Variation of coating transmission under intense laser irradiation

A model that considers both thermal expansion and thermo-optical effects is developed to investigate the transmission variation of optical coatings when they are exposed to an intense laser beam. Our results indicate that a higher gradient of the transmission spectrum curve at a certain wavelength leads to a more evident variation of the coating transmission. Three customized Hf O2–Si O2 multilayer coatings with different transmission spectra are used to measure the transmitted power under the irradiation of a 1080 nm continuous-fiber laser. Excellent agreement is found between the experimental result and the theoretical prediction. Our result is helpful for the improvement of such devices in the application of high-power laser systems.     

X-ray emission from aluminium under intense, ultrashort irradiation

We present studies of X-ray emission from aluminium under picosecond and femtosecond irradiation in the intensity range 10¹²-10¹⁵ W cm^-2. We use a new and simple method to measure spectrally resolved absolute X-ray yields. It is shown that the X-ray yields can be obtained for arbitrary levels of X-ray flux. We present details of the variation of the absolute yields as a function of wavelength, intensity, polarization and pulse duration of the incident laser radiation. Electron temperatures in the keV range are observed at 10¹⁵ W cm^-2 with femtosecond laser pulses. Received 12 August 1999 and Received in final form 29 November 1999     

Anisotropic explosions of hydrogen clusters under intense femtosecond laser irradiation

We report on measurements of ion energy distributions from hydrogen clusters irradiated by intense laser pulses of duration 40 and 250 fs. Contrary to the predictions of a simple Coulomb explosion model, we observe a pronounced spatial anisotropy of the ion energies from these explosions with the highest energy ions ejected along the laser polarization direction. The origin of the anisotropy is distinct from that previously seen in clusters of high Z atoms such as Ar and Xe. Furthermore, a measured increase in H+ ion energy when longer, lower intensity pulses are employed suggests that multiple-pass, vacuum heating of the cluster electrons is important in the deposition of energy by the laser.     

激光辐照下PV型HgCdTe探测器反常响应机理

 利用PV型探测器开路电压的表达式,并考虑探测器的温度变化建立模型,对激光辐照下PV型HgCdTe探测器开路电压的变化进行了理论计算。当激光较弱时,计算结果与实验结果符合得很好。当激光较强时,对于辐照过程当中探测器输出变化的一般性趋势以及激光完全停照后的热弛豫过程,该模型也能给出较好的解释;但对于激光开始辐照时输出下跳和激光停止辐照时输出上跳的反常现象,该模型不能给出合理的解释。分析认为,该模型较好地描述了晶格温升对探测器输出的影响,但是它没有考虑热载流子效应;当激光较强时,热载流子效应不可忽略,特别是激光开始辐照和激光停止辐照时,载流子与晶格的温度差有比较明显的快速变化,从而导致了探测器的反常响应。     

Size dependence of the maximum energy of protons from Coulomb explosion of methane clusters under intense femtosecond laser irradiation

An investigation of the cluster size dependence of the maximum energy of protons ejected from explosion of methane clusters in an intense femtosecond laser field has been conducted on the basis of the cluster size estimation by Rayleigh scattering measurements. The interaction of a 2 × 1016-W/cm2 intense laser pulse (790 nm, 60 fs) with the methane clusters revealed that the clusters were Coulomb exploded and the maximum energy (Emax) of the protons produced was linearly proportional to the square of the cluster radius (r2c). In a cluster size range, with the methane cluster radii up to about 3 nm, the established relation of Emax and rc2 was found to be Emax (keV) = 3.3 + 0.75r2c (nm2), in good agreement with the simulation results. This demonstrated that Coulomb explosion of ionic clusters (C+4H4+)n took place following the cluster vertical ionization in the laser-cluster interaction.     

Electronic subband structure in two-dimensional electron gases under intense laser radiations

We present a theoretical study on the influence of intense terahertz (THz) electromagnetic (EM) radiations on the electronic structures in a modulation-doped two-dimensional (2D) semiconductor system. We find that in the presence of a THz EM radiation field polarised linearly along the 2D-plane of a 2D electron gas (2DEG), the electron density of states will be modified strongly by the intensity and the frequency of the radiation field. As a consequence, the Fermi-energy and the electron density in the system will be affected markedly by the EM radiations. The results have indicated that the photon-modified electronic subband structure in a 2DEG device can be observed by using the recently developed free-electron laser sources.     

