控制双激光脉冲的宽度提高N2分子的取向

提出了通过控制双激光脉冲宽度的方法来提高N2分子取向程度.利用数值方法求解了N2分子刚性转子模型在双激光脉冲作用下的薛定谔方程,计算了双原子分子N2在总强度固定的两束激光脉冲作用下.不同脉冲宽度对于N2分子取向的影响.研究结果表明,通过调整两束激光脉冲的宽度,选择合适的延迟时间能够有效提高N2分子的取向程度.     

Dynamic Alignment of D2 Enhanced by Two Few-cycle Pulses

利用数值方法求解D2分子刚性转子模型在超短飞秒激光脉冲作用下的薛定谔方程,计算了双原子分子D2在两束固定强度的飞秒激光脉冲作用下,延迟时间对于分子取向的影响.结果表明,只要选取合适的延迟时间,就能很好改善分子取向;保持两束激光强度不变,通过调整两束脉冲的宽度,选择合适的延迟时间能够进一步有效提高D2分子的取向程度.     

两束短脉冲激光驱动O_2分子的取向特性

利用数值方法模拟O2分子在短脉冲激光场中的转动波包,从理论上研究了在两束短脉冲激光驱动下O2分子的取向程度.在相同的环境温度下,与单脉冲激光驱动相比,通过优化两束激光脉冲的强度比和时间延迟,可以有效提高O2分子的取向程度.研究表明,当两束激光脉冲的强度比约为1:3,时间延迟为11.73 ps时,O2分子可以达到很好的取向效果.     

飞秒激光脉冲对N2分子非绝热准直的调控

介质内分子在飞秒激光场中的准直会诱导介质内部折射率发生变化并产生光谱调制效应.本文实验上采用泵浦-探测方法测量了N₂中探测光波长偏移量的时间演化,提取了N₂分子的准直度信息;理论上通过求解含时薛定谔方程,计算了分子非绝热准直的时间演化,探究了分子非绝热准直和克尔效应两种机制共同影响所诱导的双折射效应对探测光光谱的调制作用.实验和理论结果符合良好,证实了光谱测量法可以用来表征分子的准直度.进一步采用双脉冲泵浦方式对分子准直进行调控,发现双脉冲泵浦可以有效增强分子的准直度.通过调节双脉冲间的延迟时间,即在分子的一个转动恢复周期及半个转动恢复周期处引入第二束泵浦脉冲,可以分别控制分子准直的增强与消失,起到“准直开关”的作用.双脉冲调控方式同样适用于其他多个分子体系,具有一定的普适性.     

啁啾飞秒激光脉冲形成的光纤光栅的Bragg反射特性

在800nm飞秒脉冲激光照射下光敏玻璃As₂S₃光纤具有双光子吸收 现象，当一束超短激光脉冲与另一束啁啾超短激光脉冲在As₂S₃光纤相遇时，干涉图案将 永久地记录一个空间啁啾光栅.研究了该光栅结构与入射超短脉冲激光脉冲参数的关系，数值计算表明该光栅具有较大的Bragg反射带宽. 关键词： 啁啾光纤光栅 飞秒激光脉冲 双光子吸收效应     

飞秒与皮秒激光脉冲的主动同步及和频产生宽带超短激光的研究

利用主动同步方式实现了飞秒钛宝石激光器与皮秒Nd:YVO₄激光器的同步，得到了时间抖动低于1ps的同步精度.在此基础上进行了两束激光的和频实验研究，产生了波长为460nm的宽带蓝光飞秒脉冲激光.这一技术不仅代表了实现不同波长、不同脉冲宽度激光同步的一般方法，而且也证明了通过和频、差频两束不同激光产生新波长超短脉冲激光的一种全新技术方案. 关键词： 主动同步 超短脉冲 和频 差频     

Selective Alignment of D2 Induced by Two Ultrashort Laser Pulses

通过求解D₂分子在飞秒激光场中的含时薛定谔方程,研究了室温下D₂分子在超快飞秒激光驱动下的的转动波包动力学. 选择用第一束超短飞秒脉冲与温度为300 K的D₂分子系综相互作用产生一个相干转动波包,用第二束超短飞秒脉冲在波包的1/4和3/4恢复周期选择操纵D₂分子取向. 研究结果表明,通过选择两束超短飞秒脉冲的延迟时间,可以有效控制D₂分子转动波包中奇偶态的相对布居,从而选择性的控制D₂分子取向.     

