Backward Terahertz Radiation from a Two-Color Femtosecond Laser Filament

A change in the quasistatic magnetic susceptibility in thin plates of iron borate (FeBO₃), which is a weak ferromagnet, has been revealed at adsorption of water molecules. The measurements have been performed at room temperature with the use of the magneto-optical Faraday effect. The change of the susceptibility in saturated water vapors is about 30%. The observed effect is reversible. The time of establishing the susceptibility after the introduction of water vapors is 1.5 min, which is twice as large as the time of establishing the susceptibility after the evacuation. The effect is explained by the appearance of uniaxial surface magnetic anisotropy in the basal plane because of the adsorption of water molecules.     

Fragmentation of CO in Femtosecond Laser Fields

Fragmentation of CO in a linearly polarized femtosecond laser field within the intensity order of 10^14 W.cm^-2 at 820nm is investigated experimentally by using velocity mapping technique. According to the observed kinetic energy and angular distributions of different charged fragment ions, fragmentation channels of CO are proposed. The angular distributions provide helpful information for assigning the dissociation channels.     

Terahertz Radiation from Graphite Excited by Femtosecond Laser Pulses

We experimentally investigate the terahertz （THz） generation from a graphite surface induced by femtosecond laser pulses, and systematically study the dependence on the excitation power, the crystal orientation of graphite, polarization state and incident angle of optical beam. New evidence related to excitation and detection geometry is found and presented, which supports the THz generation mechanism of transient photocarrier transporting along the basal plane normal. Our observation also suggests other probable contributions by in-plane charge oscillations. The results may be helpful to explore and understand the photoelectric properties of graphite and other allotropes of carbon.     

Multiphoton Ionization of Potassium Atoms in Femtosecond Laser Fields

We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields.In the 800 nm laser field,the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift.The resonance feature is clearly shown as an Autler-Townes(AT) splitting and is mapped out in the electron kinetic energy spectrum.In a 400 nm laser field,although one photon resonance is possible with the 5p state,no splitting is observed.The different transition amplitudes between 4s-4p and 4s-5p explain the observed results.Due to the AT effect,an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed.A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component.The observed ATsplitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.     

Broadband-Isolated Attosecond Pulse Generation by Two-Color Elliptically Polarized Laser Pulses

We numerically solve the two-dimensional Schrödinger equation and investigate isolated attosecond pulse generation by combining a left and a right elliptically polarized pulse together. It is found that the plateau of the harmonic spectra can be extended and the quantum paths can be controlled by adjusting ellipticity. A broadband width of about 220 eV is generated for the case that ellipticity equals 0.3, and time-frequency time–frequency analysis shows that only one long quantum path contributes to high-order harmonic generation. By superposing a bandwidth of 60 eV near the cutoff of the harmonic plateau, an attosecond pulse with the duration of about 62 as could be generated.     

Emergence of Classical Radiation Fields through Decoherence in the Scully-Lamb Laser Model

The quantum theory of the laser of Scully and Lamb is used to determine the longest-lived states of the quantized field in an idealized, single-mode laser cavity. It is shown that quasiclassical states (states with well-defined phase and amplitude) are naturally selected. A quantum trajectory analysis provides some insight as to why this is so.     

Generation of Terahertz Radiation under the Femtosecond Laser Excitation of a Multilayer Structure Based on a-Si:H/a-SiC:H/c-Si

JETP Letters - Coherent terahertz radiation has been generated in p–n heterostructures based on a-Si:H/a-SiC:H/c-Si excited by 800-nm femtosecond laser pulses at room temperature. Terahertz...     

双色飞秒脉冲场作用下吡啶分子的弛豫动力学研究

利用自制的飞秒激光系统和飞行时间质谱仪结合泵浦探测的方法对吡啶分子的弛豫动力学进行了研究.用265和398nm的飞秒激光分别作为泵浦光和探测光,通过(1+2′)的共振增强多光电子电离的办法测量了C5H5N+离子强度随时间演化的情况,从而测得吡啶分子在265nm激发的S1电子态有一寿命为(3.3±0.1)ps的快速弛豫过程.     

