Electron Single and Double-slit Diffraction in the Temporal Domain

Few-cycle pulses with stable and controllable relative phase between the carrier wave and the envelope （CE phase） have become available as research tools. The actual shape of the electric field of such a pulse strongly depends on this phase, and so do the physical processes the pulse generates. Owing to its pronounced nonlinearity, above-threshold ionization （ATI） provides an excellent example. In particular, the rescattering-induced high- energy part of the ATI spectrum exhibits a dramatic dependence on the value of the CE phase. Moreover, the backward/forward symmetry of the ATI spectrum generated by a, long pulse is broken by a few-cycle pulse. Therefore, analyzing the spectrum in two opposite directions provides a very accurate means of measuring the CE phase.     

Origination of gamma-ray burst pulses associated with the Doppler effect of spherical fireballs or uniform jet

Ryde and Petrosian have pointed out that the rise phases of gamma-ray burst （GRB） pulses originate from the widths of the intrinsic pulses and their decay phases are determined by the curvature effect of the expanding fireball surface based on their simplified formula. In this paper we investigate in detail the issue based on the formula in Ref.[20], which is derived based on a model of highly symmetric expanding fireballs, where the Doppler effect is the key factor to be concerned about, and no terms are omitted in their derivation. Our analyses show that the decay phases of the observed pulses originate from the contributions from both the curvature effect of the expanding fireball and the two timescales of the local pulses, and the rise phases of the observed pulses only come from the two timescales of the local pulses. Associated with a local pulse with both rise and decay portions, the light curve of GRBs in the rise portion is expected to undergo a concave phase and then a convex one, whereas that in the decay portion is expected to evolve by an opposite process. And the ratio of the concave timescale to the convex one in the rise phase of the observed pulse linearly increases with the ratio of the rising timescale to the decay one of the local pulse （Trd）, whereas the ratio of the convex timescale to the concave timescale in its decay phase linearly decreases with Trd. The two correlations are independent of the local pulse forms and the rest-frame radiation forms. But the different forms of local pulses and the different values of Trd gives rise to the diversity of the light curve pulse shapes. We test a sample of 86 GRB pulses detected by the BATSE instrument on board the Compton Gamma Ray Observatory and find that the characteristics do exist in the light curve of GRBs.     

Pulse Amplification in Dispersion-Decreasing Fibers with Symbolic Computation

The pulse amplification in the dispersion-decreasing fiber （DDF） is investigated via symbolic computation to solve the variable-coefficient higher-order nonlinear Schrfdinger equation with the effects of third-order dispersion, self-steepening, and stimulated Raman scattering. The analytic one-soliton solution of this model is obtained with a set of parametric conditions. Based on this solution, the fundamental soliton is shown to be amplified in the DDF. The comparison of the amplitude of pulses for different dispersion profiles of the DDF is also performed through the graphical analysis. The results of this paper would be of certain value to the study of signal amplification and pulse compression.     

Supercontinuum spectra generation in the single-mode optical fibre with concave dispersion profile

In this paper, a new method is proposed to generate broad supercontinuum （SC） spectra in the single-mode optical fibre with concave dispersion profile, We numerically simulate pulse evolutions and discuss physics mechanism in detail for SC spectrum generation in the optical fibre with concave dispersion profile. Furthermore, general criteria are presented for specifying the shape of SC spectrum by introducing normalized parameters, which are related to the fibres and the initial pump pulses. The results show that the flat and broad SC spectra are indeed generated in our proposed optical fibre.     

Self-compression of femtosecond pulses in argon with a power close to the self-focusing threshold

Self-compression of femtosecond pulses in noble gases with an input power close to the self-focusing threshold has been investigated experimentally and theoretically. It is demonstrated that either multiphoton ionization （MPI） or space-time focusing and self-steepening effects can induce pulse shortening, but they predominate at different beam intensities during the propagation. The latter effects play a key role in the final pulse self-compression. By choosing an appropriate focusing parameter, action distance of the space-time focusing and self-steepening effects can be lengthened, which can promote a shock pulse structure with a duration as short as two optical cycles. It is also found that, for our calculation cases in which an input pulse power is close to the self-focusing threshold, either group velocity dispersion （GVD） or multiphoton absorption （MPA） has a negligible influence on pulse characteristics in the propagation process.     

