Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase(CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional(2D) particle-in-cell(PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.     

脉冲啁啾对于阿秒脉冲的影响

采用单电子近似和软核势模型, 通过数值求解一维含时薛定谔方程, 理论研究了当脉冲分别带有正、负啁啾的情况下所产生的阿秒脉冲.分析了不同脉冲啁啾特性对阿秒脉冲的强度和宽度的影响. 研究结果表明, 无论是正啁啾还是负啁啾, 随着啁啾量的增加, 都将使激光脉冲由产生单个阿秒脉冲趋向于产生阿秒脉冲链. 正啁啾和负啁啾对于阿秒脉冲宽度的影响是不同的, 负啁啾对于阿秒脉冲宽度影响很小, 适当的负啁啾有利于缩小阿秒脉冲的宽度; 而正啁啾脉冲产生的阿秒脉冲较无啁啾时展宽, 且随着啁啾量的增加, 其阿秒脉冲宽度迅速增大.     

Cover Picture: Contrib. Plasma Phys. 10/2019

Contour plots showing the field amplitude for a negatively chirped, un‐chirped, and positively chirped pulse after propagating a certain distance in an underdense plasma. Note that all the subfigures have the same scale for the transverse and longitudinal axes as that shown for the bottom four subfigures. Fig. 1 of the paper by Gang Li et al.     

Laser energy deposition in relativistic interactions with underdense plasmas

It is well established that, at sub-relativistic intensities, the absorption of laser light by underdense plasmas decreases with increasing pulse intensity as interaction enters a non-linear regime. On the other hand, as the relativistic interaction regime is reached, further absorption mechanisms can be activated which can account for a substantial energy transfer. Using the particle code WAKE, we performed numerical simulations of the relativistic interaction of intense laser pulses with underdense plasmas in conditions that can be experimentally tested. Our simulations show that, while the relativistic laser intensity generates a population of fast electrons, a considerable fraction of the pulse energy goes into a population of thermal electrons. These findings open new possibilities for a direct observation of relativistic interaction processes using high resolution soft X-ray techniques.     

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.     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm     

A high-energy, chirped laser system for optical Stark deceleration

We describe a high-energy, frequency chirped laser system designed for optical Stark deceleration of cold molecules. This system produces two, pulse amplified beams of up to 700 mJ with flat-top temporal profiles, whose frequency and intensity can be well controlled for durations from 20 ns–10 μs. The two beams are created by amplifying a single, rapidly tunable Nd:YVO₄ microchip type laser at 1064 nm, which can be frequency chirped by up to 1 GHz over the duration of the pulse. Intensity modulation induced by relaxation oscillations in the microchip laser during the frequency chirp are virtually eliminated by injection locking a free running semiconductor diode laser before pulsed amplification.     

Nonlinear pulse propagation and phase velocity of laser-driven plasma waves

Laser evolution and plasma wave excitation by a relativistically intense short-pulse laser in underdense plasma are investigated in the broad pulse limit, including the effects of pulse steepening, frequency redshifting, and energy depletion. The nonlinear plasma wave phase velocity is shown to be significantly lower than the laser group velocity and further decreases as the pulse propagates owing to laser evolution. This lowers the thresholds for trapping and wave breaking and reduces the energy gain and efficiency of laser-plasma accelerators that use a uniform plasma profile.     

Pulse compression of a high-order harmonic

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser were experimentally shown to be negatively chirped by using an LiF plate as a positive dispersive medium. The chirp of the harmonic pulse originates from the intensity-dependent atomic dipole phase, which is estimated to be proportional to 25 U_p, where U_p is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were consistent with the negative chirp of the fifth harmonic.     

宽带啁啾脉冲激光非线性传输过程中的时空微扰研究

基于非线性薛定谔方程推导出大啁啾脉冲的传输方程,利用微扰理论,分析了啁啾脉冲激光的时空不稳定性. 理论上比较清楚的阐述了宽带脉冲激光在大啁啾情况下,其时空噪声的相互影响与相互作用情况及脉冲啁啾对噪声微扰调制的影响,发现在相同γI₀(非线性系数与峰值强度乘积)的情况下脉冲啁啾对噪声调制的增长没有直接影响. 最后我们在实验上利用非线性介质对啁啾脉冲的空间小尺度自聚焦过程进行了部分验证,同时也在数值上对宽带啁啾脉冲的时间噪声调制的增长进行了模拟分析,发现实验结果和模拟分析结论 关键词： 时空不稳定性 宽带脉冲激光 啁啾     

Observation of Raman forward scattering and electron accelerationin the relativistic regime

Raman forward scattering (RFS) is observed in the interaction of a high intensity (>10¹⁸ W/cm²) short pulse (<1 ps) laser with an underdense plasma (n_e~10¹⁹ cm ^-3). Electrons are trapped and accelerated up to 44 MeV by the high-amplitude plasma wave produced by RFS. The laser spectrum is strongly modulated by the interaction, showing sidebands at the plasma frequency. Furthermore, as the quiver velocity of the electrons in the high electric field of the laser beam becomes relativistic, various effects are observed which can be attributed to the variation of electron mass with laser intensity     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

Sub-picosecond chirped laser pulse-induced airflow and water condensation in a cloud chamber

Sub-picosecond chirped laser pulse-induced airflow and water condensation were investigated in a cloud chamber.The results indicate that the positively chirped sub-picosecond laser pulses generate a more uniform intensity distribution inside the plasma column, leading to a weaker airflow and an elliptic-shaped snow pile. The negatively chirped sub-picosecond laser pulses generate a spark-like intensity distribution inside the plasma column, which produces a wider range of airflow and a round snow pile. The amount of snow weight and the concentration of NO_3~- are found to be dependent on the intensity distribution inside the plasma column. The visibly stronger plasma column generates much more snow and a higher concentration of NO_3~-. These experimental results provide a reference for sub-picosecond laser-induced water condensation in realistic atmospheric conditions.     

