Gaussian Profile Laser Research Articles

Study of particle acceleration by Laguerre–Gaussian ultra intense laser plasma interactions

In this work, simulations of multi-petawatt lasers in the range of ∼0.2 PW–∼100 PW with varying Laguerre–Gaussian (LG) azimuthal modes as well as linearly polarized (LP) and circularly polarized (CP) laser beams striking near critical density targets were studied by using three-dimensional particle in cell (PIC) codes. Particle acceleration mechanisms have a dependence on laser polarization and it affects the energy gained by the particles. It is known that laser pulses can be polarized helically by applying the LG distribution function to the fundamental Gaussian laser profile. In this study, differently polarized laser beams with varying powers were employed to study laser driven particle acceleration and compares accelerated charged particles’ energy spectra and angular distribution. It is seen that LG laser beams can accelerate higher energetic particles due to higher conversion efficiency compared to LP and CP laser beams. It is also seen that LG laser beams can collimate ions with a narrower spread compared to LP and CP beams. Furthermore, ions can have a smaller divergence angle with increasing azimuthal mode index when the laser is LG polarized. We also studied the energy deposition of these particles in a water cell obtained by the PIC codes for different laser parameters by using Geant4 Monte Carlo simulations which suggests that LG laser beam can be useful for the future hadron therapy applications.

Measurements of plasma density profile evolutions with a channel-guided laser

Abstract The discharged capillary plasma channel has been extensively studied as a high-gradient particle acceleration and transmission medium. A novel measurement method of plasma channel density profiles has been employed, where the role of plasma channels guiding the advantages of lasers has shown strong appeal. Here, we have studied the high-order transverse plasma density profile distribution using a channel-guided laser, and made detailed measurements of its evolution under various parameters. The paraxial wave equation in a plasma channel with high-order density profile components is analyzed, and the approximate propagation process based on the Gaussian profile laser is obtained on this basis, which agrees well with the simulation under phase conditions. In the experiments, by measuring the integrated transverse laser intensities at the outlet of the channels, the radial quartic density profiles of the plasma channels have been obtained. By precisely synchronizing the detection laser pulses and the plasma channels at various moments, the reconstructed density profile shows an evolution from the radial quartic profile to the quasi-parabolic profile, and the high-order component is indicated as an exponential decline tendency over time. Factors affecting the evolution rate were investigated by varying the incentive source and capillary parameters. It can be found that the discharge voltages and currents are positive factors quickening the evolution, while the electron-ion heating, capillary radii and pressures are negative ones. One plausible explanation is that quartic profile contributions may be linked to plasma heating. This work helps one to understand the mechanisms of the formation, the evolutions of the guiding channel electron-density profiles and their dependences on the external controllable parameters. It provides support and reflection for physical research on discharged capillary plasma and optimizing plasma channels in various applications.

Femtosecond Laser Modification of Silica Optical Waveguides for Potential Bragg Gratings Sensing.

The optimum femtosecond laser direct writing of Bragg gratings on silica optical waveguides has been investigated. The silica waveguide has a 6.5 × 6.5 µm2 cross-sectional profile with a 20-µm-thick silicon dioxide cladding layer. Compared with conventional grating inscribed on fiber platforms, the silica planar waveguide circuit can realize a stable performance as well as a high-efficiency coupling with the fiber. A thin waveguide cladding layer also facilitates laser focusing with an improved spherical aberration. Different from the circular fiber core matching with the Gaussian beam profile, a 1030-nm, 400-fs, and 190-nJ laser is optimized to focus on the top surface of the square silica waveguide, and the 3rd-order Bragg gratings are inscribed successfully. A 1.5-mm long uniform Bragg gratings structure with a reflectivity of 90% at a 1548.36-nm wavelength can be obtained. Cascaded Bragg gratings with different periods are also inscribed in the planar waveguide. Different reflection wavelengths can be realized, which shows great potential for wavelength multiplexing-related applications such as optical communications or sensing.

Laguerre–Gaussian laser beam guiding and its second harmonics in plasma having density ramp

This paper presents the optical guiding of a laser beam in plasma by using a preformed plasma channel. The density ramp in plasma density due to the plasma pressure has also been considered. The effect of ponderomotive force has been taken into account which originates due to the intensity gradient present in the laser beam. This force produces a plasma gradient by expelling plasma electrons from a high-field to a low-field region, providing heavy ions remain immobile. Plasma oscillations result from a gradient in plasma density that excites an electron plasma wave. The equation governing the plasma wave excitation has been found by using linear perturbation theory. An in-phase mixing of an incident laser beam with this plasma wave generates its second harmonics. Laguerre–Gaussian laser profile has been used for harmonic production. Moment theory has been used to obtain a differential equation for beam waist, which has been solved numerically by Runge–Kutta's fourth-order method. The effect of different modes of Laguerre–Gaussian profile, beam intensity, plasma density, channel depth, and slope of density ramp has been explored.

