Self-focusing Of Beam Research Articles

Linear self-focusing of continuous UV laser beam in photo-thermo-refractive glasses.

The experimental and theoretical study of continuous UV laser beam propagation through thick silver-containing photo-thermo-refractive glass is presented. It is shown for the first time that self-action of UV Gaussian beam in glass results in its self-focusing. The observed linear effect is non-reversible and is caused by the transformation of subnanosized charged silver molecular clusters to neutral state under UV laser radiation. Such transformation is accompanied by the increase of molecular clusters polarizability and the refractive index increase in irradiated area. As a result, an extended positive lens is formed in glass bulk. In a theoretical study of linear self-focusing effect, the "aberration-free" approximation was used, taking into account spatial distribution of induced absorption.

The interaction of intense, ultra-short microwave beams with the plasma generated by gas ionization

Results of the non-linear interaction of an extremely short (0.6 ns) high power (∼500 MW) X-band focused microwave beam with the plasma generated by gas ionization are presented. Within certain gas pressure ranges, specific to the gas type, the plasma density is considerably lower around the microwave beam axis than at its periphery, thus forming guiding channel through which the beam self-focuses. Outside these pressure ranges, either diffuse or streamer-like plasma is observed. We also observe high energy electrons (∼15 keV), accelerated by the very high-power microwaves. A simplified analytical model of this complicated dynamical system and particle-in-cell numerical simulations confirm the experimental results.

On Demand Spatial Beam Self-Focusing in Hexagonal Multicore Fiber

Combination of the classical effect of light self-focusing and recently emerged multi-core fiber technology offers new opportunities for the spatio-temporal control and manipulation of high-power light radiation. Here we apply genetic algorithm to design a system enabling self-focusing of light in various fiber cores on demand. The proposed concept is general and can be applied and adapted to any multi-core fiber or 2D array of coupled waveguides paving a way for numerous applications.

Exponential density transition based self-focusing of Gaussian laser beam in collisional plasma

In this work, nonlinear aspects of a high intensity Gaussian laser beam propagating in plasma having upward density ramp of tangent and exponential profiles are studied. We have employed the nonlinearity in dielectric function of plasma due to nonuniform heating of carriers along the wavefront of the laser beam which is important in collisional plasmas. The differential equation governing the dimensionless beam width parameter is achieved by using Wentzel–Kramers–Brillouin (WKB) and paraxial approximations and solved it numerically by using Runge-Kutta fourth order method. Effect of exponential density ramp profile on self-focusing is systematically carried out and compared with tangent density ramp profile. It is found that exponential plasma density ramp causes the laser beam to become more focused than tangent density ramp over several Rayleigh length and gives reasonably interesting results.

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas with linear density and temperature ramps

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas are investigated in the presence of various laser intensities and linear density and temperature ramps. Considering the ponderomotive force and using the momentum transfer and energy equations, the nonlinear electron density is derived. Taking into account the paraxial approximation and nonlinear electron density, a nonlinear differential equation, governing the focusing and defocusing of the laser beam, is obtained. Results show that in the absence of ramps the laser beam is focused between a minimum and a maximum value of laser intensity. For a certain value of laser intensity and initial electron density, the self-focusing process occurs in a temperature range which reaches its maximum at turning point temperature. However, the laser beam is converged in a narrow range for various amounts of initial electron density. It is indicated that the σ2 parameter and its sign can affect the self-focusing process for different values of laser intensity, initial temperature, and initial density. Finally, it is found that although the electron density ramp-down diverges the laser beam, electron density ramp-up improves the self-focusing process.