Bubble Generation in Germanate Glass Induced by Femtosecond Laser

We report the observation of bubble generation and migration in a germanate glass during irradiation by a femtosecond laser of high repetition rate.Bubbles are formed around the focal area of the laser beam,and their movement indicates the presence of thermal gravity convection in the glass melt,which is beyond the existing theoretical model about temperature field of focal area.Inside the bubbles,oxygen molecules are observed by the confocal Raman micro-spectroscopy.The generation of molecular oxygen and bubbles is explained in terms of the spatial separation of Ge and O ions and micro-explosion inside the glass melt.     

Resonance Selective Excitation of Terahertz Plasma Waves in a Semiconductor Quantum Well Using Two-Color Laser Radiation

A new method of coherent resonance excitation of plasma instability in a 2D quantum well driven by twocolor laser radiation due to photoinduced generation of electron–hole pairs is proposed. It is shown that under the resonance condition, where the frequency and wavenumber of the beat wave obey the 2D plasmon dispersion relation, a spacecharge wave is excited efficiently. A selfconsistent treatment of the problem is presented and the smallsignal response of the 2D system in the THz frequency region is found. This new method provides the selective resonance excitation of plasmons, tuning of their frequency, and efficient control of plasma wave parameters crucial for THz semiconductor technology.     

Excitonic Absorption of Semiconductor Nanorings under Terahertz Fields

The optical absorption of GaAs nanorings （NRs） under adc electric field and a terahertz （THz） ac electric field applied in the plane containing the NRs is investigated theoretically. The NRs may enclose some magnetic flux in the presence of a magnetic field perpendicular to the NRs plane. Numerical calculation shows that the excitonic effects are essential to correctly describe the optical absorption in NRs. The applied lateral THz electric field, as well as the dc field leads to reduction, broadening and splitting of the exciton peak. In contrast to the presence of a dc field, significant optical absorption peak arises below the zero-field bandgap in the presence ofa THz electric field at a certain frequency. The optical absorption spectrum depends evidently on the frequency and amplitude of the applied THz field and on the magnetic flux threading the NRs. This promises potential applications of NRs for magneto-optical and THz electro-optical sensing.     

采用附加共振高频场提高强场高次谐波的产生效率

提高高次谐波效率一直是强场物理研究中的一个重要课题,对此人们采取了很多方法,如采用双色场,实现相位匹配,附加一个强电场或强磁场等。本文在研究双色场高次谐波的过程中,发现原子的能级结构对于高次谐波的产生效率有极大的影响,据此提出利用附加共振高频场来大幅度提高高次谐波的产生效率。     

纳秒强光场下呋喃的激光电离中高价离子的产生

利用 2 5ns脉冲Nd :YAG 5 32nm的激光 ,在 10 11W/cm2 的光场强度下 ,用飞行时间质谱对不同条件下呋喃的激光电离过程进行了研究 .当利用氩作为载气时 ,首次观察到了较强的Cn + (n =2～ 4 )和Om + (m =2～ 3)离子信号 .通过测定峰分裂和数值模拟计算的方法可以确定其中C2 + 、C3 + 和C4+ 的最可几平动能分别为 2 1、6 3和 10 0eV ,O2 + 和O3 + 的最可几平动能分别为 2 0和 4 0eV .质谱峰型的分裂现象和激光延迟实验结果表明 ,这些高价离子可能来源于呋喃团簇的库仑爆炸过程 .     

激光波长对双色激光等离子体产生太赫兹波的影响

基于瞬态光电流模型研究了激光波长对双色激光等离子体产生太赫兹波的影响.通过理论计算证明:太赫兹信号随激光波长的增大而增强,其变化趋势不会因为双色激光强度、脉宽、相位差、强度比的变化而改变;太赫兹波的频谱不会因为激光波长的改变发生变化.分析了双色激光波长影响太赫兹辐射强度的原因,并利用自由电子密度和漂移电流密度阐明了该影响的内在物理机制.     

