飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明：飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。     

Ultrafast dynamics process of platinum nanoparticles under femtosecond laser

The energy relaxation dynamics process of Pt nanoparticles with a diameter of 25 nm after a 400-nm femtosecond laser excitation was investigated using pump-probe technology with 130 fs laser pulse. Experimental results show that the process includes a fast decay with time constant of 850 fs and a slow decay with time constant of 9 ps. Theoretical discussion indicates that the former results from electron-phonon coupling process, while the latter is from phonon-phonon coupling process.     

Nanoparticle plume dynamics in femtosecond laser ablation of gold

This paper describes some recent results on femtosecond laser ablation of gold. We have studied both the fast vapour/plasma and slow nanoparticle plumes using Langmuir probe, time-resolved ICCD imaging and time-resolved optical absorption measurements. The nanoparticle plume dynamics was analysed by comparing the optical emission absorption measurements with an adiabatic isentropic model of ablation plume expansion, leading to an estimate of the amount of material in the nanoparticle plume.     

Ultrafast dynamics control on ablation of Cu using shaped femtosecond pulse trains

The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.     

多脉冲飞秒激光对锗材料的烧蚀效应

 开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm^-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

多脉冲飞秒激光对锗材料的烧蚀效应

开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

Ultrafast dynamics of thermionic emission on Au film under femtosecond laser excitation

A high-fidelity numerical model for investigations of the ultrafast heating is highly desirable for simulating the pulsed laser damage and the ultrafast electron emission characteristics. However, realization of accurate predictions of thermal dynamics and thermionic electron emission remains challenging due to the high non-equilibrium state, in which the equilibrium heating parameters are invalid. Here, we report an axisymmetric two-dimensional (2-D) high-fidelity numerical model for predictions of the thermionic emission with respect to the temperature-dependent dynamics parameters. The spatio-temporal temperature evolution dynamics and the thermionic emission rate characteristics on Au film target are demonstrated, whose credibility is approved by the Au film ablation threshold experiments.     

聚合物飞秒激光加工的分子动力学建模与仿真

通过合理选择聚合物分子结构模型和高精度势能函数,建立了聚合物飞秒激光烧蚀加工的分子动力学仿真模型,并使用该模型研究了聚乙烯和聚苯乙烯飞秒激光烧蚀加工中的激光能量辐照和扩散过程。研究结果表明：激光烧蚀加工中聚合物材料的去除方式包括单链热激发引起的表面蒸发和单链热运动引起的内部热膨胀。聚苯乙烯单链的微观变形方式包括单链的整体移动和单链自身结构的变化。聚乙烯和聚苯乙烯的分子结构差异对聚合物单链变形行为和材料去除体积等烧蚀加工结果具有显著的影响。     

单脉冲和多脉冲飞秒激光对单层光学薄膜的损伤

 利用掺钛的蓝宝石飞秒激光系统输出的单脉冲和多脉冲飞秒激光(中心波长800 nm,脉宽50 fs,靶面聚焦直径Ф 40 μm),分别对BK7玻璃基底上厚约500 nm的单层HfO₂和单层ZrO₂薄膜进行辐照,得到了这两种薄膜在1-on-1和1 000-on-1测试方法下的激光损伤阈值。实验发现,两种方法下HfO₂单层膜的阈值均比ZrO₂单层膜的阈值高。从简化的Keldysh多光子离化理论出发,认为HfO₂薄膜材料的带比ZrO₂的宽是导致上述结果的主要原因。同时,同一种薄膜的多脉冲下的阈值比单脉冲下的低,原因是多脉冲下,飞秒激光对光学薄膜的损伤存在累积效应。     

单脉冲和多脉冲飞秒激光对单层光学薄膜的损伤

利用掺钛的蓝宝石飞秒激光系统输出的单脉冲和多脉冲飞秒激光(中心波长800 nm,脉宽50 fs,靶面聚焦直径Ф 40 μm),分别对BK7玻璃基底上厚约500 nm的单层HfO2和单层ZrO2薄膜进行辐照,得到了这两种薄膜在1-on-1和1 000-on-1测试方法下的激光损伤阈值。实验发现,两种方法下HfO2单层膜的阈值均比ZrO2单层膜的阈值高。从简化的Keldysh多光子离化理论出发,认为HfO2薄膜材料的带比ZrO2的宽是导致上述结果的主要原因。同时,同一种薄膜的多脉冲下的阈值比单脉冲下的低,原因是多脉冲下,飞秒激光对光学薄膜的损伤存在累积效应。     

Ultrafast electron dynamics of material surfaces under the action of femtosecond laser pulses

Electric collector investigations of the singleand multi-shot femtosecond laser ablation of optical-quality surfaces of different materials, including aluminum, copper, titanium, silicon, and graphite, show that the emission of erosion plasma is significantly lower than the energy density of laser ablation of these materials and replaces the dominant electron emission at lower energy densities. I–V characteristics and cumulative dependences of the collector signal are studied in the emission mode. The observed dependences of the electron and plasma emission signals on the laser pulse energy density are discussed.     

Molecular dynamics simulations of cluster distribution from femtosecond laser ablation in aluminum

Femtosecond laser ablation and plume evolution of aluminum is investigated for various inhomogeneous laser pulses. For the simulations of the atoms the molecular dynamics code IMD is used. The ablated gas-phase is scanned by a cluster algorithm (DBSCAN), from which we gain a cluster size distribution of the ablated material. Per single pulse, only a small portion of the total volume evaporates into the gas phase. Therefore??to have reasonable statistics??we have to deal with huge samples (6×10⁷?atoms). The ablation threshold is determined by comparing the depth of the holes to the applied fluence. Angular and velocity distributions of the plume are compared to experiments.     

