arXiv subjects

Lars Bojer Madsen

Publications and source records attributed to Lars Bojer Madsen.

At least 37 records · Page 2Linked to original sources

arXiv2024

Noninteger high-harmonic generation from extended correlated systems

The spectra produced by high-harmonic generation (HHG) typically exhibit well-defined peaks at odd integers times the laser frequency. However, in recent investigations of HHG from correlated materials, spectra exhibit signals at noninteger harmonics which do not conform to the well-known symmetry-based selection rules for HHG-spectra. Here, we use the Fermi-Hubbard model to study HHG from a linear chain of atoms. This model allows us to study both the correlated and uncorrelated phases through a specification of the amount of onsite electron-electron repulsion. The presence of signal at noninteger harmonics can be interpreted as originating from the population of multiple Floquet states. We show how this coupling to different Floquet states depends on the characteristics of the driving pulse and the strength of the electron-electron interaction in the system.

physics.atom-ph↗

arXiv2024

Scattering of ultrashort electron wave packets: optical theorem, differential phase contrast and angular asymmetries

Recent advances in electron microscopy allowed the generation of high-energy electron wave packets of ultrashort duration. Here we present a non-perturbative S-matrix theory for scattering of ultrashort electron wave packets by atomic targets. We apply the formalism to a case of elastic scattering and derive a generalized optical theorem for ultrashort wave-packet scattering. By numerical simulations with 1-fs wave packets, we find in angular distributions of electrons on a detector one-fold and anomalous two-fold azimuthal asymmetries. We discuss how the asymmetries relate to the coherence properties of the electron beam, and to the magnitude and phase of the scattering amplitude. The essential role of the phase of the exact scattering amplitude is revealed by comparison with results obtained using the first-Born approximation. Our work paves a way for controlling electron-matter interaction by the lateral and transversal coherence properties of pulsed electron beams.

physics.atom-ph↗

arXiv2023

Electron-correlation induced nonclassicallity of light from high-harmonic generation

We study the effect of electron-electron correlations on the quantum state of the light emitted from high-harmonic generation (HHG). The quantum state of the emitted light is obtained by using a fully quantum mechanical description of both the optical modes as well as the electronic system. This is different from the usual semiclassical description of HHG, which only treats the electronic target system quantum mechanically. Using the generic Fermi-Hubbard model, the strength of the electron-electron correlation can be treated as a parameter enabling us to investigate the two limiting cases of a completely uncorrelated phase and a correlated Mott-insulating phase. In the completely uncorrelated phase, the model reduces to a single-band tight-binding model in which only intraband currents contribute to the spectrum. In this limit, we analytically find that the emitted light is in a classical coherent state. In the Mott-insulating phase, a consideration of the photon statistics and squeezing of the emitted photonic state shows that the inter-Hubbard-subband current generates nonclassical light. In this sense, we show that electron-electron correlation can induce the generation of nonclassical states of light.

quant-ph↗

arXiv2023

Advanced momentum sampling and Maslov phases for a precise semiclassical model of strong-field ionization

Recollision processes are fundamental to strong-field physics and attoscience, thus models connecting recolliding trajectories to quantum amplitudes are a crucial part in furthering understanding of these processes. We report developments in the semiclassical path-integral-based Coulomb quantum-orbit strong-field approximation model for strong-field ionization by including an additional phase known as Maslov's phase and implementing a new solution strategy via Monte-Carlo-style sampling of the initial momenta. In doing so, we obtain exceptional agreement with solutions to the time-dependent Schrödinger equation for hydrogen, helium, and argon. We provide an in-depth analysis of the resulting photoelectron momentum distributions for these targets, facilitated by the quantum-orbits arising from the solutions to the saddle-point equations. The analysis yields a new class of rescattered trajectories that includes the well-known laser-driven long and short trajectories, along with novel Coulomb-driven rescattered trajectories. By virtue of the precision of the model, it opens the door to detailed investigations of a plethora of strong-field phenomena such as photoelectron holography, laser-induced electron diffraction and high-order above threshold ionization.

