Harmonic generation at air–soil interface by femtosecond laser filament

We observe the third-harmonic generation and second-harmonic generation together with element fluorescence from the interaction of a femtosecond laser filament with a rough surface sample(sandy soil) in non-phasematched directions. The harmonics prove to originate from the phase-matched surface harmonics and air filament, then scatter in non-phase-matched directions due to the rough surface. These harmonics occurr when the sample is in the region before and after the laser filament, where the laser intensity is not high enough to excite the element fluorescence. The observed harmonics are related to the element spectroscopy, which will benefit the understanding of the interaction of the laser filament with a solid and be helpful for the application on filament induced breakdown spectroscopy.     

激光驱动晶体发射低阶谐波强度随激光波长的变化规律

通过数值求解激光驱动下电子在一维周期势场中运动的薛定谔方程,研究了晶体在激光场中发射的低阶谐波强度随激光波长的变化规律,结果表明,晶体发射低阶谐波强度随激光波长的变化规律与晶体发射高次谐波第一平台区域的变化规律不同.已有的研究表明晶体发射高次谐波第一平台区域的强度会随激光波长的增加而衰减,而我们发现晶体发射低阶谐波的强度会随激光波长的增加而增加.通过对晶体发射低阶谐波的时频分析、晶体价带能量变化与激光光子能量的关系,解释了晶体发射低阶谐波强度随激光波长增加而增加的原因.     

Phase characteristics of high-order harmonics in the cut-off region

The behavior of the phases of high-order harmonics generated upon interaction of intense laser radiation with matter is investigated. Some specific features typical of the harmonic phases in the high-frequency part of plateau (cut-off harmonic phases) are found. First, the phase difference between neighboring harmonics is a constant value. The width of the spectral range in which this regularity occurs determines the minimum duration of the attosecond pulse obtained from these harmonics by the so-called amplitude gating technique. Second, it is shown that the phase of each harmonic in the cut-off region depends linearly on the laser intensity. The proportionality coefficient is the same for all harmonics in this region and proportional to the cube of laser wavelength. Thus, this dependence is especially important for generating high-order harmonics by lasers with a wavelength of few micrometers.     

Influence of an ambipolar field and target boundary blurring on transition radiation from fast electrons of laser plasma

It is shown that the blurring of the rear side of a thin laser target leads to a decrease in the intensity of higher harmonics in the spectrum of coherent transition radiation and that the scale of the boundary inhomogeneity can be estimated from the amplitude ratio of harmonics. Deceleration of the electron flow in an ambipolar electric field at the rear boundary of a target leads to a decrease in the intensity of lower harmonics in the spectrum of coherent transition radiation, and the strength of the ambipolar field can be estimated from the amplitude ratio of harmonics. A change in the permittivity of a dielectric laser target with frequency can lead to an increase in the intensity of some harmonics in the spectrum due to the Vavilov-Cherenkov mechanism.     

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.     

Laser ablation of polymers using 395 nm and 790 nm femtosecond lasers

We compared a Ti:sapphire fs laser (790 nm) with a second harmonics (395 nm) fs laser, and then mixed them for ablating polyethylene (PE). Compared to the 790 nm fs laser, the 395 nm fs laser harmonics could etch PE faster. However, isolated carbon was formed on the ablated surface, in addition to C=O and C=C-H bonds. When we mixed a faint beam of the 395 nm fs laser harmonics with the 790 nm fs laser, the etching depth became even deeper. Moreover, the chemical composition of the ablated surface remained unchanged. At a total laser fluence of 80 mJ/cm², the most suitable laser fluences for the 395 nm fs laser harmonics and the 790 nm fs laser were found to be approximately 2 and 78 mJ/cm² respectively. PACS 81.65.Cf     

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.     

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.     

Control of the intensity of a single harmonic generated in laser plasma by varying the chirp of intense femtosecond pulses

A significant increase, as well as a decrease, in the intensity of single higher harmonics of femtosecond radiation of a Ti:sapphire laser that are generated in a weakly excited laser plasma on the surface of GaAs, Cr, InSb, In, and stainless steel is demonstrated. The intensities of some higher harmonics in different regions of their plateaulike distribution varied from a 23-fold excess over the intensity of the neighboring harmonics to the complete absence in the extreme ultraviolet spectrum. The specific features of a 200-fold increase in the intensity of the 13th harmonic generated in indium plasma are analyzed. These variations in the intensity distribution of harmonics in the plateau region are obtained by varying the chirp of laser pulses.     

利用43飞秒的强激光脉冲实现单个阿秒脉冲输出的新机理

研究了多个光周期的长脉冲(43 fs)激光与氖原子相互作用产生的高次谐波的辐射特性. 通过计算机数值模拟，发现当激光场的强度超过饱和光强时，原子将会在几个光周期内被迅速电离，使得原子的高次谐波谱在截止区呈现出平滑的多平台结构. 同时，在高频波段离子产生的高次谐波的影响将变得十分显著. 通过选取合适的子平台进行频率叠加，在消除了离子的高次谐波的影响后，可以获得单个的阿秒软X射线脉冲. 关键词： 高次谐波 阿秒脉冲     

不同取向角下CO2分子波长依赖的垂直谐波效率

通过数值计算, 研究了强激光场中CO₂ 分子在不同波长和取向角下产生的高次谐波辐射的效率. 发现CO₂ 分子的垂直谐波效率在较小的和中间的取向角时倾向于与平行谐波效率可比或更高, 而在较大的取向角时, 垂直谐波效率远低于平行谐波效率. 进一步的分析表明, CO₂ 分子的结构对其垂直谐波效率有重要的影响, 且该影响与波长有关. 建议对于较复杂的分子, 应该在分子的轨道成像实验中考虑垂直谐波的贡献.     

