脉冲形状对氢原子高次谐波的影响

在含时密度泛函理论框架下,采用经典和量子相结合的模型,模拟了氢原子在超强飞秒激光脉冲作用下的高次谐波产生现象,并研究了激光脉冲形状对氢原子高次谐波的影响.结果表明,氢原子的高次谐波谱具有典型的原子谐波谱的特征;具有“下降－平台－截止”的结构和偶次谐波被禁闭仅出现奇次谐波的选择性特征;不同的脉冲形状会导致高次谐波的截止频率和谱线强度的变化.     

拉伸分子高次谐波产生的阿秒控制

通过数值求解一维含时薛定谔方程,本文研究了高频阿秒脉冲对拉伸双原子分子模型与低频飞秒激光脉冲相互作用产生高次谐波谱的影响.研究表明,若阿秒脉冲在低频脉冲的第二个光学周期截止处注入时,只第二个平台截止位置IP 5.6UP(IP为电离势,UP为有质动力能)附近谐波的效率得到显著提高;若阿秒脉冲的包络峰值处于低频脉冲第三个光学周期T/4(为飞秒激光脉冲的光学周期)附近时,则谐波谱出现了明显的整体抬高现象.对此,本文根据拉伸分子谐波谱的四种产生机制及阿秒脉冲在飞秒激光不同相位注入时的电子电离曲线,对这些谐波谱的结构给出了合理解释.     

组合脉冲的相对相位对谐波谱截止位置的影响

通过数值求解一维含时薛定谔方程,本文研究了组合脉冲与氢原子模型相互作用的高次谐波谱,组合脉冲由基频钛宝石激光和它的二次谐波构成. 研究发现,当两束脉冲之间的相对相位取值不同时,谐波谱的截止位置呈现较大差异. 对此,采用半经典的“三步”模型给出了合理的分析. 本文的数值结果表明,尽管双色谐波谱的结构复杂,截止频率不能用一个简单的公式表示,但通过“三步”模型计算同样能准确预言出截止频率.     

碳化硅晶体产生中红外飞秒激光研究获进展

中红外激光(3~5μm)在环境监控、气体分子识别、相干断层成像、军事等领域有着重要应用,特别是近年来在高次谐波产生单个阿秒脉冲的研究中,由于周期量级中红外飞秒激光能获得更高截止能量的谐波阶次,有望获得更短的阿秒脉冲和更高的时间分辨率,因此倍受人们的青睐。但受限于激光增益介质,目前较难在室温下直接获得中红外波段的飞秒激光,为此广泛应用的方案是基于非线性晶     

中红外飞秒激光场中氮分子高次谐波的多轨道干涉特性研究

通过系统研究氮分子高次谐波产生过程中的电子超快动力学过程,实验上发现在中红外飞秒强激光场驱动下高次谐波谱的截止区附近存在清晰的谐波谱极小值.进一步研究表明谐波谱极小值对应的光子能量强烈依赖于驱动激光脉冲的光强和波长,而与分子取向角无关,由此推断该极小值来源于氮分子最高占据分子轨道和次最高占据分子轨道产生的高次谐波之间的相消干涉.本研究结果将对极端强场条件下多轨道电子超快动力学研究起到积极推动作用.     

超强飞秒激光脉冲作用下氢分子的高次谐波行为——基于含时密度泛函理论的模拟

在含时密度泛函理论框架下,采用经典和量子相结合的模型,模拟了氢分子在超强飞秒激光脉冲作用下的高次谐波行为. 结果表明氢分子的高次谐波谱具有“下降—平台—截止”的结构和偶次谐波被禁闭仅出现奇次谐波的选择性特征;不同的激光偏振方向会导致高次谐波的截止频率和谱线强度的变化. 关键词： TDDFT 超强飞秒激光 高次谐波 2分子')" href="#">H₂分子     

激光驱动晶体发射高次谐波的特性研究

通过数值模拟激光驱动下电子在周期性势阱中的动力学行为, 研究了晶体在激光场中发射高次谐波的特性. 研究发现在一定的激光波长和光强驱动下, 晶体发射的谐波谱会呈现出双平台结构, 经分析后得知第一个平台主要来自于最低导带与价带间的电流(电子-空穴对复合), 第二个平台主要来源于较高导带与价带间的电流(电子-空穴对复合), 且两个平台的截止位置处的能量都与激光场的振幅呈线性关系. 在少周期激光驱动下, 晶体谐波第二平台的截止位置与激光的载波相位呈单调变化, 由此我们提出可以利用晶体谐波第二平台的截止位置来确定少周期激光的载波相位. 进一步研究发现, 在啁啾激光驱动下, 晶体发射谐波谱的第二平台有较大变化, 第二平台的发射效率会随啁啾参数而改变, 能够通过改变啁啾激光场来提高晶体谐波第二平台的发射效率.     

