Effects of the evolution of two cross‐focused Gaussian laser beams on the terahertz generation in thermal collisional plasma

Generation of the terahertz (THz) radiation based on the beating of two cross‐focused high intensity Gaussian laser beams in a warm rippled density plasma is numerically investigated, taking into account the ponderomotive force, Ohmic heating, and collisional nonlinearities. The beat ponderomotive force as a result of cross‐focusing of beams induces a vertical velocity component that by coupling with the rippled density gives rise to a nonlinear current deriving THz radiation. The effect of laser beams spot size evolution and plasma parameters on the THz generation is studied. It was found that there exist special electron temperature and laser intensity ranges with “turning points” where the generation of THz radiation reaches its maximum value and outside of these ranges, it disappears. The results also indicated that increasing the background electron density as well as taking into account the collision frequency help THz generation. Moreover, the maximum yield of THz radiation occurs when the beat wave frequency approaches the plasma frequency.     

Self‐focusing of a cosh‐Gaussian laser beam in magnetized plasma under relativistic‐ponderomotive regime

The combined effect of relativistic and ponderomotive nonlinearities on the self‐focusing of an intense cosh‐Gaussian laser beam (CGLB) in magnetized plasma have been investigated. Higher‐order paraxial‐ray approximation has been used to set up the self‐focusing equations, where higher‐order terms in the expansion of the dielectric function and the eikonal are taken into account. The effects of various lasers and plasma parameters viz. laser intensity (a₀), decentred parameter (b), and magnetic field (ω_c) on the self‐focusing of CGLB have been explored. The results are compared with the Gaussian profile of laser beams and relativistic nonlinearity. Self‐focusing can be enhanced by optimizing and selecting the appropriate laser‐plasma parameters. It is observed that the focusing of CGLB is fast in a nonparaxial region in comparison with that of a Gaussian laser beam and in a paraxial region in magnetized plasma. In addition, strong self‐focusing of CGLB is observed at higher values of a₀, b, and ω_c. Numerical results show that CGLB can produce ultrahigh laser irradiance over distances much greater than the Rayleigh length, which can be used for various applications.     

Excitation of an upper hybrid wave by two intense laser beams and particle acceleration

This Letter presents an investigation of the excitation of an upper hybrid wave (UHW) by cross focusing of two intense laser beams in a collisionless hot magnetoplasma, when relativistic and ponderomotive nonlinearities are operative. The electric vectors of the two beams are polarized along uniform static magnetic field and the beams propagate perpendicular to the static magnetic field. Analytical expressions for the beam width of the laser beams, electric vector and power of the excited UHW and energy gain have been obtained. The UHW generation at the difference frequency and particle acceleration has also been studied. The nonlinear coupling between intense laser beams and UHW is so strong that UHW gets excited and a large fraction of the laser beam energy gets transferred to UHW and this UHW accelerates electrons. It has been shown that the presence of a magnetic field affects significantly the power of the UHW and energy gain by the electron in the presence of the UHW.     

Electron Acceleration by Beating of Two Intense Cross-Focused Hollow Gaussian Laser Beams in Plasma

This paper presents propagation of two cross-focused intense hollow Gaussian laser beams(HGBs) in collisionless plasma and its effect on the generation of electron plasma wave(EPW) and electron acceleration process,when relativistic and ponderomotive nonlinearities are simultaneously operative. Nonlinear differential equations have been set up for beamwidth of laser beams, power of generated EPW, and energy gain by electrons using WKB and paraxial approximations. Numerical simulations have been carried out to investigate the effect of typical laser-plasma parameters on the focusing of laser beams in plasmas and further its effect on power of excited EPW and acceleration of electrons. It is observed that focusing of two laser beams in plasma increases for higher order of hollow Gaussian beams,which significantly enhanced the power of generated EPW and energy gain. The amplitude of EPW and energy gain by electrons is found to enhance with an increase in the intensity of laser beams and plasma density. This study will be useful to plasma beat wave accelerator and in other applications requiring multiple laser beams.     

Generation of electromagnetic waves in the terahertz frequency range by optical rectification of a Gaussian laser pulse in a plasma in presence of an externally applied static electric field

We propose a theoretical model for the generation of electromagnetic waves in the terahertz (THz) frequency range by the optical rectification of a Gaussian laser pulse in a plasma with an applied static electric field transverse to the direction of propagation. A Gaussian laser pulse can exert a transverse component of the quasi‐static ponderomotive force on the electrons at a frequency in the THz range by a suitable choice of the laser pulse width. This nonlinear force is responsible for the density oscillation. The coupling of this oscillation with the drift velocity acquired by electrons due to the applied static electric field leads to the generation of a nonlinear current density. A spatial Gaussian intensity profile of the laser beam enhances the generated THz yield by many folds as compared to a uniform spatial intensity profile.     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

强激光脉冲在部分离化等离子体中的传播特性

采用变分法给出了强激光脉冲在部分离化等离子体中传播的参数演化方程,其中考虑了相对论自聚焦、有质动力激发尾波场以及部分离化非线性极化强度的影响。通过非线性动力学分析的方法,得到了强激光脉冲以恒定焦斑半径传播时的激光、等离子体参数匹配条件。研究表明:在部分离化等离子体中,激光自聚焦随着电离程度的增大而增强;有质动力激发的尾波场进一步增强激光脉冲的自聚焦;相对于等离子体密度而言,激光强度对激光脉冲的自聚焦更有影响。     

Enhanced spatiotemporal laser-beam smoothing in gas-jet plasmas

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.     

