Photoemission of a single-electron wave packet in a strong laser field

The radiation emitted by a single-electron wave packet in an intense laser field is considered. A relation between the exact quantum formulation and its classical counterpart is established via the electron's Wigner function. In particular, we show that the wave packet, even when it spreads to the scale of the wavelength of the driving laser field, cannot be treated as an extended classical charge distribution, but rather behaves as a pointlike emitter carrying information on its initial quantum state. We outline an experimental setup dedicated to put this conclusion to the test.     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.     

Attosecond plasma wave dynamics in laser-driven cluster nanoplasmas

We introduce a microscopic particle-in-cell approach that allows bridging the microscopic and macroscopic realms of laser-driven plasma physics. As a first application, resonantly driven cluster nanoplasmas are investigated. Our analysis reveals an attosecond plasma-wave dynamics in clusters with radii R is approximately equal to 30 nm. The plasma waves are excited by electrons recolliding with the cluster surface and travel toward the center, where they collide and break. In this process, energetic electron hot spots are generated along with highly localized attosecond electric field fluctuations, whose intensity exceeds the driving laser by more than 2 orders of magnitude. The ionization enhancement resulting from both effects generates a strongly nonuniform ion charge distribution. The observed nonlinear plasma-wave phenomena have a profound effect on the ionization dynamics of nanoparticles and offer a route to extreme nanoplasmonic field enhancements.     

Semiclassical dynamics of electron wave packet states with phase vortices

We consider semiclassical higher-order wave packet solutions of the Schr?dinger equation with phase vortices. The vortex line is aligned with the propagation direction, and the wave packet carries a well-defined orbital angular momentum (OAM) variant Planck's over 2pil (l is the vortex strength) along its main linear momentum. The probability current coils around the momentum in such OAM states of electrons. In an electric field, these states evolve like massless particles with spin l. The magnetic-monopole Berry curvature appears in momentum space, which results in a spin-orbit-type interaction and a Berry/Magnus transverse force acting on the wave packet. This brings about the OAM Hall effect. In a magnetic field, there is a Zeeman interaction, which, can lead to more complicated dynamics.     

Time-resolved quantum dynamics of double ionization in strong laser fields

Quantum calculations of a (1+1)-dimensional model for double ionization in strong laser fields are used to trace the time evolution from the ground state through ionization and rescattering to the two-electron escape. The subspace of symmetric escape, a prime characteristic of nonsequential double ionization, remains accessible by a judicious choice of 1D coordinates for the electrons. The time-resolved ionization fluxes show the onset of single and double ionization, the sequence of events during the pulse, and the influences of pulse duration and reveal the relative importance of sequential and nonsequential double ionization, even when ionization takes place during the same field cycle.     

Steering the electron in H2(+) by nuclear wave packet dynamics

By combining carrier-envelope phase (CEP) stable light fields and the traditional method of optical pump-probe spectroscopy we study electron localization in dissociating H2(+) molecular ions. Localization and localizability of electrons is observed to strongly depend on the time delay between the two CEP-stable laser pulses with a characteristic periodicity corresponding to the oscillating molecular wave packet. Variation of the pump-probe delay time allows us to uncover the underlying physical mechanism for electron localization, which are two distinct sets of interfering dissociation channels that exhibit specific temporal signatures in their asymmetry response.     

Quantum theory of a free electron laser for strong fields

A quantum mechanical derivation of the gain of the free electron laser is presented which starts from the exact solutions of the Klein-Gordon equation in the presence of the magnetic wiggler field and the stimulating plane wave field. The gain is obtained from the transition amplitude of the electron, which is expressed in terms of the Green's function. For comparatively weak fields the usual small signal gain results, whereas for strong fields saturation behaviour is found.Supported by Deutsche Forschungsgemeinschaft     

Neutrino wave packet propagation in gravitational fields

We discuss the propagation of neutrino wave packets in a Lense–Thirring metric using a gravitational phase approach. We show that the neutrino oscillation length is altered by gravitational corrections and that neutrinos are subject to helicity flip induced by stellar rotation. For the case of a rapidly rotating neutron star, we show that absolute neutrino masses can be derived, in principle, from rotational contributions to the mass-induced energy shift, without recourse to mass generation models presently discussed in the literature.     

Attosecond pulse trains generated using two color laser fields

We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a pi phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime.     

Isolated attosecond electron wave packet diffraction

Wave-particle duality is one of the most fundamental and mysterious natures of matters.Here,we present an interesting scheme of isolated electron wave packet diffraction with a few-cycle laser pulse and an extreme ultraviolet (XUV) pulse.The diffraction fringes are clearly present in the laser dressed XUV photoelectron spectra,strongly resembling the Airy diffraction pattern of optical waves.This phenomenon suggests a great potential of attosecond diffractometry.According to this scheme we also propose a simple method to determine the XUV pulse duration from the photoelectron spectra with a rather high resolution.     