Characterization of High-current Pulsed Electron Beam Interaction with AISI 1045 Steel and the Microstructure Evolution

In this paper, the process of pulsed electron beam interaction with AISI 1045 steel is the main focus under investigation. The electron beam is generated by “pseudo-spark” discharge device and characterized by high current (hundreds of amperes to thousands of amperes), short pulse duration (hundreds of nanoseconds to thousands of nanoseconds), high frequency (hundreds of Hz to thousands of Hz) and self-focused beam diameter (typically 0.5-5mm), which are more favorable for metal surface treatment compared with the conventional thermal-emission electron beam diode. In this work, both the microstructure evolution of the metal target and the temporal evolution of the target surface plasma and vapor are investigated by a diversity of diagnostic methods. The influence of the plasma phase on the formation of surface irregularities are analyzed and discussed. The experimental setup is introduced, surface microstructure and chemical composition of AISI 1045 steel pre and post pseudo spark electron beam treatment are investigated by using scanning electron microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS, also called EDX). The mechanisms of the metal surface profile evolution are proposed based on the experimental results. It is demonstrated that the pseudo-spark based electron beam parameters can be optimized to decrease the surface irregularities within the proper pulse irradiation time and plasma density range.

Effect of q-parameter on relativistic self-focusing of q-Gaussian laser beam in plasma

The exponential function in an expression of intensity profile of Gaussian laser beam has been replaced by algebraic function having q exponent in it. In preliminary theoretical investigations, it has been observed that q-Gaussian intensity distribution function plays vital role in the dynamics of laser beam propagating in relativistic plasma. In present theoretical investigation, the usual parabolic equation approach under paraxial and WKB approximations has been adopted, taking into account relativistic nonlinearity. We have established the nonlinear differential equation for the beam-width parameter f and solved it numerically by using fourth order Runge-Kutta method. The effect of q parameter on the variation of beam width parameter f with normalized distance of propagation ξ has been presented graphically and discussed.

Weakly relativistic self-focusing of Gaussian laser beam in magnetized cold quantum plasma

AbstractIn the present paper, we have studied self-focusing of Gaussian laser beam in weakly relativistic magnetized cold quantum plasma. When interparticle distance is comparable to the de Broglie wavelength of charged particles, we cannot neglect the quantum contribution of plasma constituents. Therefore, propagation characteristics are studied by taking in to account quantum contribution in the presence of static magnetic field applied along the beam propagation. Our results show that the magnetic field plays a key role in achieving additional focusing, it modifies the quiver motion of electrons by adding cyclotron frequency to the natural frequency of oscillating electrons during laser beam propagation. The results are compared with those of weakly relativistic quantum plasma and weakly relativistic magnetized plasma. The self-focusing is found to be more pronounced when axial magnetic field is increased in the present model. We have setup the non-linear differential equation for the evolution of beam-width parameter by well-known paraxial ray approximation and solved it with the help of computational technique.

Early and strong relativistic self-Focusing of cosh-Gaussian laser beam in cold quantum plasma

Relativistic self-focusing of cosh-Gaussian laser beam in the cold quantum plasma has been investigated theoretically. For a decentered parameter b=0.9, we present a comparative analysis for self-focusing of cosh-Gaussian laser beam in relativistic cold quantum (RCQ) and classical relativistic (CR) case. It is observed that as the beam penetrates deeper inside the RCQ plasma, the self-focusing ability of the laser beam enhances and shifted towards lower value of normalized propagation distance due to quantum contribution. The beam width parameter as a function of normalized propagation distance for various values of relative density parameter of the medium and intensity parameter has also been studied. The self-focusing is observed to be stronger with the increase in relative density parameter. Observation of early and strong self-focusing for higher values of relative density parameter and intensity parameter are reported. The present study might be very useful in the applications like the generation of inertial fusion energy driven by lasers, laser driven accelerators, scribing type of applications in electronics etc.

Analysis of illumination uniformity affected by small-scale self-focusing of a pump beam in the radial smoothing scheme.

