Morten Førre

Exact nondipole Kramers-Henneberger form of the light-atom hamiltonian: an application to atomic stabilization and photoelectron energy spectra.

The exact nondipole minimal-coupling Hamiltonian for an atom interacting with an explicitly time- and space-dependent laser field is transformed into the rest frame of a classical free electron in the laser field, i.e., into the Kramers-Henneberger frame. The new form of the Hamiltonian is used to study nondipole effects in the high-intensity, high-frequency regime. Fully three-dimensional nondipole ab initio wave packet calculations show that the ionization probability may decrease for increasing field strength. We identify a unique signature for the onset of this dynamical stabilization effect in the photoelectron spectrum.

Strong orientation effects in ionization of H+2 by short, intense, high-frequency light pulses

We present three-dimensional time-dependent calculations of ionization of arbitrarily spatially oriented H+2 by attosecond, intense, high-frequency laser fields. The ionization probability shows a strong dependence on both the internuclear distance and the relative orientation between the laser field and the internuclear axis. The physical features are explained in terms of two-center interference effects.

Numerical study of two-photon ionization of helium using an ab initio numerical framework

Few-photon-induced breakup of helium is studied using a newly developed ab initio numerical framework for solving the six-dimensional time-dependent Schroedinger equation. We present details of the method and calculate (generalized) cross sections for the process of two-photon nonsequential (direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a process that has been very much debated in recent years and is not yet fully understood. In particular, we have studied the convergence property of the total cross section in the vicinity of the upper threshold ({approx} 54.4 eV) versus the pulse duration of the applied laser field. We find that the cross section exhibits an increasing trend near the threshold, as has also been observed by others, and show that this rise cannot solely be attributed to an unintended inclusion of the sequential two-photon double ionization process caused by the bandwidth of the applied field.

High-order harmonic generation in graphene flakes exposed to circularly polarized femtosecond pulses

We calculate high-order harmonic spectra from graphene based on the strong-field approximation using circularly polarized infrared laser pulses. We allow for the plane of polarization to be tilted with respect to the two-dimensional graphene sheet, demonstrating that the structure of the harmonic spectra strongly depends on the tilt angle.

A master equation approach to double ionization of helium

It is demonstrated how a numerical approach based on absorbing boundaries may be used to describe the process of non-sequential two-photon double ionization of helium. Contrary to any method based on solving the Schrodinger equation alone, this numerical scheme is able to reconstruct the remaining particles as one particle is absorbed. This may be used to distinguish between single and double ionization. A model of reduced dimensionality, which describes the process at a qualitative level, has been used. The results have been compared with a more conventional method in which the time-dependent Schrodinger equation is solved and the final wavefunction is analysed in terms of projection onto eigenstates of the uncorrelated Hamiltonian, i.e. with no electron–electron interaction included in the final states. It is found that the two methods indeed produce the same total cross sections for the process.

Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.

Dynamics of a single Rydberg shell in time dependent external fields

Probabilities for adiabatic or near-adiabatic state transformation within a highly excited shell of Li(n = 25) were studied experimentally and theoretically for a time dependent electric field, (t), and a constant magnetic field, . The fields were sufficiently weak and the time dependence slow enough such that only states belonging to the chosen shell were involved. The studies show that the dynamics are governed by the approximate hydrogenic character of the system in most cases, but for some specific time dependences it is influenced strongly by core interactions as expressed through the quantum defects, δl. The s-state is effectively decoupled from the rest of the n = 25 manifold due to a very large quantum defect. However the quantum defects of the p, d and f states are shown to play a decisive role in the dynamics. The core interactions lead to avoided crossings, non-adiabatic state transformations, and possibly even phase-interference effects. When a resonance condition pertaining to the hydrogenic character of the system is fulfilled, a linear Stark state is transformed completely into a circular Stark state oriented along f.

Quantitative modeling of spin relaxation in quantum dots

Physics Department, Harvard University, 02138 Cambridge MA, USA(Dated: December 16, 2011)We use numerically exact diagonalization to calculate the spin-orbit and phonon-induced triplet-singlet relaxation rate in a two-electron quantum dot exposed to a tilted magnetic field. Our schemeincludes a three-dimensional description of the quantum dot, the Rashba and the linear and cubicDresselhaus spin-orbit coupling, the ellipticity of the quantum dot, and the full angular descriptionof the magnetic field. We are able to find reasonable agreement with the experimental results ofMeunier et al. [Phys. Rev. Lett. 98, 126601 (2007)] in terms of the singlet-triplet energy splittingand the spin relaxation rate, respectively. We analyze in detail the effects of the spin-orbit factors,magnetic-field angles, and the dimensionality, and discuss the origins of the remaining deviationsfrom the experimental data.

One-photon double ionization of helium: A heuristic formula for the cross section

Without a formal derivation, we propose a formula for the total and single-differential cross in the problem of one-photon double ionization of an atom. The formula is benchmarked against accurate experimental data for the total cross section of helium. Furthermore, a direct comparison with ab initio calculations for the double ionization of Li+ suggests that the framework is valid for the entire helium isoelectronic sequence. To this end, we introduce a formula for the double ionization of lithium, as well as for the triple ionization of lithium and beryllium.

Electron transport in a double quantum dot controlled by magnetic switching

We investigate the possibility of performing single-electron controlled transport in coupled quantum dots based on magnetic switching. From numerical solution of the time-dependent Schrodinger equation it is shown that certain combinations of static and switched magnetic fields can result in a situation where an initially localized wavefunction can be transferred from one of the dot centres to the other one with unit probability.