Dynamics of femtosecond laser pulse induced damage in multilayers

We report the single-shot damage thresholds of MgF2/ZnS onmidirectional reflector for laser pulse durations from 50 A to 900 fs. A coupled dynamic model is applied to study the damage mechanisms, in which we consider not only the electronic excitation of the material, but also the influence of this excitation-induced changes in the complex refractive index of material on the laser pulse itself. The results indicate that this feedback effect plays a very important role during the damage of material. Based on this model, we calculate the threshold fluences and the time-resolved excitation process of the multiplayer. The theoretical calculations agree well with our experimental results. (c) 2005 Elsevier B.V. All rights reserved.

Recent progress in femtosecond holography

The recent advancements of femtosecond (fs) holography are introduced. The experimental requirements and the time resolution are presented. Applications of femtosecond holography to signal processing, and other femtosecond holographic techniques such as femtosecond holographic imaging and microprocessing are detailed. A potential alternative of femtosecond holography is proposed, based on the sectional interference of reference pulse with the time stretched signal pulse. (c) 2005 Elsevier B.V. All rights reserved.

Measurement of two-photon excitation cross-section of a new symmetric-type fluorene derivative

A new dye, 2,7-bis(4-methoxystyryl)-9,9-bis(2-ethylhexyl)-9H-fluorene, has been synthesized, which is a D-pi-D symmetrical-type fluorene derivative. The two-photon absorption (TPA) of this new dye has been experimentally studied by comparable two-photon-induced fluorescence method. This new dye has a TPA cross-section of 84 x 10(-50) cm(4) s/photon at 790 nm/13 fs. (c) 2004 Elsevier GmbH. All rights reserved.

Attosecond X-ray pulse obtained from linear Thomson scattering

Linear Thomson scattering by a relativistic electron of a short pulse laser has been investigated by computer simulation. Under a laser field with a pulse of 33.3-fs full-width at half-maximum, and the initial energy of an electron of gamma(0) = 10, the motion of the electron is relativistic and generates an ultrashort radiation of 76-as with a photon wave length of 2.5-nm in the backward scattering. The radiation under a high relativistic energy electron has better characteristic than under a low relativistic energy electron in terms of the pulse width and the angular distribution. (c) 2005 Elsevier GrnbH. All rights reserved.

Ionization induced scattering of femtosecond intense laser pulses in cluster plasmas

The 45 degrees scattering of a femtosecond (60 fs) intense laser pulse with a 20 nm FWHM (the full width at half maximum) spectrum centered at 790 nm has been studied experimentally while focused in argon clusters at intensity similar to 10(16) W/cm(2). Scattering spectra under different backing pressures and laser-plasma interaction lengths were obtained, which showed spectral blueshifting, beam refraction and complex modulation. These ionization-induced effects reveal the modulation of laser pulses propagating in plasmas and the existing obstacle in laser cluster interaction at high laser intensity and high electron density.

再生放大器宽带光谱整形的双折射片设计

提出了一种用于再生放大器中展宽光谱的双折射片的设计方法，利用这种方法设计了厚度为761微米的石英双折射片，并将其插入钛宝石再生放大器中，有效抑制了增益窄化效应，实验的结果和理论计算一致。使用设计的石英双折射片后光谱带宽由使用前的28nm展宽到了62nm，最后压缩脉冲为23fs。

Standing electron plasma wave mechanism of void array formation inside glass by femtosecond laser irradiation

We investigate the mechanism of formation of periodic void arrays inside fused silica and BK7 glass irradiated by a tightly focused femtosecond (fs) laser beam. Our results show that the period of each void array is not uniform along the laser propagation direction, and the average period of the void array decreases with increasing pulse number and pulse energy. We propose a mechanism in which a standing electron plasma wave created by the interference of a fs-laser-driven electron wave and its reflected wave is responsible for the formation of the periodic void arrays.

