Strong Laser Pulses Research Articles

Auger effect in the presence of strong x-ray pulses

We study the role of propagation of strong x-ray free-electron laser pulses on the Auger effect. When the system is exposed to a strong x-ray pulse the stimulated emission starts to compete with the Auger decay. As an illustration we present numerical results for Ar gas with the frequency of the incident x-ray pulse tuned in the $2{p}_{3/2}--4s$ resonance. It is shown that the pulse propagation is accompanied by two channels of amplified spontaneous emission, $4s--2{p}_{3/2}$ and $3s--2{p}_{3/2}$, which reshape the pulse when the system is inverted. The population inversion is quenched for longer propagation distances where lasing without inversion enhances the Stokes component. The results of simulations show that the propagation of the strong x-ray pulses affect intensively the Auger branching ratio.

Alignment structures of rotational wavepacket created by two strong femtosecond laser pulses

We experimentally and theoretically study the alignment structures of the rotational wavepacket created by linear molecules and two strong femtosecond laser pulses. In the experiment, we observe that the alignment structures depend on the time delay between the two laser pulses. In the theory, we find that the alignment structures are composed of the self-coupling term and the cross-coupling term. The contributions of these two terms are separately calculated. Their coherent superposition reproduces the alignment structures observed in the experiment.

Ionization of atomic hydrogen in strong infrared laser fields

We have used the matrix iteration method of Nurhuda and Faisal [Phys. Rev. A 60, 3125 (1999)] to treat ionization of atomic hydrogen by a strong laser pulse. After testing our predictions against a variety of previous calculations, we present ejected-electron spectra as well as angular distributions for few-cycle infrared laser pulses with peak intensities of up to ${10}^{15}$ W/cm${}^{2}$. It is shown that the convergence of the results with the number of partial waves is a serious issue, which can be managed in a satisfactory way by using the velocity form of the electric dipole operator in connection with an efficient time-propagation scheme.

Deflection of Field-Free Aligned Molecules

We consider deflection of polarizable molecules by inhomogeneous optical fields, and analyze the role of molecular orientation and rotation in the scattering process. We show that by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.

Emission and its back-reaction accompanying electron motion in relativistically strong and QED-strong pulsed laser fields

The emission from an electron in the field of a relativistically strong laser pulse is analyzed. At pulse intensities of J>or=2x10(22) W/cm(2) the emission from counterpropagating electrons is modified by the effects of quantum electrodynamics (QED), as long as the electron energy is sufficiently high: E>or=1 GeV . The radiation force experienced by an electron is for the first time derived from the QED principles and its applicability range is extended toward the QED-strong fields.

The excitation and ionization of the hydrogen atom in strong laser fields

Abstract The probability of excitation and ionization of the hydrogen atom by short strong laser pulses is calculated by solving numerically the time-dependent Schrödinger equation. The probability of excitation to different states is investigated, which helps to identify the important ionization mechanisms. The ionization probability density was also calculated and a good agreement with the other theoretical results was found.

Quantum Kinetic Approach to Time‐Resolved Photoionization of Atoms

AbstractTheoretical approaches to the photoionization of few‐electron atoms are discussed. These include nonequilibrium Greens functions and wave function based approaches. In particular, the Multiconfiguration Time‐Dependent Hartree‐Fock method is discussed and applied to model a one‐dimensional atom with four electrons. We compute ground state energies and the time‐dependent photoionization by the field a strong laser pulse with two different frequencies in the ultraviolet (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observing the Transition from Stark-Shifted, Strong-Field Resonance to Nonadiabatic Excitation

Time-dependent Hartree−Fock simulations for a linear triatomic molecular monocation (CO2+) interacting with a 5 fs, 800 nm, strong field laser pulse were performed to explore the excitation mechani...

Time-dependent theory of Auger decay induced by ultra-short pulses in a strong laser field

A quantum mechanical theory of a laser-assisted Auger process in atoms excited by an ultra-short (attosecond) electromagnetic pulse in the field of a few-cycle strong laser pulse is presented. It is based on the non-stationary Schrödinger equation, which describes the photoionization of an inner atomic shell and the decay of the created vacancy, while the Auger electron is treated in the strong field approximation. As an example, the photoionization of the Ne 1s shell with the subsequent KLL Auger transition is considered. The spectra and angular distributions of photoelectrons and Auger electrons are calculated and discussed. The photoelectron spectra show a typical picture of streaking in the laser field. In contrast, the Auger electron spectrum contains sideband structure, which is however, different from the conventional equidistant sideband structure, which had been discussed for longer pulses and smaller Auger electron energies. The predicted sideband structure strongly depends on the delay time between the laser pulse and the x-ray pulse. It is sensitive to the carrier-envelope phase as well. A simplified description of the Auger electron spectra in laser-assisted Auger decay, which gives the sideband structure in close agreement with the results of the exact theory, is suggested.

X-ray absorption in neon modulated by a strong laser pulse

We have measured the absorption of x-rays in neon gas in the presence of a strong laser pulse. The femtosecond x-rays were tuned to energies near the neon 1s-3p resonance, and the laser intensity of 1013 W/cm2 was below the intensity required to alone ionize neon. We observed strong modification of the x-ray absorption when the neon was subjected to laser light that was temporally overlapped with the x-rays.

Angular distribution in multi-photon ionization of hydrogen in intense laser fields

We have studied the multi-photon ionization of atomic hydrogen by a strong laser pulse. Particular emphasis is placed on the angular distribution of the ejected electrons. Rapid changes in the β parameters across the photolines may complicate the comparison of theory and experiment in practical scenarios.

