Ab-initio multi-scale simulation of high-harmonic generation in solids

High-harmonic generation by a highly non-linear interaction of infrared laser fields with matter allows for the generation of attosecond pulses in the XUV spectral regime. This process, well established for atoms, has been recently extended to the condensed phase. Remarkably well pronounced harmonics up to order ~30 have been observed for dielectrics. We present the first ab-initio multi-scale simulation of solid-state high-harmonic generation. We find that mesoscopic effects of the extended system, in particular the realistic sampling of the entire Brillouin zone, the pulse propagation in the dense medium, and the inhomogeneous illumination of the crystal have a strong effect on the formation of clean harmonic spectra. Our results provide a novel explanation for the formation of clean harmonics and have implications for a wide range of non-linear optical processes in dense media

Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors

We calculate the time evolution of the transient reflection signal in an MoS2 monolayer on a SiO2/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electron–phonon and hole–phonon scattering lead to a gradual relaxation of the carriers toward small Active Excitonic Regions around K, enhancing the dielectric screening. The accompanying time-dependent band gap renormalization dominates over Pauli blocking and the excitonic binding energy renormalization. This explains the delayed buildup of the transient reflection signal of the probe pulse, in excellent agreement with recent experimental data. Our results show that the observed delay is not a unique signature of an exciton formation process but rather caused by coordinated carrier dynamics and its influence on the screening

Messung der Ionentransmission durch dünne freitragende Folien mittels eines Energieanalysators

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersZusammenfassung in deutscher SpracheDie hier verfasste Masterarbeit befasst sich mit der Vermessung von Ladungsverlust und Energieverlust von Ionen in verschiedenen Ladungszuständen nach Transmission durch dünne, frei stehende Schichten. Zu diesem Zweck wurde ein Experiment am Institut für Angewandte Physik der TU Wien aufgebaut, analysiert und in Betrieb genommen sowie eine Messzeit am Partnerexperiment am Helmholtz-Zentrum Dresden-Rossendorf verbracht. Die Wechselwirkungen von Ionen mit herkömmlichen Festkörpern wird im Allgemeinen über einen Gleichgewichtsladungszustand, der sich nach wenigen Nanometern durch das Zusammenspiel von Elektronenverlust und Elektronenaufnahme einstellt, beschrieben. Effekte die vor der Oberfläche des Materials passieren, können hier vernachlässigt werden. Verringert man nun die Schichtdicke auf wenige Atomlagen, so ändert sich die Situation: Wechselwirkungen, die vor der Oberfläche stattfinden können durchaus messbaren Einfluss auf die Energie- und die Ladungsänderung eines Projektils haben. Außerdem ist die Wechselwirkungszeit innerhalb des Materials zu gering, als dass sich ein Gleichgewichtsladungszustand einstellen könnte. Die Messung von Energieverlust und Ladungsverlust eines geladenen Projektils nach Transmission durch dünne freistehende Schichten ermöglicht somit eine direkte Messung fundamentaler Prozesse in der Ionen-Festkörper Wechselwirkung. Im ersten Teil dieser Arbeit werden die theoretischen Grundlagen der Ionen-Oberflächen und der Teilchen-Festkörper Wechselwirkung beschrieben. Teil zwei widmet sich den experimentellen Anlagen in Wien und in Dresden. Der Schwerpunkt liegt auf der im Zuge dieser Arbeit installierten Elektronik und der Ansteuerung des Experiments in Wien. Danach werden Simulationen in SIMION, Details zur Inbetriebnahme des Experiments und zur Auswertung der Spektren diskutiert. Im letzten Teil finden sich die vorläufigen Resultate der Messungen sowie ein Ausblick auf zukünftige Projekte.In the course of this masters thesis, charge loss and energy loss of ions in different charge states after transmission through atomically thin free-standing foils have been investigated. For this reason, an experiment was set up, analysed and taken into operation at the Institute of Applied Physics at TU Wien. As well as that, a series of measurements was performed at the Helmholtz-Zentrum Dresden-Rossendorf. The interaction of ions with bulk materials is generally described using an equilibrium charge state of the projectile which is established after a few nanometres inside the solid due to electron loss and gain processes. Effects of pre-surface interaction can be neglected in this description. The situation changes when the layer thickness is reduced to just a few atomic layers: pre-surface interactions may contribute measurably to the energy and charge change of the projectile. Additionally, the limited interaction time inside the material prohibits the establishment of an equilibrium charge state. Measurements of energy loss and charge loss of an ion after transmission through thin free-standing foils thus allows for a direct determination of fundamental processes governing the ion-condensed matter interaction. The theoretical concepts of ion-surface and ion-condensed matter interaction are discussed in the first part of this thesis. Part two describes the experimental setups in Vienna and in Dresden, focusing on the electronics and the remote control of the experiment in Vienna that were installed in the course of this thesis. After that, simulations performed in SIMION, details on the commissioning of the setup and on the evaluation of the data are discussed. In the last part, preliminary results of the measurements are presented together with an outlook on further investigations.6

Charge-state-dependent energy loss of slow ions. I. Experimental results on the transmission of highly charged ions

We report on energy loss measurements of slow (v ≪ v0) highly charged (Q > 10) ions upon transmission through a 1nm thick carbon nanomembrane. We emphasize here the scaling of the energy loss with velocity and charge exchange/loss. We show that a weak linear velocity dependence exists, whereas charge exchange dominates the kinetic energy loss especially in the case of large charge capture. A universal scaling of the energy loss with charge exchange and velocity is found and discussed in this paper. A model for charge state dependent energy loss for slow ions is presented in part B of this series [Wilhelm, R.A., Mo ̈ller, W., Phys. Rev. A., submitted (2015)]

Ultrafast electronic response of graphene to a strong and localized electric field

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Ultrafast electronic response of graphene to a strong and localized electric field

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