Generation of even harmonics in a relativistic laser plasma of atomic clusters

It is shown that the irradiation of atomic clusters by a superintense femtosecond laser pulse gives rise to various harmonics of the laser field. They arise as a result of elastic collisions of free electrons with atomic ions inside the clusters in the presence of the laser filed. The yield of even harmonics whose electromagnetic field is transverse is attributed to the relativism of the motion of electrons and the consideration of their drift velocity associated with the internal ionization of atoms and atomic ions of a cluster. These harmonics are emitted in the same direction as odd harmonics. The conductivities and electromagnetic fields of the harmonics are calculated. The generation efficiency of the harmonics slowly decreases as the harmonic number increases. The generation of even harmonics ceases when the drift velocity of electrons becomes equal to zero and only the oscillation velocity of electrons is nonzero. The results can also be applied to the irradiation of solid-state targets inside a skin layer.     

High-order optical harmonic generation from solid surfaces

During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Plasma radiation spectra in the presence of static electric and high-frequency radiation fields

Harmonics generation of high-frequency radiation in a plasma embedded in a constant electric field is investigated theoretically. It is shown that the electron directed motion due to the static electric field yields the appearance in the plasma emission spectrum of high-frequency radiation even harmonics. The conditions are established when the even harmonics generation is as effective as that of the odd ones. At variance with the odd harmonics, the even harmonics polarization plane is found to rotate with respect to that of the fundamental field. The basic dependencies concerning the rotation angle and the generation efficiency on the plasma and field parameters are established.Received: 3 May 2004, Published online: 14 September 2004PACS: 52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.) - 52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)     

Single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse

A new scheme for single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse is proposed. It is shown that both even and odd harmonics are generated from the asymmetric molecules, and the attosecond pulses are produced every full cycle of the driving laser field rather than each half-cycle. By filtering the harmonics in the cutoff of the spectrum, a single attosecond pulse can be obtained with a multicycle laser pulse with a duration of 2 times longer than the few-cycle pulse conventionally used.     

正交偏振双色激光场作用下高次谐波的椭偏特性研究

通过数值求解二维含时薛定谔方程,理论研究了正交偏振双色激光场作用下H2+分子高次谐波发射和孤立阿秒脉冲的产生. 数值结果显示,改变激光场的强度比,可以调控高次谐波强度以及谐波椭偏率. 结果表明随着场强比的增加,y方向的谐波强度不断提高. 当场强比为1:2.5时,x方向和y方向分别仅有奇次谐波和偶次谐波产生,并且偶次谐波的强度比奇次谐波强度高2-3个数量级. 此时,奇次谐波的椭偏率最大可以达到0.3,偶次谐波的椭偏率接近于0. 此外,通过改变分子的准直角可以控制谐波的椭偏率,基于这一现象,利用椭圆偏振高次谐波的平台区合成了椭偏率为0.05的椭圆偏振阿秒脉冲链.     

Odd harmonic generation of ultra-intense laser pulses reflectedfrom an overdense plasma

A method of generating odd harmonics from an intense laser incident upon a sharp vacuum-overdense plasma interface is presented. One- and two-dimensional simulations are used to investigate the interaction of ultra-intense laser pulses with a sharp vacuum-plasma interface. With an intensity greater than 10¹⁸ W/cm² , these pulses have a pressure greater than 10³ Mbar creating large density oscillations and relativistic electron velocities at the surface. This results in efficient odd harmonic generation. The author present is a physical model for this harmonic generation mechanism, along with some scaling of the power in the third harmonic. This is compared with relativistic particle-in-cell simulations that include mobile and immobile ions, as well as a variety of upper shelf densities. A discussion on how ion profile modifications influence the harmonics is also presented     

基于半解析自洽理论研究相对论激光脉冲驱动下阿秒X射线源的产生

发展了一种描述相对论激光脉冲与稠密等离子体相互作用产生阿秒X射线源的半解析自洽理论.该理论模型不仅可以获得等离子体界面的振荡轨迹、振荡面电场和磁场等物理参数,而且能够精确计算出激光脉冲驱动下阿秒X射线源的频谱,结果与粒子模拟程序一致.理论计算结果表明阿秒X射线源的辐射特性与等离子体界面随时演化过程相关,在周期量级激光场驱动下等离子体界面振荡振幅呈现中心不对称,通过改变激光场的载波包络相位实现对等离子体界面振荡的控制,获得准单阿秒X射线源.     