Cross-focusing of two laser beams in a plasma

Cross-focusing of two copropagating laser beams in a plasma is investigated using paraxial ray theory. If the lasers have a frequency difference equal to the electron plasma frequency, they can drive a large amplitude plasma wave. The ponderomotive force due to the plasma wave forces the plasma electrons outwards thereby generating a parabolic density profile giving rise to cross-focusing. The results show a decrease in threshold for focusing by two orders of magnitude as compared to focusing due to the ponderomotive force of the laser beams.     

Plasma-field structures during relativistic laser interaction with overdense plasmas at finite electron temperatures

A simple model taking into account the effect of electron temperature is derived to define the plasma-field structures which may arise during relativistically intense laser interaction with overdense plasmas. We show that there exist multilayer solutions with electron cavitation, allowing for both relativistic and ponderomotive nonlinearities. The influence of finite electron temperature on such structures is studied. Examples of these plasma-field structures for the cases of an infinite plasma and a plasma layer are presented.     

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre–Gaussian(LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation,catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian(FG)mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Self-focusing of laser beams in a plasma by ponderomotive forces

Self-focusing of a laser beam in a plasma is treated in terms of the ponderomotive acceleration due to the gradient of the light intensity. The focusing of radiation within the first minima of diffraction sets a lower limit to the laser power which is of the order of 1 MW for the usual lasers if cut-off density and a plasma temperature of about 10 eV are assumed.     

Unconventional photon blockade induced by the self-Kerr and cross-Kerr nonlinearities

We study the use of the self-Kerr and cross-Kerr nonlinearities to realize strong photon blockade in a weakly driven, four-mode optomechanical system. According to the Born−Oppenheimer approximation, we obtain the cavity self-Kerr coupling and the inter-cavity cross-Kerr coupling, adiabatically separated from the mechanical oscillator. Through minimizing the second-order correlation function, we find out the optimal parameter conditions for the unconventional photon blockade. Under the optimal conditions, the strong photon blockade can appear in the strong or weak nonlinearities.     

Relativistic Derivation of the Ponderomotive Force Produced by Two Intense Laser Fields

The ponderomotive force plays a fundamental role in the absorption of laser light on self-consistent plasma density profiles, in multiple-photon ionization, and in intense field electrodynamics. The relativistic corrections to the ponderomotive force of a transversely polarized electromagnetic wave lead to an approximately 20-percent reduction in the single particle ponderomotive force produced by a 10-?m 1016-W/cm2 laser field. Recent experimental investigations are based on using two intense laser fields to produce desired laser-matter interactions. This paper presents the first derivation of the nonlinear relativistic ponderomotive force produced by two intense laser fields. The results demonstrate that relativistic ponderomotive forces are not additive.     

Modulation instability by intense laser beam in magnetized plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through magnetized plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in magnetized plasma by the Lorentz transformation. The Karpman equation is firstly generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in magnetized plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the peak growth rate of self-modulation instability is increased due to the axial magnetization of plasma. It is also shown that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

强激光在窄通道中传播的理论研究

在考虑相对论和有质动力非线性以及全局电量守衡的前提下,分析了强激光在冷等离子体窄通道中稳定传播的情况。采用较为简化的二维理论模型,给出了描述激光和通道横向结构的解,对不同通道宽度、通道密度、激光强度和电磁模式等进行了讨论,分析了其对激光在等离子体通道中传播的影响。分析发现,在存在预通道的情况下,当等离子体通道的密度大于临界密度很多时(例如20倍临界密度),即使是在激光波长量级的通道中,激光仍然可以传播。通道越宽,等离子体密度越小;激光强度越大越容易传播。在同样的通道和传输情况下,TE0模传输所需要的激光强度比TE1模要小。     

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应用电子和多光子集团非弹性碰撞模型和冷等离子体模型，研究了飞秒强激光与线性等离子体发生多光子非线性Compton散射时，散射激光与入射激光形成的飞秒耦合激光场对线性等离子体层中光场和电子密度分布的影响。研究发现，在耦合激光的有质动力作用下，电子密度分布和离子密度分布比Compton散射前的偏离更加严重，电子密度的变化比离子密度的变化更快，产生的静电场更强。即使耦合激光场非常弱，电子的运动仍表现出相对论效应，仍有静电场存在。     

Second Harmonic Generation of Self Focused Cosh‐Gaussian Laser Beam in Collisionless Plasma

This paper presents an investigation of self‐focusing of a Cosh‐Gaussian (ChG) laser beam and its effect on second harmonic generation in collisionless plasma. In the presence of ChG laser beam the carriers get redistributed from high field region to low field region on account of ponderomotive force as a result of which a transverse density gradient is produced in the plasma which in turn generates an electron‐plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and hence generates its second harmonics. Moment theory has been used to derive differential equation governing the evolution of spot size of ChG laser beam propagating through collisionless plasma. The differential equation so obtained has been solved numerically. The effect of decentered parameter, intensity of ChG laser beam and density of plasma on self‐focusing of the laser beam and second harmonic yield has been investigated. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)