Adaptive control of femtosecond laser ablation plasma emission

The influence of temporal pulse shaping on plasma plume generated by ultrafast laser irradiation of aluminum is investigated. Time resolved plasma emission spectroscopy is coupled with a temporal shaping procedure in a closed loop. The ionic emission is enhanced relative to the neutral one via an adaptive optimization strategy. The plasma emission efficiency in case of optimized and ultrashort temporal shapes of the laser pulses are compared, evidencing an enhancement of the ionization degree of the plasma plume. Simplified temporal shapes of the femtosecond laser pulses are extracted from the optimized shape and their corresponding effect on laser induced plasma emission is discussed.     

Terahertz-pulse emission through laser excitation of surface plasmons in a metal grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.     

Pulse shape and molecular orientation determine the attosecond charge migration in Caffeine

The recent reduction of laser pulse duration down to the attosecond regime offers unprecedented opportunities to investigate ultrafast changes in the electron density before nuclear motion sets in. Here, we investigate the hole dynamics in the Caffeine molecule that is induced by an ionizing XUV pulse of 6 fs duration using the approximate time-dependent density functional theory method TD-DFTB. In order to account for ionization in a localized atomic orbital basis we apply a complex absorbing potential to model the continuum. Propagation of the time-dependent Kohn–Sham equations allows us to extract the time-dependent hole density taking the pulse shape explicitly into account. Results show that the sudden ionization picture, which is often used to motivate an uncorrelated initial state, fails for realistic pulses. We further find a strong dependence of the hole dynamics on the polarization of the laser field. Notwithstanding, we observe fs charge migration between two distant functional groups in Caffeine even after averaging over the molecular orientation.     

Recombination emissions and spectral blueshift of pump radiation from ultrafast laser induced plasma in a planar water microjet

We report spectroscopic investigations of an ultrafast laser induced plasma generated in a planar water microjet. Plasma recombination emissions along with the spectral blueshift and broadening of the pump laser pulse contribute to the total emission. The laser pulses are of 100 fs duration, and the incident intensity is around 10¹⁵ W/cm². The dominant mechanisms leading to plasma formation are optical tunnel ionization and collisional ionization. Spectrally resolved polarization measurements show that the high frequency region of the emission is unpolarized whereas the low frequency region is polarized. Results indicate that at lower input intensities the emission arises mainly from plasma recombinations, which is accompanied by a weak blueshift of the incident laser pulse. At higher input intensities strong recombination emissions are seen, along with a broadening and asymmetric spectral blueshift of the pump laser pulse. From the nature of the blueshifted laser pulse it is possible to deduce whether the rate of change of free electron density is a constant or variable within the pulse lifetime. Two input laser intensity regimes, in which collisional and tunnel ionizations are dominant respectively, have been thus identified.     

Study of Organized Media Using Time-Resolved Fluorescence Spectroscopy

Many photophysical processes which occur on an ultrafast time scale in ordinary liquids become significantly retarded in organized assemblies, by two to three orders of magnitude. Recently many groups have applied ultrafast laser spectroscopy and theoretical methods to elucidate this dramatic phenomenon. Although the implications of this phenomenon in biology and chemistry are not yet fully understood, it has been demonstrated that ultrafast time-resolved fluorescence spectroscopy is a very powerful tool to study the microscopic properties of the organized assemblies and that water or other liquids confined inside these assemblies are fundamentally different from the corresponding liquid in bulk.     

Pulse width effect in ultrafast laser ionization imaging

The effects of different laser pulse widths on laser-induced ionization imaging of microstructures embedded in transparent materials are investigated. It is shown that a femtosecond laser-induced ionization probe can detect the variation of elemental composition of the sample materials with a higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the structural change inside optical materials, which can be well explained by the different roles of multiphoton ionization and avalanche ionization involved in material breakdown. These results also suggest that an optimum diagnosis could be obtained if well-selected laser parameters are employed in ultrafast laser ionization imaging.     

利用双色激光场下阈上电离谱鉴别H3^2+两种不同分子构型

我们最近证明,利用红外和深紫外双色激光场,SF6分子的结构信息可以通过其电离谱上的相干条纹获得[arXiv,1912.08499(2019)].在本文中,我们利用该方法考察了两种不同几何结构的分子离子H32+ 在激光场中的直接阈上电离(ATI)过程.通过与单色激光场中电离谱的比较发现,双色激光场的电离谱可以分辨分子的不同几何结构.由相干条件导出的公式可以很好地解释直接ATI动能谱和动量谱中的干涉条纹.此外,还发现通过改变分子核间距或改变激光强度可以改变电离谱的形状.由此可以推断,双色激光场诱导的ATI谱具有鉴别分子不同构型的能力,对复杂分子的几何结构成像具有一定的参考意义.     

