arXiv subjects

Eva Lindroth

Publications and source records attributed to Eva Lindroth.

At least 37 records · Page 2Linked to original sources

arXiv2019

Fano's propensity rule in angle-resolved attosecond pump-probe photoionization

In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. Here we generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular-dependence of the photoionization time delays and the delay-dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.

physics.atom-ph↗

arXiv2018

Pulse analysis by delayed absorption from a coherently excited atom

In this tutorial we provide a short review of attosecond pulse characterization techniques and a pedagogical account of a recently proposed method called Pulse Analysis by Delayed Absorption (PANDA) [Pabst and Dahlström, Phys. Rev. A, 94, 13411 (2016)]. We discuss possible implementations of PANDA in alkali atoms using either principal quantum number wave packets or spin-orbit wave packets. The main merit of the PANDA method is that it can be used as a pulse characterization method that is free from atomic latency effects, such as scattering phase shifts and long-lived atomic resonances. Finally, we propose that combining the PANDA method with angle-resolved photoelectron detection should allow for experimental measurements of attosecond delays in photoionization from bound wave packets on the order of tens of attoseconds.

quant-ph↗

arXiv2018

A Schrödinger equation for relativistic laser-matter interactions

A semi-relativistic formulation of light-matter interaction is derived using the so called propagation gauge and the relativistic mass shift. We show that relativistic effects induced by a super-intense laser field can, to a surprisingly large extent, be accounted for by the Schr{ö}dinger equation, provided that we replace the rest mass in the propagation gauge Hamiltonian by the corresponding time-dependent field-dressed mass. The validity of the semi-relativistic approach is tested numerically on a hydrogen atom exposed to an intense XUV laser pulse strong enough to accelerate the electron towards relativistic velocities. It is found that while the results obtained from the ordinary (non-relativistic) Schr{ö}dinger equation generally differ from those of the Dirac equation, merely demonstrating that relativistic effects are significant, the semi-relativistic formulation provides results in quantitative agreement with a fully relativistic treatment.

quant-ph↗

arXiv2017

Attosecond transient absorption of a bound wave packet coupled to a smooth continuum

We investigate the possibility to use transient absorption of a coherent bound electron wave packet in hydrogen as an attosecond pulse characterization technique. In recent work we have shown that photoionization of such a coherent bound electron wave packet opens up for pulse characterization with unprecedented temporal accuracy --- independent of the atomic structure --- with maximal photoemission at all kinetic energies given a wave packet with zero relative phase [Pabst and Dahlström, Phys. Rev. A, 94, 13411 (2016)]. Here, we perform numerical propagation of the time-dependent Schrödinger equation and analytical calculations based on perturbation theory to show that the energy-resolved maximal absorption of photons from the attosecond pulse does not uniquely occur at zero relative phase of the initial wave packet. Instead, maximal absorption occurs at different relative wave packet phases, distributed as a non-monotonous function with a smooth $-π/2$ shift across the central photon energy (given a Fourier-limited Gaussian pulse). Similar results are found also in helium. Our finding is surprising because it implies that the energy-resolved photoelectrons are not mapped one-to-one with the energy-resolved absorbed photons of the attosecond pulse.

physics.optics↗

arXiv2017

Attosecond delays in laser-assisted photodetachment from closed-shell negative ions

We study laser-assisted photodetachment time delays by attosecond pulse trains from the closed-shell negative ions F- and Cl-. We investigate the separability of the delay into two contributions: (i) the Wigner-like delay associated with one-photon ionization by the attosecond pulse train and (ii) the delay associated with exchange of an additional laser photon in the presence of the potential of the remaining target. Based on the asymptotic form of the wave packet, the latter term is expected to be negligible because the ion is neutralized leading to a vanishing laser-ion interaction with increasing electron-atom separation. While this asymptotic behavior is verified at high photoelectron energies, we also quantify sharp deviations at low photoelectron energies. Further, these low-energy delays are clearly different for the two studied anions indicating a breakdown of the universality of laser-ion induced delays. The fact that the short-range potential can induce a delay of as much as 50 as can have implications for the interpretation of delay measurements also in other systems that lack long-range potential.

physics.atom-ph↗

arXiv2017

Alternative gauge for the description of the light-matter interaction in a relativistic framework

