Search

Ultra-short extreme ultraviolet pulses from the free-electron laser FLASH are characterized using terahertz-field driven streaking. Measurements at different ultra-short extreme ultraviolet wavelengths and pulse durations as well as numerical simulations were performed to explore the application range and accuracy of the method. For the simulation of streaking, a standard classical approach is used which is compared to quantum mechanical theory, based on strong field approximation. Various factors limiting the temporal resolution of the presented terahertz streaking setup are investigated and discussed. Special attention is paid to the cases of very short (similar to 10 fs) and long (up to similar to 350 fs) pulses.

In this joint experimental and theoretical study we demonstrate coherent control of the optical field emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that an external THz field or a minor dc bias, orders of magnitude smaller than the optical bias owing to the laser field, allows one to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultrafast nanoscale vacuum diode for lightwave electronics. Using time-dependent density functional theory calculations we elucidate the main physical mechanisms behind the observed effects and show that an applied dc field significantly modifies the optical field emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device, which allows us to establish a paradigm for controlling nanocircuits at petahertz frequencies.

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low kinetic energy (few eV) sidebands, the photoelectron energy varies as a function of the emission angle. Such behavior has not yet been observed in sideband creation and has not been predicted in commonly used theoretical descriptions such as strong field approximation and soft photon approach. Describing the photoionization with a time-dependent Schrodinger equation allows a qualitative description of the observed effect, as well as the prediction of fine structure in the sideband distribution.

We report on the theoretical study of the second and third harmonic generation in plasmonic dimer nanoantennas with narrow gaps. Our study is based on the time dependent density functional theory. This allows us to address the nonlinear response of a tunneling junction in a subnanometric plasmonic gaps with a quantum calculation, which goes beyond conventional classical local and nonlocal treatments. We demonstrate that the nonlinear electron transport in a plasmonic junction, associated to the corresponding strong field enhancement in the narrow gap, allows to reach orders of magnitude enhancement in the efficiency of the second and third harmonic generation. Depending on the size of the junction and the frequency of the fundamental incident wave, we show that the frequency conversion in plasmonic dimer gaps can be determined by (i) the intrinsic nonlinearity of each individual nanoparticle, (ii) the nonlinear ac tunneling current across the gap, and (iii) the resonant excitations of the plasmon modes of the dimer. The study of the nonlinear response of plasmonic gaps within a full quantum treatment allows us to understand the fundamental mechanisms of nonlinearity in nanoplasmonics.

The optical response of small charged metallic nanodisks of one atomic monolayer thickness is analyzed under the excitation by an incident plane wave and by a localized pointlike dipole. Using the time-dependent density functional theory (TDDFT) and classical electrodynamical calculations we identify the bright and dark plasmon modes and study their evolution under external charging of the nanostructure. For neutral nanodisks, despite their monolayer thickness, the in-plane optical response, as obtained from TDDFT, is in agreement with classical electromagnetic results. The optical response for an incident wave polarized perpendicular to the nanostructure cannot be retrieved classically as it reflects a discrete energy structure of electronic levels. This latter situation appears most sensitive to external charging while the energy of the in-plane plasmon with dipolar character is nearly charge independent.

The nonlinear response of metallic nanoparticles is obtained from quantum time dependent density functional theory calculations. Without any aprioristic assumption our calculations allow us to identify high-order harmonic generation in canonical plasmonic structures such as spherical single particles and dimers. Furthermore, we demonstrate that under currently available experimental conditions, the application of an external polarizing field to the nanoparticles allows to actively control even-order harmonic generation in otherwise symmetry forbidden situations. Our quantum calculations provide quantitative access to the high-order response of metallic nanoantennas, which is of utmost importance in the design, control, and exploitation of optoelectronic devices as well as in the generation of extreme ultraviolet radiation.

We use a time-dependent density functional theory approach to study the optical response of a hybrid nanostructure where the junction between thin metallic films is functionalized with a quantum well (QW) structure. We show that an unoccupied QW-localized electronic state opens the possibility of the active electrical control of the photoassisted electron transport through the junction and of the absorption at optical frequencies. Control strategies based on an applied bias or an external THz field are demonstrated.

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.

Using the time-dependent density functional theory, we perform quantum calculations of the electron dynamics in small charged metallic nanoparticles (clusters) of spherical geometry. We show that the excess charge is accumulated at the surface of the nanoparticle within a narrow layer given by the typical screening distance of the electronic system. As a consequence, for nanoparticles in vacuum, the dipolar plasmon mode displays only a small frequency shift upon charging. We obtain a blue shift for positively charged clusters and a red shift for negatively charged clusters, consistent with the change of the electron spill-out from the nanoparticle boundaries. For negatively charged clusters, the Fermi level is eventually promoted above the vacuum level leading to the decay of the excess charge via resonant electron transfer into the continuum. We show that, depending on the charge, the process of electron loss can be very fast, on the femtosecond time scale. Our results are of great relevance to correctly interpret the optical response of the nanoparticles obtained in electrochemistry, and demonstrate that the measured shift of the plasmon resonances upon charging of nanoparticles cannot be explained without account for the surface chemistry and the dielectric environment.

The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 1012 A cm−2, the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.