Wavelength-dependence of double optical gating for attosecond pulse generation

Both polarization gating （PG） and double optical gating （DOG） are productive methods to generate single attosecond （as） pulses. In this paper, considering the ground-state depletion effect, we investigate the wavelength-dependence of the DOG method in order to optimize the generation of single attosecond pulses for the future application. By calculating the ionization probabilities of the leading edge of the pulse at different driving laser wavelengths, we obtain the upper limit of duration for the driving laser pulse for the DOG setup. We find that the upper limit duration increases with the increase of laser wavelength. We further describe the technical method of choosing and calculating the thickness values of optical components for the DOG setup.     

Threshold of Non－Destructive Pulse for Pt Tip of STM on Graphite Surface

The experimental relationship between width and amplitude of the non-destructive threshold pulse for Pt tip of a scanning tunneling microscope (STM) on graphite surface have been studied strictly and systematically in a wide range of pulse width for the first time. The threshold curve of amplitude versus width indicates that the amplitude of threshold pulse will increase with the decrease of the pulse width. A more rigorous explanation is suggested to interpret the dependence of threshold pulse amplitude on width. Fitted with the experimental data,a new empirical formula is given, extrapolated from which the threshold pulse amplitude will rise to 50 V when the pulse width decreases to 10ns.     

Numerical simulation for characterizing femtosecond optical pulses with the SPIDER algorithm

In this article based on the spectral phase interferometry for direct electric-field reconstruction (SPIDER), the femtosecond pulses with various phase characters are numerically simulated. The spectral phases and amplitudes of the transform-limited pulse, the linear chirped pulse, the cubic dispersion pulse, the quartic dispersion pulse, the self-phase modulation pulse and the pulses with the combination of different chirped characters are retrieved. These characterized pulses are applicable to the real-time measurement as samples for diagnosing the chirped characters of pulses quickly.     

Accurate determination of the absolute phase and temporal-pulse phase of few-cycle laser pulses

A Fourier analysis method is used to accurately determine not only the absolute phase but also the tempuralpulse phase of an isolated few-cycle （chirped） laser pulse. This method is independent of the pulse shape and can fully characterize the light wave even though only a few samples per optical cycle are available. It pavas the way for investigating the absolute phase-dependent extreme nonlinear optics, and the evolutions of the absolute phase and the temporal-pulse phase of few-cycle laser pulses.     

Effects of initial frequency chirp on the linear propagation characteristics of the exponential optical pulse

In this paper, the linear propagation characteristics of the exponential optical pulse with initial linear and nonlinear frequency chirp are numerically studied in a single mode fibre for β2 〈 0. It can be found that the temporal full width at half maximum and time-bandwidth product of exponential pulse monotonically increase with the increase of propagation distance and decrease with the increase of linear chirp C for C 〈 0.5, go through an initial decreasing stage near ζ= 1, then increase with the increase of propagation distance and linear chirp C for C 〉 0.5. The broadening of pulses with negative chirp is faster than that with positive chirp. The exponential pulse with linear chirp gradually evolves into a near-Gaussian pulse. The effect of nonlinear chirp on waveform of the pulse is much greater than that of linear chirp. The temporal waveform breaking of exponential pulse with nonlinear chirp is first observed in linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the exponential optical pulse with the frequency chirp are given by use of the numerical analysis method.     

Dynamics of traveling reaction pulses

The growth of activator losses is accompanied by the decay of a traveling reaction pulse. In a ring reactor, this propagation threshold is present simultaneously with a threshold related to the ring diameter. The results of numerical experiments with pulses of an exothermal reaction reveal the transition from pulse propagation to a homogeneous hot regime, established regimes with periodic variations of the pulse velocity, and oscillatory decay of the pulse. When the medium becomes “bistable” as a result of the variation in parameters, this factor does not prevent the propagation of pulses, but leads to changes in the pulse structure.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

流式细胞仪脉冲补偿恢复算法研究

流式细胞仪数据采集过程中,触发阈值的设定导致脉冲信号信息完整性的缺失,从而造成后续脉冲参数提取误差增大。针对以上问题,根据流式细胞仪脉冲信号类高斯特性,利用MATLAB分别采用斜线法、高斯拟合法、抛物线法以及灰色预测法对阈值以下的脉冲缺失信息进行补偿恢复。同时,分别对4种方法补偿恢复后的脉冲进行脉冲峰值、脉宽、脉冲面积三个参数的提取,并进行误差比对。仿真结果表明,触发阈值的设定主要对脉宽及脉冲面积产生影响;灰色预测法的脉冲补偿恢复误差最小,可用于流式细胞仪底层在线脉冲补偿恢复。     

