椭圆偏振强激光场中准直的乙炔分子的高次谐波发射

We perform an experimental study on high-order harmonic generation (HHG) of aligned acetylene molecules induced by a 35-fs 800-nm strong laser field, by using a home-built HHG spectrometer. It is observed that the molecular HHG probability declines with increasing the laser ellipticity, which is in consistence with the deduction from the well-known tunneling-plus-rescattering scenario. By introducing a weak femtosecond laser pulse to nonadiabatically align the molecules, we investigated the molecular orbital effect on the HHG in both linearly and elliptically polarized driving laser fields. The results show that the harmonic intensity is maximum for the molecular axis aligned perpendicularly to the laser electric field. It indicates that both the highest occupied molecular orbitals (HOMO) and HOMO-1 contribute to the strong-field HHG of acetylene molecules. Our study should pave the way for understanding the interaction of molecules with ultrafast strong laser fields.     

Minimum structure of high-harmonic spectrafrom aligned O2 and N2 molecules

We experimentally investigated the high-order harmonic generation (HHG) from aligned O₂ and N₂ molecules in a linearly polarized laser field, and presented the dependence of the harmonic spectrum on the driving laser intensity and molecular alignment angle. The minimum position of HHG of O₂ varies with changing the laser intensity, which is caused by multi-orbital interference. However, the location of the observed minimum structure in N₂ harmonic spectrum remained unchanged upon changing the laser intensity. The mechanism of the spectral minimum for N₂ case is regarded as a Cooper-like minimum in HHG associated with the molecular electronic structure. This work indicates that harmonic spectroscopy can effectively uncover information about molecular structure and electron dynamics.     

Ionic Angular Distributions Induced by Strong-Field Ionization of Tri-Atomic Molecules

Angular distributions of fragment ions from ionization of several tri-atomic molecules(CO_2, OCS, N_2O and NO_2) by strong 800-nm laser fields are investigated via a time-of-flight mass spectrometer. Anisotropic angular distributions of fragment ions, especially those of atomic ions, are observed for all of the molecules studied.These anisotropic angular distributions are mainly due to the geometric alignment of molecules in the strong field ionization. Distinct different patterns in ionic angular distributions for different molecules are observed. It is indicated that both molecular geometric structure and ionization channels have effects on the angular distributions of strong field ionization/fragmentation.     

Controlling high harmonic generation with molecular wave packets

We show that, by controlling the alignment of molecules, we can influence the high harmonic generation process. We observed strong intensity modulation and spectral shaping of high harmonics produced with a rotational wave packet in a low-density gas of N2 or O2. In N2, where the highest occupied molecular orbital (HOMO) has sigma(g) symmetry, the maximum signal occurs when the molecules are aligned along the laser polarization while the minimum occurs when it is perpendicular. In O2, where the HOMO has pi(g) symmetry, the harmonics are enhanced when the molecules are aligned around 45 degrees to the laser polarization. The symmetry of the molecular orbital can be read by harmonics. Molecular wave packets offer a means of shaping attosecond pulses.     

脉冲激光偏振方向对氧分子高次谐波的影响——基于含时密度泛函理论的模拟

采用含时密度泛函方法,结合赝势模型和电子交换相关作用的广义梯度近似,模拟了氧分子在超强飞秒激光脉冲作用下的高次谐波产生现象,并研究了激光脉冲偏振方向对氧分子高次谐波的影响.结果表明氧分子的高次谐波谱具有典型原子谐波谱的特征;当θ=45°(激光偏振方向与分子轴向夹角)时,谐波谱强度最大.这与文献报道的实验结果基本一致.     

Ellipticity dependence of high-order harmonic generation from aligned molecules

We report ellipticity dependence of high-order harmonic generation (HHG) from aligned N2, O2, and CO2 molecules. Experimentally, we find that the ellipticity dependence is sensitive to molecular alignment and to the shape and symmetry of the valence orbitals. It is also found that the destructive interference in the recombination process affects the ellipticity dependence. Theoretically, we extend the original Lewenstein model to a more generalized model, which can be applicable to HHG from molecules, by introducing an electron acceleration parameter xi(theta) and by combining the molecular orbital method. The present observations are successfully explained by our model.     

High-order harmonic generation from organic molecules in ultra-short pulses

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×10¹³-5×10¹⁵ W cm^-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful tool for understanding the electron dynamics of molecules exposed to an intense field. Received 14 September 2000 and Received in final form 6 December 2000     

理论研究激光强度对H2+分子高次谐波产生的影响

本文通过求解非波恩-奥本海默近似条件下的一维含时薛定谔方程来研究脉冲强度对H₂⁺分子高次谐波的影响.由于干涉相消,在啁啾激光场的条件下在谐波谱中出现了干涉最小值.我们用全量子方法计算的时频分布图分析了分子高次谐波产生中的微观机制.此外,数值计算结果显示随着脉冲强度的增加电子的有质动力势随之增加,同时干涉最小值的位置移向谐波的高阶部分.     