Effects on the proton production from explosions of hydrogen clusters subjected to intense femtosecond laser fields

The dependence of the maximum and average energies of protons, which were produced in the interaction of an intense laser pulse (similar to 1 x 10(16) W cm(-2), 65 fs) with hydrogen clusters in a gas jet backed up to 80 bar at liquid nitrogen temperature (similar to 80 K), on the backing pressure has been studied. The general trend of the proton energy dependence on the square of the average cluster radius, which is determined by a calibrated Rayleigh scattering measurement, is similar to that described by theory under the single size approximation. Calculations are made to fit the experimental results under a simplified model by taking into account both a log-normal cluster size distribution and the laser intensity attenuation in the interaction volume. A very good agreement between the experimental proton energy spectra and the calculations is obtained in the high- energy part of the proton energy distributions, but a discrepancy of the fits is revealed in the low-energy part at higher backing pressures which are associated with denser flows. A possible mechanism which would be responsible for this discrepancy is discussed. Finally, from the fits, a variation of the cluster size distributions was revealed to be dependent on the gas backing pressure as well as on the evolving time of the gas flow of clusters.

Distribution of the microcrystallites generated in borate glass irradiated by femtosecond laser pulses

This paper deals with the distribution of generated microcrystallites in borate glass irradiated by 120 fs laser pulses at a central wavelength of 800 nm. Raman spectroscopy is used to investigate the distribution of the high and low temperature phases of barium metaborate crystals generated in the borate glass. In combination with a microexplosion model, bond-breaking induced by laser irradiation is served as the origin of the formation of BBO crystals. Depending on the laser fluence and cooling conditions, the distribution mechanisms have been discussed. (c) 2006 Elsevier B.V. All rights reserved.

Femtosecond laser ablation of crystals SiO2 and YAG

Damage threshold of crystals SiO2 and YAG against 60-900 fs, 800 nm laser pulses are reported. The breakdown mechanisms were discussed based on the double-flux model and Keldysh theory. We found that impact ionization plays the important role in the femtosecond laser-induced damage in crystalline SiO2, while the roles of photoionization and impact ionization in YAG crystals depend on the laser pulse durations. (C) 2007 Elsevier B.V. All rights reserved.

Parasitic lasing suppression in high gain femtosecond petawatt Ti: sapphire amplifier

New parasitic lasing suppression techniques are developed and high gain amplification is demonstrated in a petawatt level Ti:sapphire amplifier based on the chirped pulse amplification (CPA) scheme. Cladding the large aperture Ti: sapphire with refractive-index matched liquid doped with absorber suppresses the transverse lasing. The acousto-optic programmable dispersive filter (AOPDF) is used to realize side-lobe suppression in the temporal profile of the compressed pulse. The 800 nm laser output with peak power of 0.89 PW and pulse width of 29.0 fs is demonstrated. (c) 2007 Optical Society of America.

Temporal confinement and enhancement of high-order harmonic emission with a synthesized laser field

We demonstrated that a synthesized laser field consisting of an intense long (45 fs, multi-optical-cycle) laser pulse and a weak short (7 fs, few-optical-cycle) laser pulse can control the electron dynamics and high-order harmonic generation in argon, and generate extreme ultraviolet supercontinuum towards the production of a single strong attosecond pulse. The long pulse offers a large amplitude field, and the short pulse creates a temporally narrow enhancement of the laser field and a gate for the highest energy harmonic emission. This scheme paves the way to generate intense isolated attosecond pulses with strong multi-optical-cycle laser pulses.

Generation of an extreme ultraviolet supercontinuum in a two-color laser field

We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.

Mechanisms of femtosecond laser-induced damage in magnesium fluoride

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump-probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF2. (C) 2006 Elsevier Ltd. All rights reserved.

不同烧蚀条件下飞秒激光脉冲诱导ZnO纳米结构研究

烧蚀条件对飞秒激光脉冲诱导氧化锌纳米结构有重要影响。研究了800nm,150fs,250kHz的飞秒激光脉冲分别在空气中,去离子水中以及无水乙醇中垂直聚焦于氧化锌晶体表面,诱导形成不同形态的纳米结构。实验结果表明,在空气中利用飞秒激光脉冲辐照样品表面,形成了周期为180nm的纳米线;在去离子水中辐照诱导形成了由氧化锌纳米线聚集而成的"纳米球";在无水乙醇中形成出现分叉结构的纳米线。拉曼光谱分析辐照前后晶体晶相结果表明,形成的纳米结构相对于辐照前特征峰437cm-1强度有所下降,在570cm-1处的峰值则显著增强。分析了在各种烧蚀条件下诱导形成纳米结构的演化过程以及物理机理。