Study of the Filamentation Phenomenon of Femtosecond Laser Radiation

The filamentation phenomenon during propagation of femtosecond laser radiation in transparent solid media is experimentally studied. An experimental scheme for measuring the filament parameters using spherical and cylindrical optics is developed and assembled. The sample is structured in the filamentation mode, which makes it possible to obtain a specified distribution of laser radiation intensity.     

Filamentation control in the temperature gradient argon gas

A novel technique of controlling the evolution of the filamentation was experimentally demonstrated in an argon gas-filled tube. The entrance of the filament was heated by a furnace and the other end was cooled with air, which resulted in the temperature gradient distribution along the tube. The experimental results show that multiple filaments are merged into a single filament and then no filament by only increasing the temperature at the entrance of the filament. Also, the filament can appear and disappear after increasing the local temperature and input pulse energy in turn. This technique offers another degree of freedom to control the filamentation and opens a new way for multi-mJ level monocycle pulse generation through filamentation in the noble gas.     

Filamentation of interacting femtosecond laser pulses in air

The filamentation of two co-propagated femtosecond (fs) laser pulses in air is studied by numerical simulation. Depending on the different initial separation distances, relative phase shift and crossing angles, simulations show attraction, fusion, repulsion and collision of the two pulses. A long plasma channel can be formed by two in-phase pulses with small separation distance and cross angle. The coupling of two laser beam becomes weaker when the separation distance or cross angle between two beams is larger. In this case, the filamentation of each pulse develops independently. Our simulation results will be helpful for understanding the effect of the initial amplitude and phase modulation of laser pulse on the filamentation characteristics.     

Filamentation instability of helicons

Nonlinear interaction of a large amplitude ducted helicon (whistler wave) with slow plasma motion is considered. The low-frequency response accounts for a finite ion gyroradius effect to the usual MHD Alfvén waves. It is found that this coupling leads to a filamentation instability which grows at a rate faster than those reported earlier.     

Temperature Controlled Filamentation in Argon Gas

Temperature controlled filamentation is experimentally demonstrated in a temperature gradient gas-filled tube. The proper position of the tube is heated by a furnace and two ends of the tube are cooled by air. The experimental results show that multiple filaments are shrunken into a single filament or no filament only by increasing the temperature at the beginning of the filament. This technique offers another degree of freedom of controlling the filamentation and opens a new way for intense monocycle pulse generation through gradient temperature in a noble gas.     

Filamentation of ultrashort light pulses in a liquid scattering medium

We have studied filamentation of 1-ps laser pulses in a scattering medium (aqueous suspension of 2-μm polystyrene microspheres) and compared filamentation dynamics to that in pure water. Our results indicate that light scattering does not alter filamentation dynamics in general, but rather results in farther position of the nonlinear focus, shorter filament length, and the development of speckle structure in the peripheral part of the beam. The experimental observations are qualitatively reproduced by the numerical model which accounts for diffraction, self-focusing, multiphoton absorption, and light scattering introduced through a stochastic diffusion and diffraction term.     

Femtosecond Laser Filamentation in Liquid Spray

Russian Physics Journal - The paper presents the femtosecond laser filamentation, which splits the liquid spray into two jets. The dependence is suggested for the acoustic signal and the...     

Filamentation of a rotating electron beam in a magnetized plasma

Filamentation of a nonrelativistic rotating electron beam in a magnetized plasma is investigated by solving the kinetic equation and finding its dielectric permitivity. The period and the establishment time of the filamentation structure and threshold for instability development are obtained. It will be shown that only when the external magnetic field strength becomes smaller than a characteristic value, filamentation appears.     

强激光部分离化等离子体成丝不稳定性

从部分离化等离子体和通常的全离化等离子体的差异是存在束缚电子出发,分析强激光在部分离化等离子体中的传播和折射指数,其束缚电子加强成丝不稳定性的发展.对钕玻璃三倍频激光金靶等离子体的原子成丝不稳定性进行了计算和分析.结果表明强激光部分离化等离子体的原子成丝不稳定性显著高于相对论成丝不稳定性. 关键词： 激光等离子体相互作用 部分离化等离子体 成丝不稳定性     

Filamentation of an annular laser beam in a short wavelength laser produced plasma

We report on the first observations of periodic structures caused by the filamentation instability. These structures were observed on 0.35 μm laser irradiated planar target experiments. Comparison of these observations with self-focusing theory indicates that even though thermal filamentation may initiate the instability, ponderomotive force effects eventually dominate the actual filamentation process.     

