超短激光脉冲在水中的非线性传输特性研究

基于扩展的非线性薛定谔方程及与其耦合的电子密度演化方程,理论研究了超短激光强脉冲在水中的非线性传输特性.理论模型综合考虑了衍射、正常群速度色散,多光子吸收、自聚焦及激光诱导产生的等离子体对光脉冲的自散焦效应.采用有限差分法模拟得到超短脉冲在传输过程中轴向上光功密度和等离子体密度的时空分布.系统分析了超短脉冲在水中传输的动力学过程,讨论了非线性自聚焦和等离子体自散焦效应在脉冲传输过程中的竞争关系.同时研究了不同入射激光能量,脉宽和聚焦条件对等离子体丝的时宅结构和脉冲能流横向分布的影响.该研究将有助于理解和推动超短光脉冲在激光医学、激光安全防护和水中激光加工中的应用.     

超短强激光作用下空气的三次谐波辐射

进行了飞秒强激光与自由空气和充气毛细管相互作用产生三次谐波的实验研究,成功地获得了功率达0.17GW、重复频率为10Hz的紫外三次谐波信号,谐波的转换效率达到0.04%。对自由空气和充气毛细管中的谐波转换效率和谐波空间分布特性进行了比较。实验表明微毛细管虽然使谐波转换效率略有降低,但能有效地改善谐波的空间分布,使其更好地满足实际应用的需要。     

飞秒超短脉冲激光加热金属平面靶

从能流分析出发,对飞秒超短脉冲激光与金属平面靶相互作用的机制进行了理论研究,对其中主要物理过程的能流损耗作了详尽的分析,并根据一维,双温热扩散模型推导了自由电子温度随时间变化的函数关系.并从理论上推导了超短脉冲近似假设成立的脉宽范围和在此条件下自由电子所能达到的最高温度表达式.     

Nonlinear Refraction of Dielectric Single Crystals under Excitation with Ultrashort Near-Infrared Laser Pulses

Optics and Spectroscopy - Nonlinear refraction in a number of crystals (KGW, KGW:Eu3+, Ba(NO3)2, BaWO4, PbMoO4, and CdWO4) that are promising for excitation of multifrequency stimulated Raman...     

Laser Ablation of Metals by Low-Density Picosecond Pulses

Thresholds of laser ablation ofmetals for the picosecond range of laser pulse duration are determined. Within the two-temperature metal model, the space-time dependences of the electron and lattice temperature are obtained. It is shown that ablation parameters become dependent on the pulse duration at the pulse duration τ _p ≥ τ _ie , where τ _ie is the cooling time of the electron gas. For noble metals, such pulse durations are τ _p ≈ 3 ? 10 ps.     

Ablation of (111) and (001) Crystal Plates by Ultrashort Laser Pulses with Rotated Linear Polarization

JETP Letters - The dependence of the threshold fluence for surface ablation of such crystalline materials as silicon and diamond (with orientations (001) and (111)) on the direction of linear...     

超短激光脉冲中时空耦合效应的研究进展

概述了超短激光脉冲中时空耦合效应的研究进展。将时空耦合效应分为一阶时空耦合和高阶时空耦合两大类,并详细介绍了两类时空耦合效应的研究历史和现状。其中一阶时空耦合效应相对较为简单,对其的理论及实验研究也较为充分。相比一阶时空耦合效应,高阶时空耦合效应的起源更广泛、种类更丰富、时空结构更复杂。目前,对高阶时空耦合效应的研究也刚处于起步阶段。最后,展望了该领域的研究趋势。     

Attosecond Dynamics of Photoexcitation of the Hydrogen Atom by Ultrashort Laser Pulses

The dynamics of photoexcitation of the hydrogen atom in the discrete and continuous spectra under the action of laser pulses in the attosecond range of time and pulse durations has been analyzed using perturbation theory. It is shown that over time interval shorter than or on the order of pulse duration, the time dependence of the photoexcitation probability is generally oscillating by nature. It has been established that for certain values of parameters, the envelope of this dependence has a peak, the position of which is determined by the pulse duration and carrier frequency.     

Elastic Scattering of Ultrashort Laser Pulses on Noble Gas Atoms

Russian Physics Journal - The work deals with a theoretical investigation of elastic scattering of ultrashort electromagnetic pulses (USP) on the noble gas atoms in the far ultra-violet and soft...     

超短脉冲激光对硅膜的热力效应分析

为了研究超短脉冲激光辐照下半导体材料的热力效应,在热电子崩力和自洽场两种模型的基础上,得到了完全耦合的非线性热弹方程组。在单轴应力条件下,采用有限差分法,讨论了不同脉宽的超短脉冲激光辐照下,硅膜内载流子温度、晶格温度、热应力以及热电子崩力随时间及膜深度的变化情况。结果表明,脉冲宽度对硅膜的热力损伤过程起重要作用。能量密度一定时,载流子和晶格达到热平衡所需时间随脉冲宽度的增加而增加;热电子崩力呈现明显的双峰结构,同时脉冲宽度对第二个峰值的影响较大;脉宽越窄,热应力的峰值越大,越容易对材料造成损伤,为激光加工和光电器件的损伤提供了理论参考。     