In the radial smoothing (RS) scheme, small-scale self-focusing (SSSF) can degrade beam quality of a pump beam with a Gaussian pulse train, resulting in the degradation of the smoothing performance of the RS scheme. Considering SSSF of the pump beam in an optical Kerr medium (OKM), the propagation model of the laser beam in the RS scheme has been developed, and the effects of the characteristics of the pump beam and the thickness of the optical Kerr medium on the RS performance have further been numerically simulated and analyzed. The results show that SSSF decreases the illumination uniformity in the RS scheme by inducing distorted wavefront modulation of the pump beam. Consequently, the beam quality of the pump beam should be controlled to avoid the degradation of the RS performance affected by SSSF. In addition, the peak intensity of the pump beam and the thickness of the OKM should be reasonable to ensure RS performance while mitigating the impact of SSSF.

Self-focusing of asymmetric cosh-Gaussian laser beams propagating through collisionless magnetized plasma

AbstractIn this paper, self-focusing of asymmetric cosh-Gaussian laser beams in collisionless magnetized plasma has been studied. The non-linearity in dielectric constant considered herein is mainly due to the ponderomotive force. The non-linear coupled differential equations for the beam width parameters in transverse dimensions of the beam have been obtained by using WKB and paraxial approximations under parabolic equation approach. The numerical computation is completed by using fourth-order Runge–Kutta method. The effect of unlike decentered parameters in both transverse dimensions of the beam on self-focusing of cosh-Gaussian beams has been presented. Further, the effect of the static magnetic field and polarization modes of the laser has been explored.

Third-order-nonlinear effects in single crystals with arbitrary orientation and in ceramics

The influence of cubic crystal orientation on the generation of cross-polarization, self-phase modulation, and laser beam self-focusing is investigated. The orientations at which these effects are maximal and minimal have been found. The qualitative and quantitative difference of these effects in ceramics from those in single crystals and glass has been determined. Random small-scale (of order grain size) spatial modulation of laser beam polarization and phase has been predicted. This effect has no analogs in glasses or in single crystals. It is shown that, from the viewpoint of nonlinear phase incursion average over beam cross-section, ceramics is equivalent to glass whose nonlinearity is determined by a combination of the elements of ceramics tensor χ(3).

Relativistic self-focusing of finite Airy-Gaussian beams in collisionless plasma using the Wentzel‐Kramers‐Brillouin approximation

In this paper, we present a preliminary study of relativistic self-focusing of finite Airy-Gaussian beams in collisionless plasma taking into consideration the relativistic nonlinearity in the dielectric constant. The differential equation for the beam width parameter of the finite Airy-Gaussian beam is derived by using Wentzel-Kramers-Brillouin (WKB) and paraxial approximations through parabolic wave equation approach. The behavior of beam width parameter with distance of propagation is investigated at different parameters of plasma and those of the considered beams family. Our results detect that the beam is more focused at higher values aperture coefficient and higher intensity parameter. Furthermore, a strong oscillatory behavior of the beam width parameter and better focusing during laser propagation in plasma is apparent.

On validity of paraxial theory for super-Gaussian laser beams propagating in a plasma

In the present paper, we have investigated a situation where a high intensity laser beam passes through a gas and ionizes this gas by tunnel ionization. Here the electric field of the laser provides a sufficient velocity to the electrons to surpass the Coulomb barrier of the atom. Owing to the ionization the plasma density enhances which affects the laser beam propagation. The use of paraxial ray approximation theory for the present case of super-Gaussian lasers reveals the self-focusing of the beams and frequency upshifting. The predicted self-focusing of the laser beams is contrary to the expected outcome of defocusing of these beams in the plasma, indicating that the paraxial theory may not be valid for the case of super-Gaussian lasers even for the inclusion of most of the near axis region in the theory.

Generation of terahertz radiation by intense hollow Gaussian laser beam in magnetised plasma under relativistic-ponderomotive regime

This paper explores the self-focusing of hollow Gaussian laser beam (HGLB) in collisionless magnetized plasma and its effect on the generation of THz radiation in the presence of relativistic-ponderomotive nonlinearity. The relativistic change of electron mass and electron density perturbation due to the ponderomotive force leads to self-focusing of the laser beam in plasma. Nonlinear coupling between the intense HGLB and electron plasma wave leads to generation of THz radiation in plasma. Resonant excitation of THz radiation at different frequencies of laser and electron plasma wave satisfies proper phase matching conditions. Appropriate expressions for the beam width parameter of the laser beam and the electric vector of the THz wave have been evaluated under the paraxial-ray and Wentzel-Kramers Brillouin approximations. It is found that the yield of THz amplitude depends on the focusing behaviour of laser beam, magnetic field, and background electron density. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on self-focusing of the laser beam and further its effect on the efficiency of the generated THz radiation.

