An electrohydrodynamics model for non-equilibrium electron and phonon transport in metal films after ultra-short pulse laser heating

The electrons and phonons in metal films after ultra-short pulse laser heating are in highly non-equilibrium states not only between the electrons and the phonons but also within the electrons. An electrohydrodynamics model consisting of the balance equations of electron density, energy density of electrons, and energy density of phonons is derived from the coupled non-equilibrium electron and phonon Boltzmann transport equations to study the nonlinear thermal transport by considering the electron density fluctuation and the transient electric current in metal films, after ultra-short pulse laser heating. The temperature evolution is calculated by the coupled electron and phonon Boltzmann transport equations, the electrohydrodynamics model derived in this work, and the two-temperature model. Different laser pulse durations, film thicknesses, and laser fluences are considered. We find that the two-temperature model overestimates the electron temperature at the front surface of the film and underestimates the damage threshold when the nonlinear thermal transport of electrons is important. The electrohydrodynamics model proposed in this work could be a more accurate prediction tool to study the non-equilibrium electron and phonon transport process than the two-temperature model and it is much easier to be solved than the Boltzmann transport equations.     

Laser short pulse heating: Influence of pulse intensity on temperature and stress fields

Laser short-pulse heating of gold surface is considered and the influence of laser pulse intensity on the temperature and stress fields is investigated. Laser step input pulses with different pulse lengths and the same energy content are employed in the simulations. The electron kinetic theory approach employing thermomechanical coupling is introduced to model the non-equilibrium energy transport in the electron and lattice sub-systems. Thermal stress development in the lattice sub-system and temperature rise in the lattice and electron sub-systems are computed. It is found that electron temperature rises rapidly while lattice site temperature rise is gradual in the early heating period, which is more pronounced for high intensity pulses. Thermal stress component in the axial direction is compressive and its magnitude is considerably less than the yielding limit of the substrate material.     

飞秒激光辐照下单晶硅薄膜中超快能量输运的数值模拟

利用载流子输运模型对飞秒激光辐照下单晶硅亚微米薄膜中的能量输运过程进行数值模拟。研究了不同辐照能量密度和不同激光波长对载流子密度和温度超快变化过程的影响规律。结果表明,在800nm激光辐照下,不同入射能量密度仅影响载流子密度和温度响应的峰值,但达到峰值的时刻不变。平衡态的恢复过程受入射能量密度影响很小。在不同波长激光辐照下,光子能量越大,载流子密度和温度达到峰值所用时间越短,对应峰值越大,但衰减速度也越快。当入射光子能量大于单晶硅的直接带隙时,快速衰减时间常数可以与载流子能量弛豫时间相当。     

Laser heating of a moving slab: Influence of laser intensity parameter and scanning speed on temperature field and melt size

Laser melting of a moving slab is considered, and the temperature field and the phase-change in the heated region are simulated in-line with experimental conditions. The influence of laser power intensity parameter (β) and laser scanning speed on temperature field and melt depth is examined. An experiment is carried out to compare the melt layer thickness with the predictions. It is found that increase in laser power intensity parameter enhances melt size along the x-axis, despite the fact that peak temperature in the melt-pool reduces. This is more pronounced at a low laser scanning speed (0.05 m/s).     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

Radiative transfer due to atmospheric water vapor: Global considerations of the earth's energy balance

A simple analytical formulation is presented for describing radiative transfer due to atmospheric water vapor. The radiative model is then applied to a global energy balance for earth, and the net infrared flux to space is expressed in terms of the mean surface temperature and atmospheric lapse rate. Water vapor and clouds are assumed to be the only sources of infrared opacity. When compared with empirical information, and for a global mean surface temperature of 288 K, the radiative model indicates a cloud top altitude for a single effective cloud of 6·8 km. Alternatively, when applied to a more realistic three-cloud formulation, the model predicts a comparable value of 6·5 km for an average cloud top altitude.With respect to changes in mean surface temperature, again comparing with empirical results, a discussion relating to the model suggests that the cloud top altitude decreases with decreasing surface temperature, which results in the surface temperature being roughly twice as sensitive to changes in factors such as planetary albedo than for the conventional assumption of a fixed cloud top altitude. Implications of this are discussed with respect to possible albedo changes due to atmospheric particulate matter as well as cloudiness as a climate feedback mechanism.     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced predissociative fluorescence

2 (8) and Q₂(11) in OH. Received: 6 February 1997/Revised version: 28 July 1997     

Photoinduced energy transfer in blend films of hole and electron transport materials

The photoluminescence properties of the blend films consisting of the hole transport and electron transport materials, PVK and Alq₃, are studied by steady-state and time-resolved photoluminescence (PL) spectroscopy. Both the relative intensity and the photoluminescence lifetime are intensively dependent of the weight ratios of PVK and Alq₃. The detailed analysis of experiment data provides clear evidence for a Förster energy transfer from excited PVK, as donor, to Alq₃, as acceptor, based on nonradiative resonant transfer mechanism, and allows the determination of Förster radius and the concentration dependent energy transfer efficiency.     