多波长激光同时辐照下熔石英元件的损伤研究

对比研究了3ω单独辐照、3ω＋2ω和3ω＋1ω双波长同时辐照下熔石英元件的初始损伤和损伤增长规律,重点研究3ω能量密度在其阈值附近时,低能量密度的2ω和1ω对初始损伤和损伤增长的影响,分析了波长间的能量耦合效应。结果表明：双波长同时辐照下,当2ω和1ω能量密度远低于其自身阈值时,它们对初始损伤几率和损伤增长阈值的影响可以忽略,但也会参与初始损伤和损伤增长过程,会增加初始损伤程度和损伤增长系数。基于飞秒双脉冲成像的冲击波速度测量表明,3ω和1ω同时辐照下,波长间的能量耦合效应会促进激光能量向材料沉积的效率。     

Acousto-optical domain generation in CdS under intense laser irradiation and accompanying phenomena

This paper is concerned with the investigation of a longitudinal electric field (1 V/cm) arising in a homogeneous piezo-electric crystal at a high level of optical excitation. The sign of the effect corresponds to free carrier drag (electrons — in the case of CdS) in the direction of light propagation. The magnitude of the effect and its kinetics have been related to the light intensity, temperature, crystal orientation and the distance between the fest points. The effect is assumed to be a result of generation of a phonon packet, referred to by the authors as an acousto-optical domain, and carrier drag by this domain. Calculations based on the Boltzmann kinetic equation for the electron distribution function give a reasonable phonon density inside the domain, necessary for generating the electric field observed. Estimates based on the calculated phonon density show considerable mechanical stresses to exist in the domain area. These can result in the destruction of the crystal when there is an increase in light intensity; so the effect observed can be directly related to the problem of optical strength.     

Cluster-model study on the K-shell excited states of crystalline lithium under intense laser irradiation

Molecular-orbital calculations are performed to elucidate electronic structures and optical properties of lithium clusters in which several K-shell electrons are simultaneously excited to the valence levels. It is shown that relaxation of valence electrons around localized core holes influences the photoabsorption near-edge spectra significantly. The spectra in the excited state are modified from those in the ground state due to the presence of initial core holes. Potential energy surfaces are calculated for core-ionized Li₉ ^z+ clusters, which exhibit bound states for z≤3. The present cluster calculations would serve as prototypical models of laser-excited hollow atom solids with applications to X-ray optics.     

纳秒激光诱导熔石英光学玻璃的损伤增长

利用Nd:YAG激光器研究了纳秒激光诱导熔石英光学玻璃的初始损伤及损伤增长,对比研究了损伤程度和损伤形貌随激光波长、能量密度、脉冲数及位置的变化规律,并对损伤机制进行了分析和讨论。研究结果表明：初始损伤受损伤先驱的物理化学性质和激光参数的影响,而损伤增长规律与初始损伤程度、激光参数和位置有关;后表面的损伤随脉冲数的增加呈指数关系增长,前表面则呈线性关系;裂纹的产生及其在后续脉冲辐照下的发展是后表面损伤增长的主要原因,高温等离子体表面刻蚀是前表面损伤增长的主要原因     

纳秒激光诱导熔石英光学玻璃的损伤增长

利用Nd:YAG激光器研究了纳秒激光诱导熔石英光学玻璃的初始损伤及损伤增长,对比研究了损伤程度和损伤形貌随激光波长、能量密度、脉冲数及位置的变化规律,并对损伤机制进行了分析和讨论。研究结果表明:初始损伤受损伤先驱的物理化学性质和激光参数的影响,而损伤增长规律与初始损伤程度、激光参数和位置有关;后表面的损伤随脉冲数的增加呈指数关系增长,前表面则呈线性关系;裂纹的产生及其在后续脉冲辐照下的发展是后表面损伤增长的主要原因,高温等离子体表面刻蚀是前表面损伤增长的主要原因     

Sputtering-induced nanometre hole formation in Ni3Al under intense electron beam irradiation

The irradiation damage of polycrystalline Ni₃Al thin foils of stoichiometric composition by a stationary nanoscale 200?keV field emission gun (FEG) electron probe in a FEI Tecnai F20 (S)TEM has been investigated. At current densities greater than 10⁷?A/m², nanometre holes are produced quickly with both ?001? and ?110? incident electron beam directions. EDX spectra from the irradiated volume have been collected simultaneously during the hole forming process. From the EDX results, preferential surface sputtering of aluminium from Ni₃Al has been demonstrated. To understand the underlying physical process of sputtering, modelling based on a combination of molecular dynamics and Monte Carlo simulation has been performed. It appears to reproduce faithfully the overall film sputtering and hole formation processes, but is not capable of predicting the detailed geometry of the hole. It predicts that the sputtering cross-section of Al atoms is much higher than that of Ni atoms, resulting in a very small concentration of Al at the surface. This, together with the increase of surface area during hole formation, explains the preferential Al loss observed from the specimen. Calculated sputtering rates agree well with experiment, and are of the order of magnitude of 10^?8?atoms/electron.     

