极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外（XUV）阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展．讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应．采用彤作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰．在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍．并且

Electron quantum path tuning and isolated attosecond pulse emission driven by a waveform-controlled multi-cycle laser field

We investigate high-order harmonic emission and isolated attosecond pulse (IAP) generation in atoms driven by a two-colour multi-cycle laser field consisting of an 800 nm pulse and an infrared laser pulse at an arbitrary wavelength. With moderate laser intensity, an IAP of similar to 220 as can be generated in helium atoms by using two-colour laser pulses of 35 fs/800 nm and 46 fs/1150 nm. The discussion based on the three-step semiclassical model, and time-frequency analysis shows a clear picture of the high-order harmonic generation in the waveform-controlled laser field which is of benefit to the generation of XUV IAP and attosecond electron pulses. When the propagation effect is included, the duration of the IAP can be shorter than 200 as, when the driving laser pulses are focused 1 mm before the gas medium with a length between 1.5 mm and 2 mm.

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed. (C) 2008 Optical Society of America.

Femtosecond laser-induced periodic surface structure on ZnO

Three different ZnO nanostructures include nanoparticles, ripples and regular nanogratings were successfully prepared by femtosecond laser irradiation under different experimental conditions. The in-situ observation of the second harmonic generation (SHG) excited in ZnO crystals before, during, and after the formation of the nanostructures was investigated. The obtained results show that the formed nanostructures contribute to the enhancement of the SHG. We propose that the second harmonics in the sample surface plays an important role in the formation of nanostructures. (c) 2007 Published by Elsevier B.V.

Formation of X-waves at fundamental and harmonics by infrared femtosecond pulse filamentation in air

We experimentally observe the formation of X-waves at fundamental, third harmonic, and fifth harmonic wavelengths by infrared (central wavelength at similar to 1500 nm) femtosecond laser pulse filamentation in air. By fitting the angularly resolved spectra of the fundamental and harmonic waves using X-wave relations, we confirm that all the X-waves have nearly the same group velocity, indicating that they are locked in space and time during their propagation in filament.

Phase control of higher spectral components in the presence of a static electric field

We investigate the higher spectral component generations driven by a few-cycle laser pulse in a dense medium when a static electric field is present. Our results show that, when assisted by a static electric field, the dependence of the transmitted laser spectrum on the carrier-envelope phase (CEP) is significantly increased. Continuum and distinct peaks can be achieved by controlling the CEP of the few-cycle ultrashort laser pulse. Such a strong variation is due to the fact that the presence of the static electric field modifies the waveform of the combined electric field, which further affects the spectral distribution of the generated higher spectral components.

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm. centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr-5(PO4)(3)F laser. The broad gain bandwidth can completely support similar to 6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm. (c) 2005 Elsevier B.V. All rights reserved.

Optimization of single attosecond x-ray pulses by genetic algorithm control of the chirp and initial phase of 5 fs laser pulses

We show that the peak intensity of single attosecond x-ray pulses is enhanced by 1 or 2 orders of magnitude, the pulse duration is greatly compressed, and the optimal propagation distance is shortened by genetic algorithm optimization of the chirp and initial phase of 5 fs laser pulses. However, as the laser intensity increases, more efficient nonadiabatic self-phase matching can lead to a dramatically enhanced harmonic yield, and the efficiency of optimization decreases in the enhancement and compression of the generated attosecond pulses. (c) 2006 Optical Society of America.

Ultraviolet-infrared femtosecond laser-induced damage in fused silica and CaF2 crystals

The damage in fused silica and CaF2 crystals induced by wavelength tunable femtosecond lasers is studied. The threshold fluence is observed to increase rapidly with laser wavelength lambda in the region of 250-800 nm, while it is nearly a constant for 800

Optimal feedback control of two-photon fluorescence based on genetic algorithm

An optimal feedback control of two-photon fluorescence in the ethanol solution of 4-dicyanomethylene-2-methyl-6-p-dimethyl-amiiiostryryl-4H-pyran (DCM) using pulse-shaping technique based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence of the DCM ethanol solution is enhanced in intensity of about 23%. The second harmonic generation frequency-resolved optical gating (SHG-FROG) trace indicates that the effective population transfer arises from the positively chirped pulse. The experimental results appear the potential applications of coherent control to the complicated molecular system.

Optimal feedback control of two-photon fluorescence in Coumarin 515 based on genetic algorithm

An optimal feedback control of two-photon fluorescence in the Coumarin 515 ethanol solution excited by shaping femtosecond laser pulses based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence intensity can be enhanced by similar to 20%. Second harmonic generation frequency-resolved optical gating traces indicate that the optimal laser pulses are positive chirp, which are in favor of the effective population transfer of two-photon transitions. The dependence of the two-photon fluorescence signal on the laser pulse chirp is investigated to validate the theoretical model for the effective population transfer of two-photon transitions. The experimental results appear the potential applications in nonlinear spectroscopy and molecular physics. (c) 2005 Elsevier B.V. All rights reserved.

Formation mechanisms of uniform arrays of periodic nanoparticles and nanoripples on 6H-SiC crystal surface induced by femtosecond laser ablation

Uniform arrays of periodic nanoparticles with 80-nm period are formed on 6H-SiC crystal irradiated by circularly polarized 400-nm femtosecond laser pulses. In order to understand the formation mechanism, the morphology evolvement as a function of laser pulse energy and number is studied. Periodic nanoripples are also formed on the sample surface irradiated by linearly polarized 400-, 510- and 800-nm femtosecond laser pulses. All these results support well the mechanism that second-harmonic generation plays an important role in the formation of periodic nanostructures.

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femto-second laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by similar to 16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase. (c) 2006 Elsevier B.V. All rights reserved.

飞秒激光在6H SiC晶体表面制备纳米微结构

激光诱导周期性纳米微结构在多种材料包括电介质、半导体、金属和聚合物中观察到。研究了800 nm和400 nm飞秒激光垂直聚焦于6H SiC晶体表面制备纳米微结构。实验观察到800 nm和400 nm线偏光照射样品表面分别得到周期为150 nm和80 nm的干涉条纹, 800 nm圆偏振激光单独照射样品表面得到粒径约100 nm的纳米颗粒。偏振相互垂直的800 nm和400 nm激光同时照射晶体得到粒径约100 nm的纳米颗粒阵列, 该纳米阵列的方向随400 nm激光强度增加而向400 nm偏振方向偏转。利