Direct simulation monte carlo of pulsed laser ablation of metals with clusterization processes in vapor

A mathematical model is proposed to describe laser ablation of metals in vacuum under the action of nanosecond laser pulses of moderate intensity taking into account the processes of cluster formation and decay in the vapor cloud. To describe the laser radiation absorption and metal heating, the thermal model based on the unsteady one-dimensional heat equation with a volume heat source is used, and the method of statistical modelling is employed for modelling vapor expansion and the processes of cluster formation. The efficiency of the proposed complex model is considered by the example of pulsed laser ablation of a niobium target. The work was supported financially by INTAS (grant No. 03-51-5208).     

Femtosecond pump-probe investigation of ultrafast silver nanoparticle deformation in a glass matrix

Single-color femtosecond pump-probe experiments were performed to investigate the time dependence of laser-induced ultrafast desorption and deformation processes of silver nanoparticles in glass. After laser excitation at wavelengths close to the surface plasmon resonance, transient extinction changes were found to exhibit dynamics on quite different time scales ranging from sub-picoseconds to some hundred ps. The slowest observed decay component is identified as characteristic for the deformation/desorption processes. Possible mechanisms for these processes are discussed. Received: 3 April 2000 / Revised version: 3 July 2000 / Published online: 20 September 2000     

Time-resolved laser saturation spectroscopy. Theory of the free induction decay of two-level saturation resonances

We give the theory of the free induction decay of two-level saturation resonances in the time-resolved laser saturation spectroscopy. The saturating and probe fields may be either co- or counter-propagating. The change-signal line-shape is analysed in detail in two cases of practical importance, namely when the saturating field is either strong or weak. The time evolution of the change-signal exhibits many interesting features which include Ramsey-type fringes, oscillatory decay, narrow resonances, line-narrowing etc. It is shown that by analysing the decay of the change-signal in time one can distinguish (i) various laser interaction processes such as population effects, coherence effects, three-photon Raman-type process, dynamic Stark splitting, power-broadening etc., and (ii) various relaxation mechanisms such as phase-changing collisions, etc. Work supported in part by D.R.E.T. (Paris).     

基于皮秒时间分辨的癌细胞荧光寿命研究

研究了用于癌症诊断与治疗的光敏剂血卟啉(hematoporphyrin derivative,HPD)的超快光动力学过程,采用超短脉冲激光光谱技术和皮秒时间相关单光子计数系统,测量了经血卟啉培养的活体癌细胞与正常细胞的皮秒时间分辨荧光光谱及荧光峰值强度随时间衰变曲线,观测到：癌细胞与正常细胞样品荧光寿命的快成分分别为150和300 ps;癌细胞与正常细胞的荧光峰值强度经12 h分别衰减10%和55%。经对测量所得的荧光衰减曲线进行分析,计算出癌细胞与正常细胞的荧光寿命分别为824和1798 ps;血卟啉在癌细胞与正常细胞样品中滞留时间分别为17天和6天。结果表明癌细胞与正常细胞对血卟啉亲和性及对血卟啉滞留的稳定性有显著差异,测量结果确认了荧光光谱技术诊断与治疗癌症的可行性,并对实时监测生物样品微弱超快荧光具有重要的指导意义和临床应用价值。     

Ultra-Fast Decay Process of Electrons in LiNbO3 Crystal Induced by Femtosecond Pulse

Temporal evolution of absorption induced by single femtosecond pulse （13Ors, 800nm） with high intensity in LiNbO3 is obtained using the probe shadow imaging technique in order to investigate light-induced electron relaxation processes. By saturating the polaron density with a high intensity laser pulse, ultra-fast decay process on picosecond time scale is observed. The decay time constant is about 141 ps and it is attributed to the direct interband electron-hole recombination process.     

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stückelberg-like oscillations of measurable quantities, such as the electron emission yield. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Laser-assisted muon decay

We show theoretically that the muon lifetime can be changed dramatically by embedding the decaying muon in a strong linearly polarized laser field. Evaluating the S-matrix elements taking all electronic multiphoton processes into account we find that a CO(2) laser with an electric field amplitude of 10(6) V cm(-1) results in an order of magnitude shorter lifetime of the muon. We also analyze the dependencies of the decay rate on the laser frequency and intensity.     