在单晶硅中用相干光耦合方法压缩短脉冲和超短脉冲

提出一种新型的相干光耦合压缩短脉冲和超短脉冲的方法。以硅为非线性材料，经计算表明：在材料的破坏阈值之内，出射信号光脉冲的脉宽可被压缩到入射光脉宽的７０％。计算了泵浦能量密度，泵浦光与信号光夹角等与脉宽压缩量的关系。     

用干涉自相关导数谱图明确超短脉冲啁啾方向

提出了新型的确定超短激光脉冲啁啾方向的方法,采用对二次十涉自相关(IAC)中的基频信号求一阶导数,得到干涉自相关导数谱图(D-IAC).利用该导数谱图对啁啾方向有很高灵敏度的特性,通过对高斯型强度分布的线性啁啾、平方啁啾及立方啁啾脉冲进行数值计算与模拟,得到导数谱图的峰与谷分别对应啁啾的负与正.找到了判定啁啾正负的方向因子,即正弦函数因子,并通过实验证实用十涉自相关导数谱图可明确超短激光脉冲啁啾方向,且干涉白相关导数谱图能用丁超短激光脉冲啁啾方向的检测.     

Polarization-selective Excitation of Dye Luminescence on a Gold Film by Structured Ultrashort Laser Pulses

Donut-shaped visible (515 nm) femtosecond laser pulses with linear, radial and azimuthal polarizations after tight focusing (numerical aperture NA = 0.65) have been used for spectrally adjusted excitation of plasmonenhanced rhodamine 6G dye luminescence on the surface of a nanocrystalline 45-nm thick gold film with its plasmon resonance near the laser line. Significant luminescence output increments—1.7-fold for radial polarization compared to the azimuthal one, and almost 3-fold—compared to the linear one, have been observed. This effect is related to significant transformation of the transverse electrical field component into longitudinal one for radial polarization (up to 33% in intensity, according to our calculations), and corresponding increase in excitation efficiency of plasmons in the smaller nanocrystallites throughout the film.     

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.     

Structural Anisotropy of Amorphous Silicon Films Modified by Femtosecond Laser Pulses

It is demonstrated that the surface-relief orientation in the form of one-dimensional gratings with a period of 1.20 ± 0.02 μm formed under processing of hydrogenated-silicon films by femtosecond laser pulses (1.25 μm) with an energy density of 0.15 J/cm² is determined by the direction of the polarization vector of the radiation and total laser exposure. Based on the results of the analysis of Raman spectra, the presence of a nanocrystalline phase of silicon with a volume fraction between 15 and 67% (depending on processing conditions) is detected. The observed processes of micro- and nanostructuring are caused by excitation of the surface plasmon–polaritons and nanocrystallization in the near-surface region in the field of high-power femtosecond laser pulses, respectively. In addition, formation of polymorph modifications of silicon Si-III and Si-XII under femtosecond laser processing with a number of pulses exceeding 500 is discovered. Anisotropy of the Raman signal for the above polymorph modifications is revealed.     

大气湍流中超短高斯激光脉冲的时间域展宽模型研究

穿过大气湍流的高斯激光脉冲在时间域的展宽行为与高斯脉冲通过理想高斯滤波器后的时域展宽行为相似。因此,为了激光通信信道仿真的需要,提出用级联巴特沃斯滤波器来逼近理想高斯滤波器的方法对这种时间域上的展宽行为进行建模。逼近后的高斯滤波器的3dB频率点与大气湍流的物理参数(包括折射率结构常数和湍流外尺度)和传播几何路径长度联系在一起,从而将高斯脉冲的展宽与所通过的大气湍流环境参数相结合并得到了模拟高斯脉冲展宽的解析表达式。该模型和由模型参数表示的高斯脉冲展宽的解析表达式不仅在湍流弱起伏区有效,而且在从弱起伏到中等起伏再到强起伏的整个区域中都有效。     

Radiative Losses of Plasma on Interaction of Ultrashort Laser Pulses with Matter

Radiative losses of plasma produced by ultrashort laser pulses near the surface of solid targets were studied by the methods of highresolution xray spectroscopy and using the numerical simulation of gasdynamics and radiation processes. The experiments were carried out at the NEODIM laser facility in the Central Research Institute for Machine Building. The flux density was as high as 10¹⁷ W/cm² when the radiation was focussed onto the surface of solid targets from Al and Cu. The timeintegrated discrete spectra of plasma of the multiply charged ions were recorded in the soft xray band from 0.8 to 3.0 keV. The experimental spectra were compared with the results of numerical calculations within the framework of the model, which considered both hydrodynamic and radiationcollisional processes. The association of the radiation dynamics of the individual groups of lines with the spatial and temporal dynamics of the laser torch was revealed. Such an association permits one to judge the nonstationary processes in the expanding plasma even by the spectra integrated over time. From comparison of the experimental results and model calculations, it follows that the line emission of a relatively narrow (in the charge number) group of ions constitutes the major portion of radiation losses.     

Transformations of the Spectrum of an Optical Phonon Excited in Raman Scattering in the Bulk of Diamond by Ultrashort Laser Pulses with a Variable Duration

JETP Letters - A spontaneous Raman signal at the frequency of the triply degenerate optical phonon of diamond is observed in the photoluminescence spectrum under the pre-filamentation excitation of...