Amplitude and phase reconstruction of electron wave packets for probing ultrafast photoionization dynamics

Ultrafast atomic processes, such as excitation and ionization occurring on the femtosecond or shorter time scale, were explored by employing attosecond high-harmonic pulses. With the absorption of a suitable high-harmonic photon a He atom was ionized, or resonantly excited with further ionization by absorbing a number of infrared photons. The electron wave packets liberated by the two processes generated an interference containing the information on ultrafast atomic dynamics. The attosecond electron wave packet, including the phase, from the ground state was reconstructed first and, subsequently, that from the 1s3p state was retrieved by applying the holographic technique to the photoelectron spectra comprising the interference between the two ionization paths. The reconstructed electron wave packet revealed details of the ultrafast photoionization dynamics, such as the instantaneous two-photon ionization rate.     

Comparative analysis of the strong-field ionization of a quantum system with the Coulomb and short-range potentials

The dynamics of a hydrogen atom and a 3D model quantum system with a short-range potential is investigated using the direct numerical integration of the nonstationary Schrödinger equation in a wide range of laser intensities and frequencies. The simulation data are compared with the predictions of variants of the Keldysh-type theories. It is demonstrated that, in the low-frequency (tunnel) limit, the ionization rates of the systems with the Coulomb and short-range potentials and the same values of the ionization potential significantly differ from each other whereas, in the high-frequency (single-photon) limit, we do not observe a substantial difference between the ionization rates. Specific features of the angular distribution of the photoelectron emission and the photoelectron energy spectra are investigated in detail. In addition, the ionization suppression is studied for both Coulomb- and short-range-potential atoms. The stabilization is due to the dramatic reconstruction of the atom in the presence of a strong laser field and the formation of a new system (Kramers-Henneberger atom) that exhibits an increasing resistance to the ionization upon an increase in the laser intensity. In the two-photon ionization regime, the stabilization phenomenon is substantially more pronounced for the system with the Coulomb potential. This results from the effective excitation of the Rydberg states of the dressed atom in the strong-field limit.     

Non-Hermitian electronic theory and applications to clusters

Electronically excited cations, generated by inner-valence ionization of small molecules, relax in general by dissociation and photon emission. Autoionization is forbidden for energetic reasons. The situation changes fundamentally in an inner-valence ionized cluster, which releases its excess energy by emitting an electron. This novel process, referred to as Intermolecular Coulombic Decay, is characterized by an efficient energy transfer between monomers in the cluster. The decay is ultrafast, taking place on a femtosecond time scale. Theoretical tools are developed to predict the properties, in particular lifetimes, of molecular systems undergoing electronic decay. These methods are applied to study the relaxation of inner-valence holes in clusters. In order to enable a treatment of the scattering and the many-particle problem with standard electronic correlation methods for bound states, a complex absorbing potential is added to the Hamiltonian. Conceptual as well as practical aspects of this procedure are discussed in detail.     

Multiphoton ionization in dielectrics: comparison of circular and linear polarization

Ionization mechanisms in bulk dielectrics irradiated by single intense 50-fs-laser pulses are investigated by ultrafast time-resolved imaging interferometry. Polarization-sensitive 6-photon ionization is shown to be the dominant ionization mechanism in fused silica and sapphire at intensities around 10 TW/cm2. For both materials the cross sections of 6-photon ionization are found to be significantly higher for linear polarization than for circular. Our experimental results corroborate an earlier theoretical prediction on the dominance of linear polarization in high-order multiphoton ionization.     

基于光场离化电流机制产生强太赫兹辐射的参数优化研究

基于超短激光脉冲与气体作用通过光场离化电流产生太赫兹(THz)辐射的模型, 研究了用双色激光脉冲的方法产生强THz辐射的优化参数条件. 数值计算表明, 导致THz辐射产生的离化电流主要是由一阶电离过程产生的, 高阶离化对该电流产生的贡献很小. 通过调节基频光与倍频光的配比、相位差都能增大离化电流, 从而可以提高THz辐射振幅. 将激光波长拓展到中红外波段, 也有利于提高离化电流. 此外,改变作用气体的种类也能改变离化电流. 在激光和密度参数相等的情况下, 在氦气中可以产生高于氮气中2倍左右的离化电流.     

Interferometric characterization of ultrashort deep ultraviolet pulses using a multiphoton ionization mass spectrometer

The temporal characterization of a femtosecond laser pulse in the deep ultraviolet region using an interferometric autocorrelation scheme is demonstrated. Two-photon ionization of a molecule in a time-of-flight mass spectrometer was used as a nonlinear detector to obtain an autocorrelation trace. This setup proved useful in not only providing a temporal characterization of a pulse but also investigating the ultrafast dynamics of photochemical processes.     