Polarization singularities in the self-focusing of an elliptically polarized laser beam in an isotropic phase of nematic liquid crystal close to the temperature of phase transition

ABSTRACTNumerical simulations of initially homogenously polarized Gaussian light beam self-focusing in isotropic phase of nematic liquid crystal, the temperature of which is close to the temperature of nematic-isotropic phase transition, were carried out. Here, we show that light polarization singularities (C-lines) of both polarization handednesses are nucleated near these transversal cross-sections of the beam, in which local extrema of peak intensities of corresponding circularly polarized components of the beam are attained. These lines are closed loops, surrounding the beam axis. They appear in wide range of beam and nonlinear medium parameters. Minor fluctuations of beam polarization and amplitude profiles do not destroy these singularities.

Relativistic effects on evolution of a q-Gaussian laser beam in magnetoplasma: Application of higher order corrections

In this research work, the authors have investigated the propagation characteristics of a q-Gaussian laser beam in a magnetoplasma by considering the relativistic nonlinearity. The field distribution in the medium is expressed in terms of beam width parameter f and q parameter. Higher order terms in the expansion of the dielectric function and the eikonal have been taken into account. The behavior of the beam width parameter with a dimensionless distance of propagation ξ for various values of q is examined. The phenomenon of self-trapping is also studied under variety of parameters. Further, the effect of the magnetic field on the self-focusing of the laser beam has also been explored.

Magnetic field effect on the self-focusing of an intense laser pulse interacting with a bulk medium of graphite nanoparticles

The motivation of the present work is the study of self-focusing of an intense laser beam propagating through a magnetized bulk medium consisting of metallic nanoparticles. Using a perturbative method, the wave equation describing the nonlinear interaction of the intense laser beam with metallic magnetized nanoparticles is derived. Evolution of the laser spot size for the circular polarization with the Gaussian profile is considered. A graphite nanoparticle medium is introduced as a good candidate for self-focusing of electromagnetic waves with the wavelength in the micrometer range where plasmon resonance occurs. In such a frequency area, using a few hundred Tesla external magnetic field can substantially improve the focusing property of the medium. The threshold power for self-focusing at least two orders of magnitude is less than that for the identical case of the plasma medium. In addition, a typical external magnetic field for improving the focusing property of the medium is much lower than other conventional nonlinear media like plasma.

Effect of ripple on self-focusing of a cosh-Gaussian laser beam in collisionless plasma

In this paper, the effect of presence of ripple on the cosh-Gaussian beam in collisionless plasma considering relativistic nonlinearity is investigated. Using the Wentzel Kramers Brillouin (WKB) paraxial-ray approximation method and eikonal method, the basic sets of equations has been derived. It is observed that when the initial power of the ripple is greater than the critical power for self-focusing of cosh-Gaussian beam the ripple gets focused over the beam.

Analysis of Raman scattering of self-focused Gaussian laser beam in plasma without WKB approximation

The self-focusing and propagation of intense Gaussian laser beams in plasmas are investigated, and the explicit form of intensity of self-focused laser beams is derived without the use of WKB approximation. Propagation of self-focused laser beams in plasmas is strongly affected by Raman scattering and Brillouin scattering that are expected for hohlraum targets in inertial confinement fusion. The intensity of Raman and Brillouin scattered waves is derived in paraxial approximation where the effect of plasma temperature and Landau damping is considered through the kinetic theory of plasmas. The effect of plasma temperature and its density, as well as laser wavelength and its intensity, on self-focusing and spatial growth of scattered waves is considered.