Temperature and thickness dependence of low temperature transport in amorphous silicon thin films: A comparison to amorphous germanium

The temperature and thickness dependence of the low temperature electrical transport of amorphous silicon thin films measured in-situ in ultra-high vacuum are presented. As for a-Ge there is an extended range of $\text{ln ? ∝ T}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}$ for thick films with $\text{ln ? ∝ S}{}_2\text{T}{}^{\mathrm{<mtext>?1</mtext><mtext>3</mtext>}}$ for thin films with $\text{S}{}_2\text{∝ d}{}^{\mathrm{<mtext>?1</mtext><mtext>3</mtext>}}$ as expected from a variable range tunneling mechanism.     

Influence of the acoustic phonon scattering anisotropy on the distribution function and the electronic transport inn-Si

The excess noise of dc current-carrying thin metal wires in gases of different kind and pressure has been investigated. The noise depends on the kind of the gas. Therefore its origin must be in the gas or in the boundary between the metal and the gas. By means of a simple theory the power spectrumW _s of the random heat current _s flowing into the gas (surface source) can be calculated from the observed noise voltage. At frequencies above 100 HzW _s is given by a Nyquist formula, at lower frequencies an additional term 2 · 10^–3 · ₀ ² /N_hf is superimposed (₀ = dc heat current,f=frequency). The numberN _h characterizes this Hooge-contribution. A lot ofN _h -values for different metals, gases, pressures and heating currents have been measured.N _h is smaller for light gases and small wire diameters. For heavy gasesN _h is about ten times larger. A possible qualitative explanation is given. At higher pressures and heating currents thef ^–1 dependence turns over into af ^–3 one. This may be due to gas convection.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Influence of pulse laser energy on laser-induced voltage in La2/3Ca1/3MnO3:Ag0.04 films

With various pulse laser energy (E _pulse), La_2/3Ca_1/3MnO₃:Ag _x [La_1?x Ca _x MnO₃ (LCMO):Ag _x , x = 0.04, wt%] films were prepared on vicinal cut LaAlO₃ substrates by the pulsed laser deposition technique. It is found that laser-induced voltage (LIV) of LCMO:Ag_0.04 films was improved and enhanced by E _pulse. With E _pulse increasing, figure of merit (F _m) and anisotropic Seebeck coefficient (ΔS) of LCMO:Ag_0.04 films reached the maximum value of 109.8 mV/ns and 0.29 μV/K for E _pulse = 300 mJ. The results suggested that the LIV enhancement of LCMO:Ag_0.04 films was due to Seebeck tensor improvement, the high crystallization and oxygen balance in LCMO:Ag_0.04 films.     

双色激光场中激光强度和脉宽对线性多原子分子离子电离的影响

 利用强激光场电离和离解分子来研究分子激发态的波包结构是强场物理的重要研究方向。利用短时指数传播子对称分割法和快速傅里叶变换技术,数值求解了一维含时Schr-dinger方程,探讨了双色激光场中激光的基波和谐波强度之间的不同配比以及脉宽对线性多原子分子离子电离的影响。理论计算结果表明：基波和谐波的相对相位为π时,尽管随着激光的基波和谐波强度之间配比的变化,电离几率随原子间距变化的趋势基本保持不变,但在一定的激光基波强度下（1.2×10¹³~1.2×10¹⁵ W/cm²),激光基波强度的变化可以明显改变电离几率随原子间距变化的趋势。另外,激光脉冲的持续作用可以增强分子的电离,取原子个数为5,基频光波长为800 nm,基波与谐波的强度配比为4,频率配比为2,当其作用时间达到75 fs时,电离基本接近饱和。采用外静电场电离模型能够合理地解释这些现象。     

双色激光场中激光强度和脉宽对线性多原子分子离子电离的影响

利用强激光场电离和离解分子来研究分子激发态的波包结构是强场物理的重要研究方向。利用短时指数传播子对称分割法和快速傅里叶变换技术,数值求解了一维含时Schr-dinger方程,探讨了双色激光场中激光的基波和谐波强度之间的不同配比以及脉宽对线性多原子分子离子电离的影响。理论计算结果表明：基波和谐波的相对相位为π时,尽管随着激光的基波和谐波强度之间配比的变化,电离几率随原子间距变化的趋势基本保持不变,但在一定的激光基波强度下（1.2×1013~1.2×1015 W/cm2),激光基波强度的变化可以明显改变电离几率随原子间距变化的趋势。另外,激光脉冲的持续作用可以增强分子的电离,取原子个数为5,基频光波长为800 nm,基波与谐波的强度配比为4,频率配比为2,当其作用时间达到75 fs时,电离基本接近饱和。采用外静电场电离模型能够合理地解释这些现象。     

