Strong Femtosecond Laser Pulses Research Articles

Optical measurement of the work function and the field reduction factor of metallic needle tips.

Quintessential parameters for needle tip-based electron sources are the work function, the tip apex radius, and the field reduction factor. They determine the static emission properties and strongly influence laser-triggered photoemission experiments at these needle tips. We present a simple method based on photoemission with two different commonly available continuous-wave laser diodes to determine both parameters in situ. We demonstrate our technique at tungsten needle tips. In a first application, use the method to in situ monitor changes of the emitter caused by illumination with strong femtosecond laser pulses. After illumination, we observe an increase in the work function caused by laser-induced changes to the apex of the tip. These changes are reversible upon field evaporation and are accompanied by a change in the spatial electron emission distribution. We believe that this simple in situ work function determination technique is applicable to any metal and in many experimental settings.

Modeling of two-photon polymerization in the strong-pulse regime

This paper reports a study of two-photon polymerization induced by femtosecond laser pulses having microjoule pulse energy and kilohertz repetition rates. Light-matter interaction and polymerization kinetics are modeled in highly confined spatiotemporal scales. The model employs a non-diffractive Bessel beam, considers the effects of temperature-dependent species diffusions, and regards propagation and termination kinetic constants as functions of double-bond conversion. The model is validated by comparing the size of features predicted from simulations to those generated experimentally. The model is used to investigate how the time and energy required to create a single volume element ("voxel") change under various conditions of irradiation. The results show that polymerizing a single voxel requires a minimum exposure time that is constant across a range of irradiation conditions, and is largely determined by the chemical kinetics. In the regime where the pulse energy is low (< 10 μ J), it is more energy-efficient to use fewer pulses having higher energy within the same total exposure time. However, this trend reverses in the regime where the pulse energy is high (10 μ J - 30 μ J), because radical-radical recombination becomes significant, which wastes absorbed energy. This work advances the understanding of two-photon polymerization in the strong-pulse regime and is a step toward increasing throughput to a level suitable for industrial applications. • Two-photon polymerization is induced by few and strong femtosecond laser pulses • A Bessel beam is utilized to rapidly fabricates high aspect ratio structures • Polymerizing a single voxel requires a minimum exposure time • It is more energy-efficient to use fewer pulses when the pulse energy is low (<10 μ J) • Radical-radical recombination is severe when the pulse energy is high (>10 μ J)

Engineering quantum wave-packet dispersion with a strong nonresonant femtosecond laser pulse

A non-dispersing wave packet has been attracting much interest from various scientific and technological viewpoints. However, most quantum systems are accompanied by anharmonicity, so that retardation of quantum wave-packet dispersion is limited to very few examples only under specific conditions and targets. Here we demonstrate a conceptually new and universal method to retard or advance the dispersion of a quantum wave packet through 'programmable time shift' induced by a strong non-resonant femtosecond laser pulse. A numerical simulation has verified that a train of such retardation pulses stops wave-packet dispersion.

Experimentally validated modeling of the optical energy deposition in highly ionized ambient air by strong femtosecond laser pulses

The deposition of femtosecond laser optical energy in gases leads to the emission of secondary electromagnetic and acoustic radiation. These optoacoustic components have a complex nonlinear dependency on the laser beam characteristics, such as the pulse energy, duration, wavelength and the focusing conditions, as well as on the optical and elastic characteristics of the gaseous medium. The initial interaction times are governed by the high electronic excitation and ionization. These phenomena result in a self-modulation of the laser pulse, significantly affecting the optical energy deposition on the medium. Such complex nonlinear phenomena are very difficult to be studied via analytical equations. To address this, a multiphysics Particle-In-Cell model is applied for the evaluation of the optical energy deposition and plasma generation from tightly focused femtosecond pulses in ambient air. The computational domain of the model is built to describe optical energy deposition in its full spatiotemporal scale. The model is validated by experimental results of the absorbed energy. The agreement between the computational and experimental results provides the basis for the future development of an advanced microstructural Finite Element Method model, which, combined with the Particle-In-Cell model, will have the ability of delivering detailed insights for all the sub-domains and timescales varying from nano- to femto-seconds of the laser-induced breakdown phenomenon.Graphic

Phase-locking mechanism in non-sequential double ionization

Here, we identify a new mechanism where the early stage of electron ionization in the field of strong femtosecond laser pulse determines the final yield of non-sequential double ionization. By using simple trajectory methods we prove that the powerful short wavelength laser field causes an injection locking between its phase and the phases of the electron trajectories, which is next responsible for the enhanced or suppressed ionization for given intensities in the knee region. It is shown that both ionization and entanglement of the final electron state can be easily controlled by introducing frequency chirp of the laser field. Our methods allow one to quantify the quantum correlations due to the different mechanisms of strong field ionization.