Mapping attosecond electron wave packet motion

Attosecond pulses are produced when an intense infrared laser pulse induces a dipole interaction between a sublaser cycle recollision electron wave packet and the remaining coherently related bound-state population. By solving the time-dependent Schr?dinger equation we show that, if the recollision electron is extracted from one or more electronic states that contribute to the bound-state wave packet, then the spectrum of the attosecond pulse is modulated depending on the relative motion of the continuum and bound wave packets. When the internal electron and recollision electron wave packet counterpropagate, the radiation intensity is lower. We show that we can fully characterize the attosecond bound-state wave packet dynamics. We demonstrate that electron motion from a two-level molecule with an energy difference of 14 eV, corresponding to a period of 290 asec, can be resolved.     

Many-electron dynamics of a Xe atom in strong and superstrong laser fields

We report on detailed investigations of ionization dynamics of a Xe atom exposed to intense 800-nm pulses of 20-fs duration in the extensive intensity range from 10(13)-10(18) W/cm(2). Ion yields of Xe+-Xe20+ were observed as a function of laser intensity and compared with the results from a single active electron based Ammosov-Delone-Krainov model. Unexpected ionization probabilities for lower charge states and no interplay between the inner and outer shells by screening are inferred. Suppression of nonsequential ionization towards higher intensity and few optical cycle regimes is also proved.     

Cold collisions in strong laser fields: partial wave analysis of magnesium collisions

We have developed Monte Carlo wave function simulation schemes to study cold collisions between magnesium atoms in a strong red-detuned laser field. In order to address the strong-field problem, we extend the Monte Carlo wave function framework to include the partial wave structure of the three-dimensional system. The average heating rate due to radiative collisions is calculated with two different simulation schemes which are described in detail. We show that the results of the two methods agree and give estimates for the radiative collision heating rate for ²⁴Mg atoms in a magneto-optical trap based on the ¹S₀–¹P₁ atomic laser cooling transition.     

Gas dynamics in strong gravitational fields

The steady and axially symmetric flow of a perfect fluid is studied in the context of general relativistic gas dynamics. It is assumed that the flow occurs in the background field of a rotating black hole (or any compact object). The hydrodynamic equations are referred to a locally nonrotating frame and their characteristics are found. The equations describing oblique shock waves are also obtained.     

Attosecond laser station

The attosecond laser station(ALS) at the Synergetic Extreme Condition User Facility(SECUF) is a sophisticated and user-friendly platform for the investigation of the electron dynamics in atoms, molecules, and condensed matter on timescales ranging from tens of femtoseconds to tens of attoseconds. Short and tunable coherent extreme-ultraviolet(XUV)light sources based on high-order harmonic generation in atomic gases are being developed to drive a variety of endstations for inspecting and controlling ultrafast electron dynamics in real time. The combination of such light sources and end-stations offers a route to investigate fundamental physical processes in atoms, molecules, and condensed matter. The ALS consists of four beamlines, each containing a light source designed specifically for application experiments that will be performed in its own end-station. The first beamline will produce broadband XUV light for attosecond photoelectron spectroscopy and attosecond transient absorption spectroscopy. It is also capable of performing attosecond streaking to characterize isolated attosecond pulses and will allow studies on the electron dynamics in atoms, moleculars, and condensed matter. The second XUV beamline will produce narrowband femtosecond XUV pulses for time-resolved and angle-resolved photoelectron spectroscopy, to study the electronic dynamics on the timescale of fundamental correlations and interactions in solids, especially in superconductors and topological insulators. The third beamline will produce broadband XUV pulses for attosecond coincidence spectroscopy in a cold-target recoil-ion momentum spectrometer, to study the ultrafast dynamics and reactions in atomic and molecular systems. The last beamline produces broadband attosecond XUV pulses designed for time-resolved photoemission electron microscopy, to study the ultrafast dynamics of plasmons in nanostructures and the surfaces of solid materials with high temporal and spatial resolutions simultaneously. The main object of the ALS is to provide domestic and international scientists with unique tools to study fundamental processes in physics, chemistry,biology, and material sciences with ultrafast temporal resolutions on the atomic scale.     

Non-linear wave packet dynamics of coherent states

We have compared the non-linear wave packet dynamics of coherent states of various symmetry groups and found that certain generic features of non-linear evolution are present in each case. Thus the initial coherent structures are quickly destroyed but are followed by Schrödinger cat formation and revival. We also report important differences in their evolution.     

Attosecond electron bunches

Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.