RE-doped bulk lasers, fibre lasers, disk lasers, and microlasers are reviewed and discussed.