Three-dimensional Wigner-function description of the quantum free-electron laser

A free-electron laser (FEL) operating in the quantum regime can provide a compact and monochromatic x-ray source. Here we present the complete quantum model for a FEL with a laser wiggler in three spatial dimensions, based on a discrete Wigner-function formalism taking into account the longitudinal momentum quantization. The model describes the complete spatial and temporal evolution of the electron and radiation beams, including diffraction, propagation, laser wiggler profile and emittance effects. The transverse motion is described in a suitable classical limit, since the typical beam emittance values are much larger than the Compton wavelength quantum limit. In this approximation we derive an equation for the Wigner function which reduces to the three-dimensional Vlasov equation in the complete classical limit. Preliminary numerical results are presented together with parameters for a possible experiment.     

Photoinjector beam quality improvement by shaping the wavefront of a drive laser with oblique incidence

To increase the quantum efficiency (QE) of a copper photocathode and reduce the thermal emittance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.     

Photoinjector beam quality improvement by shaping the wavefront of a drive laserwith oblique incidence

To increase the quantum efficiency (QE) of a copper photocathode and reduce the thermal emittance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.     

Electron radiation effects on InAs/GaAs quantum dot lasers

The InAs/GaAs quantum dot laser diodes and corresponding quantum dot samples are irradiated by 1 MeV electron. The laser performance and quantum dot photoluminescence intensity at room temperature are enhanced over a fluence range of 4 × 10¹³ cm^?2. The radiation-induced defects increase the efficiency of carrier transfer to the quantum dots, which results in the improvement of photoluminescence performance under low level displacement damage. The contact resistant of quantum dot lasers decreases because the ohmic contact is also improved by electron irradiation.     

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

Characteristics of thermionic electron emission during femtosecond laser ablation of gold film are studied numerically. For the rigorous calculation of electron and lattice temperatures, an enhanced two-temperature model with transient thermal and optical properties is developed and it is demonstrated that the model predicts the damage threshold fluences closely matching experimental data. From the calculated electron emission characteristics, quantum efficiency and electron current density are estimated.     

基于螺线管扫描法进行电磁叠加场中热发射度测量的仿真分析

在一种猝发高重频的X射线自由电子激光（XFEL）装置中,由于受到光阴极注入器内补偿螺线管与电子枪之间特殊结构的限制,阴极附近电场与磁场为叠加状态。实验中需要对阴极热发射度进行测量,而测量热发射度常用的螺线管扫描法基于几何发射度不变的前提,无法直接应用于电磁场叠加的结构。针对这一问题,考虑到归一化过程可以剔除电场对发射度的影响,基于此,研究归一化相空间中应用的螺线管扫描法,并通过仿真计算与分析,最终证明该方法适用于电磁叠加场中阴极热发射度的测量。     

Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

Non-Thermal Laser Ablation Model for Micro-Surgical Applications

We have developed a non-thermal laser ablation model which may reduce thermal damage to neighboring structures. Based on this model, the three critical parameters for a well controlled non-thermal microsurgery are (1) the laser wavelength with its photon energy matching closely the bond dissociation energy, (2) the energy fluence must be above threshold to avoid thermal process due to non-radiative relaxation from the excited electronic states to vibrational, (3) ultra short laser pulses (few fs) to completely eliminate thermal and direct biomolecular reactions. In this model the UV laser photon dissociates the molecular bonds which leads to the splitting of longer polymer chains into small fragments. The excess energy if any may appear as kinetic energy in the polymer-fragments. The extreme rapidity of the bond breaking process reduces heat conduction. The model establishes a relationship between ablation depth per pulse, the absorption coefficient, the incident laser energy fluence, and the threshold energy fluence. The ablation depths per pulse were calculated for the polymers Polymethyl methacrylate (PMMA) and polyimide for various commercially available UV lasers. It has been found that the minimum ablations depth occurs at 193 nm for both PMMA and polyimide. This assures a well defined incision with minimal thermal damage to the surrounding structures at this wavelength. There exists a definite threshold energy fluence for non-thermal ablation for any given biomolecule and below the threshold the non-radiative relaxation process may cause thermal ablation. New ultra fast lasers (few femtoseconds) (fs) will completely eliminate thermal diffusion as well as direct biomolecular reactions.     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