Phase transitions in femtosecond laser ablation

In this study we simulate an interaction of femtosecond laser pulses (100 fs, 800 nm, 0.1-10 J/cm²) with metal targets of Al, Au, Cu, and Ni. For analysis of laser-induced phase transitions, melting and shock waves propagation as well as material decomposition we use an Eulerian hydrocode in conjunction with a thermodynamically complete two-temperature equation of state with stable and metastable phases. Isochoric heating, material evaporation from the free surface of the target and fast propagation of the melting and shock waves are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical spallation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are taken into account but the main issue of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results are in agreement with available experimental findings.     

266 nm飞秒激光烧蚀单晶硅的分子动力学模拟

 基于Stillinger-Weber(SW)势和“x-分区”模型,用分子动力学方法模拟了266 nm飞秒激光烧蚀单晶硅的过程,给出了烧蚀过程的物理图像,烧蚀过程中材料内部缺陷的产生与发展最终导致整层材料被移除。对比研究了烧蚀材料中不同区域粒子的运动轨迹,结果体现了在固、液、气不同状态下粒子的运动特征。模拟了激光诱导应力波的传播,其速度为8.18 km/s。     

Ultrafast electron dynamics on the silicon surface excited by an intense femtosecond laser pulse

The electron dynamics on the silicon surface during the pump ultrashort infrared laser pulse is studied by time-resolved optical microscopy and electron-emission measurements. It is found that the optical response of the material under the conditions where a dense electron-hole plasma is formed is determined by the renormalization of the band spectrum of the material rather than by intraband transitions of photoexcited carriers. Nonlinear Auger recombination in the plasma enhanced by the plasma-induced renormalization of the band gap and accompanied by the generation of hot charge carriers stimulates intense prompt emission of such carriers from the surface of the photoexcited material, whose work function decreases owing to the large plasma-induced renormalization of the energies of higher conduction bands.     

Evolution and termination of a femtosecond laser filament in air

The evolution of the filamentation of a femtosecond laser pulse in air is measured. The divergence of the filament core is almost constant over a long distance, encompassing a zone with efficient ionization followed by another where ionization is much weaker. At the end, the core diverges out linearly with a low divergence due to self-spatial filtering.     

Spectroscopic analysis of femtosecond laser plasma filament in air

We report a spectroscopic analysis of a filament generated by a femtosecond laser pulse in air. In the filament spectra, the characteristic Stark broadened atomic oxygen triplet centered at 777.4 nm has been observed. The measured electron impact Stark broadening parameter of the triplet is larger than the theoretical value by Griem [H.R. Griem, Plasma Spectroscopy, McGraw Hill, New York, 1964] by a factor 6.7. Using the experimental value , the plasma densities derived from Stark broadening agree well with those most recently obtained from Théberge et al.’s measurement of the nitrogen fluorescence calibrated by longitudinal diffraction [F. Théberge, W. Liu, P.T. Simard, A. Becker, S. L. Chin, Phys. Rev. E 74 (2006) 036406]. However, the Stark broadening approach is much simpler and can be used to non-invasively measure the filament plasma density distribution in air under different propagation conditions.     

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied. Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse.Besides, the energy of 400-nm pulse drops after interaction with the plasma, which is found to be another effect leading to the attenuation. To reveal the physical origin behind the energy loss, we measure fluorescence emissions of the interaction area. The fluorescence is hardly detected with the weak 400-nm laser pulse, and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown. However, when the 400-nm pulse propagates through the plasma filament, the fluorescence at 391 nm from the first negative band system of N_2~+ is enhanced, while that from the second positive band of neutral N_2 at 337 nm remains constant. Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma, which results in the enhanced fluorescence. Furthermore, the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.     

Electron dynamics and prompt ablation of aluminum surface excited by intense femtosecond laser pulse

Thin aluminum film homogeneously heated by intense IR femtosecond laser pulses exhibits on the excitation timescale consequent fluence-dependent rise and drop of the IR-pump self-reflectivity, followed by its final saturation at higher fluences F > 0.3 J/cm². This prompt optical dynamics correlates with the initial monotonic increase in the accompanying laser-induced electron emission, which is succeeded by its non-linear (three-photon) increase for F > 0.3 J/cm². The underlying electronic dynamics is related to the initial saturation of IR resonant interband transitions in this material, followed by its strong instantaneous electronic heating via intraband transitions during the pump pulse resulting in thermionic emission. Above the threshold fluence of 0.3 J/cm², the surface electronic heating is balanced during the pump pulse by simultaneous cooling via intense plasma removal (prompt ablation). The relationship between the deposited volume energy density in the film and its prompt electronic temperature derived from the self-reflection measurements using a Drude model, demonstrates a kind of electron “liquid–vapor” phase transition, driven by strong cubic optical non-linearity of the photo-excited aluminum.     

Neutrals ejection in intense femtosecond laser ablation

Time-resolved shadowgraphs and holograms of intense femtosecond laser ablation of aluminum are reported. The different bending trends of interference fringes are found in the ablations of aluminum and other materials at probe wavelengths of 400 and 800 nm, for the first time to our knowledge, which could be attributed to the ejection of massive neutral droplets induced by critical-point phase separation during the second material ejection.