利用光子晶体光纤产生超连续飞秒激光光谱

将单脉冲能量约为2．5nJ、脉宽为25fs、对应峰值功率为0．1MW的的800nm钛宝石激光耦合到长为10mm，芯径为1．8μm的光子晶体光纤中，产生了耦合效率约为17％、谱宽覆盖可见光及近红外波段的超连续光谱。     

Laser-Induced Continuum Structure of NO Molecules in Two-Colour Femtosecond Pulsed Laser Fields

The method of quantum wave packet dynamics is used to study the multiphoton ionization of NO molecules via a two-photon Raman coupling and a laser-induced continuum structure （LICS） state in two-colour strong femtosecond pulsed laser fields. Time-and energy-resolved photoelectron energy spectra are calculated for describing three photoionization channels. The population transfers through the LICS and the Raman coupling passages are discussed.     

中红外可调谐大能量飞秒脉冲激光产生

可调谐中红外飞秒光纤激光器具有非常普遍的应用,从而引起了人们的广泛关注。目前,非线性光纤中的拉曼孤子自频移效应是实现大范围可调谐飞秒脉冲激光的理想方法之一。然而,非线性光纤中其他高阶非线性效应的产生通常会限制拉曼孤子脉冲的能量提升。本文提出了利用有源掺杂光纤作为非线性介质和增益介质实现可调谐大能量中红外飞秒激光脉冲的方法。在理论上研究了有源掺杂非线性光纤中高阶孤子劈裂和孤子自频移效应的产生,以及线性增益对波长移动拉曼孤子能量、脉宽、光谱的影响。结果表明,通过为波长红移的低能量拉曼孤子提供线性增益,孤子脉冲的能量得到了显著提升且保持了其单脉冲特性,脉冲宽度为45 fs,且孤子脉冲的波长可通过所提供的增益进行大范围调谐。因此,利用有源掺杂光纤作为非线性介质是实现大能量可调谐中红外飞秒脉冲激光的一种有效方法。     

超宽带近红外和蓝光飞秒激光脉冲产生的实验研究

增益介质中泵浦光束提供的软光阑效应对于实现克尔透镜稳定锁模及其超宽带光谱脉冲的产生具有非常重要的作用.实验上首先对半导体泵浦全固化钛宝石飞秒激光器的锁模动态特性进行了研究,在4 W绿光泵浦状态下获得了平均输出功率为570 mW、中心波长在794 nm～835 nm范围内调谐、光谱带宽最大可达135 nm的近红外光脉冲输出,其相应的时域变换极限脉冲宽度均小于10 fs.另外,将光束聚焦在超薄BBO晶体上,获得了中心波长在418 nm～429 nm之间调谐、光谱宽带时域变换极限小于15 fs的蓝光飞秒脉冲.     

Role of XUV Photons in Atomic High-Order Above-Threshold Ionization Processes in IR+XUV Two-Color Laser Fields

We investigate the above-threshold ionization of an atom in a combined infrared(IR) and extreme ultraviolet(XUV) two-color laser field and focus on the role of XUV field in the high-order above-threshold ionization(HATI)process. It is demonstrated that, in stark contrast to previous studies, the XUV laser may play a significant role in atomic HATI process, and in particular, the XUV laser can accelerate the ionized electron in a quantized way during the collision between the electron and its parent ion. This process cannot be explained by the classical three-step model. Our results indicate that the previously well-established concept that HATI is an elastic recollision process is broken down.     

Internal Conversion Process of Chlorophyll a in Solvents in Femtosecond Pump-Probe Laser Fields

采用约化密度矩阵理论研究了溶液中叶绿素a分子的内转换过程. 内转换时间可以通过模拟实验荧光亏蚀光谱得到. 计算得到的叶绿素a分子在乙酸乙酯、四氢呋喃和二甲基甲酰胺溶剂中的内转换时间分别是141、147和241 fs. 理论分析表明：荧光亏蚀光谱曲线的振荡行为主要是由分子布居在耦合势能面间的振荡引起的. 分析了两个电子态之间非绝热耦合对内转换时间及荧光亏蚀光谱的影响,还讨论了叶绿素a分子与溶液耦合作用对内转换时间的影响.