Ultrafast thermalization characteristics in Au film irradiated by temporally shaped femtosecond laser pulses

The ultrafast thermalization processes of Au film irradiated by multi-pulse sequences with variable temporal separation were investigated by numerical simulations. Two temporally sequential thermal relaxation mechanisms including the two temperature relaxation and the thermal diffusion relaxation were demonstrated. With inclusion of the electron ballistic effect, we obtained the full 2-D temperature fields evolution dominated by the two distinct relaxation mechanisms. It is proposed that the laser thermalization processes can be greatly promoted through choosing the optimized temporal separation of the pulse train. We also found that the two temperature relaxation period can be modulated by changing the pulse train parameters such as the temporal separation, the pulse durations and the pulse number in a train.     

Insight into the thermionic emission regimes under gold film thermal relaxation excited by a femtosecond pulse

We theoretically investigated different thermal relaxation participating in the ultrafast thermionic emission processes on gold film surface with a femtosecond pulse excitation. The thermionic emission regimes under the two temperature relaxation and the thermal diffusion relaxation were demonstrated. The simulations showed that the thermionic emission properties can be defined in the regime under two temperature relaxation by reducing the laser fluence, or widening the pulse duration or increasing the laser wavelength. It was also found that there exists a transition between the two distinct thermionic emission regimes under peculiar laser parameters of laser fluence, pulse duration and laser wavelength. The results were explained as significant intervene of laser irradiation parameters into gold film thermal relaxation processes.     

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.     

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.     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

Heating process and damage threshold analysis of Au film coated on Cu substrate for femtosecond laser

The heating processes of a two-layer film assembly of Au padded with Cu irradiated by femtosecond laser pulse are studied using a two-temperature model. It is found that the chosen substantially influence the energy transport, and consequently the temperature variation, and thermal equilibrium time. At the same laser fluence, the different thickness of gold film leads to a change of gold surface temperature. By choosing the thickness of the gold layer in the two-layer film assemblies, the damage threshold of the gold film can be maximized. The results can be used to optimize the damage threshold of gold coating optical components.     

激光辐照下皮肤组织光热响应有限元分析

为了更好地选择临床激光医疗曝光参量,采用有限元数值计算方法,模拟了脉冲激光与连续激光对人皮肤组织的光热作用及导致的温度变化效应,比较了两者的不同,得到了热响应时间及热弛豫时间与组织深度的关系,即组织越深(0～60 μm),其热响应时间(0～4 ms)与热弛豫时间(0.4～12.1 ms)越长;分析了激光脉宽长短对组织升温的影响;建立了评价脉冲间热损失的评价函数δ,并以此对脉冲间隔的选取作了探讨.     

Plasmonic enhancement of the vanadium dioxide phase transition induced by low-power laser irradiation

Nanocomposites consisting of gold nanoparticle (NP) arrays and vanadium dioxide (VO₂) thin films are noteworthy for the tunability of both their thermal and optical properties. The localized surface plasmon resonance (LSPR) of the Au can be tuned when its dielectric environment is modulated by the semiconducting-to-metal phase transition (SMT) of the VO₂; the LSPR itself can be altered by changing the shape of the NPs and the pitch of the NP array. In principle, then it should be possible to choose a combination of VO₂ film and Au LSPR properties that maximizes the overall optical response of the nanocomposite. To demonstrate this effect, transient transmission measurements were conducted on lithographically fabricated arrays of Au NPs of diameter 140?nm, array spacing 350 nm, and covered with a 60?nm thick films of VO₂ via pulsed laser deposition. Both Au::VO₂ nanocomposites and bare VO₂ film were irradiated with a shuttered 785?nm pump laser, and their optical response was probed at 1550?nm by a fixed-frequency diode laser. The Au::VO₂ nanocomposite exhibited an increased effective absorption coefficient 1.5 times that of the plain film and required 37?% less laser power to induce the SMT. The time-dependent temperature rise in the film as a function of laser intensity was calculated from these measurements and compared with both analytic and finite-element models. Our results suggest that Au::VO₂ nanocomposites may be useful in applications such as thermal-management coatings for energy efficient ??smart?? windows.     