We present a generalized velocity gauge form of the relativistic laser-matter interaction. In comparison with the (equivalent) regular minimal coupling description, this new form of the light-matter interaction results in superior convergence properties for the numerical solution of the time-dependent Dirac equation. This applies both to the numerical treatment and, more importantly, to the multipole expansion of the laser field. The advantages of the alternative gauge is demonstrated in hydrogen by studies of the dynamics following the impact of superintense laser pulses of extreme ultraviolet wavelengths and sub-femtosecond duration.

physics.atom-ph↗

arXiv2017

Relativistic ionization dynamics for a hydrogen atom exposed to super-intense XUV laser pulses

We present a theoretical study of the ionization dynamics of a hydrogen atom exposed to attosecond laser pulses in the extreme ultra violet region at very high intensities. The pulses are such that the electron is expected to reach relativistic velocities, thus necessitating a fully relativistic treatment. We solve the time dependent Dirac equation and compare its predictions with those of the corresponding non-relativistic Schr{ö}dinger equation. We find that as the electron is expected to reach about 20% of the speed of light, relativistic corrections introduces a finite yet small decrease in the probability of ionizing the atom.

physics.atom-ph↗

arXiv2016

Envelope induced ionization dynamics beyond the dipole approximation

When atoms and molecules are ionized by laser pulses of finite duration and increasingly high intensities, the validity of the much used dipole approximation, in which the spatial dependence and magnetic component of the external field are neglected, eventually breaks down. We report that when going beyond the dipole approximation for the description of atoms exposed to ultraviolet light, the spatial dependence of the pulse shape, the envelope, provides the dominant correction, while the spatial dependence of the carrier is negligible. We present a first order beyond-dipole correction to the Hamiltonian which accounts exclusively for nondipole effects stemming from the carrier-envelope of the pulse. We demonstrate by {\it ab initio} calculations for hydrogen that this approximation, which we will refer to as the {\it envelope approximation}, reproduces the full interaction beyond the dipole approximation for absolute and differential observables and proves to be valid for a broad range of high-frequency fields. This is done both for the Schr{ö}dinger and the Dirac equation. Moreover, it is demonstrated that the envelope approximation provides an interaction-term which gives rise to faster numerical convergence in terms of partial waves compared to its exact counterpart.

quant-ph↗

arXiv2015

Time delay anisotropy in photoelectron emission from the isotropic ground state of helium

Time delays of electrons emitted from an isotropic initial state and leaving behind an isotropic ion are assumed to be angle-independent. Using an interferometric method involving XUV attosecond pulse trains and an IR probe field in combination with a detection scheme, which allows for full 3D momentum resolution, we show that measured time delays between electrons liberated from the $1s^2$ spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the linear polarization axis of the ionizing XUV light. Such time-delay anisotropy, for which we measure values as large as 60 attoseconds, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon in the field of the parent-ion. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general, significant, and initially unexpected effect that must be taken into account in all such photoionization measurements.

physics.atom-ph↗

arXiv2013

Photoionization of helium by attosecond pulses: extraction of spectra from correlated wave functions

We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wavepacket resulting from the integration of the time-dependent Schroedinger equation in a finite-element discrete variable representation basis. These techniques are: projection on products of hydrogenic bound and continuum states, projection onto multi-channel scattering states computed in a B-spline close-coupling basis, and a technique based on exterior complex scaling (ECS) implemented in the same basis used for the time propagation. These methods allow to monitor the population of continuum states in wavepackets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wavepackets for two-photon double ionization.

physics.atom-ph↗

arXiv2010

Ionization branching ratio control with a resonance attosecond clock

We investigate the possibility to monitor the dynamics of autoionizing states in real time and control the yields of different ionization channels in helium by simulating XUV-pump IR-probe experiments focused on the N=2 threshold. The XUV pulse creates a coherent superposition of doubly excited states which is found to decay by ejecting electrons in bursts. Prominent interference fringes in the photoelectron angular distribution of the $2s$ and $2p$ ionization channels are observed, along with sizable out-of-phase quantum beats in the yields of the corresponding parent ions.

physics.atom-ph↗

arXiv2010

Estimation of the spatial decoherence time in circular quantum dots

We propose a simple phenomenological model to estimate the spatial decoherence time in quantum dots. The dissipative phase space dynamics is described in terms of the density matrix and the corresponding Wigner function, which are derived from a master equation with Lindblad operators linear in the canonical variables. The formalism was initially developed to describe diffusion and dissipation in deep inelastic heavy ion collisions, but also an application to quantum dots is possible. It allows us to study the dependence of the decoherence rate on the dissipation strength, the temperature and an external magnetic field, which is demonstrated in illustrative calculations on a circular GaAs one-electron quantum dot.