Improved two-temperature model including electron density of states effects for Au during femtosecond laser pulses

The electron temperature dependences of the electron–phonon coupling factor and electron heat capacity based on the electron density of states are investigated for precious metal Au under femtosecond laser irradiation. The thermal excitation of d band electrons is found to result in large deviations from the commonly used approximations of linear temperature dependence of the electron heat capacity, and the constant electron–phonon coupling factor. Results of the simulations performed with the two-temperature model demonstrate that the electron–phonon relaxation time becomes short for high fluence laser for Au. The satisfactory agreement between our numerical results and experimental data of threshold fluence indicates that the electron temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons should not be neglected under the condition that electron temperature is higher than 10⁴ K.     

Femtosecond pulsed laser damage characteristics of 7% Y2O3-ZrO2 thermal barrier coating

Ablation characteristics of 7% yttria stabilized zirconia thermal barrier coating, deposited by the electron beam physical vapor deposition method, was studied using a femtosecond pulsed laser. The single-pulse damage threshold fluence for the coating was determined to be 1.64 J cm^-2. The threshold fluence was not appreciably dependent on the initial roughness of the coating surface. An incubation effect, i.e., a decrease in the damage threshold fluence with an increase in number of pulses incident at a given location, was observed for this coating. Based on the damage morphology generated by single and multi-pulse interactions, a mechanism for the incubation effect was proposed. PACS 70.20.Ds; 42.62.Cf; 52.38.Mf; 42.62.Cf; 42.55.Px     

Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J?cm^?2 single-shot irradiation under UHV condition and 0.25–2.5 J?cm^?2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J?cm^?2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J?cm^?2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.     

Identification of non-thermal and thermal processes in femtosecond laser-ablated aluminum

Non-thermal and thermal processes due to femtosecond laser ablation of aluminum (Al) at low, moderate, and high-fluence regimes are identified by Atomic Force Microscope (AFM) surface topography investigations. For this purpose, surface modifications of Al by employing 25 fs Ti: sapphire laser pulses at the central wavelength of 800 nm have been performed to explore different nano- and microscale features such as hillocks, bumps, pores, and craters. The mechanism for the formation of these diverse kinds of structures is discussed in the scenario of three ablation regimes. Ultrafast electronic and non-thermal processes are dominant in the lower fluence regime, whereas slow thermal processes are dominant at the higher fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower fluence regime (0.06–0.5 J cm^?2 single-shot irradiation under ultrahigh vacuum condition and 0.25–2.5 J cm^?2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm^?2 multiple-shot irradiation), and a high-fluence regime 2.5–3.5 J cm^?2 multiple-shot irradiation. For the lower fluence (gentle ablation) regime, around the ablation threshold, the unique appearance of individual, localized Nano hillocks typically a few nanometers in height and less than 100 nm in diameter are identified. These Nano hillock-like features can be regarded as a nonthermal, electronically induced phase transition process due to localized energy deposition as a result of Coulomb explosion or field ion emission by surface optical rectification. At a moderate-fluence regime, slightly higher than ablation threshold multiple-pulse irradiation produces bump-formation and is attributed to ultrafast melting (plasma formation). The high-fluence regime results in greater rates of material removal with highly disturbed and chaotic surface of Al with an appearance of larger protrusions at laser fluence well above the ablation threshold. These nonsymmetrical shapes due to inhomogeneous nucleation, cluster formation, and resolidification of a metallic surface after melting are attributable to slow thermal processes (ps time scale).     

Emittance measurements of a laser-wakefield-accelerated electron beam

The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the "pepper-pot" method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as (2.7+/-0.9) pi mm mrad for (55+/-2) MeV electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.     

