Building up the screening below the femtosecond scale

Efficient computational modeling of electronic stopping power of organic polymers for proton therapy optimization

This comprehensive study delves into the intricate interplay between protons and organic polymers, offering insights into proton therapy in cancer treatment. Focusing on the influence of the spatial electron density distribution on stopping power estimates, we employed real-time time-dependent density functional theory coupled with the Penn method. Surprisingly, the assumption of electron density homogeneity in polymers is fundamentally flawed, resulting in an overestimation of stopping power values at energies below 2 MeV. Moreover, the Bragg rule application in specific compounds exhibited significant deviations from experimental data around the stopping maximum, challenging established norms.

Dynamic optical response of solids following 1-fs-scale photoinjection

Photoinjection of charge carriers profoundly changes the properties of a solid. This manipulation enables ultrafast measurements, such as electric-field sampling^1,2, advanced recently to petahertz frequencies^3-7, and the real-time study of many-body physics^8-13. Nonlinear photoexcitation by a few-cycle laser pulse can be confined to its strongest half-cycle^14-16. Describing the associated subcycle optical response, vital for attosecond-scale optoelectronics, is elusive when studied with traditional pump-probe metrology as the dynamics distort any probing field on the timescale of the carrier, rather than that of the envelope. Here we apply field-resolved optical metrology to these dynamics and report the direct observation of the evolving optical properties of silicon and silica during the first few femtoseconds following a near-1-fs carrier injection. We observe that the Drude-Lorentz response forms within several femtoseconds-a time interval much shorter than the inverse plasma frequency. This is in contrast to previous measurements in the terahertz domain^8,9 and central to the quest to speed up electron-based signal processing.

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Since the first demonstration of the generation of attosecond pulses (1 as = 10^-18s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron-nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

Distinguishing attosecond electron-electron scattering and screening in transition metals

Electron-electron interactions are the fastest processes in materials, occurring on femtosecond to attosecond timescales, depending on the electronic band structure of the material and the excitation energy. Such interactions can play a dominant role in light-induced processes such as nano-enhanced plasmonics and catalysis, light harvesting, or phase transitions. However, to date it has not been possible to experimentally distinguish fundamental electron interactions such as scattering and screening. Here, we use sequences of attosecond pulses to directly measure electron-electron interactions in different bands of different materials with both simple and complex Fermi surfaces. By extracting the time delays associated with photoemission we show that the lifetime of photoelectrons from the d band of Cu are longer by ∼100 as compared with those from the same band of Ni. We attribute this to the enhanced electron-electron scattering in the unfilled d band of Ni. Using theoretical modeling, we can extract the contributions of electron-electron scattering and screening in different bands of different materials with both simple and complex Fermi surfaces. Our results also show that screening influences high-energy photoelectrons (≈20 eV) significantly less than low-energy photoelectrons. As a result, high-energy photoelectrons can serve as a direct probe of spin-dependent electron-electron scattering by neglecting screening. This can then be applied to quantifying the contribution of electron interactions and screening to low-energy excitations near the Fermi level. The information derived here provides valuable and unique information for a host of quantum materials.

Laser-pump/X-ray-probe experiments with electrons ejected from a Cu(111) target: space-charge acceleration

A comprehensive investigation of the emission characteristics for electrons induced by X-rays of a few hundred eV at grazing-incidence angles on an atomically clean Cu(111) sample during laser excitation is presented. Electron energy spectra due to intense infrared laser irradiation are investigated at the BESSY II slicing facility. Furthermore, the influence of the corresponding high degree of target excitation (high peak current of photoemission) on the properties of Auger and photoelectrons liberated by a probe X-ray beam is investigated in time-resolved pump and probe measurements. Strong electron energy shifts have been found and assigned to space-charge acceleration. The variation of the shift with laser power and electron energy is investigated and discussed on the basis of experimental as well as new theoretical results.

Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models

What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu(111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

Dynamic screening probed by core-resonant double photoemission from surfaces

The universal response of a sudden created core hole, predicted to occur on an attosecond (10(-18)  s) time scale, lacks an experimental demonstration. With a two-dimensional coincidence spectrometer, we demonstrate an extensive energy sharing between the Ag 4p photoelectron and the N2,3VV Auger electron exceeding 10 eV. This energy width provides access to the time scale of the emission process. This is the fingerprint of the dynamic fluctuation process 4p(-1)⇌4d(-2)4f. The shakeup induced interband transitions from the Ag(100) surface are also identified by comparing the coincidence spectrum with the M4,5VV Auger transitions.