飞秒紫外激光脉冲振荡的实验研究

本文讨论了采用空间光脉冲光谱的啁啾特性和选择聚焦透镜焦距相结合的技术大大提高二次谐波转换效率和产生紫外飞秒光脉冲的实验研究.采用一类相位匹配的BBO晶体,当飞秒钛宝石光脉冲平均功率为560mW时,二次谐波输出功率为352mW,二次谐波转换效率高达63%;采用一类相位匹配的LBO晶体时,获得高光束质量的倍频蓝光输出,输出平均功率为170mW,转换效率大于30%.运用LBO倍频产生的蓝光脉冲和剩余的基频光脉冲进行了三次谐波的振荡研究.三倍频晶体采用BBO,通过优化设计倍频光与基频光之间的空间模匹配及精确时间延迟,得到飞秒紫外光输出,输出功率为2mW,中心波长约为280nm,重复率为100MHz.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

多周期双色场方案下附加脉冲频率对阿秒脉冲产生的影响

通过数值求解一维含时薛定谔方程,研究了多周期双色场与氢原子相互作用过程中不同波段的附加脉冲波长对体系高次谐波及阿秒脉冲产生的影响.研究发现,当双色场的附加脉冲波长处于红外波段1600 nm时,比起处于可见光波段的533 nm附加脉冲,谐波谱截止位置得到大幅延展,相应地拓宽了超连续带宽,小波变换后获得了78 as的孤立短脉冲.由合成电场的光学振荡频率解释了附加脉冲波段对谐波谱的影响,同时该结论有利于在获得孤立阿秒脉冲的研究中选择合适的附加脉冲频率.     

Tunable ultraviolet source from fifth and seventh harmonic generated by mid-infrared pulses filamentation in air

The seventh harmonic generation by tunable mid-infrared femtosecond pulses filamentation in air is observed first time. Pumped by a focused mid-infrared pulses, which pulse duration is ～50 fs and wavelength is tunable from 1300 to 2000 nm, the tunable region of the fifth and seventh harmonic generation can cover the near ultraviolet region from 180 to 400 nm.     

Extended phase matching of high harmonics driven by mid-infrared light

We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.     

中红外组合激光场产生低于50阿秒的单个脉冲

研究了氦离子在中红外组合激光场作用下的高次谐波辐射和孤立阿秒脉冲的产生. 研究结果表明,当主脉冲强度相对低时,谐波截止在657阶次处,形成一个宽度为287 eV的连续谱. 当主脉冲强度相对高时,可使谐波截止拓展到1795阶次,连续谱加宽到834 eV. 在两种情况下,我们实现了长量子路径选取,并且产生转换效率较高的连续谱. 特别是对于相对高的激光强度,叠加450—1590阶次内任意87 eV的谐波都可以直接得到50 as以内的单个脉冲.     

Programmable amplitude- and phase-modulated femtosecond laser pulses in the mid-infrared

We present a scheme to produce programmable phase- and amplitude-modulated femtosecond laser pulses in the mid-infrared regime of 3-10mum by difference frequency mixing. The 80-fs signal output of an optical parametric amplifier is shaped with a liquid-crystal mask and mixed in an AgGaS(2) crystal with a temporally stretched idler pulse. Without changing the mechanical alignment, we produce programmable amplitude modulations and chirped pulses at lambda=3mum with energy as high has thas 1muJ . This scheme, further, allows the generation of controllable pulse sequences. The results are in good agreement with theoretical simulations.     

Ionization Mechanism of the Generation of Tunable Ultrashort Pulses in the Mid-Infrared Range

The generation of pulses in the mid-infrared range during the ionization of a gas by an intense two-color femtosecond field containing quasimonochromatic components with the frequency ratio close to 2 has been studied. The generation occurs at the detuning frequency equal to the difference between the higher frequency and the doubled lower frequency. It has been shown that the efficiency of the generation is determined both by the magnitude of the detuning frequency and by its sign and is higher at negative detunings. The duration of the generated pulse is determined by the ionization duration, which is much smaller than the duration of the ionizing field, which ensures obtaining few-cycle and subcycle pulses in the mid-infrared range.     

New trends in high-order harmonics generation using the mid-infrared pulses propagating through the laser-produced plasmas

Various recently emerged applications of mid-infrared pulses for high-order harmonic generation in laser plasmas produced on the surfaces of various materials are reviewed. Among them are (a) the highorder harmonic generation in Ag, Sn, fullerene, and graphene nanoparticle-containing plasmas using tunable two-color mid-infrared pulses, (b) efficiency dependence on the generating particle properties in the case of two-color high-harmonic generation of the longer-wavelength radiation in the plasmas, and (c) high-order sum and difference frequency generation using tunable two- and three-color commensurate and incommensurate mid-infrared pumps of graphite plasma. These studies demonstrate the perspectives of the frequency conversion of mid-infrared radiation in the extended laser-produced plasmas.     

A systematic investigation of high harmonic generation using mid-infrared driving laser pulses

We report on a systematic investigation of the influences of gas pressure,focal position and focusing geometry on high harmonic generation by use of mid-infrared femtosecond laser pulses. We also discuss the spatial characteristics of harmonics under different focusing conditions. By optimizing the parameters,we experimentally observed the generation of 1 kHz,low divergence coherent X-ray beams in the water-window region.     

Mid-infrared pulses generated from the mixing output of an amplified, dual-wavelength Ti:sapphire system

We demonstrate the generation of femtosecond mid-infrared pulses by direct difference-frequency mixing in a AgGaS(2) crystal of two intense pulses from a dual-wavelength oscillator that have been simultaneously amplified in a single regenerative amplifier. A pulse energy of as much as 1.6 muJ was achieved at wavelengths of 9-11 mum.     

PPLN晶体差频测量飞秒激光脉冲的载波包络相移

在飞秒激光频率梳系统中，通常采用自参考技术测量飞秒激光脉冲的载波包络相移，但该技术需要采用光子晶体光纤进行光谱扩展从而增加了系统的不稳定性，这种技术已经制约了高稳定度的飞秒激光频率梳的发展.采用PPLN晶体差频法测量了宽谱钛宝石振荡器输出的7fs激光脉冲的载波包络频移，得到了大于30dB的拍频信号，为研制无光纤的新一代高稳定度光学频率梳奠定了基础.