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 temperature relaxation evolution in Au film under femtosecond laser pulses irradiation

Ultrafast temperature relaxation processes in Au film including two temperature relaxation and thermal diffusion relaxation with femtosecond laser pulse excitation were investigated numerically by Finite Element Method (FEM). With the temperature dependent thermal parameters, the full 2D temperature field evolution in picosecond and nanosecond domains were obtained. It is proposed that the heat transfer depth can be alternatively localized or enhanced by the distinct temperature relaxation mechanisms. Moreover, the effect of laser parameters and Au film thickness and surface reflectivity on the two temperature relaxation time were analysed.     

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     

短脉冲激光对硅基探测器的热作用模型选择

从傅里叶模型和非傅里叶模型的基本方程出发,通过有限差分方法对方程进行数值求解。分别分析了10,1．0,0．1,0．01 ns这4种脉宽的脉冲激光作用于硅材料时两种传热模型温度曲线的相对变化;讨论了热弛豫时间对非傅里叶模型数值结果的影响。结果表明:脉宽小于或等于100 ps的激光作用于硅材料时,表层温度上升缓慢,会发生载流子效应,非傅里叶模型可以合理地反映这种现象;对于一般材料,载流子效应发生的条件是脉宽小于或等于材料热弛豫时间,此时应当用非傅里叶模型描述加热过程。     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Kinetics of Relaxation of Stored Light Sums in Silver Chloride

Experimental investigation of the time and temperature dependences of the process of relaxation of excited silver chloride has been carried out by the method of a photostimulated burst of luminescence. It is found that the relaxation process can be divided into two components, the rates of which differ by an order of magnitude. The activation energies of these processes of 0.03 and 0.06 eV correspond to the thermal ionization energies of electrons from shallow traps. Thermally activated relocalization of charges from shallow to deep traps in the process of relaxation has been revealed. The depth of a corresponding trap appeared equal to 0.17 eV. A model of a crystal phosphor has been suggested that contains five levels in the forbidden band: a hole center of recombination (luminescence), a deep electron trap, two shallow electron traps located at a depth of 0.03 and 0.06 eV from the bottom of the conductivity band, and a hole trap located at a height of 0.17 eV from the valence band top. Within the framework of this model, we can qualitatively explain the time and temperature dependences of stored light sums. We have shown the possibility of applying the method of a photostimulated burst of luminescence to investigation of the processes of relaxation of electron excitations in crystals to establish the relaxation mechanisms.     

Investigation of MWS Relaxation in Nylon 1010 using Dielectric Relaxation Spectroscopy

Maxwell–Wagner–Sillars (MWS) relaxation in nylon 1010, arising from charge carriers accumulated at the interphase between amorphous and crystalline regions, has been investigated by means of dielectric relaxation spectra. In the frequency spectra of nylon 1010, dielectric permittivity showed high values at low frequencies originating from charge carrier movement. For the MWS relaxation, the dielectric strength was independent of temperature. The results revealed that there is a transition temperature, located between 110 and 120°C, resulting in the separation of two different charge carrier movement mechanisms. Below and above this transition temperature, the temperature dependence of the MWS relaxation time follows the Vogel–Tammann–Fulcher type, showing that the charge carrier transport is governed by the motion of the polymer chains. The change of charge carrier movement mechanisms is due to the onset of polymer chain motion in the interphase.     