Influence of short-and long-range interaction forces on the efficiency of collisional vibrational energy transfer in the vibrational quasi-continuum

Based on measurements of the time-resolved delayed fluorescence, the influence of universal interactions on the collisional vibrational energy transfer is studied in mixtures of vibrationally excited polyatomic molecules (acetophenone, benzophenone, and anthraquinone) with inert bath gases (Ar, C₂H₄, SF₆, CCl₄, and C₅H₁₂). From the dependences of the decay rates of delayed fluorescence on bath gas pressure, the efficiencies of the establishment of vibrational (V-V relaxation) and thermal (V-T relaxation) equilibrium after excitation of molecules into the vibrational quasi-continuum of a triplet state are estimated. The main emphasis is on studying the dependences of the efficiency of collisional vibrational energy transfer on temperature and the molecular characteristics of collision partners. It is found that the efficiency increases with the complication of the structure of bath gases for the V-V process and decreases upon the increasing of their mass for the V-T process. For the temperature range 273–553 K, the negative temperature dependence of the V-V transfer probability and the positive (Landau-Teller) dependence of the V-T transfer probability were obtained. It is concluded that the above regularities reflect the dominant role of long-range attractive forces in initiating the quasi-resonant V-V transfer and of short-range repulsive forces in the V-T transfer of vibrational energy.     

Compton散射下固体温度变化对光声信号强度的影响

应用多光子非线性Compton散射模型,研究了固体温度变化对光声信号强度的影响。结果表明,在Compton散射光和入射光形成的耦合光强度和频率不变时,随固体温度的升高,光声信号强度非线性迅速增强;固体温度不变时,随耦合激光强度和频率的增大,光声信号强度几乎趋于0。     

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应用多光子非线性Compton散射模型,研究了固体温度变化对光声信号强度的影响。结果表明,在Compton散射光和入射光形成的耦合光强度和频率不变时,随固体温度的升高,光声信号强度非线性迅速增强;固体温度不变时,随耦合激光强度和频率的增大,光声信号强度几乎趋于0。     

二维移相光栅光强分布的计算及在制备有序纳米硅阵列中的应用

从菲涅耳衍射积分的一般形式出发,结合二维（2D）移相光栅掩模(PSGM)的具体参数,通过数值计算得到了作用于样品表面的光强分布.实验上,采用激光干涉晶化的方法,利用周期为400 nm的2D-PSGM调制KrF准分子激光器的脉冲激光束斑的能量分布,在氢化非晶硅（a-Si:H）薄膜上直接制备了2D的有序纳米硅（nc-Si）阵列.测试结果表明：2D的nc-Si阵列的周期和PSGM的相一致,晶化区域与理论模拟的结果符合得很好. 关键词： 纳米硅 激光干涉晶化 移相光栅 菲涅耳衍射     

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应用光声理论、相对论量子理论和电子与多光子集团非线性Compton散射模型,研究了Compton散射对掺杂固体中光声信号强度的影响。结果表明,当掺杂固体中发生电子与多光子集团之间的多光子非线性Compton散射时,在忽略光声转换效率的变化和基质晶格对介电函数贡献的情况下,耦合激光能量和磁感应强度是影响光声信号强度的两个主要因素,其中耦合磁感应强度起主导作用。Compton散射使光声信号强度随耦合光的磁感应强度和能量的增大而迅速增大。能量与光声信号声压之间存在对数线性关系,但它相对于Compton散射前的曲线下移。     

Compton散射对固体中激光能量与光声信号强度关系的影响

应用光声理论、相对论量子理论和电子与多光子集团非线性Compton散射模型,研究了Compton散射对掺杂固体中光声信号强度的影响。结果表明,当掺杂固体中发生电子与多光子集团之间的多光子非线性Compton散射时,在忽略光声转换效率的变化和基质晶格对介电函数贡献的情况下,耦合激光能量和磁感应强度是影响光声信号强度的两个主要因素,其中耦合磁感应强度起主导作用。Compton散射使光声信号强度随耦合光的磁感应强度和能量的增大而迅速增大。能量与光声信号声压之间存在对数线性关系,但它相对于Compton散射前的曲线下移。