超短脉冲强激光与固体靶作用产生的高次谐波红移

用一维粒子模拟程序研究了超短脉冲强激光与超临界密度等离子体平板作用产生的高次谐波。分析了振荡镜面模型所不能解释的伴随高次谐波出现的频率红移现象。通常激光与固体靶作用中的高次谐波是由于激光从振荡靶面的反射产生的.除此之外，相对论光强的激光与固体靶作用还应考虑光压对靶的烧蚀推进作用，这时激光相当于从一个移动的振荡靶面反射，所以产生了带有红移的高次谐波. 关键词： 红移 超短脉冲强激光 高次谐波 粒子模拟     

利用OMA谱仪测量飞秒激光的谐波光谱

本文给出了利用光学多道分析(OMA)谱仪测量飞秒激光谐波光谱的一种方法 .该方法是利用OMA谱仪(谱分辨0.1nm)加CCD(1152×1242)相机探测设备,用消色差的相机镜头作为空间分辨,在固体靶前表面测量了激光的二次谐波(2ω_0)光谱.结果显示:在平行于靶面的方向和接近于法线方向分别观测到了2ω_0光谱,但在接近于法线方向的谐波光谱出现了精细结构,并得到2ω_0谐波谱的分裂间隔约为3.183 nm.分析认为,自生磁场的产生和作用是导致二次谐波光谱精细结构及谐波谱分裂的主要原因.     

由激光等离子体细丝引起的二次谐波侧向空间分辨谱

本文从实验上,用窄频带和宽频带激光,在无预脉冲和有预脉冲两种情况下,辐照平面Al靶.在垂直入射激光方向观察到高光谱分辨(～0.2?)和空间分辨(～2μm)的二次谐波空间分辨谱.通过分析各种现象,基本上证明了在垂直入射激光方向的二次谐波辐射是由激光与等离子体细丝相互作用所引起的,而不是由平面波与等离子体相互作用引起的,理论和实验基本一致.     

相对论圆偏振激光与固体靶作用产生高次谐波

超短超强激光与固体靶表面等离子体相互作用可以通过高次谐波的方式产生从极紫外到软X射线波段的相干辐射,获得飞秒甚至阿秒量级的超短脉冲,可用于观测原子或分子中的电子运动等超快动力学过程.本文实验研究了相对论圆偏振飞秒激光与固体靶相互作用的高次谐波产生过程,实验结果表明,在较大入射角下,圆偏振激光也可以有效地产生高次谐波辐射.通过预脉冲控制靶表面的预等离子体密度标长,发现高次谐波的产生效率随密度标长的增加而单调下降.进一步通过二维粒子模拟程序,分析了激光的偏振以及预等离子体密度标长对高次谐波产生的影响,很好地解释了实验观测结果.     

激光与固体靶作用产生高次谐波的振荡镜面模型

利用振荡镜面模型解析地研究了激光与固体靶相互作用产生高次谐波的物理过程.比较了利用镜面模型的结果与粒子模拟及实验结果的异同，说明了振荡镜面模型的局限性. 关键词： 高次谐波 振荡镜面模型 激光     

1.053μm激光打靶产生的二次谐波

测量了基频激光打靶条件下不同角度的二次谐波光谱,得到了谐波光强随发射角度的变化关系以及能量转换效率随激光强度变化的曲线.实验现象可能与参量衰变级联造成的Langmuir湍流有关 关键词： 二次谐波发射 激光等离子体 共振吸收 离子声衰变不稳定性     

激光与二维多势阱系统相互作用下的谐波发射

采用二维渐近边界条件和辛算法数值求解激光与多势阱系统相互作用的二维含时Schr(o|¨)dinge方程的无穷空间初值问题.计算二维多势阱系统分别在线偏振激光和圆偏振激光作用下的谐波发射,得到不同模型发射谐波谱的特点,说明多势阱环境有利于高次谐波的发射.     

5fs驱动激光脉冲的高次谐波选择性优化

研究了在紧聚焦实验条件下, 采用5 fs激光脉冲与氩气相互作用产生的高次谐波特性. 通过优化系统色散、气体靶气压与位置等参数, 观察到在60–73 eV波段范围内高次谐波光谱接近一个量级的增强. 进一步通过对单原子模型和传播方程的数值求解及模拟相位匹配实验参数下高次谐波的产生过程, 发现相位匹配在所观察到的实验现象中起着关键作用, 得到了理论上与实验规律一致的结果. 关键词： 极紫外激光 高次谐波 超快光谱     

短脉冲强激光与固体靶作用在非相对论激光参数区的高次谐波

利用"表面电流"模型, 探讨了激光参数在非相对论区条件下激光与固体靶作用产生的高次谐波的若干性质, 也讨论了在非相对论区条件下劳仑兹力项的修正对高次谐波效率的影响.     