数值模拟飞秒激光加热金属的热电子发射

研究了超短超强激光脉冲与薄膜靶相互作用中产生的电子热发射.当超短激光脉冲与薄膜靶相互作用时,首先入射超短脉冲激光对吸收深度内的自由电子进行热激发,接下来热激发电子将能量传递到附近的晶格,再通过电子和晶格二体系的热传导,以及电子晶格间的热耦合,将能量传递到材料的内部.因此,电子在皮秒级甚至更短的时间内不能与晶格进行能量耦合,使电子温度超出晶格温度很多,电子热发射就变得非常明显了.用双温方程联合Richardson-Dushman方程的方法对飞秒脉冲激光照射金属靶的电子热发射进行了研究,结果发现电子热发射对飞     

直流光阴极注入器电子束壁损失初步研究

真空度被认为是影响GaAs阴极寿命的最重要参数之一,电子束在阴极附近的壁损失会导致真空度下降。基于高斯分布模型,研究了中国工程物理研究院自由电子激光相干强太赫兹源(FEL-THz)直流光阴极注入器电子束在束线管壁上的损失情况。通过理论分析、数值计算、束流动力学模拟、热力学分析及电子束初步出束实验研究,证明了FEL-THz电子束壁损失能达到W量级,必然引起真空度下降和阴极寿命缩短,这是目前限制出束的重要原因之一。研究表明,为维持电子束持续稳定工作,应将注入器阳极后的管道尺寸扩大到至少45mm。     

Theoretical studies of ultrafast ablation of metal targets dominated by phase explosion

Ultrashort pulse laser ablation of metallic targets is investigated theoretically through establishing a modified two-temperature model that takes into account both the temperature dependent electron–lattice coupling and the electron–electron-collision dominated electron diffusion processes for higher electron temperature regime. The electron–lattice energy coupling rate is found to reduce only slowly with increasing pulse duration, but grow rapidly with laser fluence, implying that the melting time of metallic materials decreases as the laser intensity increases. By taking phase explosion as the primary ablation mechanism, the predicted dependences of ablation rates on laser energy fluences for different laser pulse widths match very well with the experimental data. It is also found that during phase explosion the ablation rate is almost independent of the pulse width, whereas the ablation threshold fluence increases with the pulse duration even for femtosecond pulses. These theoretical results should be useful in having proper understanding of the ablation physics of ultrafast micromachining of metal targets. PACS 52.50.Jm; 61.80.Az; 72.15.Cz; 79.20.Ap; 79.20.Ds     

Laser fluence, repetition rate and pulse duration effects on paint ablation

The efficiency (mm³/(J pulse)) of laser ablation of paint was investigated with nanosecond pulsed Nd:YAG lasers (λ = 532 nm) as a function of the following laser beam parameters: pulse repetition rate (1-10,000 Hz), laser fluence (0.1-5 J/cm²) and pulse duration (5 ns and 100 ns). In our study, the best ablation efficiency (η ≅ 0.3 mm³/J) was obtained with the highest repetition rate (10 kHz) at the fluence F = 1.5 J/cm². This ablation efficiency can be associated with heat accumulation at high repetition rate, which leads to the ablation threshold decrease. Despite the low thermal diffusivity and the low optical absorption of the paint (thermal confinement regime), the ablation threshold fluence was found to depend on the pulse duration. At high laser fluence, the ablation efficiency was lower for 5 ns pulse duration than for the one of 100 ns. This difference in efficiency is probably due to a high absorption of the laser beam by the ejected matter or the plasma at high laser intensity. Accumulation of particles at high repetition rate laser ablation and surface shielding was studied by high speed imaging.     

基于紧聚焦超高斯激光脉冲的大电量低发散度电子束产生

在激光尾场加速中,光学注入是一种有效的可控电子注入机制。然而,低电量、大发散度的电子束特性无法满足实际应用的需要。为获得大电量、高品质电子束提出采用紧聚焦的超高斯激光作为注入脉冲的新型注入方案。研究发现,相比于普通高斯激光,紧聚焦的超高斯激光不仅能够将电子束发散度降低近一个数量级,而且能够保持电子束电荷量不变。通过哈密顿理论模型证实,离轴电子是发散度的主要来源,而紧聚焦的超高斯激光极大地限制了离轴电子的注入,因此有效地降低了电子束的发散度。