用变分法研究超短超强脉冲在介质中的成丝

利用变分法将非线性薛定谔方程在形式上简化,把光束成丝等价于粒子在势阱中的运动,直观反映了自聚焦、衍射和多光子电离等机制对光束成丝的影响.实验得到了不同入射功率、光束口径、波面曲率半径等初始条件下脉冲在空气介质中的成丝过程和时间波形变化.通过半解析方法得到的结论能够定性解释成丝和多次聚焦现象等实验结果.     

超强线极化激光在e-p等离子体中传播的成丝不稳定性

基于超强线极化激光在正负电子对(e-p)等离子体中的传播方程,讨论了相互作用过程中的成丝不稳定性。得到了电磁波的非线性色散关系和不稳定性增长率,并讨论了传播过程中激光强度和等离子体温度对成丝不稳定性的影响。结果表明,成丝不稳定性的强弱主要由相互作用的非线性效应与受有质动力作用后等离子体密度的减小之间的竞争所决定。     

Filamentation and stimulated Brillouin scattering in a turbulent plasma

This paper presents a theory of filamentation and stimulated Brillouin scattering (SBS) of high-frequency electromagnetic radiation in a weakly collisional plasma with ion-acoustic turbulence. When the square of the wavelength of the plasma perturbations is less than the product of the two mean free path lengths of an electron with respect to its collisions with turbulent fluctuations and with electrons, the influence of cold highly collisional electrons on the parametric instabilities becomes apparent. It is shown that the plasma turbulence lowers the filamentation threshold, and the SBS threshold can be either lowered or raised. The dependence of the SBS and filamentation thresholds on the electron mean free path length in the turbulent plasma and on the anisotropy of the plasma turbulence is determined. The corresponding dependence of the spatial scale of the most efficiently growing filaments and their spatial growth rate are found. Zh. éksp. Teor. Fiz. 113, 629–645 (February 1998)     

Zinc Oxide Nanoparticles Induce Cell Filamentation in Escherichia coli

Zinc oxide nanoparticles (ZnO NPs) induce morphological transformation of Escherichia coli from its native rod‐shape of ≈2–4 μm to filamentous cells of 20–40 μm in length. The transient response can only be observed at up to 3.5 h proliferation, beyond which the cytotoxic effect is neutralized and the rod‐shape is restored. The filamentation is part of the bacterium SOS response to the Trojan horse‐type internalization of undissolved ZnO solids. In the absence of ZnO solids, no cell filamentation can be observed from the leached soluble zinc fraction or dissolved zinc salt.     

Influence of Initial Pulse Chirp on Rainbow-Like Supercontinuum Generation from Filamentation in Air

Supereontinuum （SC） generation from laser filamentation in air is found to depend strongly on the pulse duration. Rainbow-like SC generation is observed only for a pulse of appropriate negative chirp that agrees with the predictions put forward by Golubtsov et al. [Quantum Electron. 33 （2003） 525]. The conversion efficiency of an 800-nm laser light to rainbow-like SC is found to be the highest for 257fs pulses with an initial negative chirp. A larger chirp will lead to filamentation surviving at longer distance.     

Filamentation length of powerful laser pulses

From a theoretical model for the propagation of high-power laser pulses in air over long distances, we derive analytical estimates for the length of an infrared light filament as a function of the pulse duration and beam energy. For a fixed energy per pulse, a maximum filamentation length is shown to be obtained for a specific pulse duration. These estimates are in agreement with the results of numerical simulations and measurements available in the literature.     

Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows,which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic(CH) foils with 1ns-pulsed laser beams of total energy of 1.7 k J in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.     

激光自聚焦和成丝模拟中的并行计算方法

已编制完成三维非线性流体力学与激光传播耦合并行模拟程序,用以研究高强度激光在稀疏等离子体中的自聚焦和成丝不稳定性形成机制.介绍该三维并行程序中所使用的数值方法和并行算法.     

Filamentation Instability in a Current‐Carrying Plasma in the Presence of Quantum Effects

The filamentation instability of a current‐carrying plasma under the diffusion condition is investigated taking into account the Bohm potential and the Fermi electron pressure. Using quantum hydrodynamic equations, the dispersion relation and growth rate of the instability is obtained. It is found that the filamentation instability, in the presence of quantum effects, depends on various characteristic parameters such as: electron Fermi velocity, plasma number density, ion thermal velocity and electron drift velocity. Moreover, the wavelength region in which the instability occurs is more restricted and the minimum size of filaments is larger, in comparison with the classical case. It is also found that the growth rate of the instability is smaller in the presence of quantum effects. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Filamentation of an Ultrashort Laser Pulse in a Medium with Artificial Nonlinearity

JETP Letters - The introduction of a small amount of subwavelength gold nanoparticles (extinction coefficient ∼0.01–1 cm−1) to water allows the efficient control of filamentation...