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.     

Origin of anomalous spectra of dynamic alignments observed in N2 and O2

Recent pump-probe experiments with intense femtosecond laser pulses and diatomic molecules N2 and O2, have revealed the presence of Raman-forbidden anomalous series and lines in the Fourier spectrum of HHG (high harmonic generation) signals. A theoretical analysis of the problem is made by deriving a general expression of the angle dependent HHG operator that governs the dynamic alignment signals in linear molecules, and applying them to the experiments in N2 and O2. A unified interpretation of the origin of the observed Raman-allowed and the anomalous spectral features is given. The results are also used to estimate the molecular temperature in the experiments.     

Measurement of the polarization of high-order harmonics from aligned N(2) molecules by spatial interferometry

The polarization of high-harmonics from aligned N(2) molecules was measured by observing the visibility of spatial interference between two high-harmonics generated separately. The minimum visibility was observed at an angle of 60 degrees between the polarization of the harmonic generation laser field and the molecular orientation. In this case, the angular shift of harmonic polarization is 15 degrees from the molecular orientation. Our measurement of the visibility variation matches the theoretical prediction based on the harmonic field calculation for aligned N(2) molecules.     

Strong field ionization to multiple electronic states in water

High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H(2)O and D(2)O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by solving the time-dependent Schr?dinger equation to simulate strong field ionization and high harmonic generation from the water isotopes. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.     

New results in above-threshold ionization and high-order harmonic generation of atomic and molecular systems

Numerical results obtained applying the strong-field approximation to atomic and molecular processes in intense laser fields are presented. In particular, the forward-backward asymmetry in high-order above-threshold ionization (HATI) of Ar atoms by a few-cycle laser pulse is considered. It is shown that the resonantlike enhancements observed in the focal-averaged spectra disappear with the decreasing laser pulse duration. For HATI and high-order harmonic generation (HHG) by molecular systems, we present new results for diatomic molecules. The choice of gauge and the problem of dressing of the initial bound state are discussed. The appearance of the interference minima in molecular HATI and HHG spectra and their role in the characterization of the molecular structure are considered using the example of the Ar₂ molecule.     

Controlling continuum wavepacket interference by two-color laser field in over-the-barrier ionization regime

Continuum wavepacket interference is investigated by numerically solving the time-dependent Schröodinger equation for the interaction of hydrogen atoms with laser fields. The obtained wavepacket evolution indicates that, in the over-the-barrier ionization regime (10¹⁶ W/cm²), the continuum–continuum (CC) interference of ionizing electrons becomes the main process in highorder harmonics generation (HHG), compared with continuum-bound (CB) transition, as reported by Kohler et al. [Phys. Rev. Lett. 105(20), 203902 (2010)].We propose a two-color laser field scheme for controlling the quantum trajectories of ionizing electrons and for extending the CC harmonic energy. As a result, a high energy platform occurs in the HHG spectrum, which entirely originates from the CC harmonics, with a cutoff adjustable by the relative phase of the two-color fields. This provides further understanding of the dynamic feature of atoms and molecules in super intense laser fields and provides an opportunity to image the atomic or molecular potential.     

非线性拉曼激光雷达测量CO2气体的研究

提出了利用气体的受激拉曼散射(SRS)效应激光雷达光源来探测大气中的CO2气体的新方法,设计出探测大气中CO2气体含量的非线性拉曼增益激光雷达,用Nd:YAG激光器(1064 nm)的三倍频光(354.7 nm)通过分别装有CO2气体和N2气体的拉曼管,分别得到CO2气体和N2气体的受激拉曼散射的一阶斯托克斯线(S1),并用S1线作为雷达的种子发射光源.通过实验得到拉曼管中的气压与S1能量的变化关系,对其优化条件和物理机制进行了分析.该实验方法已经成功测出了大气中CO2气体的回波电压信号.     