半绝缘GaAs光电导开关的瞬态热效应

实验用波长1064 nm,触发光能为1.0 mJ的激光脉冲触发电极间隙为4 mm的半绝缘GaAs光电导开关,当光电导开关的偏置电压达到3800 V时,开关进入非线性(lock-on)工作模式,在偏置电场和触发光能不变的条件下,开关输出稳定的非线性电脉冲,1500次触发后GaAs开关表面出现因丝状电流引起损伤的痕迹.分析认为:在一定触发光能和电场阈值条件下,开关芯片内存在两种瞬态热效应:热弛豫效应和光激发电荷畴-声子曳引效应.热弛豫时间很短,在皮秒甚至亚皮秒量级,热弛豫过程导致了热传导的弛豫行为;当光激发电荷畴以10⁷ cm/s的速度从阴极向阳极渡越时,在这两种效应的作用下使得开关芯片瞬态温度变化发生了弛豫振荡现象.光激发电荷畴-声子曳引效应在位错运动方向上传播,声子流携带的热能集中在移动的平面内,使得移动区域温度升高,移动轨迹经多次叠加累积呈现出丝状的损伤痕迹. 关键词： 半绝缘GaAs光电导开关 热弛豫效应 光激发电荷畴-声子曳引效应 丝状电流     

Coherent control of fragmentation of methyl iodide by shaped femtosecond pulse train

Coherent control of fragmentation of CH_3I using shaped femtosecond pulse train is investigated.The dissociation processes can be modulated by changing the separation of the shaped pulse train, and the yield of I~+under the irradiation of the optimal pulse is significantly increased compared with that using the transform-limited pulse.We discuss the control mechanism of dissociation processes with coherent interference in time domain.A three-pulse control model is proposed to explain the counterintuitive experimental results.     

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.     

准静态颗粒介质的弹性势能弛豫分析

颗粒体系是典型的多体相互作用体系, 具有多重的能量亚稳态. 对于准静态颗粒体系, 引入构型颗粒温度T_c描述弹性势能涨落. 本文认为平衡的体系具有一定的构型颗粒温度T_a, 其量值反映了其结构特征. 当外界扰动激发的构型颗粒温度超出T_a时, 产生不可逆过程. 通过对应力松弛过程的分析, 发现(T_c-T_a)激发了弹性弛豫, 且(T_c-T_a)越大则松弛过程中应力变化越大, 最终构型颗粒温度T_c→T_a时,宏观应力松弛结束,体系达到新的能量亚稳态.     

光纤中受激Brillouin散射动态弛豫振荡特性及其抑制方法

为了避免在高功率光纤放大器和光纤相位共轭镜等实际应用中因受激Brillouin散射（SBS）造成的光纤损伤,根据描述SBS动态弛豫振荡特性的振幅耦合方程,利用有限差分的数值模拟方法研究了光纤中SBS的动态弛豫振荡特性,并对其抑制进行了初步探讨.得到了光纤中SBS弛豫振荡在不同的脉冲上升时间的时空三维图;同时利用方波和脉冲光作抽运光进行了相应实验研究,实验结果与理论模拟结果符合很好.结果表明,增大抽运光脉冲上升时间可以有效抑制因SBS而产生的弛豫振荡,进而避免因其造成的光纤损伤. 关键词： 受激Brillouin散射 动态弛豫振荡特性 有限差分法     

Direct measurement of dipole-dipole/CSA cross-correlated relaxation by a constant-time experiment

Relaxation rates in NMR are usually measured by intensity modulation as a function of a relaxation delay during which the relaxation mechanism of interest is effective. Other mechanisms are often suppressed during the relaxation delay by pulse sequences which eliminate their effects, or cancel their effects when two data sets with appropriate combinations of relaxation rate effects are added. Cross-correlated relaxation (CCR) involving dipole-dipole and CSA interactions differ from auto-correlated relaxation (ACR) in that the signs of contributions can be changed by inverting the state of one spin involved in the dipole-dipole interaction. This property has been exploited previously using CPMG sequences to refocus CCR while ACR evolves. Here we report a new pulse scheme that instead eliminates intensity modulation by ACR and thus allows direct measurement of CCR. The sequence uses a constant time relaxation period for which the contribution of ACR does not change. An inversion pulse is applied at various points in the sequence to effect a decay that depends on CCR only. A 2-D experiment is also described in which chemical shift evolution in the indirect dimension can share the same constant period. This improves sensitivity by avoiding the addition of a separate indirect dimension acquisition time. We illustrate the measurement of residue specific CCR rates on the non-myristoylated yeast ARF1 protein and compare the results to those obtained following the conventional method of measuring the decay rates of the slow and fast-relaxing (15)N doublets. The performances of the two methods are also quantitatively evaluated by simulation. The analysis shows that the shared constant-time CCR (SCT-CCR) method significantly improves sensitivity.     