激光场中锂原子发射的电离阈值以下低阶谐波的研究

利用单电子近似下锂原子精确的模型势,通过数值求解三维含时薛定谔方程的方法,研究了锂原子在激光场中发射低阶谐波的性质,研究结果表明,对于锂原子电离阈值以下的低阶谐波,除了通常的奇次谐波外,还有侧峰结构出现,运用同步压缩变换技术(SST)分析这些低阶谐波的辐射特性,我们发现这些侧峰是由原子在激光场中的缀饰态之间的跃迁和无场下激发态到基态的跃迁组成的,如果当激光脉冲结束后,仍有发射的侧峰,其位置与主峰之间的能量差可以用来估算原子激发态在激光场中的最大Stark移动.     

Optical high harmonic generation in C60

High harmonic generation (HHG) requires a strong laser field, but in C60 a relatively weak laser field is sufficient. Numerical results presented here show that, while its low order harmonics result from the laser field, its high order ones are mainly from the multiple excitations. Since high order harmonics directly correlate with electronic transitions, the HHG spectrum accurately measures transition energies. Therefore, C60 is not only a promising material for HHG, but may also present an opportunity to develop HHG into an electronic structure probing tool.     

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 harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.     

Interference and spectral broadening of higher harmonics of laser radiation in plasma torches containing ions,nanoparticles, and fullerenes

We analyze the interference between two processes of higher harmonic generation (HHG) in plasma containing mixtures of different materials (silver and gold nanoparticles, as well as graphite and boron). We find that, for mixtures and individual ingredients, the limiting orders of generated harmonics of laser radiation approximately coincide with one another. At the same time, for plasma torches formed by the ablation of mixtures of materials, the HHG efficiency is considerably reduced compared to the case of the frequency transformation of laser radiation in individual ingredients of these mixtures as a result of destructive interference in the former case. We demonstrate a considerable spectral broadening of harmonics generated in laser plasma with pulses passed through filaments formed in air. In this case, the HHG efficiency increases fourfold (from 3 × 10⁻⁶ to 1.2 × 10⁻⁵) compared to the case of radiation free of phase and frequency modulation. The generation of harmonics is also observed upon the passage of 120-fs laser pulses through plasma containing fullerenes. In this case, the limiting value of generated harmonics achieves the 33rd order. The efficiency of harmonics in fullerene plasma considerably exceeds a similar process in silver plasma.     

Generation of harmonics during the interaction of ultrashort superstrong laser pulses with solid targets

Generation of even and odd harmonics in the skin layer formed during the interaction of a short relativistic laser pulse with solid targets is considered. The complex motion of free electrons in the skin layer along the electric field vector and along the direction of propagation of a laser wave is analyzed. The Fourier expansion of the trajectory of this motion is used to obtain the components of the conductivity tensor and of the amplitude of the transverse electromagnetic field of harmonics propagating along the electric field. Even harmonics appear due to relativistic effects. The efficiency of generation of even and odd harmonics at the leading front of a laser pulse is calculated.     

Generation of higher harmonics in laser plasma formed on the surface of a silver target

The results of investigation of the higher harmonics (from the 21st to the 61st order) of radiation of a femtosecond laser (793 nm, 48 fs) that are generated in silver plasma are presented. The changes in the shape and spectra of harmonics for different chirps of the laser radiation are studied. It is found that the wavelengths of harmonics generated in the region of the plateaulike distribution of the converted radiation intensity can be tuned. It is shown that the conditions of phase matching between harmonics and laser radiation are different for different modulations of the femtosecond pulse spectrum. The tuning range of the spectrum of the converted radiation in the region of lower harmonics, which lie in the beginning region of the plateau, is shown to amount to 0.8 nm.     

利用啁啾激光调制分子谐波信号

数值研究了啁啾激光驱动H_2~+辐射谐波的特点.结果表明,(1)在低强度和高强度激光驱动下,谐波光谱分别呈现红移和蓝移现象.引入正负向啁啾参数后,谐波频移和谐波辐射强度都可以得到调控.(2)谐波光谱在高强度激光驱动下呈现非奇次谐波.理论分析表明,非奇次谐波产生的原因是双H核频移不一致所导致的不对称相消干涉引起的.     

Even-order high-harmonic generation from solids in velocity gauge

We present a velocity-gauge model for the generation of even-order high harmonics, and reveal that the even-order harmonics originate from the multiple-step transitions among the energy bands in momentum space, while the odd-order harmonics are mainly from direct transitions. The lower valence band is found vital for the generation of even harmonics. Relative intensity of even-order harmonics versus the odd orders is calculated and shows a growing trend as the laser field amplitude increases.