Monte Carlo Wave Packet Theory of Dissociative Double Ionization

Nuclear dynamics in strong-field double ionization processes is predicted using a stochastic Monte Carlo wave packet technique. Using input from electronic structure calculations and strong-field electron dynamics the description allows for field-dressed dynamics within a given molecule as well as transitions between several different charge states. The description is computationally efficient and applicable to a wide range of systems. As a proof of principle, theoretical nuclear kinetic energy release spectra for H2 (D2) in strong near-infrared laser pulses of 40 fs duration are compared to experiments and very good agreement is obtained.

Laser-assisted molecular orientation in gaseous media: new possibilities and applications

It was shown recently by us that an isotropic distribution of molecules in gaseous media can be drastically effected via their orientation-dependent selective excitation by a strong femtosecond multicomponent laser pulse. In the present paper, we analyze the specific effects accompanying the dynamical orientation of molecules driven this way. It is demonstrated that the peculiarities of the post-pulse transient angular distribution of molecules allow original proposals for the generation of pulsed terahertz radiation and also for the determination of the molecular rotational constants.

Manipulating electronic couplings and nonadiabatic nuclear dynamics with strong laser pulses

We demonstrate the possibility of manipulating the coupling between two (optically bright and optically dark) excited electronic states and of controlling the ensuing nuclear wave packet dynamics via a strong laser pulse, which couples the ground and the bright electronic state. The control of the wave packet dynamics is implemented through the creation of a highly nonequilibrium distribution in the bright-dark vibronic manifold. The distribution is produced due to the combined effect of the external pulse (through Rabi cycling) and the system itself (through the electronic interstate coupling). The induced wave packet dynamics persists long after the pulse is over, both in the isolated and in the dissipative system. The effects are robust and are achieved by varying the strength and duration of a Gaussian pulse, that is, no careful tuning of the pulse shape is required. Possible applications of our results include the strong-pulse control of electron transfer as well as the enhancement and detection of intramolecular electronic coupling via strong-pulse spectroscopy.

Molecular Rotational Excitation by Strong Femtosecond Laser Pulses

We study the rotational wave packet created by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. The applicable condition of the Delta-Kick method is obtained by comparing the laser intensity and pulse duration dependences of the wave packet calculated with different methods. The wave packet evolution is traced analytically with the Delta-Kick method. The calculations demonstrate that the rotational populations can be controlled for the rotational wave packet created by two femtosecond laser pulses. The evolution of the rotational wave packet with controlled populations produces interference patterns with exotic spatial symmetries. These calculations are validated by comparing the theoretical calculations with our experimental measurements for the rotational wave packet created by thermal ensemble CO(2) and two strong femtosecond laser pulses. Potential applications in molecular science are also discussed for the rotational wave packet with controlled populations and spatial symmetries.

Laser control of conical intersections: Quantum model simulations for the averaged loss-gain strategies of fast electronic deactivation in 1,1-difluoroethylene

The enhancing and inhibition of population transfer via a conical intersection is demonstrated with quantum model calculations on the 1,1-difluoroethylene system. Averaged loss-gain strategies are achieved using strong laser pulses, which either trap the wave packet in the excited state, or accelerate the wave packet in the vicinity of the conical intersection.

Explicit time-propagation method to treat the dynamics of driven complex systems

We describe the efficient implementation of an explicit method to solve systems of stiff differential equations either on a grid or within a spectral approach. This method is based on an ansatz that approximates the solution. This ansatz depends on stiffness parameters that are shown to be related to the eigenfrequencies of the system. The accuracy and the performance of the method are tested in three different cases. First, we treat a highly stiff single differential equation, where explicit schemes converge rather slowly. Then, we solve the stationary Schr\"odinger equation associated to the quantum reflection of an ultracold atom by a surface. Finally, we consider the interaction of atomic hydrogen with a strong low-frequency laser pulse whose duration is of the order of 25 fs. We focus on the calculation of the above-threshold ionization electron spectrum, a problem which, under such realistic physical conditions, is computationally very demanding.

Strong field double ionization of H 2: Insights from nonlinear dynamics

The uncorrelated (“sequential”) and correlated (“nonsequential”) double ionization of the H 2 molecule in strong laser pulses is investigated using tools of nonlinear dynamics. We focus on the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate these ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitative changes in ionization occur. We also show how these findings depend on the internuclear distance.

Proof-of-concept of real-world quantum key distribution with quantum frames

We propose a fibre-based quantum key distribution system, which employs polarization qubits encoded into faint laser pulses. As a novel feature, it allows sending of classical framing information via sequences of strong laser pulses that precede the quantum data. This allows synchronization, sender and receiver identification and compensation of time-varying birefringence in the communication channel. In addition, this method also provides a platform to communicate implementation specific information such as encoding and protocol in view of future optical quantum networks. We demonstrate in a long-term (37 h) proof-of-principle study that polarization information encoded in the classical control frames can indeed be used to stabilize unwanted qubit transformation in the quantum channel. All optical elements in our setup can be operated at Gbps rates, which is a first requirement for a future system delivering secret keys at Mbps. In order to remove another bottleneck towards a high rate system, we investigate forward error correction based on low-density parity-check codes.

Dynamics of emitting electrons in strong laser fields

A new derivation of the motion of a radiating electron is given, leading to a formulation that differs from the Lorentz–Abraham–Dirac equation and its published modifications. It satisfies the proper conservation laws. Particularly, it conserves the generalized momentum, eliminating the symmetry-breaking runaway solution. The equation allows a consistent calculation of the electron current, the radiation effect on the electron momentum, and the radiation itself, for a single electron or plasma electrons in strong electromagnetic fields. The equation is then applied to a simulation of a strong laser pulse interaction with a plasma target. Some analytical solutions are also provided.