Generation of non-integer high-order harmonics and significant enhancement of harmonic intensity

High-order harmonics from helium atom in the orthogonally two-color(OTC) laser field are investigated by solving the two-dimensional time-dependent Schrodinger equation.Non-integer high-order harmonics are obtained in some ratio of frequencies of two components.Pure odd and even harmonics from atoms could be separated in two components by adjusting the ratio of frequencies in OTC scheme,and the resolution of harmonics is improved at the same time.The physical mechanism is explained by the periodicity of dipole.With the same intensity of the incident laser,the intensity of the high-order harmonics from the OTC field scheme is improved by three orders of magnitude compared to the monochromatic laser field scheme.A theoretical scheme is provided for experimentally achieving improving energy resolution and separation of pure odd and even harmonics in atoms.Also,we provide a means for improving harmonic intensity.     

Selection of odd or even harmonics by a multi-color laser field with macroscopic phase-matching

We experimentally and theoretically demonstrate that the property(odd or even) of generated harmonics can be selected by manipulating the macroscopic phase-matching conditions based on a three-color laser field. Only odd or even harmonics can be made dominant by changing the focal position and adjusting the gas pressure. These results indicate that the odd-even property of the generated harmonics can be controlled by using the multi-color laser field with macroscopic phase-matching.     

Even-harmonic generation due to beyond-Born-Oppenheimer dynamics

We calculate the harmonic spectrum generated by a model HD molecule in a strong laser pulse. The unequal nuclear masses lead to the emission of even harmonics, i.e., photon frequencies which are even multiples of the laser frequency. The effect does not occur within the Born-Oppenheimer approximation. In the high-frequency region, the even harmonics are almost of the same order of magnitude as the odd ones.     

Fine splits of photon emission spectrum of hydrogen atom caused by transitions between different dressed states in intense high frequency laser field

The photon emission spectrum of the hydrogen atoms in an intense high-frequency laser pulse is simulated by using one-dimensional soft Coulomb potential. Regular fine structures appear on the two sides of both the odd and even multiples of photon energy of the laser field besides the ordinary odd harmonic peaks. It is proved that the splits of the fine structures are responsible for hyper-Raman lines and the energy spacing between the odd harmonic lines is equal to the difference in energy between the eigenstates with the same parity of the time averaged Krameters Henneberger （KH） potential. By analysing the features of the fine structures, we also verify that the so-called even order harmonics under the stabilization condition are indeed hyper-Raman lines caused by the transitions between the dressed atomic states with different values of parity.     

Charged particle motion in the field of a short laser pulse of relativistic intensity

The electron motion in the field of the laser radiation of relativistic intensity was analyzed using the Lorentz force. In the laser pulse field, an initially rest electron does not move along trajectories such as “figure eight”. At relativisitic intensities, the electron oscillations in an optical field are significantly anharmonic.     

激光驻波中运动电子的谐波自发辐射

 采用单电子模型分析了电子在线极化激光驻波中的动力学及谐波自发辐射谱,数值计算了电子在驻波中的运动情况及辐射谱。结果表明:电子在波节和波腹处入射后,其辐射谱出现不同的特征;电子在波节处垂直磁场入射后,在洛伦兹力作用下快速振动并向前运动,其向后辐射的光谱发生红移,向前辐射的光谱发生蓝移,谱线出现展宽;当激光强度或者电子初始能量增大时,这些效应更加突出,以至于产生更高阶谐波,形成连续谱;而电子在波腹处以平行电场的方向入射后,仅在电场作用下作直线运动,其自发辐射谱线没有发生移动和展宽。     

Spontaneous emission of atoms in a strong laser field

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.     