cond-mat.other↗

arXiv2010

Effects of screened Coulomb impurities on autoionizing two-electron resonances in spherical quantum dots

In a recent paper (Phys. Rev. B {\bf 78}, 075316 (2008)), Sajeev and Moiseyev demonstrated that the bound-to-resonant transitions and lifetimes of autoionizing states in spherical quantum dots can be controlled by varying the confinment strength. In the present paper, we report that such control can in some cases be compromised by the presence of Coulomb impurities. It is demonstrated that a screened Coulomb impurity placed in the vicinity of the dot center can lead to bound-to-resonant transitions and to avoided crossings-like behavior when the screening of the impurity charge is varied. It is argued that these properties also can have impact on electron transport through quantum dot arrays.

cond-mat.mes-hall↗

arXiv2010

Fundamental Gates for a Strongly Correlated Two-Electron Quantum Ring

We demonstrate that conditional as well as unconditional basic operations which are prerequisite for universal quantum gates can be performed with almost 100% fidelity within a strongly interacting two-electron quantum ring. Both sets of operations are based on a quantum control algorithm that optimizes a driving electromagnetic pulse for a given quantum gate. The demonstrated transitions occur on a time scale much shorter than typical decoherence times of the system.

cond-mat.mes-hall↗

arXiv2010

Many-Body perturbation theory calculations on circular quantum dots

The possibility to use perturbation theory to systematically improve calculations on circular quantum dots is investigated. A few different starting points, including Hartree-Fock, are tested and the importance of correla- tion is discussed. Quantum dots with up to 12 electrons are treated and the effects of an external magnetic field are examined. The sums over excited states are carried out with a complete finite radial basis set obtained through the use of B splines. The calculated addition energy spectra are compared with experiments and the implications for the filling sequence of the third shell are discussed in detail.

cond-mat.mes-hall↗

arXiv2010

Controlled Operations in a Strongly Correlated Two-Electron Quantum Ring

We have analyzed the electronic spectrum and wave function characteristics of a strongly correlated two-electron quantum ring with model parameters close to those observed in experiments. The analysis is based on an exact diagonalization of the Hamiltonian in a large B-spline basis. We propose a new qubit pair for storing quantum information, where one component is stored in the total electron spin and one multivalued "quMbit" is represented by the total angular momentum. In this scheme the controlled NOT (CNOT) quantum gate is demonstrated with near 100% fidelity for a realistic far infrared electromagnetic pulse.

cond-mat.mes-hall↗

arXiv2010

Performance of the coupled cluster singles and doubles method on two-dimensional quantum dots

An implementation of the coupled-cluster single- and double excitations (CCSD) method on two-dimensional quantum dots is presented. Advantages and limitations are studied through comparison with other high accuracy approaches for two to eight confined electrons. The possibility to effectively use a very large basis set is found to be an important advantage compared to full configuration interaction implementations. For the two to eight electron ground states, with a confinement strength close to what is used in experiments, the error in the energy introduced by truncating triple excitations and beyond is shown to be on the same level or less than the differences in energy given by two different Quantum Monte Carlo methods. Convergence of the iterative solution of the coupled cluster equations is, for some cases, found for surprisingly weak confinement strengths even when starting from a non-interacting basis. The limit where the missing triple and higher excitations become relevant is investigated through comparison with full Configuration Interaction results.

cond-mat.mes-hall↗

arXiv2004

The nonrelativistic limit of Dirac-Fock codes: the role of Brillouin configurations

We solve a long standing problem with relativistic calculations done with the widely used Multi-Configuration Dirac-Fock Method (MCDF). We show, using Relativistic Many-Body Perturbation Theory (RMBPT), how even for relatively high-$Z$, relaxation or correlation causes the non-relativistic limit of states of different total angular momentum but identical orbital angular momentum to have different energies. We show that only large scale calculations that include all single excitations, even those obeying the Brillouin's theorem have the correct limit. We reproduce very accurately recent high-precision measurements in F-like Ar, and turn then into precise test of QED. We obtain the correct non-relativistic limit not only for fine structure but also for level energies and show that RMBPT calculations are not immune to this problem.

physics.atom-ph↗