Attosecond time-resolved photoemission from core and valence states of magnesium

We report on laser-assisted attosecond photoemission from single-crystalline magnesium. In strong contrast to the previously investigated transition metal tungsten, photoelectron wave packets originating from the localized core level and delocalized valence-band states are launched simultaneously from the solid within the experimental uncertainty of 20 as. This phenomenon is shown to be compatible with a heuristic model based on free-particle-like propagation of the electron wave packets generated inside the crystal by the attosecond excitation pulse and their subsequent interaction with the assisting laser field at the metal-vacuum interface.

Dynamic screening of a localized hole during photoemission from a metal cluster

: Recent advances in attosecond spectroscopy techniques have fueled the interest in the theoretical description of electronic processes taking place in the subfemtosecond time scale. Here we study the coupled dynamic screening of a localized hole and a photoelectron emitted from a metal cluster using a semi-classical model. Electron density dynamics in the cluster is calculated with time-dependent density functional theory, and the motion of the photoemitted electron is described classically. We show that the dynamic screening of the hole by the cluster electrons affects the motion of the photoemitted electron. At the very beginning of its trajectory, the photoemitted electron interacts with the cluster electrons that pile up to screen the hole. Within our model, this gives rise to a significant reduction of the energy lost by the photoelectron. Thus, this is a velocity-dependent effect that should be accounted for when calculating the average losses suffered by photoemitted electrons in metals.

Bridging quantum and classical plasmonics with a quantum-corrected model

Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems.

Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

A flexible apparatus for attosecond photoelectron spectroscopy of solids and surfaces

We describe an apparatus for attosecond photoelectron spectroscopy of solids and surfaces, which combines the generation of isolated attosecond extreme-ultraviolet (XUV) laser pulses by high harmonic generation in gases with time-resolved photoelectron detection and surface science techniques in an ultrahigh vacuum environment. This versatile setup provides isolated attosecond pulses with photon energies of up to 140 eV and few-cycle near infrared pulses for studying ultrafast electron dynamics in a large variety of surfaces and interfaces. The samples can be prepared and characterized on an atomic scale in a dedicated flexible surface science end station. The extensive possibilities offered by this apparatus are demonstrated by applying attosecond XUV pulses with a central photon energy of ∼125 eV in an attosecond streaking experiment of a xenon multilayer grown on a Re(0001) substrate.

Time-dependent electron phenomena at surfaces

Femtosecond and subfemtosecond time scales typically rule electron dynamics at metal surfaces. Recent advance in experimental techniques permits now remarkable precision in the description of these processes. In particular, shorter time scales, smaller system sizes, and spin-dependent effects are current targets of interest. In this article, we use state-of-the-art theoretical methods to analyze these refined features of electron dynamics. We show that the screening of localized charges at metal surfaces is created locally in the attosecond time scale, while collective excitations transfer the perturbation to larger distances in longer time scales. We predict that the elastic width of the resonance in excited alkali adsorbates on ferromagnetic surfaces can depend on spin orientation in a counterintuitive way. Finally, we quantitatively evaluate the electron-electron and electron-phonon contributions to the electronic excited states widths in ultrathin metal layers. We conclude that confinement and spin effects are key factors in the behavior of electron dynamics at metal surfaces.

Time-dependent screening of a point charge at a metal surface

The space-time evolution of the dynamical screening charge density caused by a suddenly created point charge at the Cu(111) surface is investigated in the linear response approximation. Considering a thin slab as a model for the Cu(111) surface, we investigate the confinement effects on dynamical screening as well. The results have been obtained on the basis of self-consistent evaluation of the energy-momentum-dependent response function, taking into account the realistic surface band structure of Cu(111). At the initial stage, we observe fast long-range charge density oscillations due to excitation of the surface plasmon modes. Then we observe the propagation of the shock wave of the electron-hole excitations along the slab with velocity determined by the Fermi velocity of bulk Cu. At longer times, we have identified the propagation along the two slab surfaces of a much slower (with velocity ∼ 0.3 au, close to the Fermi velocity of the Cu(111) surface state) charge disturbance due to acoustic surface plasmon. The role of the energy band gap in the direction perpendicular to the surface in establishing the screening is also addressed.

Extreme ultrafast dynamics of quasiparticles excited in surface electronic bands

We develop a many-body description of the nonadiabatic dynamics of quasiparticles in surface bands valid on an extremely ultrashort time scale by combining the formalism for the calculation of quasiparticle survival probabilities with the self-consistent treatment of the electronic response of the system. Applying this approach to the benchmark Cu(111) surface, we assess the behavior and intervals of preasymptotic electron and hole dynamics in surface bands and locate the transition to the asymptotic regime of the exponential quasiparticle decay characterized by the corrected Fermi golden rule-type of transition rate. The general validity of these findings enables distinguishing the various regimes of ultrafast electron dynamics that may be revealed in time resolved experiments.