Photoluminescence and energy transfer processes in rare earth ion doped oxide thin films with substrate heating

Tm-Er codoped amorphous aluminum oxide thin films were prepared by pulsed laser deposition. Broadband photoluminescence in the wavelength region of 1400-1700 nm comprised of two emissions at around 1532 and 1620 nm was observed. PL performance was investigated as a function of the substrate-heating temperature. Possible energy transfer processes involved in the heat treatment were discussed and nonradiative decay rates were evaluated, by comparing the inverse of measured lifetimes with the calculated radiative decay rates. Our results suggest that Tm-Er codoped Al₂O₃ thin film might be potential candidate as broadband light sources and amplifiers.     

激光氧气等离子体光谱的时间演化研究

报道了脉冲激光在一个大气压的纯氧(纯度为99.999 9％)中所产生的等离子体光谱进行时间演化特性研究的结果。激光器的工作波长为1.06 μm,光谱探测范围为300～900 nm。实验结果表明,激光氧气等离子体光谱均表现为连续谱和线状谱的迭加,它们的演化过程各不相同。基于各种光谱成分的时间演化特征,分析了激光氧气等离子体在衰变期间可能存在的一些物理过程及其演化特征,讨论了激光氧气等离子体寿命较长的主要原因。这些结果有助于加深对激光大气等离子体特性和机理的了解,为进一步开展延长激光大气等离子体寿命方法的研究提供实验依据。     

用时间分辨光谱研究红细胞悬液的荧光发射

实验获得了激光照射红细胞悬液的荧光光谱,并分别监测不同荧光峰值波长处强度随时间的衰变过程,测试了其相应的荧光寿命。结果表明,在波长为407nm的激光照射下,红细胞悬液向外发射中心波长分别位于596,628,692nm的荧光光谱,各荧光峰对应衰变过程的平均荧光寿命分别为1.97,13.31,14.58ns。利用荧光强度和吸收率的加和性表示了混合物的总吸收率和总荧光发射强度,通过理论计算获得了红细胞悬液中锌卟啉、原卟啉和其他游离物参与荧光发射的相对含量和相对强度在不同荧光峰位的变化关系,进一步解释了不同峰位处荧光发射强度和平均荧光寿命的变化原因。     

方波激发下Er~(3+)上转换红光激发机理的研究

用方波电源驱动808 nm激光二极管(LD)激发Er3+掺杂的亚碲酸盐氟氧化物玻璃,测量4F9/2能级上转换发光的上升和衰减,根据上升的时间常数确定中间能级的寿命,从而确定4F9/2能级粒子数积累的过程。通过建立速率方程模型分析4F9/2能级的上升和衰减特性与中间能级的关系,确定808 nm LD激发下上转换红光的激发机理,同时提供了一种通过上转换发光,用光电倍增管测量红外能级寿命的方法。     

Decay kinetics of the defect-based visible luminescence in ZnO

Photoluminescence spectra and decays under pulsed N₂ (337 nm) laser excitation were measured for hydrothermally grown bulk and liquid-phase epitaxy (LPE)-grown film ZnO samples within 9-300 K. Temperature dependence of integrated spectra over the exciton and visible spectral regions was evaluated using a model involving standard energy barrier processes. Decay curves measured within a broad time window (10 ns-1 ms) and with extreme signal/background ratio (five orders of magnitude) point to complex decay mechanism in which the exponential and inverse power-law processes can coexist. There is no straightforward interconnection between the observed temperature dependence of integrated visible photoluminescence intensity and its decay shape over the 9-300 K temperature interval.     

