Laser-filamentation-induced condensation and snow formation in a cloud chamber

Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced spectacular snow formation in a cloud chamber. An intense updraft of warm moist air is generated owing to the continuous heating by the high-repetition filamentation. As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and cyclone like action on a macroscopic scale. This indicates that high-repetition filamentation plays a significant role in macroscopic laser-induced water condensation and snow formation.     

Synthesis of FeCo nanoparticles by pulsed laser deposition in a diffusion cloud chamber

Magnetic FeCo nanoparticles were successfully synthesized in a diffusion cloud chamber setup within pulsed laser deposition (PLD) equipment. The variation of morphology and size of FeCo nanoparticles with the number of laser pulses, ambient gas pressure and temperature gradient was studied. It was observed that the morphology of the nanoparticles changes from “cloud-like” fractal clusters to particle chains; average particle size increased at higher argon gas pressure. Increasing the temperature gradient considerably reduced the agglomeration of the nanoparticles. Nanoparticles deposited using the diffusion cloud chamber are found to be crystalline.     

The cloud chamber as a tool in cloud physics

Since 1875 when Coulier and, a little later, Aitken made the first simple expansion cloud chambers to study the condensation of water vapour on atmospheric particles, and particularly since 1895 when C. T. R. Wilson made his famous instrument with the initial object of studying optical phenomena in clouds, the cloud chamber has been one of the principal tools for investigating condensation phenomena and for simulating natural cloud-forming processes. The cloud physicist commonly employs expansion-, mixing- and diffusion-cloud chambers (he has yet to find a use for the bubble chamber) for studying the spontaneous condensation of water vapour; condensation upon ions and atmospheric particles; the production, nucleation and crystallization of super-cooled clouds; the properties of ice-forming nuclei; and the growth of ice crystals from the vapour. The purpose of this article is to review some of the more important aspects of this work.     

Vortices formation induced by femtosecond laser filamentation in a cloud chamber filled with air and helium

We report on the experimental observation of the airflow motion induced by an 800 nm, 1 k Hz femtosecond filament in a cloud chamber filled with air and helium. It is found that vortex pairs with opposite rotation directions always form both below and above the filaments. We do not observe that the vortices clearly formed above the filament in air just because of the formation of smaller particles with weaker Mie scattering.Simulations of the airflow motion in helium are conducted by using the laser filament as a heat source, and the simulated pattern of vortices and airflow velocity agree well with the experimental results.     

Sub-picosecond ultraviolet laser filamentation-induced bulk modifications in fused silica

We present experiments with sub-picosecond ultraviolet laser pulses (248 nm, 450 fs) tightly focused in the bulk of fused-silica samples. The high laser intensities attained generate plasma through multi-photon absorption and electron avalanche processes in the bulk of the material. Depending on the initial experimental conditions three distinct types of structural changes in the material are observed, from small changes of the refractive index to birefringence, and even cracks and voids. We also observe the creation of micro-channels, up to 115 m in length, inside the material due to self-guiding and filamentation of the laser pulses in the transparent material. The selective change of the refractive index is a promising method for the fabrication of photonic structures such as waveguides and three-dimensional integrated optical devices. PACS  52.38.Hb; 42.65.-k; 42.70.-a     

Phase transition model of water flow irradiated by high-energy laser in a chamber

In the absorption chamber of a high-energy laser energy meter, water is directly used as an absorbing medium and the interaction of the high-power laser and the water flow can produce a variety of physical phenomena such as phase transitions. The unit difference method is adopted to deduce the phase transition model for water flow irradiated by a high-energy laser. In addition, the model is simulated and verified through experiments. Among them, the experimental verification uses the photographic method, shooting the distribution and the form of the air mass of water flow in different operating conditions, which are compared with the simulation results. The research shows that it is achievable to reduce the intensity of the phase transition by increasing the water flow, reducing the power intensity of the beam, shortening the distance the beam covers, reducing the initial water temperature or adopting a shorter wavelength laser. The study＇s results will provide the reference for the design of a water-direct-absorption-type high-energy laser energy meter as well as an analysis of the interaction processes of other similar high-power lasers and water flow.     

水云增长过程中的云滴谱及散射特性分析

基于大气物理学研究了水云云滴增长过程中的粒谱及散射特性.研究结果表明,凝结增长使粒谱半高宽和有效半径不断增加,碰并增长使粒谱出现多峰分布,有效半径增加.在凝结增长和碰并增长共同作用下,有效半径的平均增长速率为8 nm/s.凝结增长和碰并增长单独作用下,消光系数和散射系数随时间呈线性变化.在二者共同作用下,除3.2 mm波长外,消光系数和散射系数随时间呈指数增长;1.064, 2.2, 3.7, 12和22μm波长的不对称因子逐渐趋于稳定,200μm的不对称因子呈指数增长,3.2 mm的不对称因子基本保持不变;1.064和2.2μm波长的雷达比在20 sr附近波动,3.7μm波长的雷达比呈大幅振荡.云滴增长过程中,水云在1.064, 2.2和3.7μm波长的单次散射反照率逐渐降低,在12μm, 22μm, 200μm和3.2 mm波长的单次散射反照率逐渐增加,波长指数的绝对值逐渐减小.研究结果可为天气预报、地气辐射平衡研究和遥感数据校正提供重要的参考.     