Probing subcycle spectral structures and dynamics of high-order harmonic generation in crystals

Subcycle spectral structures and dynamics of high-order harmonic generation (HHG) processes of atoms and molecules driven by intense laser fields on the attosecond time scale have been originally studied theoretically and experimentally. However, the time scale of HHG dynamics in crystals is in the order of sub-femtosecond, and the carrier dynamics of HHG in crystals driven by subcycle laser pulses are largely unexplored. Here we perform a theoretical study of subcycle structures, spectra, and dynamics of HHG of crystals in mid-infrared laser fields subject to excitation by a subcycle laser pulse with a time delay. The HHG spectra as a function of time delay between two laser fields are calculated by using a single-band model for the intra-band carrier dynamics in crystal momentum space and by solving the time-dependent Schrödinger equation in velocity gauge for the treatment of multi-band crystal systems. The results exhibit a complex time-delay-dependent oscillatory pattern, and the enhancement and suppression of the HHG related to subcycle pulse are observed at the given time delay in either single-band or multi-band crystal systems. To understand oscillation structures with respect to the dependence for the subcycle laser fields, the time-frequency characteristics of the HHG as well as the probability density distribution of the radiation are analyzed in detail.     

Kinetic energy release of diatomic and linear triatomic molecules in intense femtosecond laser fields

The kinetic energy release of fragment ions produced by the interaction of femtosecond laser pulse radiation with diatomic and linear triatomic molecules N_2, CO, CO_2 and CS_2 is investigated. In the case of linear polarization, angles at which the kinetic energy release of ions has the maximum value are different from the alignment of molecules though the kinetic energy release of fragment atomic ions depends on the angle between the laser polarization vector and the detection axis of the time-of-flight. For the diatomic molecules, the critical internuclear distance in multielectron dissociative ionization with a circularly polarized light is larger than that with a linearly polarized light. For linear triatomic molecules, our data indicate that a concerted Coulomb explosion process is a universal phenomenon in the interaction of molecules with intense laser fields, even in the circularly polarized regime. During two C－O (or C－S) bonds breaking simultaneously, the C ion obtained larger energy in circular polarization than that in the linear polarization. Different variations of kinetic energy release between the diatomic and the linear triatomic molecules are discussed.     

激光强度依赖的阈下谐波产生机制

利用广义伪谱方法精确数值求解了氢原子在强激光场中的三维含时薛定谔方程,获得了强激光中氢原子的含时波函数,利用时间依赖的偶极矩的傅里叶变换得到了高次谐波谱,研究了氢原子在强激光场中发射低于电离阈值的谐波谱对激光强度的依赖性.研究发现,激光强度在低于电离阈值的谐波产生的通道选择的过程中扮演着重要角色,主要有两种量子通道对阈下谐波的产生有贡献,即广义的短轨道和长轨道,其中长轨道对激光场强度比较敏感.结合小波时频变换、经典轨道分析、以及强度依赖的量子通道选择分析,本文阐明了其背后的物理机制.     

Strain effect on the orientation-dependent harmonic spectrum of monolayer aluminum nitride

In this study, we theoretically investigate the strain effect on the orientation-dependent high-order harmonic generation(HHG) of monolayer aluminum nitride(Al N) by solving the multiband semiconductor Bloch equations in strong laser fields. Our simulations denote that the efficiency of the orientation-dependent HHG is considerably enhanced when a 15% biaxial tensile strain is applied to Al N, which is attributed to the downshifting energy level of the conduction band. Furthermore, the odd-even feature in the orientation-dependent high harmonic spectra owing to the strain is considerably different when compared with that in the case without strain. The enhanced quantum interference between different energy bands in strained Al N around the Γ-M direction is responsible for the observed odd-even distributions of the orientation-dependent HHG. This study helps to better understand the HHG in solids by tuning their electronic structures.     

不同核轴取向的O2的高次谐波

分子的高次谐波是强场超快物理的重要研究课题. 采用建立在形式散射理论基础上的频域方法计算了O₂在线偏振激光场下的高次谐波, 探讨了核轴被准直在与激光传输方向垂直的平面内时, 高次谐波随核轴与光电场偏振方向所成夹角θ₀的依赖关系. 结果表明: 各次谐波都是在θ₀约为45°时强度最大, 并有较宽的峰值宽度; 当偏离此角度, 高次谐波的强度变小; 到达平行或垂直取向时, 降到最低. 分析表明, 这是由于高次谐波的强度取决于分子基态的电子在动量空间中的电场方向的布居. 针对核轴被准直在激光传输方向与电场偏振方向所确定的平面内的情况, 计算了高次谐波随θ₀的依赖关系, 结果与前一种情况基本相同. 分析发现, 当核轴被准直固定后, 分子绕核轴旋转的角度ψ没有固定, 所以最后的高次谐波强度需要对不同的ψ 时的高次谐波的贡献求和平均. 平均后相当于波函数相对于核轴旋转对称, 从而导致O₂的高次谐波仅与θ₀有关, 而与核轴被准直在哪个面上无关.