Investigation of self-focusing a Gaussian laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless warm quantum plasma

This article investigates nonlinear self-focusing of an intense right hand circularly polarized Gaussian profile laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless and unmagnetized warm quantum plasma. The nonlinear propagation equation for laser pulse in plasma has been derived. Then, the evolution differential equation for laser spot-size was obtained with considering the parabolic equation approach under the Wentzel-Kramers-Brillouin and paraxial ray approximations. This differential equation was solved numerically by fourth-order Runge-Kutta method. It is shown that our solution confirms the results of the self-focusing of the laser pulse in a weakly relativistic ponderomotive regime in cold quantum plasma in extreme conditions. Numerical results indicate that self-focusing of the laser pulse in the presence of relativistic and ponderomotive nonlinearity inside warm quantum plasma is improved in comparison with relativistic and ponderomotive cold quantum plasma.

Scanning strategies effect on temperature, residual stress and deformation by multi-laser beam powder bed fusion manufacturing

Laser beam Powder Bed Fusion (PBF-LB) is an increasingly attractive technology due to its capability of manufacturing complex and customised metal parts. However, the build cycle time and resulting residual stress profiles are limiting a wider uptake of PBF-LB manufacturing. The incorporation of additional laser beams in a single build chamber, i.e. multi-laser beam Powder Bed Fusion (PBF-MLB) technology, has been proposed as the next generation of PBF-LB technology. A series of computational process simulations of PBF-MLB was carried out using a 3D coupled thermomechanical model to investigate optimum multi-laser scanning strategies. The influences of twelve different scanning strategies on temperature, the final residual stresses, the z- (build) direction deflections by dual Gaussian profile laser beams PBF-MLB were investigated. It was found that the 90° layer rotation is necessary to reduce the residual stress, and a ‘dual-laser following but time-delayed’ scanning strategy had the lowest final residual stress. Another scanning strategy (45° rotation approaching beam scanning) resulted in the lowest z-direction deflection for the dual laser beams PBF-MLB manufacturing. These findings provide insight and application of PBF-MLB finite element process simulation in improving industrial additive manufacturing of metal components.

Effect of the laser transverse profile in a capillary for laser-plasma acceleration with external injection

Guiding of a high intensity laser beam in a hollow dielectric capillary is investigated with application for a laser plasma wakefield accelerator stage. For such guiding, the transverse profile of the laser intensity at the capillary entrance has a strong influence on the fluctuations of the transverse plasma wakefield during the laser propagation. The effects of these fluctuations on the acceleration of an electron bunch injected at 100 MeV is analyzed in detail. Our results show that, in the best match conditions, the usually used Gaussian transverse profile leads to strong fluctuations generated by the diffraction of the edges of the laser at the capillary entrance. We demonstrate that by reducing this diffraction through the use of a flattened Gaussian laser profile, an efficient acceleration of the electron bunch can be obtained over several Rayleigh lengths

Laguerre-Gaussian and beamlet array as second generation laser heater profiles

The microbunching instability is known to be detrimental to x-ray free electron laser performance. At the Linear Coherent Light Source, the microbunching instability is suppressed with a laser heater, which increases the uncorrelated energy spread of e-beam in the injector. While the current system has been shown to improve x-ray brightness, other laser architectures could further enhance performance. In this study, we model the interaction between a laser and e-beam with arbitrary transverse profiles and examine the effect of various laser designs on the energy distribution of the electrons after the injector and laser heater, as well as their ability to suppress microbunching instability. This simulation incorporates random transverse jitter in order to reproduce physically representative operation of the Linac Coherent Light Source. We compare Gaussian and Laguerre-Gaussian modes, and explore composite beams in the form of an array of Gaussian beamlets. We conclude that the Gaussian laser profile is highly susceptible to e-beam ellipticity and random transverse jitter. The Laguerre-Gaussian profile, a mathematically ideal solution to suppressing microbunching, is less susceptible to these effects and can provide effective suppression even with a distorted e-beam, though performance can be improved by increasing stabilization. The array of beamlets presents a solution that produces consistent and smooth energy distributions with significantly less variance in heating than the Laguerre-Gaussian profile.

Modeling of first-order photobleaching kinetics using Krylov subspace spectral methods

We solve the first order 2-D reaction–diffusion equations which describe binding-diffusion kinetics using the photobleaching scanning profile of a confocal laser scanning microscope, approximated by a Gaussian laser profile. We show how to solve the first-order photobleaching kinetics partial differential equations (PDEs) using a time-stepping method known as a Krylov subspace spectral (KSS) method. KSS methods are explicit methods for solving time-dependent variable-coefficient partial differential equations. They approximate Fourier coefficients of the solution using Gaussian quadrature rules in the spectral domain. In this paper, we show how a KSS method can be used to obtain not only an approximate numerical solution, but also an approximate analytical solution when using initial conditions that come from pre-bleach steady states and also general initial conditions, to facilitate asymptotic analysis. Analytical and numerical results are presented. It is observed that although KSS methods are explicit, it is possible to use a time step that is far greater than what the CFL condition would indicate.