Excitation of a two-level system by a chirped laser pulse

The excitation of a two-level system by a chirped laser pulse is analyzed using an analytical approach (in the perturbation-theory limit) that involves the modified rotating-wave approximation and the numerical solution to the Bloch equation. The dependence of the population of the upper energy level on the chirp is studied for various radiation intensities and pulse durations. An exact solution is compared with the result obtained using the modified rotating-wave approximation and the perturbation theory. It is demonstrated that, for a certain range of parameters, the excitation probability of a two-level system can be effectively controlled via a variation in the chirp.     

Theory and design of chirped dielectric laser mirrors

Received: 11 March 1997/Revised version: 26 April 1997     

Design of a widely tunable laser with a chirped ladder filter

We theoretically investigated a digitally tunable laser with a chirped ladder filter and a ring resonator to obtain a wide wavelength tuning range covering the whole C- or L- band. The clear relation between the tuning range and laser structure, especially the ladder filter, is described analytically. The introduction of a chirped structure into a ladder filter is effective in achieving both wide tunability and a stable lasing mode. A numerical simulation based on multimode rate equations shows that a tuning range of over 40 nm and a mode suppression ratio over 40 dB can be achieved by introducing a chirped ladder filter.     

Chirp control of femtosecond laser-filamentation-induced snow formation in a cloud chamber

Filamentation-induced water condensation and snow formation are investigated using laser pulses with different chirps and pulse widths. Chirped pulses result in the laser filamentation with different spatial lengths and intensities, which has a great impact on airflow motion and snow formation. The experiments show that snow formation mainly relates to the filament intensity distribution. Negative chirped pulses produce a greater amount of snow because of higher intensity inside the filaments as compared with the positive chirped pulses.     

Propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium

We investigate the propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium. For certain chirp strength and chirp width, an incident 2π nonlinear chirped pulse will split into optical precursors and a stable self-induced transparency soliton. This is caused by the particular Fourier spectrum that includes not only central resonant frequency components but also high-frequency and low-frequency sidebands. Moreover, the effects of chirp parameters on the evolution of nonlinear chirped pulses are also discussed.     

Nonlinear images of scatterers in chirped pulsed laser beams

The bandwidth and the duration of incident pulsed beam are proved to play important roles in modifying the nonlinear image of amplitude-type scatterer.It is found that the initially positive chirp-type bandwidth can suppress the nonlinear image,while the negative one can enhance it,and that both effects are inversely proportional to the incident pulse duration.Numerical simulations further demonstrate that the location of nonlinear image is at the conjugate plane of the scatterer and that,for negatively pre-chirped pulsed beam,the nonlinear image peak intensity can be higher than that in the corresponding monochromatic case under certain conditions.Moreover the effect of group velocity dispersion on nonlinear image is found to be similar to that of chirp-type bandwidth.     

啁啾超短脉冲光波照射下光栅Talbot效应的研究

应用傅里叶频谱分析法，给出了光栅在啁啾超短脉冲光波照射下菲涅耳衍射方程并分析了其Talbot效应. 数值计算表明，在Talbot距离处的衍射光强不仅与照射的超短脉冲光波的宽度有关，而且与脉冲光波的啁啾有关. 根据光栅在Talbot距离处衍射光强的变化，给出了一种检验超短脉冲光波是否含有啁啾的简单方法. 关键词： 光栅 菲涅耳衍射 Talbot效应 啁啾超短脉冲激光     

Symmetries and asymmetries in coherent atomic excitation by chirped laser pulses

We analyze the effects of laser-induced Stark shift and irreversible population loss on the technique of chirped-frequency adiabatic passage, and the ensuing symmetries and asymmetries in the ionization and fluorescence signals. We find that the properties of the detection signal depend critically on the fashion in which it is collected: for example, the post-pulse populations of the ground and excited states, and the ionization signal collected during the excitation, possess different symmetry properties with respect to the frequency chirp rate and the static frequency detuning. We illustrate these features with two exactly soluble analytic models, which describe simultaneous excitation and ionization of a two-state quantum system, as it typically occurs in atomic excitation with femtosecond laser pulses. We find that the ionization signal may exhibit unexpected oscillations and derive the conditions for maximizing their contrast.     

Above-threshold ionization of hydrogen atom in chirped laser fields

Above-threshold ionization(ATI) of a hydrogen atom exposed to chirped laser fields is investigated theoretically by solving the time-dependent Schr o¨dinger equation. By comparing the energy spectra, the two-dimensional momentum spectra, and the angular distributions of photoelectron for the laser pulses with different chirp rates, we show a very clear chirp dependence both in the multiphoton and tunneling ionization processes but no chirp dependence in the single-photon ionization. We find that the chirp dependence in the multiphoton ionization based ATI can be attributed to the excited bound states. In the single-photon and tunneling ionization regimes, the electron can be removed directly from the ground state and thus the excited states may not be very important. It indicates that the chirp dependence in the tunneling ionization based ATI processes is mainly due to the laser pulses with different chirp rates.