相对论高次谐波的蒙特卡罗模拟

 根据相对论条件下的经典电磁辐射理论,采用蒙特卡罗计算方法对类氢离子在强激光场下辐射的高次谐波进行了模拟计算,获得了相对论领域内激光参数（频率、脉冲波形、强度等）对高次谐波影响的各种计算结果,并对结果进行了理论分析。     

Laser coupling to nuclei via collective electronic oscillations—a simple heuristic model study

The induction field coupling of laser-driven collective electronic oscillations has been proposed as an interlevel transfer mechanism for a gamma-ray laser. Using a heuristic classical model for the electron oscillations, we verify in the model six general features which we conjecture are valid for laser-induced collective oscillations: (1) enhancement of the electric field seen by the nucleus, (2) harmonic generation, (3) generation of higher multipoles, (4) odd (even) order multipoles couple to odd (even) harmonics, (5) there exists an optimum laser intensity for each multipole order and harmonic, and (6) for a given multipole order, the optimum intensity increases with harmonic number. We briefly discuss the relevance of these results to a proposed interlevel transfer experiment in ²³⁵U.     

相对传播的双脉冲激光与薄膜靶作用产生的强单色谐波

研究了相对传播的双脉冲激光与薄膜靶的作用,观察到很强的谐波产生.其物理图像是:圆偏振高对比度强激光脉冲作用于薄膜靶,由光压推动产生的高密度等离子体靶向前运动,同时由于电荷分离场的作用,使得离子束和电子束在纵向上都有好的聚束,从而产生以相对论速度向前运动的等离子体镜;反向入射一个探测光到已被加速的等离子体镜上,由多普勒频移产生强的单色N次谐波,探测光脉冲被"压缩"至原来的1/N.还讨论了激光和等离子体参数对等离子体镜的运动和谐波级次的影响,以及相对论运动等离子体镜的稳定性对谐波的影响.     

High-order-harmonic generation: towards laser-induced phase-matching control and relativistic effects

We present a review of some recent results on high-order-harmonic generation, aiming at optimizing the photon flux to allow for future applications in extreme-ultra-violet non-linear optics. We first present new schemes to control phase matching of high harmonics in gases, by using the effect of the spatially varying atomic phase displayed by the high harmonics. An enhancement by a factor of 50 is observed in neon in conditions for which the gradient of the atomic dispersion balances the electronic dispersion. A new scheme to manipulate the laser field was demonstrated, and shown to improve phase matching. We then turn to high-harmonic generation by solid targets, and show that high harmonics generated by an intense 30-fs laser pulse remain collimated even at the threshold of the relativistic regime. Received: 5 December 2001 / Published online: 24 April 2002     

Channeling of relativistic laser pulses, surface waves, and electron acceleration

The interaction of a high-energy relativistic laser pulse with an underdense plasma is studied by means of 3-dimensional particle in cell simulations and theoretical analysis. For powers above the threshold for channeling, the laser pulse propagates as a single mode in an electron-free channel during a time of the order of 1?picosecond. The steep laser front gives rise to the excitation of a surface wave along the sharp boundaries of the ion channel. The surface wave first traps electrons at the channel wall and preaccelerates them to relativistic energies. These particles then have enough energy to be further accelerated in a second stage through an interplay between the acceleration due to the betatron resonance and the acceleration caused by the longitudinal part of the surface wave electric field. It is necessary to introduce this two-stage process to explain the large number of high-energy electrons observed in the simulations.     

欠稠密等离子体中诱发的偶次相对论谐波

在自生磁场或背景磁场的作用下,等离子体被磁化.磁化后的欠稠密等离子在超强激光的辐射下诱发相对论相干谐波辐射.由于磁场的影响,诱发的相对论相干谐波辐射中产生偶次谐波辐射.从理论上研究了这一现象,导出了谐波辐射的一般性方程,并特别研究了二次谐波辐射的有关参数 关键词： 二次谐波辐射 自生磁场 强激光 欠稠密等离子体