Solution of PDEs for First-Order Photobleaching Kinetics using Krylov Subspace Spectral Methods

We solve the first order reaction-diffusion equations which describes binding-diffusion kinetics using photobleaching scanning profile of confocal laser scanning Microscopes approximated by a Gaussian laser profile. We show how to solve the first order photobleaching kinetics partial differential equations with prebleach steady state initial conditions using a time-domain method known as a Krylov Subspace Spectral method (KSS method). KSS are explicit methods for solving time-dependent variable-coefficient partial differential equations (PDEs). KSS methods are advantageous compared to other methods because of its high resolution and its superior scalability. We will apply Gaussian Quadrature rules in the spectral domain developed by Golub and Meurant to solve PDEs. We present a simple rough analytical solution, as well as a computational solution that is first-order accurate. We then use this solution to examine short and long time behaviors.

Fibre laser welding of high-alloyed Al–Zn–Mg–Cu alloys

The theoretical fundamentals of laser weldability of metals are surveyed and relevant thermophysical parameters are identified – such as vapour pressure, keyhole pressure, beam irradiance, surface tension and viscosity. The derived approach for improving the laser weldability implies the use of a Yb fibre laser with an initial large beam diameter, a top-hat beam profile and a high laser power, which enables the formation of a large and stable keyhole during deep penetration welding. For validating the effectiveness of the developed approach, it is applied to various high-alloyed and hard-to-weld Al–Zn–Mg–Cu alloys. Defect-free welds are obtained even for AA7034 – the alloy with the highest (Zn+Mg+Cu) content commercially available. As reference, the same alloys are welded by using a conventional Nd:YAG laser with a small beam diameter, a Gaussian beam profile and medium laser power. The laser weldability deteriorates with increasing (Zn+Mg+Cu) content in terms of porosity and excess of penetration. The obtained results highlight the importance of the laser system used on the laser weldability of Al–Zn–Mg–Cu alloys.

P‐118: Low Voltage Driving Tunable Liquid Crystal Lens using Photoalignment Method

In this paper, we develop a method to prepare a liquid crystal (LC) lens using photoalignment technology. It's fabricated by utilizing variable pretilt angles of a photoalignment layer. By irradiating a Gaussian profile laser beam on a photoalignment layer, variable angles from 1° to 89° are formed. In this way, the tunable LC concave lens with negative power is fabricated and characterized. It requires low voltage control for its tunable feature, which results in low power consumption. Such lens can play a great role in many modern optical and photonic applications.

Light Induced Thermocapillary Motion in Nematic Liquid Crystals

Surface tension gradients can cause mass transfer (Marangoni effect). We investigated thermocapillary in homeotropic oriented nematic liquid crystals. The heating process was performed with a Gaussian laser profile. The upper surface of the cell is free and the bottom has hard anchoring conditions. Upon assuming the fluid is incompressible, the hydrodynamic equations that explain the system shifts can be derived from the balance of torques, Navier-Stokes and heat equation. The heat equation is written for anisotropic area. We investigated the problem of the existence of vertical magnetic field. We found that the magnetic field can affect the system.

Microlenses with focal length controlled by chemical processes

The influence of chemical processing on the optical properties of microlenses formed on a gelatin-sensitized layer was investigated. The gelatin is sensitized with tot'hema and eosin, irradiated with a Gaussian profile laser beam and subsequently chemically processed. Microlenses with a focal length of 400 μm were obtained after alcohol processing. Additionally, focal lengths could be controlled by varying the alum concentration, and lenses with focal length up to 1.2 mm were obtained. The microlenses become stable after alum processing. Their optical properties remain unchanged.

Photoacoustic detection of thin layers of explosives

Explosives may be identified by their infrared absorption spectra. Photoacoustic measurements, which measure the acoustic output generated by the explosive sample absorbing radiation from a modulated laser beam, are efficient at detecting and identifying explosives. We are investigating the standoff photoacoustic detection of surface traces of explosives on a variety of substrates using a laser Doppler vibrometer. This talk describes the detailed modeling of the measurement. A modulated infrared Gaussian profile laser source is absorbed in the layer of explosive and in the substrate. The heat absorbed raises the temperature of the explosive and the substrate leading to expansion and surface displacement. The heat absorbed by the gas adjacent to the solid generates an outward propagating acoustic wave. A laser Doppler vibrometer measures the phase shift due to the surface displacement and the compression and refraction of the acoustic wave. A cylindrically symmetric calculation of each physical step will be described. Calculations demonstrating how substrate parameters such as coefficient of thermal expansion, IR absorptivity, and Poisson's ratio effect the detection threshold of the measurements will be presented.