Adsorption of gases studied by secondary ion emission mass spectrometry

The adsorption of an active gas, like oxygen, on the surface of a metal or an alloy leads to an intensification of the positive ion emission produced by sputtering. In the present paper this phenomenon (called chemical ion emission) is observed by blowing the gas on the surface of the sample while it is sputtered. The general features of the chemical effect on M⁺ ion production are discussed in terms of coverage resulting from equilibrium between the rate of atoms sticking to the surface and the sputtering rate of adsorbed atoms, and in terms of ionization probability depending upon the coverage. Examples are given for pure metals (Al, W, Ni) and alloys (NiCr, CuBe, CuAl, …). Attention is drawn to the effects observed on nickel single crystals (100) and (110). At a critical coverage an incorporation process takes place, producing both changes in the work function and in the ionization probability. These results, largely consistent with those obtained by classical methods, show that secondary ion emission can be used for adsorption studies. Finally, investigations of metal-oxygen interaction by the dynamic method (present work) and the static method of secondary ion mass analysis will be discussed and compared.     

Sudden birth versus sudden death of entanglement for the extended Werner-like state in a dissipative environment

In this paper, we investigate the dynamical behaviour of entanglement in terms of concurrence in a bipartite system subjected to an external magnetic field under the action of dissipative environments in the extended Werner-like initial state. The interesting phenomenon of entanglement sudden death as well as sudden birth appears during the evolution process. We analyse in detail the effect of the purity of the initial entangled state of two qubits via Heisenberg XY interaction on the apparition time of entanglement sudden death and entanglement sudden birth. Furthermore, the conditions on the conversion of entanglement sudden death and entanglement sudden birth can be generalized when the initial entangled state is not pure. In particular, a critical purity of the initial mixed entangled state exists, above which entanglement sudden birth vanishes while entanglement sudden death appears. It is also noticed that stable entanglement, which is independent of different initial states of the qubits (pure or mixed state), occurs even in the presence of decoherence. These results arising from the combination of the extended Werner-like initial state and dissipative environments suggest an approach to control and enhance the entanglement even after purity induced sudden birth, death and revival.     

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     

Amplified Emissions Spectroscopy for Sensing Applications

Amplified emission spectroscopy is promising for applications of qualitative and quantitative sensing of elements and their states. This technique has the characteristics of a narrow spectral line, high spectral resolution, and high signal‐to‐noise ratio. Recently, this technology has advanced tremendously with progress in excitation methods, that is, resonance multiphoton absorption and filamentation with ultrafast lasers. The mechanism, conditions, excitation, signal processing, and applications of amplified emission spectroscopy are reviewed herein. In particular, the different formation patterns of the amplified emission spectra produced by an ultrafast laser are analyzed in detail. In addition, a brief introduction is given to the existing applications of amplified emission. Finally, both a conclusion and a perspective of the future for amplified emission are presented.     

Hot-electron-pumped K-shell X-ray laser

Amplified spontaneous inner-shell emission produced via an ultrafast burst of high-energy electrons from a femtosecond laser-produced plasma is proposed as a novel electron-pumped x-ray laser. In this scheme, a population inversion of the upper laser level is created via impact ionization of atomic inner shells by electron bombardment. Based on the requirement of a positive gain coefficient for amplifying spontaneous K _α line emission, a simple pumping threshold is found for the incident electron flux, and feasibility of the scheme is assessed for a range of low-Z elements. Published in English in the original Russian journal. Edited by Steve Torstveit. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 771–776 (25 May 1998)     

Propagation of drop coalescence in a two-dimensional emulsion: a route towards phase inversion

The phase inversion that undergoes an emulsion while being sheared is a sudden phenomenon that is still puzzling. In this Letter, we report an experimental investigation on propagative coalescence by using a microfluidic device where a calibrated two-dimensional emulsion is created and destabilized. The velocity of propagation and the probability of the coalescence are reported as a function of the size and the spatial distribution of the drops, respectively. We then discuss the efficiency of this novel scenario of phase inversion and suggest that inversion can be favored by the existence of a drop size distribution.     

Carrier phase dependence in the ionization of Rydberg atoms by short radio-frequency pulses: a model system for high order harmonic generation

We report time-resolved electron emission in experiments on ionization of rubidium Rydberg atoms (n=90) by few-cycle radio-frequency (RF) (1-10 MHz) pulses. The electron emission occurs in multiple bursts and strongly depends on the carrier-envelope phase as well as the duration and amplitude of the RF pulses. Remarkably, ionization is observed during a series of cycles with the same amplitude. Even at the low RF frequencies, ionization is not completed in a single cycle. Remixing of the states at the zero crossing of the field is believed to play an essential role. Similarities with the ionization process leading to high order harmonic generation are discussed.