physics.atom-ph↗

arXiv2023

Relativistic and Spin-Orbit Dynamics at Non-Relativistic Intensities in Strong-Field Ionization

Spin-orbit dynamics and relativistic corrections to the kinetic energy in strong-field dynamics, have long been ignored for near- and mid-IR fields with intensities $10^{13}$--$10^{14}$ W/cm$^2$, as the final photoelectron energies are considered too low for these effects to play a role. However, using a precise and flexible path-integral formalism, we include all correction terms from the fine-structure, Breit-Pauli Hamiltonian. This enables a treatment of spin, through coherent spin-states, which is the first model to use this approach in strong-field physics. We are able to show that the most energetically rescattered wavepackets, undergo huge momentum transfer and briefly reach relativistic velocities, which warrants relativistic kinetic energy corrections. We probe these effects and show that they yield notable differences for a $1600$ nm wavelength laser field on the dynamics and the photoelectron spectra. Furthermore, we find that the dynamical spin-orbit coupling is strongly overestimated if relativistic corrections to kinetic energy are not considered. Finally, we derive a new condition that demonstrates that relativistic effects begin to play a role at intensities orders of magnitude lower than expected. Our findings may have important implication for imaging processes such as laser-induced electron diffraction, which includes high-energy photoelectron recollisions.

physics.atom-ph↗

arXiv2023

High-harmonic spectroscopy of strongly bound excitons in solids

We explore the nonlinear response of ultrafast strong-field driven excitons in a one-dimensional solid with ab initio simulations. We demonstrate from our simulations and analytical model that a finite population of excitons imprints unique signatures to the high-harmonic spectra of materials. We show the exciton population can be retrieved from the spectra. We further demonstrate signatures of exciton recombination and that a shift of the exciton level is imprinted into the harmonic signal. The results open the door to high-harmonic spectroscopy of excitons in condensed-matter systems.

cond-mat.mtrl-sci↗

arXiv2023

Effects of lattice imperfections on high-harmonic generation from correlated systems

Using the one-dimensional Fermi-Hubbard model we study the effects of lattice imperfections on high-harmonic generation (HHG) from correlated systems. We simulate such imperfections by randomly modifying the chemical potential across the individual lattice sites. We control the degree of correlation by varying the Hubbard $U$. In the limit of vanishing $U$, this approach results in Anderson localization. For non-vanishing $U$, we explain the spectral observations using a qualitative picture in which correlation and the imperfections may balance each other out, causing Bloch-like transitions, i.e., transitions similar to those occurring in the case with small or vanishing $U$ and with vanishing imperfection-induced energy gaps, even though the dynamics take place under conditions of high $U$ and severe imperfections. We verify this picture by studying HHG spectra where imperfections are found to cause gain in the HHG spectra. The spectral gain is mainly in high harmonic orders for low correlation and low harmonic orders for high correlation. We explain this using the following qualitative picture. For low correlation, the addition of imperfections gives rise to significant energy gaps in the system, corresponding to high harmonics, and when the correlation is massive the addition of imperfections balances out the correlation, resulting in smaller energy gaps, meaning lower harmonic orders. We further demonstrate how in the unbalanced cases, i.e., high $U$ and relatively small amount of imperfection or vice versa, the dynamics are largely adiabatic, and how in the balanced cases, with similar magnitude of correlation and imperfection, the dynamics are qualitatively similar.

cond-mat.str-el↗

arXiv2022

Strong-field ionization and AC-Stark shifted Rydberg states in OCS

We present theoretical results for intensity-dependent above-threshold ionization (ATI) spectra from oriented OCS molecules probed by intense femtosecond laser pulses with wavelengths of 800 and 400 nm. The calculations were performed using the time-dependent Schroodinger equation within the single-active-electron approximation and including multielectron polarization effects. The results are in qualitative agreement with experimental data [Yu et al., J. Phys. B: At. Mol. Opt. Phys. 50, 235602 (2017)]. In particular, characteristic features in the ATI spectra which correspond to resonant multiphoton ionization via highly-excited Rydberg states are captured by the theory.