There is a continuous effort in the plasmonics community to achieve a control of the optical response of metallic nanostructures via a proper design of their morphology. However, in the last years there has been an increasing interest in obtaining an active control of the response by application of external bias, or polarizing fields. The modulation of the plasmon resonances in this active fashion has been recently achieved in electrochemistry by adding chemical reductants to the colloidal nanoparticle solution. Similarly to the situation in 2D materials, such as graphene, the observed shift of the plasmon frequency has been often interpreted also in 3D particles as a result of the charge doping of the nanoparticles. Based on a full quantum mechanical description, we review this long-standing misconception in electrochemistry. In this work, we study the electron dynamics and plasmon modes of small charged metallic clusters in vacuum with use of density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. We obtain the electronic structure and trace the dynamics of well-defined nanoparticle clusters that allow to isolate and identify the effects of pure charging in the optical response of metallic nanostructures, with respect to the action of an external polarization or chemical effects. In particular, we show that the frequency shift of the plasmon mode is generally small when a nanoparticle is charged, and this shift has an opposite sign to that expected from the arguments based on the change of bulk electron density. Our results can be well understood based on the theory of dynamical screening to describe the dependence of the dipolar plasmon resonance on the nanoparticle size. We also demonstrate that for negatively charged clusters, as soon as the Fermi level is promoted above the vacuum level, the extra charge decays at femtosecond (fs) time scales. Despite the small size of the systems considered in our model calculations

The ability of localized surface plasmons to squeeze light and engineer nanoscale electromagnetic fields through electron-photon coupling at dimensions below the wavelength has turned plasmonics into a driving tool in a variety of technological applications, targeting novel and more efficient optoelectronic processes. In this context, the development of active control of plasmon excitations is a major fundamental and practical challenge. We propose a mechanism for fast and active control of the optical response of metallic nanostructures based on exploiting quantum effects in subnanometric plasmonic gaps. By applying an external dc bias across a narrow gap, a substantial change in the tunneling conductance across the junction can be induced at optical frequencies, which modifies the plasmonic resonances of the system in a reversible manner. We demonstrate the feasibility of the concept using time-dependent density functional theory calculations. Thus, along with two-dimensional structures, metal nanoparticle plasmonics can benefit from the reversibility, fast response time, and versatility of an active control strategy based on applied bias. The proposed electrical manipulation of light using quantum plasmonics establishes a new platform for many practical applications in optoelectronics.

We report on the theoretical study of laser-assisted attosecond photoemission from metals. The full time-dependent quantum approach reveals the role of the resonant interband and nonresonant surface emission processes in formation of final atto-streaking spectra. The present results explain recent experimental data on magnesium and show that the valence band streaking essentially reflects the respective weight of surface and resonant bulk electron ejection. © 2013 American Physical Society.

The application of atto-second streaking spectroscopy (ASS) to direct time-domain studies of the plasmonic excitations in metallic nano-objects is addressed theoretically. The streaking spectrograms for a rectangular gold nano-antenna and spherical gold clusters are obtained within strong field approximation using classical electron trajectory calculations. The results reported here for spherical clusters are also representative of spherical nano-shells. This study demonstrates that ASS allows for detailed characterization of plasmonic modes, including near-field enhancement, frequency and decay rate. The role of the inhomogeneity of the induced electric fields is also demonstrated.

A fully quantum mechanical investigation using time-dependent density functional theory reveals that the field enhancement in a coupled nanoparticle dimer can be strongly affected by nonlinear effects. We show that both classical as well as linear quantum mechanical descriptions of the system fail even for moderate incident light intensities. An interparticle current resulting from the strong field photoemission tends to neutralize the plasmon-induced surface charge densities on the opposite sides of the nanoparticle junction. Thus, the coupling between the two nanoparticles and the field enhancement is reduced as compared to linear theory. A substantial nonlinear effect is revealed already at incident powers of 10 9 W/cm 2 for interparticle separation distances as large as 1 nm and down to the touching limit. © 2012 American Chemical Society.

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.

The energy loss by an electron ejected from a deep level of an atom adsorbed on Ag(110) surface is considered. It is shown that the dynamical effects in screening of the external field by a metal have to be taken into account and the energy loss by the ejected electron depends on the surface-plasmon dispersion and on the excitation energy. This effect can be observed with contemporary light sources.

A numerical simulation of the process of ionization of an atom adsorbed on a metal surface by the laser-assisted subfemtosecond pulse is presented. The streaking scheme is considered, when a weak subfemtosecond pulse comes together with a strong IR pulse with a variable delay between them. The process is simulated by solving numerically the nonstationary Schrödinger equation in the cylindrical coordinate system. The double differential cross-sections (DDCS) are computed for a fixed delay between the pulses, but for few different carrier frequencies of the subfemtosecond pulse. The results obtained are compared with those in the gas phase. Beside the conventional streaking shift in energy, the surface influence on the DDCS is revealed. It is shown that the corresponding variation in DDCS noticeably depends on the frequency of the subfemtosecond pulse.

The recent work of Cavalieri et al. [Nature (London) 449, 1029 (2007)] has provided the first experimental observation of electron dynamics at metal surfaces in the subfemtosecond range. We explain the experimental findings using a full time-dependent approach within a one-dimensional model that includes the main ingredients of the short time physics involved in the experiment.

A theoretical study of the influence of Cs adsorbates on the dynamics of Cu(100) image states is reported. It is applied to the coverage range where the Cs adsorbates are randomly adsorbed on the surface. In contrast to an earlier study based on a single adsorbate scattering approach and thus limited to the very low adsorbate coverages, the present work considers the image-state electron scattered by a given adsorbate in the presence of all the others. This allows the treatment of the intermediate coverage range. The core of the present approach consists of defining a coverage dependent potential representing the average interaction between the electron and the adsorbate layer together with a local perturbing potential centered on each adsorbate. A time-dependent study of electron scattering associated to a many-body treatment of inelastic interactions with bulk electrons yields the rates for population and coherence decay of the image-state electron. This leads to a discussion of the coverage dependence of the decay and dephasing rates, which exhibit a complex variation due to the simultaneous interactions of the image-state electron with many adsorbates on the surface.