激光辐照平面靶产生的二次谐波结构

利用单路钕玻璃线偏振激光辐照平面铝靶及铝箔靶,可在实验中观测到二次谐波的频移、加宽、光谱结构及空间分裂等一系列特征,谐波谱及其结构是反映冕区激光与等离子体相互作用的一个重要方面,频移与加宽反映了等离子体的温度及向外膨胀运动,这在文献[1,2]中已有报导。谱线轮廓上小的尖峰结构与垂直方向上的空间分裂,反映了谐波通过非线性介质由于不稳定所引起的时间调制与细丝自聚焦,这可从下面的讨论中看出。我们将从实验布置、实验结果、结果分析等几个方面逐一叙述。     

Whispering gallery mode emission from microtube cavity

Optical properties were studied of a novel microtube cavity of ∼8 μm diameter prepared by vacuum-assisted filtration of aluminosilicate xerogel using micro-channel glass matrix followed by thermal treatment. Periodic very narrow peaks of the emission spectra corresponding to orthogonally polarized whispering gallery modes were detected. The spectral position of different modes were analyzed using Lorenz-Mie theory. The mode assignment permits calculation of the spectral dependence of cavity Q values associated with observed peaks. Intensity-dependent, time-resolved stimulated emission experiments were performed on the single microtube cavity. The results show a strong decrease in emission lifetime with increasing excitation intensity, consistent with a mechanism of amplified stimulated emission. The text was submitted by the authors in English.     

微波与非均匀等离子体相互作用过程中坑子的形成与谐波发射

在稳态的等离子体装置中，利用微波和等离子体的相互作用，模拟了激光等离子体过程，研究了这个非线性相互作用过程中产生的坑子(caviton)，观察了坑子幅度增长，衰减及传播的演化过程。同时利用数值计算求解了在实验条件下的Zakharov方程，得到与实验较为符合的结果。实验测量了在相互作用过程中的二次谐波发射谱，并用参量衰变过程分析了二次谐波的频率红移。     

超强频率梳激光场驱动下多光子共振谐波辐射的相位突变研究

本文从理论上研究了在双色频率梳激光场驱动下多光子谐波辐射光谱中的相位突变现象。我们利用Floquet理论非微扰地模拟了频率梳激光场与原子分子等量子系统的相互作用过程。谐波辐射信号是多光子偶极跃迁相干叠加的结果,通过调节频率梳激光场间的相对相位,可以相干地控制谐波辐射信号的强度。通过对谐波信号进行傅里叶变换,可以提取不同跃迁路径的相对相位信息。我们通过改变频率梳组激光场的强度和频率组分实现多光子跃迁频率,让其跨越共振跃迁频率时,谐波相位会发生突变。从而可以观测超强激光场驱动下量子系统共振跃迁频率的斯塔克能移。     

空气中激光等离子体通道的三次谐波光谱特性研究

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12 mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道, 并在前 向观测到谱线半峰全宽（FWHM）为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时 ,通过改变可编程声光色散滤波器（AOPDF）光谱调制的位置（Hole position）,三次谐波的光谱也发生频移。     

Toward single attosecond pulses using harmonic emission from solid-density plasmas

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.     

DISAPPEARANCE OF TWO-PLASMON DECAY INSTABILITY IN PLASMAS PRODUCED BY ULTRASHORT LASER PULSES

Harmonic emission was studied from a plasma produced by ultrashort laser pulses. Unlike the harmonics from plasmas created by long (ns) laser pulses, the 3/2 harmonic emission was not observed in the interaction between plasmas and ultrashort laser pulses. A simple model is proposed to explain this phenomenon.     

Electro-optic shocks from ultraintense laser-plasma interactions

Second harmonic radiation in the form of an electro-optic shock is produced in the blowout regime of a laser wakefield in a plasma. The shock is produced by the interaction between the laser field and the electron sheath surrounding the electron cavitation region. Because the sheath is thin, phase matching is unimportant, and the radiated energy grows secularly with the interaction length. The angle of emission is given by the Cherenkov angle associated with the ratio of the second harmonic phase velocity to the fundamental phase velocity. The shock formation is investigated in three dimensions via analysis and particle-in-cell simulations.     

Dynamics of the critical surface in high-intensity laser-solid interactions: modulation of the XUV harmonic spectra

The generation of harmonics from the interaction of an intense (I>or=10(18) W cm(-2)) laser with a solid surface is investigated. Modulation of the harmonic emission spectrum with a periodicity of 2 to 4 harmonics is observed at higher laser intensities. A similar modulation is predicted by a particle-in-cell simulation. The modulation is shown to be caused by the higher modes of oscillation of the critical surface during the interaction. As a result, the dynamics of the critical surface can be inferred from the shape of the harmonic spectrum.     

Influence of intraband motion on the interband excitation and high harmonic generation

Tunnelling, acceleration, and collision of electrons are the basic events in the process of high harmonic generation(HHG) in strong-field interaction with atoms.However, the periodic array of atoms in semiconductor structure makes three steps become interatomic coherent process which leads to complicated carrier dynamics and two sources of high harmonic emission: interband polarization and intraband current.The difference of features of high harmonic generation between semiconductors and atoms is strongly linked to the unique presence of intraband motion which manifests itself a nontrivial role in intertwined two dynamics.Here, we review recent experimental and theoretical advances of understanding coupled interband and intraband mechanisms of HHG in semiconductors.Particularly we focus on the influence of intraband motion on the interband excitation, and on the subsequent HHG emission and attosecond pulse generation.     

Angularly separated harmonic generation from intense laser interaction with blazed diffraction gratings

We made numerical simulations of the generation of narrowband beams of extreme ultraviolet radiation from intense laser interaction with a blazed grating surface. Strong fifth harmonic emission into its blazed diffraction order was observed as well as heavy suppression of the fundamental frequency with comparison to a typical harmonic spectrum from a flat target. The results demonstrate a new highly efficient method of generating near-monochromatic harmonics from the fundamental with minimal effect on the pulse duration.