Damage threshold of lithium niobate crystal under single and multiple femtosecond laser pulses: theoretical and experimental study

The damage threshold of lithium niobate crystal under single and multiple femtosecond laser pulses has been studied theoretically and experimentally. Firstly, the model for the damage threshold prediction of crystal materials based on the improved rate equation has been proposed. Then, the experimental measure method of the damage threshold of crystal materials has been given in detail. On the basis, the variation of the damage threshold of lithium niobate crystal with the pulse duration has also been analyzed quantitatively. Finally, the damage threshold of lithium niobate crystal under multiple laser pulses has been measured and compared to the theoretical results. The results show that the transmittance of lithium niobate crystal is almost a constant when the laser pulse fluence is relative low, whereas it decreases linearly with the increase in the laser pulse fluence below the damage threshold. The damage threshold of lithium niobate crystal increases with the increase in the duration of the femtosecond laser pulse. And the damage threshold of lithium niobate crystal under multiple laser pulses is obviously lower than that irradiated by a single laser pulse. The theoretical data fall in good agreement with the experimental results.     

On determining the spot size for laser fluence measurements

Energy fluence, defined as pulse energy over irradiated area, is a key parameter of pulsed laser processing. Nevertheless, most of the authors using this term routinely do not realize the problems related to the accurate measurement of the spot size. In the present paper we are aiming to approach this problem by ablating crystalline Si wafers with pulses of a commercial KrF excimer laser (λ = 248 nm, τ = 15 ns) both in vacuum and at ambient atmosphere. For any pulse energy, the size of the ablated area monotonously increases with increasing number of pulses. The difference in the ablated area could be as high as a factor of three when 2000 consecutive pulses impinge on the surface. The existence and extent of the gradual lowering of multi-pulse ablation threshold queries the applicability of routinely used procedure of dividing the pulse energy with the size of the ablated area exposed into either carbon-paper or a piece of Si with one or a few pulses when determining the fluence. A more quantitative way is proposed allowing comparison of results originating from different laboratories.     

皮秒类提升脉冲串的产生与传输

通过函数和变量变换给出变系数非线性薛定谔方程的一个新的精确解,它可以用来描述非均匀光纤中的调制不稳定过程.基于这个精确解我们给出一类新的类提升脉冲串,并讨论该脉冲串的传输特性.结果表明,当类提升脉冲串能量大于某一阈值时类提升脉冲串可以稳定传输.同时我们还比较指数控制系统和理想系统中类提升脉冲串的传输,结果表明选择合适的控制系统可以抑制脉冲间的相互作用,这对增加光通信的传输比特率是非常有益的.     

飞秒激光烧蚀石英玻璃的实验与理论研究

实验研究了800nm飞秒激光作用下石英玻璃的破坏机理和烧蚀规律，给出了破坏阈值与脉冲宽度的关系.发展了雪崩击穿模型，计算了材料的烧蚀阈值与脉冲宽度的依赖关系，烧蚀深度、烧蚀体积与脉冲能量的依赖关系，研究了导带电子的扩散对材料中激光能量的沉积、分布，以及材料的破坏阈值和烧蚀规律的影响. 关键词： 飞秒激光脉冲 破坏机理 石英玻璃 电子扩散     

New single-pulse generation technique for distributed feedback dye lasers

The distributed feedback dye laser (DFDL) generates a train of picosecond pulses when pumped well above threshold. This DFDL emission can be quenched by injecting a laser pulse into DFDL. By proper timing of the quencher laser pulse, only the first DFDL pulse is generated while the successive pulses are suppressed. Operational characteristics and practical design considerations of such a quenched DFDL are given. With 2.5 ns long pump pulses from a N₂ laser, a shortest DFDL pulse of 17 ps was obtained at 380 nm.     

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.     

Memory in nonlinear ionization of transparent solids

We demonstrate a shot-to-shot reduction in the threshold laser intensity for ionization of bulk glasses illuminated by intense femtosecond pulses. For SiO2 the threshold change serves as positive feedback reenforcing the process that produced it. This constitutes a memory in nonlinear ionization of the material. The threshold change saturates with the number of pulses incident at a given spot. Irrespective of the pulse energy, the magnitude of the saturated threshold change is constant (approximately 20%). However, the number of shots required to reach saturation does depend on the pulse energy. Recognition of a memory in ionization is vital to understand multishot optical or electrical breakdown phenomena in dielectrics.