THz Radiation Generation via the Interaction of Ultra‐short Laser Pulses with the Molecular Hydrogen Plasma

In this paper, a two dimensional Particle In Cell‐Monte Carlo Collision simulation scheme is used to examine the THz generation via the interaction of high intensity ultra‐short laser pulses with an underdense molecular hydrogen plasma slab. The influences of plasma density, laser pulse duration and its intensity on the induced plasma current density and the subsequent effects on the generated THz signal characteristics are studied. It is observed that the induced current density in the plasma medium and THz spectral intensity are increased at the higher laser pulse intensities, laser pulse durations and plasma densities. Moreover, the generated THz electric field amplitude is reduced at the higher laser pulse durations. A wider frequency range for the generated THz signal is shown at the lower laser pulse durations and higher plasma densities. Additionally, it is found that the induced current density in hydrogen plasma medium is the dominant factor influencing the generation of THz pulse radiation. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO₃ crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO₃ crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.     

高强度激光与等离子体相互作用中的受激Raman级联散射、光子凝聚以及大振幅电磁孤立子的产生与加速

应用一维相对论电磁粒子模拟程序，详细研究了线性极化强激光入射到无碰撞稀疏密度长等离子体中引起的受激Raman散射、Raman级联散射、级联散射到光子凝聚、以及大振幅电磁孤立子的产生与加速. 通过研究发现：在适当的激光振幅和等离子体状态下，强的光子凝聚现象会导致大振幅电磁孤立子的产生，电磁孤立子可以以静止、向后以及向前加速的形式存在；在密度均匀的等离子体中，电磁孤立子的加速不仅依赖于激光振幅而且依赖于等离子体的长度；电磁孤立子的电磁频率大约为未扰动电子等离子体振荡频率的二分之一左右，孤立子内电磁场的电场具有半周期结构，相应电磁场的磁场以及静电场则具有一个完整的周期结构. 关键词： 粒子模拟 受激Raman散射 Raman级联散射 光子凝聚 电磁孤立子     

飞秒啁啾Gauss型脉冲在稠密Λ型三能级原子介质中的传播

利用由预估校正(PC)- 时域有限差分(FDTD)法求得的不含慢变包络近似(SVEA)和旋转波近似(RWA)的全波Maxwell-Bloch方程的数值解, 研究了飞秒啁啾Gauss型激光脉冲(以下简称啁啾脉冲)在稠密Λ型三能级原子介质中的传播.研究表明,啁啾系数(C)的正负及大小的变化对脉冲传播特性有显著的影响,而且这个影响与脉冲面积的大小密切相关.面积小于4π的啁啾脉冲,在介质中传播时不发生分裂,且啁啾脉冲逐渐演化为一个近似的无啁啾(C=0)脉冲,这一特点不随啁啾系数的改变而     

To measure chirped ultrashort laser pulse by the Talbot effect of a grating

We study the Fresnel diffraction of a phase grating under the illumination of a chirped femtosecond laser pulse. Using spatial spectrum theory, first we extract the central wavelength signal from the diffraction wave of the chirped pulse through a diaphragm. Then the central wavelength waves are directed to interfere in remote regions by two reflectors. The interference stripes at different distance will be exactly the same in interference region. Since the intensity distribution of the stripes is related to the chirp parameter of the femtosecond pulse, through measuring the stripe intensity distribution, we can indirectly detect the chirp parameter of the femtosecond pulse.     

Nonlinear Dynamics of Circularly Polarized Laser Pulse Propagating in a Magnetized Plasma with q ‐Nonextensive Velocity Distributions

The nonlinear dynamics of a circularly polarized laser pulse propagating in magnetized plasma contains hot nonextensive q ‐distributed electrons and ions is studied theoretically. A nonlinear equation which describes the dynamics of the slowly varying amplitude electromagnetic wave is obtained using the relativistic two‐fluids model. Some nonlinear phenomena include modulational instability, self‐focusing, soliton formation, and longitudinal and transversal evolutions of laser pulse in nonextensive plasma medium are investigated. Results show that the nonextensivity of particles can substantially change the nonlinearity of medium. The external magnetic field enhances the modulation instability growth rate of right‐hand polarization wave but for the left‐hand polarization the growth rate decreases. The spot size of the laser pulse is strongly affected by the plasma nonextensivity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium

We investigate the propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium. For certain chirp strength and chirp width, an incident 2π nonlinear chirped pulse will split into optical precursors and a stable self-induced transparency soliton. This is caused by the particular Fourier spectrum that includes not only central resonant frequency components but also high-frequency and low-frequency sidebands. Moreover, the effects of chirp parameters on the evolution of nonlinear chirped pulses are also discussed.