超短脉冲激光辐照金属薄膜温升效应的模拟研究

 考虑到材料的质量热容、热导率、驰豫时间等热力学参数随温度非线性变化因素的影响，利用具有人工粘性的、自适应时间步长的前向差分算法，数值求解了电子-晶格双温双曲两步热传导模型，讨论了厚度为50 nm的金膜在0.1 ps脉冲激光辐照下的温升规律。数值结果表明：薄膜前表面自由电子的温度在大约0.27 ps时达到最大值，不同厚度上自由电子达到温度平衡所需的时间大约为1.6 ps，而薄膜温度在整个厚度上达到平衡所需时间为60 ps左右。由电子温度及其温度梯度引起的热电子崩力很可能是造成材料破坏的一个主要因素。     

Formation of nanostructures by an intense femtosecond pulse laser irradiating Au film on sapphire substrate

We investigate the morphology change of Au film on sapphire substrate by irradiating with a 1 kHz femtosecond pulse laser. Under observation of a scanning electron microscope, a textured nanostructure was formed in the exposed area on Au film due to laser ablation and subsequent stress relaxation. This process was strongly determined by the laser intensity profile and the dynamics of molten liquid. With the increasing of laser pulses number, the Au film was broken down and then a few polarization-dependent nanoripples arranged in the same direction appeared on the sapphire surface, which may result from a spatial modulation of energy due to the interference between the incident light and the excited surface plasmon polaritons. In addition, we used an energy dispersive spectrometer to analyze the chemical composition of nanoripples on the surface and in the ablated crater, respectively. The changes of O and Al elements implied that a complicated chemical transformation participated in the nanoripples formation process. We believe that present results are very useful for the analysis of the interaction between femtosecond laser and solids, especially the film material.     

Laser reflectance relaxation spectroscopy: Determination of thermal diffusivity in thin films of gold

The technique of thermal modulation is applied to the study of transient thermal properties of thin films of Au. Relaxation spectra as seen in the reflected laser light give a value for the thermal diffusivity of Au as κ ? 1.2 cm²/sec, comparable to previous data for the bulk metal. Versatility and potentials of the laser reflectance relaxation spectroscopy are discussed.     

Thermalization time of thin metal film heated by short pulse laser

Based on the hyperbolic two-step heat conduction model, using the Laplace transform and numerical inverse transform method (Riemann-sum approximation method), the thermal behaviour of thin metal films has been studied during femtosecond pulse laser heating. Also the thermalization time, which is the time for the electron gas and solid lattice to reach thermal balance, has been studied in detail. The values of thermalization time for silver (Ag), gold (Au), copper (Cu) and lead (Pb) are obtained. The effects of material parameters of the thin metal film on the thermalization time are considered for the four kinds of metals by changing one of the parameters and regarding the other parameters as constant. For a typical metal material, the order of the thermalization time is of the order of hundreds of picoseconds. The thermalization time decays exponentially with the increase of phonon－electron coupling factor or electron gas thermal conductivity, and it increases linearly with the increase of the ratio of lattice heat capacity to electron gas heat capacity. However, the relaxation time of the electron gas has very little effect on the thermalization time.     

Ion crater healing and variable temperature ellipsometry as complementary probes for the glass transition in thin polymer films

Poly(methyl methacrylate) (PMMA) thin films of various tacticity and thickness were bombarded at grazing angles by 20 MeV Au ions at different temperatures. The shape of the tracks was investigated by scanning force microscopy (SFM) after annealing for various time at different temperatures and constant quenching rate. The thickness dependent glass transition temperature, T(g)(h), was estimated from the temperature of relaxation of ion-caused nanodeformations in the films. T(g)(h) obtained from the thermal healing of the holes and hillocks is found in good agreement with the one determined by variable temperature ellipsometry for PMMA film thickness of 80 nm and corresponds to the T(g) of each bulk PMMA stereoisomer. Below this thickness, some significant divergences are observed between the T(g) measured by the two techniques. We propose that the healing of ion crater hillock and the kink in the thermal expansion arise from the different nature of chains motions which are perturbed to different extents according to the main polymer chain preferential orientation in the thin film. This can be tentatively interpreted by a so-called "anisotropic" character of the glass transition.     

飞秒激光热反射系统测量金属薄膜中的热波

金属材料中的热波现象可以利用包含电子弛豫时间影响的双曲两步模型进行理论分析.通过飞秒激光热反射实验系统对金属薄膜材料进行了测量.利用偏振分光棱镜将飞秒激光分成抽运光和探测光,其中较强的抽运光用于加热金属薄膜而较弱的探测光用于探测薄膜表面反射率随时间的变化,两束光之间的光程差通过步进电机进行精确控制.利用金属薄膜反射率和电子温度的正比例关系就可以得到电子温度随时间的变化规律.实验发现在加热激光脉冲过后的电子温度下降区间会出现另一个较弱的电子温度波峰,并利用相同厚度的两块金属薄膜样品重复测量对实验结果进行了验证.理论上这一现象可以解释为金属薄膜中热波在背面反射的结果,并且实验结果和双曲两步模型给出的热波理论计算结果相符合.根据实验结果计算出热波传递速度约为5×105m/s,对应的电子弛豫时间为60fs.     

激光辐射致热单层薄膜温度响应

用热传导方程计算了在脉宽10ns、波长1.06μm的激光辐射下,TiO_2单层膜的温度场分布.结果表明:膜层的热参数增大,其峰值温度明显降低,而基板热参数变化对膜层温度响应影响很小,温度场分布由电场分布决定,1/4波长薄膜的峰值温度低于半波长薄膜的峰值温度.     

