Diagnose Parameters of Plasma Induced by Femtosecond Laser Pulse in Quartz and Glasses

Electron plasma induced by a focused femtosecond pulse (130 fs, 800 nm) in quartz, fused silica, K9 glass, and Soda Lime glass was investigated by pump-probe technology. Pump and probe shadow imaging and interferometric fringe imaging have been used to determine plasma density, relaxation time, and electron collision time in the conduction band. In these materials, the electron collision time is about several femtoseconds when the electron density is in the 10¹⁹cm⁻³ range. The electron relaxation processes are different: lifetime is about 170 fs in pure quartz and fused silica, and about 100 ps in K9 and Soda Lime glass. The modified electron band by doped ions is regarded to be responsible for the difference of decay time in these materials.     

Plasma effects in picosecond-femtosecond UV laser ablation of polymers

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths ranging from 200 fs to 200 ps was investigated. The measured ablation rates show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI).The reflected fraction of the ablating laser pulse was measured as a function of the pulse length. In the case of PMMA maximum reflectance corresponds to a minimum ablation rate.This behavior can be explained by a dynamic plasma reflection model: A fast build up of a dense plasma is followed by high obscuration for a brief transition time and a self-regulating opacity for the rest of the pulse. This model of plasma mediated ablation leads to a ^1/4-dependence of the ablation rate at fixed fluence, which fits very well to the measured data, in particular if an extension to nanosecond ablation data of PI and PMMA is considered. PACS 52.50.Jm; 61.80.Ba; 42.65.Re     

软X射线激光背光阴影成像技术的空间分辨研究

软X射线激光阴影成像技术是一种诊断高温稠密等离子体临界面附近阴影轮廓的诊断技术,具有测量视场大、空间分辨能力高的特点,具有重要的应用前景.对该技术的空间分辨具体能够达到什么程度则没有进行过系统研究.本文分别从光路几何、衍射极限、成像像差三个方面进行了仔细分析,结果表明目前采用该技术的诊断系统能够达到约2μm的空间分辨.主要受限是光路几何因素,通过增加放大倍数、选择单元尺寸更小的接收元器件等方式,有希望达到优于1μm的空间分辨.     

Electrons and hydrogen-bond connectivity in liquid water

The network connectivity in liquid water is revised in terms of electronic signatures of hydrogen bonds (HBs) instead of geometric criteria, in view of recent x-ray absorption studies. The analysis is based on ab initio molecular-dynamics simulations at ambient conditions. Even if instantaneous threadlike structures are observed in the electronic network, they continuously reshape in oscillations reminiscent of the and modes in ice (tau approximately 170 fs). However, two water molecules initially joined by a HB remain effectively bound over many periods regardless of its electronic signature.     

High-precision molecular wave-packet interferometry with HgAr dimers

Molecular wave-packet (WP) interferometry has been demonstrated in the A electronic state of the HgAr van der Waals complex with two time-delayed UV fs pulses at 254 nm. The interferograms of three vibrational levels in the WP's display almost 100% fringe contrast as a function of the interpulse delay tau, which is tuned with sub-10 as stability and resolution. It is clearly observed that the three interferograms show their dephasing and rephasing within a single vibrational period, allowing us to prepare arbitrary relative populations of the three levels by adjusting a single parameter tau.     

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.     

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.     

Mechanisms of femtosecond laser-induced damage in magnesium fluoride

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump-probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF₂.     

Surface topography (nano-sized hillocks) and particle emission of metals, dielectrics and semiconductors during ultra-short-laser ablation: Towards a coherent understanding of relevant processes

By combining new studies of the surface topography and the emission characteristics of particles during interaction of ultra-short-laser radiation with surfaces, in particular during laser ablation, three different types of general processes (sub 100 fs electronic processes like Coulomb explosion (CE) or field ion emission by surface optical rectification (SOR), processes related to electronic plasma (FEP) formation (typically a few hundred fs time scale) and thermal ablation (TA)) could be identified to explain ultra-short-laser ablation of matter at laser intensities around the ablation threshold. In particular, the identification of the unique appearance of individual, localized nano-hillocks, typically a few nm in height and with a diameter below typically 50 nm, can be regarded as characteristic for a strong localized potential energy deposition to the electronic system resulting in CE or SOR. The observation and possibility of CE even on metals has implications beyond the field of laser ablation. A remarkable result observed concerns the similarities between laser ablation and sputtering with highly charged ions.     

Experimental vibration analysis of the shadow mask

In this paper, the natural frequencies and mode shapes of the shadow mask in a multimedia monitor were measured by the modal testing technique and the amplitude fluctuation (AF) electronic speckle pattern interferometry (ESPI) method. Based on the obtained experimental results, it is advisable to investigate and optimize the assembly conditions of the shadow mask to drifting of the image due to the resonance of the shadow mask.     