physics.atom-ph↗

arXiv2022

Doping effects in high-harmonic generation from correlated systems

Using the one-dimensional Hubbard model, which is commonly used for describing, e.g., high-$T_c$ superconducting cuprates, we study high-harmonic generation (HHG) from doped, correlated materials. Doping is modeled by changing the number of electrons in the lattice from the conventional half-filling case. For relatively small Hubbard $U$, i.e., small electron-electron correlation, we find little to no effect of doping on the dynamics and the HHG spectra. For increasing $U$ the degree of doping has a marked effect on the dynamics and spectra. We explain these findings through the quasiparticle-based doublon-holon picture. The dynamics are separated into two types, firstly doublon and holon movement, and, secondly, doublon-holon pair creation and annihilation. Doping results in all configurations containing doublons or holons. Those quasiparticles can move at no extra cost in energy regardless of the correlation level. This motion at no energy cost increases the high-harmonic gain for low and medium harmonic orders. We discuss that in the high-$U$ limit, antiferromagnetic ordering becomes increasingly unlikely with increasing doping rates and explain an associated drop in the high-order harmonics relative to the case of half filling.

cond-mat.str-el↗

arXiv2022

Disappearance and reappearance of above-threshold-ionization peaks

It is shown that above-threshold ionization peaks disappear when the kinetic energy associated with the nondipole radiation-pressure-induced photoelectron momentum in the laser propagation direction becomes comparable to the photon energy, and how peaks can be made reappear if knowledge of the length and direction of the photoelectron momentum is at hand and an emission-direction-dependent momentum shift is accounted for. The reported findings should be observable with intense mid-infrared laser pulses.

physics.atom-ph↗

arXiv2022

Multielectron effect in the strong-field ionization of aligned nonpolar molecules

We revisit strong-field ionization of aligned O$_2$, CO$_2$, and CS$_2$ molecules in light of recent advances in the field of strong-field physics, in particular the inclusion of multielectron polarization in the numerical solution of the time-dependent Schr{ö}dinger equation (TDSE) within the singe-active-electron approximation. Upon inclusion of multielectron polarization and the associated induced dipole potential in the TDSE model, we obtain angular distributions of total ionization yields which are in better agreement with the available experimental results for CO$_2$ and CS$_2$. For the probed molecules, the main effect of the multielectron polarization on the photoelectron momentum distributions (PMDs) is captured by the introduction of a field at short distances that counteracts the applied external field and leads to a vanishing time-dependent interaction within a certain cut-off radius. Hence, for the PMDs we find no imprints of the long-range part of the laser-induced dipole potential.

physics.atom-ph↗

arXiv2022

Correlation effects in high-harmonic generation from finite systems

Using the Hubbard model we study how the process of high-order harmonic generation (HHG) is modified by beyond mean-field electron-electron correlation for both finite and bulk systems. A finite-size enhancement of the HHG signal is found and attributed to electrons backscattering off the lattice edges. Additionally, with increasing strength of the electron-electron correlation an enhancement of the high-frequency regime of the HHG spectrum is found. This is attributed to the on-site Coulomb repulsion between electrons giving rise to a localized quiver motion of the electrons. The finite-lattice enhancement dominates the HHG spectra from a few harmonic orders until a threshold from which the correlational enhancement dominates. This threshold is determined by the degree of correlation and decreases into the low-frequency regime for increasing electron-electron correlation. This infers that as the Mott insulator limit of high electron-electron correlation is approached, the finite-size effect on the electron dynamics becomes negligible.