高次谐波辐射发射特性研究

报告由不同脉冲宽度(半高宽,FWHM)和不同载波-包络相位(CEP,Φ)的激光产生的高次谐波辐射能量输出时间特性即发射特性的研究结果. 计算表明,由宽度为几个周期的激光产生的高次谐波辐射的截止能量明显低于由无限长脉冲宽度激光产生的截止能量ω_max=3.17U_p+I_p(其中ω_max为光子角频率,U_p和I_p分别为激光有质动力势和原子的电离能). 例如,由两周期(FWHM),Φ＝15°的激光产生的高次谐波辐射的截止能量为ω_max=2.90U_p+I_p,此时发射特性单脉冲(即分布单脉冲)具有最大的能量带宽0.86U_p. 脉冲中心位置的载波相位和时间宽度分别为0.94rad(弧度)和1.29rad. 而该激光脉冲在Φ＝-75°时能产生截止能量为ω_max=2.70U_p+I_p,最大能量带宽为0.70U_p的双分布脉冲,其中心位置分别为-0.58rad和2.43rad,宽度分别为1.22rad和1.33rad. 随着激光脉冲宽度的增加,分布单脉冲的能量带宽比时间宽度下降得更快. 对于一定宽度的激光脉冲,所产生的分布单脉冲的能量带宽和时间宽度的CEP依赖性显示出180°的周期结构. 利用这个有趣的特点,在实验上可以通过调节CEP来选择分布脉冲的能量参数,也可用来定位和控制阿秒脉冲的时间参数. 理论分析指出,只要选择合适的阿秒X射线能量带宽,CEP不稳定性对于光电子谱和测量结果的影响将大为降低,甚至在最大程度上消除这种影响. 这些研究结果不仅有助于在物理上深入了解高次谐波辐射的动力学过程,而且对于进一步在实验上优化和选择阿秒单脉冲和双脉冲具有重要的参考和指导意义. 关键词： 高次谐波产生 鞍点方法 谐波发射特性 分布脉冲     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

在相对论性激光-等离子体系统中初始物理参数对激光脉冲的影响

基于相对论性激光-等离子体动力学理论，研究了相对论性激光-等离子体系统中圆偏振入射脉冲激光和等离子体相互作用对激光脉冲宽度的影响. 具体分析了在不同初始物理参数下脉冲激光的脉冲宽度在等离子体传播过程中的变化情况，重点分析了激光脉冲在等离子中压缩. 计算结果表明增加入射激光的强度和入射脉冲宽度以及减小等离子体的初始密度，能够有效地实现脉冲宽度在等离子体中压缩；当激光脉冲的初始参数a₀=0.12和τ=70以及等离子体密度n₀=0.3时，脉冲宽度相对压缩T/τ接近于1/10，从而给出了激光压缩的理论优化参数. 关键词： 相对论性激光-等离子体 激光脉冲宽度 等离子体密度 自压缩     

Observation of enhanced field-free molecular alignment by two laser pulses

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.     

Angular distributions of CH3I fragment ions under the irradiation of single pulse and trains of ultrashort laser pulses

The angular distribution of CH₃I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×10¹³ W/cm² is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.     

Two-pulse generation of high-current relativistic electron beams and their transportation in the gaseous medium of a plasmochemical reactor

Experimental results on two-pulse generation and transportation of high-current relativistic electron beams (REBs) through the gaseous medium of a plasmochemical reactor (PR) are presented. The generation of two consecutive high-current REB pulses with a duration of 60 ns was achieved at the Tonus accelerator with modified schemes of high-voltage pulse formation. The first version of the formation scheme enabled pulse powers of 2 and 4.0–9.6 GW with a time interval between the pulses of 500 ns. The second version enabled one to generate pulses with powers of 1.8 and 16 GW and time interval between the pulses of 160 μs. The transportation parameters of an REB injected into a 1.4-m-long PR filled with an N₂: O₂ gas mixture are studied. The conductance of the plasma produced under the action of the electron beams is measured. It is shown that the schemes proposed provide more efficient (by 35–45%) transportation of the REBs in the reactor volume as compared to single-pulse high-current REBs of the same power and pulse duration.     

Single pulse extraction from a mode-locked pulse train using a dye-laser amplifier

A new method to generate high power, short light pulses with a moderate iteration frequency is described. The pulse train of the actively mode-locked Kr laser was sent into a dye-laser amplifier pumped by a N₂ laser. The trigger of the N₂ laser was generated by dividing the modulation frequency of the mode-locking into moderately low frequency (≈ 50 Hz). Only one pulse in the input train was largely amplified by one N₂ laser pulse and the intense short pulses were generated with ≈ 50 Hz iteration.     