Restricted-space ab initio models for double ionization by strong laser pulses

Double electron ionisation process occurs when an intense laser pulse interacts with atoms or molecules. Exact {\it ab initio} numerical simulation of such a situation is extremely computer resources demanding, thus often one is forced to apply reduced dimensionality models to get insight into the physics of the process. The performance of several algorithms for simulating double electron ionization by strong femtosecond laser pulses are studied. The obtained ionization yields and the momentum distributions of the released electrons are compared, and the effects of the model dimensionality on the ionization dynamics discussed.

Direct Characterization of Ultrafast Energy-Time Entangled Photon Pairs

Energy-time entangled photons are critical in many quantum optical phenomena and have emerged as important elements in quantum information protocols. Entanglement in this degree of freedom often manifests itself on ultrafast time scales, making it very difficult to detect, whether one employs direct or interferometric techniques, as photon-counting detectors have insufficient time resolution. Here, we implement ultrafast photon counters based on nonlinear interactions and strong femtosecond laser pulses to probe energy-time entanglement in this important regime. Using this technique and single-photon spectrometers, we characterize all the spectral and temporal correlations of two entangled photons with femtosecond resolution. This enables the witnessing of energy-time entanglement using uncertainty relations and the direct observation of nonlocal dispersion cancellation on ultrafast time scales. These techniques are essential to understand and control the energy-time degree of freedom of light for ultrafast quantum optics.

Optimal control of photoelectron emission by realistic waveforms

Recent experimental techniques in multicolor waveform synthesis allow the temporal shaping of strong femtosecond laser pulses with applications in the control of quantum mechanical processes in atoms, molecules, and nanostructures. Prediction of the shapes of the optimal waveforms can be done computationally using quantum optimal control theory. In this work we demonstrate the control of above-threshold photoemission of one-dimensional hydrogen model with pulses feasible for experimental waveform synthesis. By mixing different spectral channels and thus lowering the intensity requirements for individual channels, the resulting optimal pulses can extend the cutoff energies by at least up to 50% and bring up the electron yield by several orders of magnitude. Insights into the electron dynamics for optimized photoelectron emission are obtained with a semiclassical two-step model.

Acoustic Measurement of the Length of Air-plasma Filament Induced by an Intense Femtosecond Laser Pulse

The paper studies acoustic emission from air-plasma filament induced by a strong femtosecond laser pulse. Acoustic signal is detected with a sensitive directional microphone. Acoustic measurement provides a new method to determine the length of a filament. Compared with other methods, acoustic measurement is simpler, more sensitive, and with higher spatial resolution. Information of filament length is experimentally acquired through measuring acoustic pressure at different position of filament. On the basis of the relationship between acoustic signal and free-electron density in filament, profile of free-electron density is demonstrated

Population Redistribution Among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields.

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N_{2}^{+}(X^{2}Σ_{g}^{+}) state and the excited N_{2}^{+}(A^{2}Π_{u}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states occur in strong laser fields. The couplings result in a population inversion between the N_{2}^{+}(X^{2}Σ_{g}^{+}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states at wavelengths near 800nm, which is verified by our experimental observation of the amplification of a seed at ∼391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.

Controlling ultrafast currents by the nonlinear photogalvanic effect

We investigate the effect of broken inversion symmetry on the generation and control of ultrafast currents in a transparent dielectric (SiO2) by strong femtosecond optical laser pulses. Ab initio simulations based on time-dependent density functional theory predict ultrafast direct currents that can be viewed as a nonlinear photogalvanic effect. Most surprisingly, the direction of the current undergoes a sudden reversal above a critical threshold value of laser intensity of about W cm−2. We trace this switching to the transition from nonlinear polarisation currents to the tunnelling excitation regime. The latter is found to be sensitive to the relative orientation between laser polarisation and chemical bonds. We demonstrate control of the ultrafast currents by the time delay between two laser pulses. While two temporally separated laser pulses lead to currents along one direction their temporal overlap can reverse the current. We find the ultrafast current control by the nonlinear photogalvanic effect to be remarkably robust and insensitive to the laser-pulse shape and the carrier-envelope phase.

Self-consistent Bloch equation for modeling element-specific demagnetization of magnetic alloys and multilayers

Magnetization dynamics of magnetic alloys and multilayers at high temperatures are studied by solving the self-consistent Bloch equation. Upon a fast rise of the temperature, usually driven by a strong femtosecond laser pulse, the element-specific demagnetization shows rich dynamic characteristics. We find that the demagnetization time scales could differ substantially for each constitutes in the same alloy. We discuss plausible reasons for the experimental results of the laser induced magnetization switching of GdFe ferrimagnetic compounds.