physics.atom-ph↗

arXiv2022

Nondipole effects in tunneling ionization by intense laser pulses

The limit of decreasing laser frequency can not be considered independently from nondipole effects due to increase in the laser-induced continuum electron speed in this limit. Therefore, in this work, tunneling ionization in the adiabatic limit is considered for an effective field that includes effects beyond the electric-dipole term to first order in $1/c$, with $c$ the speed of light. The nondipole term describes the interaction resulting from the electric dipole-induced velocity of the electron and the magnetic field component of the laser. The impact of this term on the ionization rate, tunnel exit point, momentum at the tunnel exit and electron dynamics is discussed. In the appropriate limit, the results of a nondipole strong-field approximation approach and those of the strict adiabatic limit, where time and field strength are parameters, are consistent. The nondipole strong-field approximation approach is used to identify nonadiabatic modifications of the initial conditions. The results open up an avenue to include nondipole effects in the initial tunneling ionization step in semiclassical models of strong-field and attosecond physics.

physics.atom-ph↗

arXiv2021

Propagation time and nondipole contributions to intraband high-harmonic generation

Applying the semiclassical model, we identify two connected effects in intraband high-harmonic generation (HHG): (1) propagation time from the point of emission at the source to the point of detection, (2) beyond-electric-dipole corrections to the light-matter interaction. These effects inherit information regarding the dispersion and cause specific features in the spectra including even-ordered harmonics in systems with space- and time-inversion symmetry. They can for certain experimental geometries be measured free of the dipole background.

physics.atom-ph↗

arXiv2021

Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization

We demonstrate entanglement between the orbital angular momentum (OAM) of two photoelectrons ionized via the strongly correlated process of non-sequential double ionization (NSDI). Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. We explore detection by an entanglement witness, decomposable into local measurements, which strongly reduces the difficulty of experimental implementation. We compute the logarithmic negativity measure, which is directly applicable to mixed states, to demonstrate that the entanglement is robust to incoherent effects such as focal averaging. Using the strong-field approximation, we quantify the entanglement for a large range of targets and field parameters, isolating the best targets for experimentalists. The methodology presented here provides a general way to use OAM to quantify and, in principle, measure entanglement, that is well-suited to attosecond processes, can enhance our understanding and may be exploited in imaging processes or the generation of OAM-entangled electrons.

physics.atom-ph↗

arXiv2021

Nondipole photoelectron momentum shifts in strong-field ionization with mid-infrared laser pulses of long duration

We consider atomic strong-field ionization in the long-pulse limit for linearly polarized infrared laser fields. We show how nondipole effects in the plane formed by the propagation and polarization directions lead to (i) a shift of the origin of the rings describing the energetically allowed final electron momenta for individual above-threshold ionization channels and (ii) a redistribution in the continuum population on each ring.

physics.atom-ph↗

arXiv2021

Strong-field approximation for high-harmonic generation in infrared laser pulses in the accelerated Kramers-Henneberger frame

The strong-field approximation for high-harmonic generation in near-infrared and infrared laser pulses is formulated in the accelerated Kramers-Henneberger frame. The accompanying physical picture is discussed and the nature of the leading-order term is contrasted with that of the three-step model following the strong-field-approximation formulation in the length or velocity gauges. The theory is illustrated by high-harmonic generation spectra for atomic hydrogen.

quant-ph↗

arXiv2021

Edge-state and bulk-like laser-induced correlation effects in high-harmonic generation from a linear chain

We explore the significance of laser-induced correlation effects for high-order harmonic generation (HHG) in a linear chain model of a generic band gap material for both single-IR-pulse and VUV-pump--IR-probe HHG schemes. We examine various pump-pulse excitations, and catagorize signatures of laser-induced correlation effects into two classes related to edge-state and bulk-like effects. The relative importance of the edge-state effects decrease with increasing system-size, while the bulk-like effects remain. We identify regimes where these effects may alter the harmonic yield by an order of magnitude, but also regions where they may be neglected. We characterize the underlying laser-induced dynamics of the correlational mechanisms, using both an electronic population-description in a static field-free basis, and comparing to a bandstructure-picture with laser-induced time-dependent energy shifts. From this we provide a guideline on when to account for laser-induced electron correlations and give an estimate of the time-scale in which the laser-induced electron-electron interactions can cause correlational changes in the harmonic spectra. We find this timescale to be a few tens of femtoseconds.

cond-mat.mes-hall↗