Electron acceleration and the propagation of ultrashort high-intensity laser pulses in plasmas

Reported are interactions of high-intensity laser pulses ( lambda = 810 nm and I相似文献   

Precision measurements of the lambda(+)(c) and D0 lifetimes

We report new precision measurements of the lifetimes of the Lambda(+)(c) and D0 from SELEX, the charm hadroproduction experiment at Fermilab. Based upon 1630 Lambda(+)(c) and 10 210 D0 decays we observe lifetimes of tau[Lambda(+)(c)] = 198.1+/-7.0+/-5.6 fs and tau[D0] = 407.9+/-6.0+/-4.3 fs.     

Probing hot and dense laser-induced plasmas with ultrafast XUV pulses

In this Letter, we demonstrate the instantaneous creation of a hot solid-density plasma generated by focusing an intense femtosecond, high temporal contrast laser on an ultrathin foil (100 nm) in the 10(18) W/cm2 intensity range. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate such a medium, enables the study of the dynamics of this plasma on the 100 fs time scale. The comparison of the transmission of two successive harmonics permits us to determine the electronic density and the temperature with accuracies better than 15%, never achieved up to this date in the regime of laser pulses at relativistic intensity.     

Two-Photon Excited State Dynamics of Dark Rydberg and Bright Valence States in Furan

Two-photon absorption in systems with parity permits access to states that cannot be directly prepared by onephoton absorption. Here we investigate ultrafast internal conversion(IC) dynamics of furan by using this strategy in combination with femtosecond time-resolved photoelectron imaging. The dark Rydberg S_1 and bright valence S_2 states are simultaneously excited by two photons of 405 nm, and then ionized by two photons of 800 nm. The IC from S_2 to S_1 is clearly observed and extracted from the time dependence of the higher photoelectron kinetic energy(PKE) component. More importantly, the internal conversions to hot S_0 from directly-prepared S_1 and secondarily-populated S_1 are unambiguously identified by the time-dependence of the lower PKE component.The average lifetime of the S_2 and S_1 states is measured to be 29 fs. The internal conversions of S_2 to S_1, S_1 to hot S_0 occur on estimated timescales of 15.4 fs and 38 fs, respectively.     

Response of polyatomic molecules to ultrastrong laser- and ion-induced fields

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration. Molecular dynamics and structural information contained in the fragmentation pathways can be assessed separately. Despite the similar field strengths large differences in fragment kinetic energies are found which are related to field shape and duration with the aid of molecular dynamics simulations.     

Imaging and autocorrelation of ultrafast infrared laser pulses in the 3-11-mum range with silicon CCD cameras and photodiodes

Standard silicon photodiodes and CCD cameras are convenient and inexpensive alternatives to cryogenically cooled diodes or arrays for autocorrelation and imaging of ultrafast IR laser pulses in the wavelength range 3-11 mum . The response of these Si devices to IR pulses of duration ~100 fs is proportional to E(n) , where E is the pulse energy and n is approximately the Si electronic bandgap divided by the photon energy.     

A precise measurement of the tau lifetime

The tau lepton lifetime has been measured with the events collected by the DELPHI detector at LEP in the years 1991-1995. Three different methods have been exploited, using both one-prong and three-prong decay channels. Two measurements have been made using events in which both taus decay to a single charged particle. Combining these measurements gave . A third measurement using taus which decayed to three charged particles yielded These were combined with previous DELPHI results to measure the tau lifetime, using the full LEP1 data sample, to be .Received: 12 November 2003, Revised: 1 June 2004, Published online: 20 July 2004     

Ultra-fast laser ablation and deposition of TiO2

In this work we report on the properties of the ablation plume and the characteristics of the films produced by ultra-fast pulsed laser deposition (PLD) of TiO₂ in vacuum. Ablation was induced by using pulses with a duration of ≈300 fs at 527 nm. We discuss both the composition and the expansion dynamics of the TiO₂ plasma plume, measured by exploiting time- and space-resolved emission spectroscopy and gated imaging. The properties of the TiO₂ nanoparticles and nanoparticle-assembled films were characterized using different techniques, i.e. environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). It is suggested that most of the material decomposes in the form of nanoparticles.     

Ultrafast optical pumping of spin and orbital polarizations in the antiferromagnetic Mott insulators R(2)CuO(4)

We demonstrate that optical pumping by circularly polarized light at the charge-transfer transition can induce spin and orbital polarizations in the strongly correlated Mott insulators R(2)CuO(4) (R=Pr, Nd, Sm) providing a means of ultrafast nonlinear manipulation of spin states on time scales of less than 150 fs. We propose a model which includes both orbital- and spin-related processes possessing different spectral and temporal properties. This allows us to model the optical response of antiferromagnetic Mott insulators to circularly polarized light and estimate the spin relaxation time as tau(s) approximately 30-50 fs.     

Vacuum ultraviolet pulses of 11 fs from fifth-harmonic generation of a Ti:sapphire laser

We demonstrate that in a short Ar cell, generation of the fifth harmonic from 12 fs pulses at 810 nm directly results in ultrashort vacuum UV pulses at 162 nm. They have a spectral width of approximately 5 nm and a duration of 11+/-1 fs (1.4 times the transform limit), as measured by cross correlation with the fundamental pulses. Their energy (estimated to 4 nJ) turned out to be sufficient for use as a pump in time-resolved experiments.