Time dependence of secondary electron yield and of surface potential during charging of amorphous silica target

We study herein the charge evolution of a silica amorphous target submitted to an electronic bombardment. During the bombardment, the injected electronic charge and the primary energy as well as the surface bombardment of the target are controlled. The dynamics of the injected charge are described by combining measurements of the secondary electron and X-ray emission using a scanning electron microscope. Thus, we are able to access the evolution of the surface potential and secondary electron yield. Monte-Carlo calculations are used to simulate situations similar to our experiences and to study the effect of certain parameters (density of traps, energy of activation) inaccessible to the measures with our devices.     

An “instantaneous” distribution of the ionization-passive electrons and holes under irradiation of a dielectric by intense electron or laser beams

A method is worked out for calculation of an “instantaneous” energy distribution of the ionization-passive electrons and holes resulting from the electron-electron collisions before the onset of electron-phonon relaxation under 10⁻¹⁵–10⁻¹⁴ s irradiation of a dielectric by an intense electron or laser beam. The method is based on the solution of a system of integral-differential kinetic equations of general form. The Auger and impact ionization as well as hole recoil due to the momentum conservation law are taken into account in calculations. The “instantaneous” distribution is calculated in NaCl under irradiation of the sample by a high-density electron beam. The “instantaneous” distribution of ionization-passive electrons and holes is the initial one in solutions of all kinetic equations describing further relaxation of electron excitations in irradiated materials.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 15–22, November, 2004.     

BEPCII横向束流反馈系统的梳状滤波器

BEPCII是一个多束团、大流强的装置, 由于高频腔的高次模和电阻壁阻抗等因素，不可避免地会出现束流不稳定性。BEPCII中采用束流反馈系统来抑制束流不稳定性。横向束流反馈系统主要包括前端电子学、信号处理电子学、反馈器件和放大器等几个部分。梳状滤波器是信号处理电子学的重要部件，利用两根长短不同的电缆以及一个功分器和合成器构成了一种简单有效的梳状滤波器，其梳状深度达到-41 dB，使用这种梳状滤波器的横向反馈系统成功地抑制了束流中出现的不稳定性。     

Temperature dependences of the electron-phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation

The electron temperature dependences of the electron-phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴ K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron-phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.     

Temperature dependence of EXAFS in amorphous arsenic

EXAFS measurements at different temperatures on amorphous arsenic are reported. The low values of the EXAFS mean square relative displacement σ² for the first coordination shell are consistent with a distribution of As₄ pyramidal units throughout the amorphous network, giving a locally very ordered first shell, whose disorder is essentially dynamic in character. The temperature dependence of the vibrational contribution to the EXAFS Debye-Waller factor is well described by an Einstein oscillator model.     

熔石英介质中强紫外激光自聚焦效应研究

 针对脉冲宽度约1 ns、波长为351 nm的三倍频紫外激光,定量分析了熔石英介质中的自聚焦长度、峰值光强与强紫外激光光束质量及环境条件等因素的关系,研究了产生紫外光非线性自聚焦效应的阈值条件。研究结果表明：在强紫外激光光束质量一定的前提下,可将B积分值定义为自聚焦的阈值条件;入射强紫外激光光束质量越差,在熔石英介质中产生自聚焦的阈值条件越低;即使对于空间分布均匀的理想光束,当空气中存在灰尘时,经过一段距离的传输后,在熔石英介质中将导致强紫外激光自聚焦效应的产生,且灰尘尺寸较大时的自聚焦效应较明显,自聚焦的阈值条件也相对较低。     

Temperature dependence of the electrical resistivity in amorphous metals

In amorphous metals the electrical resistivity increases linearly in the temperature range from 2 to 40 K. This result differs fundamentally from the nonlinear behaviour known for crystalline metals and it suggests the conduction electrons not to be scattered by the vibrations of the amorphous point lattice. The temperature dependent part of the resistivity in amorphous metals is explained with scattering of conduction electrons by fluctuations ofp-electrons.