Ab-initio simulation of the ionization and fragmentation of acetylene by strong femtosecond laser pulses

The electron and nuclear dynamics of acetylene when interacting with strong short laser pulses has been simulated in the framework of real-space Time Dependent Density Functional Theory (TDDFT) and molecular dynamics. The stretching and dissociation of individual bonds are reported, and are shown to depend on the laser field intensity and orientation relative to the laser polarization. The ionization dynamics, including ionization from individual Kohn-Sham orbitals, is also reported. The orbital ionization dynamics are shown to vary with an increase in the intensity of the laser field.

Dissociative ionization of methane in an elliptical pulse shaped laser field

The effect of strong femto-second laser pulses on the dissociation probability of methane has been investigated analytically in various arrangements. The ellipticity dependence of the dissociation probability at intensities from 1014Wcm−2 to 1016Wcm−2 for Ti:Sapphire laser is presented. A reliable calculation of the dissociation probability based on 3D time-dependent Schrodinger equation with an improved model of time-dependent density-functional theory is presented. These calculations are carried out for three different cases of elliptically polarized laser pulse, optimum convolution of dual short pulses, and two-color mixed nonresonant laser pulses. It is found that the rescattering contribution of the laser pulses to the fragmental yields is more remarkable for ε=0.2, at 1015Wcm−2 laser intensity. Furthermore, by using the dual pulses combination for three proposed conditions, extracted by semi-classical approach, the dissociation rates up to 78% is achieved. In addition, by using a two-color mixed laser pulse, the total dissociation probability is improved up to 50% in comparison to a single pulse.

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)3He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD2 foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and 11B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.

Identification of strong one-step two-photon absorption in a cascadethree-level system

The ultrafast one-step and two-step two-photon absorption processes in a cascade three-level systeminduced by a strong femtosecond laser pulse is studied. The strong change of the population in the two-photonabsorption state at the zero points of the first derivative of the population in the real intermediate state showsstraightforwardly in time domain the footprint of the coherent one-step two-photon absorption.

Hose instability of relativistically strong femtosecond laser pulses with few field oscillations in a plasma under the excitation of a plasma wake wave

A detailed theoretical analysis is carried out of the hose instability of relativistically strong laser pulses propagating in a plasma, whose duration is less than the period of a plasma wake wave. An analytic expression is obtained for the displacement of the mass center of a wave pulse, and the effect of this instability on the modification of the spectrum of laser radiation is analyzed for a wide range of initial parameters. It is shown that the development of instability is characterized by a power-law (rather than exponential) time dependence along the propagation path and does not deteriorate the self-compression of laser pulses.

Laser-assisted ultrafast photoassociation in HeH2+

In this work we report control on the photoassociation of a proton and a Helium cation, or an alpha particle and an Hydrogen atom, to form HeH2+ assisted by strong femtosecond laser pulses. The results follow from the numerical solution of the time-dependent Schrödinger equation using soft-core Coulomb potentials in a (1+1)D model (one dimension for the electronic motion plus one dimension for the nuclear motion) and for a nuclear Hamiltonian describing the dynamics in four electronic states. We study the dependence of the photoassociation yield on the initial nuclear wave function for a hot collision (impact kinetic energy of the order of a eV) and on the laser parameters. We predict high sensitivity on the synchronization of the laser with the collision time and Rabi oscillations for the yield as a function of the pulse amplitude and duration, while the ionization and inelastic scattering can be minimized with pulses of 50fs or shorter.

Self-compression of relativistically strong femtosecond laser pulses during the excitation of a plasma wake wave

A new concept of the development of plasma compressor for generation of extremely short, relativistically strong femtosecond pulses at a petawatt power level is proposed. The considered compression mechanism is based on the non-stationary self-focusing of a spatially confined wave packet in transparent plasma during the excitation of a plasma wake wave with a period exceeding the laser pulse duration. It is shown that by using appropriately shaped, strong short laser pulses even shorter pulses (with durations up to one optical cycle of the field) can be generated with intensities much greater (>10 times) than the incident pulse with a significant fraction (∼25%) of incident pulse energy.

Angle-Dependent Irradiation of C4 in Femtosecond Laser Pulses

We study the angle-dependent irradiation of linear C4 in the strong femtosecond laser pulses with the help of time-dependent local density approximation applied to valence electrons, coupled non-adiabatically to molecular dynamics of ions. It is found that the excitation of C4 including the electrons and ions has a strong relation to the angle between the laser polarization and the internuclear axis of C4. The ionization yield decreases when the angle ranges from 0 to π/2. A clear suppression in the ionization yield is found when the laser polarization is perpendicular to the internuclear axis of C4. We track the dynamic motion of ionized C4 even in the relaxation time. It shows that the ionized C4 keeps on oscillating without fragmenting after the irradiation in different cases, while it is more excited when the laser polarization is along the internuclear axis. Furthermore, it is found that the change of the ELF takes place mainly in the xy plane and xz plane and the electron ejection mainly comes from the middle two atoms for the θ = 0 case.