Effect of experimental parameters and image noise on the error levels in simultaneous velocity and temperature measurements using molecular tagging velocimetry/thermometry (MTV/T)

In this work, the effect of experimental parameters on the error levels associated with simultaneous measurement of displacement and temperature using Molecular Tagging Velocimetry/Thermometry (MTV/T) was quantified via simulated images. Images were simulated using Gaussian profile laser lines. Noise was added to the images using a uniform random distribution and a Gaussian random distribution to simulate electronic noise and shot noise, respectively. The results showed that the error levels in the displacement and temperature measurements were inversely related for most experimental parameters including the laser line thickness, fluid temperature and image delay times. It is concluded that the dynamic range of the technique depends on the flow speeds and temperatures and must be determined for each experiment individually. Error levels, for 95% confidence, were found to be better than 0.3°C for temperature and 0.2 pixels for displacement for typical real-world experimental parameters.

Influence of laser beam intensity profile on propagation dynamics of laser-blow-off plasma plume

AbstractEffect of intensity profile of the ablating laser on the dynamics of laser-blow-off (LBO) plume has been studied by fast imaging technique. This work emphasizes the geometrical aspect of the LBO plume, which is an important parameter for various applications. Visualization of the expanding plume reveals that geometrical shape and directionality (divergence) of the plume are highly dependent on the laser intensity profile. Present results demonstrate that the Gaussian profile laser produces a well-collimated, low divergence plasma plume as compared to the plume formed by a top-hat profile laser. The sequence of film removal processes is invoked to explain the role of energy density profile of the ablating laser in LBO mechanism.

Powder particle temperature distribution in laser deposition technologies

PurposeThe purpose of this paper is to provide a flexible tool to predict the particle temperature distribution for traditional laser applications and for the most recent diode laser processes. In the past few years, surface processing and rapid prototyping applications have frequently implemented the use of powder delivery nozzles and high power fibre‐coupled diode lasers with highly convergent laser beams. Owing to the complexity and variety of the process parameters involved in this technology, mathematical models are necessary to understand and predict the deposition behaviour. Modeling the dynamics of the melting pool and the particle temperature distribution is critical for achieving a good deposition quality.Design/methodology/approachThis study focuses on the development of mathematical models to predict the particle temperature distribution over the melting pool. An analytical and a numerical solution are proposed for two cases of laser intensity distribution: top hat and Gaussian.FindingsThe results show that a more vertical position of powder delivery nozzle will lead to a higher and more uniform particle temperature distribution, in particular for the top‐hat intensity distribution case.Originality/valuePrevious work has dealt only with Gaussian laser spatial distributions and collimated laser beams. Therefore, they were limited to a specific class of laser processes. This work provides a flexible tool to predict the particle temperature distribution for traditional laser applications (powder delivery nozzle and Gaussian laser profile) and for the most recent diode laser processes (powder delivery nozzle and top‐hat laser distribution with highly convergent laser beam). In addition, the results demonstrate that the particle temperature does not monotonically increase while increasing the nozzle inclination as in the case of a collimated laser beam, but some particles show a minimum temperature for intermediate values of the nozzle inclination angle.

Laser ablation of metallic targets with high fluences: Self-consistent approach

Ablation of metallic target using Gaussian profile laser pulse with duration of 30 ns and fluence extending up to 100 J/cm2 is studied. A self-consistent model that involves numerical simulation to obtain the thickness of a hydrodynamic layer is proposed. The influence of the hydrodynamic layer thickness on Knudsen layer properties, and hence the rate of evaporation, is demonstrated. Self-consistent model is compared with model based on assumption about sonic speed at the Knudsen layer outer edge and validated with experimental results for Al alloy and Cu. Finally, sample calculations are performed using the proposed model to obtain depth of evaporation of Al alloy for single pulse duration.

A Closed-Form Analytic Expression for FRAP Formula for the Binding Diffusion Model

One of the most dominant methods cells use for a large class of cellular processes is reaction (or binding) diffusion kinetics, which are controlled by kinetic constants such as diffusion coefficients and on/off binding rate constants. Fluorescence recovery after photobleaching (FRAP) can be used to determine these kinetic constants in living cells. While an analytic expression for FRAP formulae for pure diffusion has been available for some time, an analytic FRAP formula for the binding diffusion model has not been reported yet. Here, we present an analytic FRAP formula for the binding diffusion model in an explicit form allowing for diffusion of the bound complex for either a uniform circle laser profile or a Gaussian laser profile.