Electric Modulus Spectroscopic Studies of the Dielectric Properties of Carbon Nanotubes/Epoxy Polymer Composite Materials

The electrical properties of epoxy polymer/carbon nanotubes composites were characterized using impedance spectroscopy in the frequency range between 1 Hz and 10 MHz and temperature range between 25°C and 105°C. We report the analysis of the experimental data using the electric modulus formalisms to understand the dielectric relaxation mechanisms. The variation of the real and imaginary parts of the electric modulus versus frequency and temperature were suggestive of two relaxation processes, associated with dipolar relaxation and CNT-polymer interfaces. The Havriliak-Negami model of dielectric relaxation was used for modelling the relaxation processes, extracting the relaxation parameters.     

Dielectric relaxations above room temperature in DMPU derived polyaniline film

Dielectric measurements carried out on drop casted from solution of emeraldine base form of polyaniline films in the temperature range 30–300 °C revealed occurrence of two maxima in the loss tangent as a function of temperature. The activation energies corresponding to these two relaxation processes were found to be ∼0.5 eV and ∼1.5 eV. The occurrence of one relaxation peak in the dispersion curve of the imaginary part of the electric modulus suggests the absence of microphase separation in the film. Thermogravimetric analysis and infrared spectroscopic measurements showed that the films retained its integrity up to 300 °C. The dielectric relaxation at higher temperatures with large activation energy of 1.5 eV is attributed to increase in the barrier potential due to decrease in the polymer conjugation as a result of wide amplitude motion of the chain segments well above the glass transition temperature.     

A Davies/Hahn multi-sequence for studies of spin relaxation in pulsed ENDOR

We extend earlier studies of the effects of relaxation on the intensities of pulsed ENDOR signals by introducing a Davies/Hahn (D/H) pulsed ENDOR multi-sequence that corresponds to a series of Davies sequences with the preparation pulse 'turned off'. In this pulse train, the Hahn [pi/2, pi] detection pulse pair of sequence n-1 both generates the echo detected for that sequence and acts as the preparation portion of sequence n, in effect replacing the pi preparation pulse of the Davies sequence. We show both theoretically, through a master-equation approach, and with both (1)H(I=1/2) and (14)N(I=1) ENDOR experiments on the non-heme Fe enzymes, superoxide reductase (SOR) (S=1/2) and AntDO (S=3/2), that under conditions of high electron-spin polarization (high microwave frequency/low temperature) the D/H multi-sequence allows simplification of ENDOR spectra by suppression of nuclear transitions associated with the m(S)=+1/2 (alpha) manifold. As such suppression depends on the sign of A, it allows determination of this sign. The suppression as a function of the time between individual sequences is found to exhibit behaviors that can be classified into three regimes of the ratio of cross-relaxation to spin-lattice relaxation rates: strong cross-relaxation (X-case); comparable rates (XL); negligible cross relaxation (L). Interestingly, the ENDOR behavior of the S=1/2 SOR center indicates it is an L case, while the S=3/2 AntDO is an L case. Overall, the D/H protocol appears to be a robust and general tool for using relaxation effects to manipulate ENDOR spectra.     

Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization

The magnetization under the spin-lattice relaxation and the nuclear magnetic resonance radiofrequency (RF) pulses is calculated for a signal RF pulse train and for a sequence of multiple RF pulse-trains. It is assumed that the transverse magnetization is zero when each RF pulse is applied. The result expressions can be grouped into two terms: a decay term, which is proportional to the initial magnetization M0, and a recovery term, which has no M0 dependence but strongly depends on the spin-lattice relaxation and the equilibrium magnetization Meq. In magnetic resonance pulse sequences using magnetization in transient state, the recovery term produces artifacts and can seriously degrade the function of the preparation sequence for slice selection, contrast weighting, phase encoding, etc. This work shows that the detrimental effect can be removed by signal averaging in an eliminative fashion. A novel fast data acquisition method for constructing the spin-lattice relaxation (T1) map is introduced. The method has two features: (i) By using eliminative averaging, the curve to fit the T1 value is a decay exponential function rather than a recovery one as in conventional techniques; therefore, the measurement of Meq is not required and the result is less susceptible to the accuracy of the inversion RF pulse. (ii) The decay exponential curve is sampled by using a sequence of multiple pulse-trains. An image is reconstructed from each train and represents a sample point of the curve. Hence a single imaging sequence can yield multiple sample points needed for fitting the T1 value in contrast to conventional techniques that require repeating the imaging sequence for various delay values but obtain only one sample point from each repetition.     

The relaxations of temporal bond polarizabilities of methylviologen adsorbed on the Ag electrode by 514.5 nm excitation: a Raman intensity study

An algorithm to elucidate the temporal bond polarizabilities from the surface enhanced Raman (SERS) intensities was employed to the case of methylviologen (MV) adsorbed on the Ag electrode. This enables us to obtain the properties of its SERS mechanisms and the effect of its adsorption. The analysis shows that the charge transfer and electromagnetic mechanisms involving in this MV SERS system possess different relaxation times for its various temporal bond polarizabilities. The physics is that the process involved in the charge transfer mechanism will take longer time than that involved in the electromagnetic mechanism since it needs more time to redistribute the charges during relaxation. The time division between these two mechanisms is figured out to be around 3 ps for this system. Adsorption also enhances the relaxation of the temporal bond polarizabilities, in general. The adsorption effect is indicated by the temporal bond polarizabilities close to the final stage of relaxation. They are, in fact, the quantities parallel to the bond electronic densities in the molecular orbital (MO) concept. For comparison, the case of MV solid was also analyzed. Copyright © 2008 John Wiley & Sons, Ltd.     

基于光子晶体光纤中脉冲俘获的超高速光开关

提出一种基于光子晶体光纤中脉冲俘获现象的超高速全光开关.信号脉冲处于正常色散区,它们彼此的时域间隔为1 ps.通过数值求解光子晶体光纤中脉冲传播满足的耦合非线性薛定谔方程,发现用孤子脉冲可俘获信号脉冲串中的任何一个脉冲,被俘获的信号脉冲的中心波长明显蓝移,在频域上和其他信号脉冲分离开来,于是让信号脉冲串在输出端通过布喇格光纤光栅,被俘获的信号脉冲将被过滤掉.数值模拟表明,用脉冲俘获实现的光开关响应速率可以达到1 THz.     

可激发气体振动弛豫时间的两频点声测量重建算法

振动弛豫时间是可激发气体分子内外自由度能量转移速率的宏观体现,它决定了声吸收谱峰值点对应的弛豫频率.本文给出了等温、绝热定压和绝热定容三种不同热力学过程下振动弛豫时间的相互关系;基于Petculescu和Lueptow[2005 Phys.Rev.Lett.94 238301]的弛豫过程合成算法,推导了单一压强下两频点声测量值的弛豫时间重建算法.该算法可应用于等温、绝热定压、绝热定容弛豫时间和弛豫频率的重建测量,并避免了弛豫时间传统声测量方法需要不断改变气体腔体压强的问题.仿真结果表明,对于室温下CO_2,CH_4,Cl_2,N_2和O_2组成的多种气体,重建的弛豫时间和弛豫频率与实验数据相符.