【作者】 汪洋；

【导师】 刘世炳；

【作者基本信息】 北京工业大学 ， 物理学， 2017， 硕士

【摘要】 当高强度飞秒激光脉冲与等离子体相互作用时,会产生一系列的非线性物理现象,例如各种相对论性散射,强的电磁辐射等。通过超强飞秒激光脉冲驱动等离子体获得相干辐射已经成为了当今的研究热点。本文对超强飞秒激光脉冲与固体靶相互作用产生高次谐波辐射进行了模拟研究,主要包括平面靶和光栅靶产生高次谐波辐射及其特征等问题。1、对于超短强激光脉冲与平面靶相互作用,谐波辐射主要沿入射光的反射方向。其机制可以归结为受入射脉冲有质动力驱动作纵向振荡的电子密度分界面对入脉冲的反射,导致反射光周期性的相位调制而包含了高频谐波成分。正入射情况下,谐波谱中只包含奇数阶次的谐波成分而斜入射情况下则包含了每一个阶次的谐波成分。由于激光脉冲强度的横向高斯分布,会导致相互作用区域的固体靶呈现一定的凹陷,这种凹陷会对反射谐波束产生不利的影响,使其发散性增大。让相互作用区域入射脉冲的波前有一定的弯曲,可以弥补固体靶凹陷带来的反射谐波束的发散性增大的问题。2、超短强激光脉冲与光栅靶相互作用作为一种新颖的高次谐波产生方案,光栅周期性结构对谐波场的相干叠加作用,会使得特定阶次的谐波辐射在特定的方向强度增强,正入射情况下,这个现象可以由光栅方程来说明。PIC模拟结果表明,光栅靶能产生沿靶面方向的强烈的谐波辐射,其强度甚至明显超过了相同作用条件下的平面靶的反射方向上的谐波强度。对表面电子的密度演化和相空间行为的分析表明,光栅靶高次谐波的产生源于表面电子在激光脉冲的有质动力驱动下沿垂直于靶面的方向振荡并受到表面结构的空间限制作用而形成高密度电子团,而这些高密度的电子团受激光电场分量的强烈驱动具有较大的沿靶面方向的动量分量,因而产生明显的沿靶面方向的辐射。在光栅周期性结构的作用下,不同光栅单元所产生的辐射场彼此相干叠加,因此形成了强烈的沿靶面方向的特定谐波分布。不同的相互作用条件(激光强度、等离子体密度)以及不同的光栅结构参数都会影响表面电子团的形成及其运动规律,以此来影响谐波产生过程。3、斜入射情况下,光栅靶仍然能够产生强烈的沿靶面方向谐波辐射。但是谐波分布的特征更加复杂:和入射脉冲异侧的靶面方向的谐波强度较高,谐波阶次构成也比较规则。而入射脉冲同侧的靶面方向的谐波强度较弱,而且谐波阶次的构成除了光栅方程的预期的阶次之外还出现了一些“反常”谐波。不同的入射角情况下都存在沿靶面方向的强烈的谐波辐射,且谐波阶次构成和整体的谐波强度依赖于入射角。在我们所研究的情况中,入射角为30~?时沿入射脉冲异侧的靶面方向的谐波强度是所有情况中最高的。然而当入射角过大时(比如60~?),无论是反射方向的谐波辐射还是沿靶面方向的谐波辐射都受到了非常强烈的抑制。对表面电子运动特征的分析表明,谐波辐射分布的不对称性起源于表面电子运动的不对称性:沿入射脉冲异侧的靶面方向,高密度的电子团得以形成,并在入射脉冲的作用下沿这个方向强烈地加速。而沿入射脉冲同侧的靶面方向,没有高密度电子团的形成,而且沿靶面方向运动的电子(团)的弥散特别明显,“相干性”较差,因此沿这个方向的谐波强度明显减弱。更多还原

【Abstract】 A series of nonlinear physical phenomena can emerge when ultraintense fem-tosecond laser pulses irradiate plasma targets,such as a number of relativistic scattering,intense electromagnetic radiations.Generation of coherent radiation by the interaction of femtosecond laser pulses with plasma medium remains an important topic all the time.In this paper,high-order harmonic generation(H-HG)by laser solid(plasma)target interaction is investigated by means of particle simulation method.Flat target scheme and grating target scheme are mainly in-volved.1.In the case where ultrashort intense laser pulse irradiates flat target surface,harmonic beam mainly emits along the reflection direction of target surface.It can be contributed to the reflection of incident light by the oscillating electron density interface driven by the ponderomotive force of incident pulse.This gives rise to the periodical phase modulation in reflected beam,producing high harmon-ic components of laser fundamental frequency.Under normal incidence,reflected harmonic spectrum just contains odd harmonic components while every odd and even harmonics can be included under oblique incidence.Due to the Gaussian distribution of transverse intensity distribution which leads to a denting target surface in the interaction area,the divergence of reflected harmonic beam may be increased.By irradiating the target surface with curved wave-front incident pulse can suppress the harmonic beam divergence caused by the denting surface.2.As a novel high harmonic generation scheme,grating targets irradiated by ul-trashort intense laser pulse can produce more intense harmonic radiation.Due to the effect of coherent superposition,harmonic intensities can be obviously en-hanced along specific directions.Under normal incidence,this phenomenon can be illustrated by the grating equation.PIC simulation results show that,intense har-monic emissions along target surface are produced,their intensities even obviously surpass the reflected harmonics from flat targets under the same interaction con-ditions.Analysis of surface electron density evolution and phase space behaviors tell us that intense near-surface harmonic emissions from grating target is linked to the coherent radiations by high density surface electron bunches formed near the grating protuberances and oscillate driven by the laser electric field.Under the effect of periodical grating structures,emissions from each bunches interfere with each other,giving rise to the specific intense near-surface harmonic distribu-tion.Different conditions(laser intensity,plasma density)and grating structure parameters can influence the formation and dynamics of surface electron bunches,which affect the harmonic generation process.3.Under oblique incidence,intense near-surface harmonics can still be produced,but the harmonic characteristics become more complex:harmonic components at different side of incident pulse are regular and their intensities are relatively high.While the harmonic emissions at the same side of incident pulse are weaker.Some“anomalous”harmonic components violating the expectations of grating equation also appeared.Near-surface harmonic components and overall intensities have strong dependence on the incident angle.In the cases investigated,near-surface harmonic intensities under 30~?incidence is the most intense.While when the incident angle becomes too large(60~?,for example),harmonic emissions along re-flection direction and near-surface directions are all obviously suppressed.Analysis of surface electron dynamics show that the asymmetric surface electron behaviors give rise to the asymmetric harmonic distribution features:along the near-surface direction at different side of incident light,highly dense electron bunches can be formed and accelerated along this direction.While along the other near-surface direction,surface electron(bunches)are severely diffused,emitting relatively weak harmonics along this direction.更多还原