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

This paper presents the use of molecular dynamics simulation in the study of laser-induced thermal desorption (LITD) of water molecules adjacent to a laser-heated Au substrate. The local structure of the water molecules is investigated by considering the densities of the oxygen and hydrogen atoms, the average number of neighbors, n_NN, and the average number of H-bonds, n_HB. At an equilibrium temperature of 300 K, the simulation results show that three adsorption water layers are formed in the immediate vicinity of the Au surface, and that each four-fold hollow site on the uppermost Au(0 0 1) surface is occupied by a single water molecule. Following laser-induced heating of the Au substrate with a sub-picosecond laser pulse of 350 fs, the substrate temperature increases to 1000 K. This causes a gradual heating of the adjacent water film, which is accompanied by a decrease in the values of n_NN and n_HB. Hence, it can be concluded that an increase in the water film temperature destroys the hydrogen-bonding network throughout the water film. Although the maximum local temperature of the water film occurs in the region immediately adjacent to the Au substrate, it is determined that the attractive energy between the Au atoms and the water molecules in this region causes the water molecules to aggregate together to form three-dimensional water clusters. Furthermore, this energy prevents the hydrogen bonds in this region from breaking apart as violently as those within the phase explosion region. Finally, it is observed that the phase explosion phenomenon occurs in the region of the water film where the values of n_NN and n_HB are at a minimum.     

The formation of different structures in the interaction between a single femtosecond laser pulse and a thin Au film

The interactions between femtosecond (fs) laser pulses and a thin Au film deposited on a silica glass substrate were systematically investigated based on experimental data. Different structures, including microholes, nanoholes, and nanobumps, are obtained when pulses with different energies are incident on the surface of a gold film. The experimental results are discussed according to specific experimental parameters. Two physical models were constructed in order to explain the experimental results. The formation of nanoholes in a silica substrate is attributed to etching by higher order harmonic generations (HHG) when the femtosecond laser pulse interacts with the generated plasma layer, while the formation of nanobumps on the surface of an Au film is attributed to the elastic and plastic characteristics of the metal film under laser pulse irradiation.     

RIBLLⅡ与CSRe中束流控制系统的设计

介绍了兰州重离子加速器冷却存储环（HIRFL-CSR）的实验环CSRe以及次级束线RIBLLⅡ中束流控制系统的设计。该系统主要采用了Java,COM,Oracle,ARM,DSP,FPGA等技术实现了对磁铁电源的实时、同步控制,已达到对束流的控制。该系统已经运行于现场的束流调试中,并在RIBLLⅡ的束流调试中运行正常、性能稳定。     

The characteristics of Au:VO2 nanocomposite thin film for photo-electricity applications

Au nanoparticles have been fabricated on normal glass substrates using nanosphere lithography (NSL) method. Vanadium dioxide has been deposited on Au/glass by reactive radio frequency (rf) magnetron sputtering. The structure and composition were determined by X-ray diffraction and X-ray photoelectron spectroscope. Electrical and optical properties of bare VO₂ and Au:VO₂ nanocomposite thin films were measured. Typical hysteresis behavior and sharp phase transition were observed. Nanopartical Au could effectively reduce the transition temperature to 40 °C. The transmittance spectrum for both Au:VO₂ nanocomposite thin film shows high transmittance under transition temperature and low transmittance above transition temperature. The characteristics present the Au:VO₂ nanocomposite thin film can be used for applications, such as “smart window” or “laser protector”.     

一种基于激光辐照热效应的薄膜参数反演方法

本文研究并建立了一种基于激光辐照热效应的薄膜参数反演方法.首先给出激光辐照薄膜产生温升问题的热传导理论模型,并利用拉普拉斯变换得到了膜层和基底温度场的解析解;然后以膜层和基底的导热系数为反演参数,基于非线性共轭梯度算法给出反演基本原理及流程,并推导得到了反演过程中灵敏度系数的解析表达式;以aluminum,silver,copper和gold四种金属薄膜为例,通过与有限元法的计算结果对比验证了温度场解析解的正确性;最后结合四种金属薄膜进行了参数反演,通过考察分析不同随机噪声等条件下的参数反演结果,验证了本文方法在薄膜参数反演精度与反演效率等方面的有效性.反演结果显示:本文方法具有较高的反演精度和效率,在迭代截止误差为10~(-7)时只需用少于20次迭代就能收敛;在测量数据中加入的随机噪声越小,反演的迭代收敛次数就越少,即使是在迭代初值与反演结果相差较大时,用包含5%随机噪声的测量数据反演也能快速收敛.本文提出的薄膜参数反演方法不仅适用于反演导热系数,也可扩展用于反演膜层反射系数或吸收率等参数,具有一定的适用性.本文方法对于激光加工或激光损伤过程中的参数反演及优化具有一定的指导意义.