Color center formation in soda lime glass and NaCl single crystals with femtosecond laser pulses

The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on different time scales, from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for a long time after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride. PACS 82.50.Pt; 78.55.Qr; 78.55.Fv; 78.47.+p     

激光驱动冲击波的增长与衰减的一维特征线解法

 本文提出一种计算激光产生冲击波的增长与衰减规律的简单方法。用该方法计算了梯形激光脉冲和高斯型脉冲在铝靶中产生冲击波的增长与衰减的情况，并与其他作者所给的结果进行了比较。     

ZnS中电子陷阱能级对光生电子弛豫过程的影响

采用微波吸收法,测量了在不同助熔剂条件及不同气氛下烧制的ZnS材料受到超短激光脉冲激发后的光电子衰减过程,并且测量了材料的热释光曲线。样品A采用过量的SrCl作为助熔剂,在1150℃下灼烧制备而成;其热释光曲线显示材料中有浅电子陷阱,电子陷阱密度小,光生电子衰减过程为双指数衰减过程,快过程寿命为45ns,慢过程寿命为312ns。样品B中加入了少量的NaCl作为助熔剂;热释光曲线显示有浅电子陷阱和深电子陷阱,且都有较高的密度,其光电子寿命为1615ns。在NH₄Br气氛中烧制样品C,热释光谱显示只有浅电子陷阱形成,光电子寿命为1413ns。结果表明材料的光电子寿命和浅电子陷阱密切相关,浅电子陷阱密度越大,光生电子寿命越长,深电子陷阱对光生电子瞬态过程影响很小。     

Ultrafast blue light emission from SiC nanowires

Cubic silicon carbide (SiC) nanowires are synthesized in a catalyst-assisted process. The nanowires with diameter of ～ 40 nm exhibit strong blue light emission at room temperature under ultraviolet (UV) femtosecond laser excitation. The photon energy of peak emission is higher than the energy bandgap of cubic SiC which shows involvement of quantum confinement effect. The ultrafast fluorescence is deconvoluted by Monte-Carlo method. The results show two ultrafast decay processes whose lifetimes are about 26 and 567 ps respectively. The mechanisms of such ultrafast processes are discussed.     

Coherent control of optical transient processes in a gas

The possibility of controlling the processes of the free-polarization decay and photon echo in a gas by means of resonant CW radiation has been shown. The photon echo and free-polarization decay are formed using the method of the Stark switching of levels in a low-intensity radiation field in the presence of an orthogonally polarized strong field. The experiments in the gas at the R(4, 3) transition in the vibrational band 0 ? 1 v₃ ¹³CH₃F in the presence of radiation from a CW CO₂ laser and the calculations by the evolution operator method show a strong effect of the high-intensity radiation field on the photon echo and free-polarization decay to its complete suppression.     

Analytical descriptions of mode switching characteristics of diode lasers

Using perturbation analysis to solve the rate equations of semiconductor lasers, the analytical expressions for the evolution of the carrier and photon densities have been derived when the laser is switched from one oscillation mode (or wavelength) to another. The solutions show that the deviations of the carrier and photon densities are damped and oscillatory. The decay rate of these deviations depends not only on the carrier lifetime, but also on the photon lifetime, which is different from that of an ordinary diode laser experiencing switch-on processes. Received: 5 December 2000 / Revised version: 5 March 2001 / Published online: 7 June 2001     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Raman investigation of silicon nanocrystals: quantum confinement and laser‐induced thermal effects

We present a detailed experimental and theoretical Raman investigation of quantum confinement and laser‐induced local thermal effects on hydrogenated nanocrystalline silicon with different nanocrystal sizes (3.6–6.2 nm). The local temperature was monitored by measuring the Stokes/anti‐Stokes peak ratio with the laser power density range from ~120 to 960 kW/cm². In combination with the three‐dimensional phonon confinement model and the anharmonic effect, which incorporates the three‐phonon and four‐phonon decay processes, we revealed an asymmetrical decay process with wavenumbers ~170 and 350 cm^–1, an increasing anharmonic effect with nanocrystal sizes, and a shortening lifetime with enhanced temperature and decreasing nanocrystal dimension. Furthermore, we demonstrated experimentally that for Si nanocrystals smaller than 6 nm, the quantum confinement effect is dominant for the peak shift and line broadening. Copyright © 2011 John Wiley & Sons, Ltd.