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O₂ and N₂ molecules irradiated by a bichromatic laser field of circular polarization are studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer the photoelectrons and perform the selective ionization of molecules.     

Gas breakdown with nanosecond CO2 laser radiation

The thresholds for CO₂ laser induced breakdown and their variation with pulse width have been measured at various pressures for Ar, N₂ and an 8/1/1 laser mixture of He/CO₂/N₂ using 3–40 ns duration pulses. These measurements indicate that excited state production plays a dominant role in determining the threshold for nanosecond duration pulses. This has been confirmed by the good agreement obtained between the measured and theoretical thresholds.     

Pulse shaping control of alignment dynamics in N2

Control on the alignment transients of impulsively aligned ensembles of N₂ molecules has been demonstrated by the use of laser pulses shaped by a spatial light modulator. An alignment experiment has been inserted in the feedback loop of an evolutionary algorithm that found optimum pulse shapes for a set of criteria. Optimum pulse shapes for the maximization of total alignment and for the control of certain aspects of the revival structures are given. The physical mechanisms responsible for the control are analysed with the help of single‐parameter control schemes and numerical simulations, which allowed us to explore the low‐temperature region. This approach sheds light on the role played by different control mechanisms for the alignment dynamics of a molecular ensemble. Copyright © 2006 John Wiley & Sons, Ltd.     

Laser on nitrogen-electronegative gas mixtures, pumped by inductive energy storage generator: Experiment and theoretical model

The advantages of inductive energy storage (IES) generators for increasing the pulse energy, power, and duration for nitrogen laser pumped by self-sustained transverse discharge have been experimentally demonstrated. A theoretical model is developed and the operation of IES-pumped laser on nitrogen-electronegative gas mixtures is numerically simulated. It is shown experimentally and theoretically that, adding electronegative gases, one can control the pulse shape of lasing on the C³II _u -B³II _g transition in nitrogen. The increase in the electric field strength in the laser gap in N₂-NF₃ and N₂-SF₆ mixtures produced 337.1-nm laser pulses consisting of two spaced peaks and 40–50-ns pulses close to rectangular. The increase in the laser active volume to 6 l (discharge cross section to 6×10 cm²) in N₂–SF₆ mixtures made it possible to obtain the maximum output energy (Q=110 mJ) and UV power (P _las =6 MW). In N₂-NF₃ mixtures, the laser pulse duration was up to ∼100 ns with an energy up to Q=30 mJ.     

Efficient lasing action from Rhodamine-110 (Rh-110) impregnated sol-gel silica samples prepared by dip method

Rhodamine-110/sol-gel samples are prepared by sol-gel technique using dip method. Concentration dependent photophysical studies of these samples have indicated about the least possibility of aggregate formation. The lasing action of Rh-110 in silica samples is studied as a function of dye concentration. An efficient laser emission is observed when the samples are transversely pumped at 337.1 nm and 1.5 Hz repetition rate using a nitrogen laser (400 μJ energy/pulse and 4 ns pulse duration). The maximum of 166% laser efficiency of dye doped sol-gel samples compared to Rhodamine-6G (Rh-6G) in methanol is achieved. The photostability is also measured by using N₂ laser at 1 Hz and it is found nearly 165 pulses. The possible reasons for the photodegradation of the dye molecules are discussed in detail.     

Femtosecond laser-induced dynamic alignment in Coulomb explosion of CO: Roles of laser wavelength, intensity and pulse duration

Dependences of dynamic alignment of CO molecules induced by intense femtosecond laser fields on laser wavelength, intensity and pulse duration are investigated by numerical simulations. A counting approach and a fourth-order Runge-Kutta algorithm are used to calculate the angular distribution and the time evolution of molecules. A two-step Coulomb explosion model of diatomic molecules in intense laser fields is used to determine the instant that CO molecular dynamic alignment is over. Our calculating results show that the linear polarizability and the damping force play an important role in the angular rotation of CO molecule in conditions of 800 nm laser wavelength and 10¹⁵ W/cm² laser intensity. The contributions of the second-order field-induced dipole moment and the higher-order correction term to molecular rotation acceleration comparing to the linear polarizability and damping force are negligible. The extent of dynamic alignment of CO molecules reduces with the increasing of laser intensity. The dynamic alignment time of CO molecules is tightly connected to the laser pulse duration. The angular distributions of CO molecules as the laser pulse length varied from 50 to 250 fs at laser intensity of 3×10¹⁴ W/cm² are shown and discussed.