Ultrafast electron dynamics and the role of screening

This thesis focuses on the ultrafast dynamics of electronic excitations in solids and how they are influenced by the screening of the Coulomb interaction between charged particles. The impact of screening on electron dynamics is manifold, ranging from modifications of electron-electron scattering rates over trapping of excess charges to massive renormalisation of electronic band structures. The timescales of these dynamical processes are directly accessible by femtosecond time-resolved photoemission and optical spectroscopy. Three exemplary systems are investigated to shed light onto these fundamental processes: Vanadiumdioxide undergoes a phase transition from a monoclinic insulator to a rutile metal. Apart from temperature, doping and other influences, the insulator-to-metal transition can also be driven by photoexcitation. This, in the past, gave rise to a controversy about the timescales of structural and electronic transition and raised the question which of them constitutes the driving mechanism. Using time-resolved photoelectron spectroscopy, it is shown that the electronic band gap of the insulator collapses instantaneously with photoexcitation and without any structural involvement. The reason is a change of screening due to the generation of photoholes. At the same time, the symmetry of the lattice potential changes, as seen by coherent phonon spectroscopy. This potential change is likely to initiate the structural phase transition from monoclinic to rutile structure. However, the initial non-equilibrium situation can be described by a metallic electronic structure with the atoms still in the monoclinic lattice positions. The SrTiO3/vacuum interface exhibits a two-dimensional electron gas (2DEG), which is delocalised within the surface plane, but localised perpendicular to it. The lower dimensionality changes the form of the screened Coulomb interaction and the phase space within the 2DEG, leading to modified hot carrier lifetimes. These are investigated by time-resolved photoemission spectroscopy: The predicted 2D behaviour is confirmed and two distinct final states within the unoccupied electronic band structure are discovered. Furthermore, the population of the 2DEG is transiently increased by photoexcitation from localised in-gap states into the 2DEG. A different type of screening by dipole moments in amorphous ice layers, is exploited to stabilise and trap electrons within the polar medium in front of a metal surface. Thereby, the mean free path of low energy electrons in amorphous ice is estimated. Moreover, the trapped electrons are used to drive a chemical reaction: A persistent modification of the surface electronic structure of the ice layer is explained via the `dielectron hydrogen evolution reaction'. Understanding the role of screening in these systems allows to explain seemingly unrelated effects, like trapping of excess electrons in ice and the insulator-to-metal transition in VO2, within the same concept

Ultrafast Exciton Formation at the ZnO(101¯0) Surface

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level EF followed by ultrafast (200 fs) formation of a surface exciton (SX). Transient screening of the Coulomb interaction reduces the SX formation probability at high excitation densities near the Mott limit. Located just below the surface, the SX are stable with regard to hydrogen-induced work function modifications and thus the ideal prerequisite for resonant energy transfer applications

Physiological responses to folate overproduction in lactobacillys plantarum WCFS1.

<p>Abstract</p> <p>Background</p> <p>Using a functional genomics approach we addressed the impact of folate overproduction on metabolite formation and gene expression in <it>Lactobacillus plantarum </it>WCFS1. We focused specifically on the mechanism that reduces growth rates in folate-overproducing cells.</p> <p>Results</p> <p>Metabolite formation and gene expression were determined in a folate-overproducing- and wild-type strain. Differential metabolomics analysis of intracellular metabolite pools indicated that the pool sizes of 18 metabolites differed significantly between these strains. The gene expression profile was determined for both strains in pH-regulated chemostat culture and batch culture. Apart from the expected overexpression of the 6 genes of the folate gene cluster, no other genes were found to be differentially expressed both in continuous and batch cultures. The discrepancy between the low transcriptome and metabolome response and the 25% growth rate reduction of the folate overproducing strain was further investigated. Folate production per se could be ruled out as a contributing factor, since in the absence of folate production the growth rate of the overproducer was also reduced by 25%. The higher metabolic costs for DNA and RNA biosynthesis in the folate overproducing strain were also ruled out. However, it was demonstrated that folate-specific mRNAs and proteins constitute 8% and 4% of the total mRNA and protein pool, respectively.</p> <p>Conclusion</p> <p>Folate overproduction leads to very little change in metabolite levels or overall transcript profile, while at the same time the growth rate is reduced drastically. This shows that <it>Lactobacillus plantarum </it>WCFS1 is unable to respond to this growth rate reduction, most likely because the growth-related transcripts and proteins are diluted by the enormous amount of gratuitous folate-related transcripts and proteins.</p

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure

Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott-Hubbard material V2O3 presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A1g coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems. \ua9 The Author(s) 2017

Antenna-assisted picosecond control of nanoscale phase transition in vanadium dioxide

Nanoscale devices in which the interaction with light can be configured using external control signals hold great interest for next-generation optoelectronic circuits. Materials exhibiting a structural or electronic phase transition offer a large modulation contrast with multi-level optical switching and memory functionalities. In addition, plasmonic nanoantennas can provide an efficient enhancement mechanism for both the optically induced excitation and the readout of materials strategically positioned in their local environment. Here, we demonstrate picosecond all-optical switching of the local phase transition in plasmonic antenna-vanadium dioxide (VO2) hybrids, exploiting strong resonant field enhancement and selective optical pumping in plasmonic hotspots. Polarization- and wavelength-dependent pump-probe spectroscopy of multifrequency crossed antenna arrays shows that nanoscale optical switching in plasmonic hotspots does not affect neighboring antennas placed within 100 nm of the excited antennas. The antenna-assisted pumping mechanism is confirmed by numerical model calculations of the resonant, antenna-mediated local heating on a picosecond time scale. The hybrid, nanoscale excitation mechanism results in 20 times reduced switching energies and 5 times faster recovery times than a VO2 film without antennas, enabling fully reversible switching at over two million cycles per second and at local switching energies in the picojoule range. The hybrid solution of antennas and VO2 provides a conceptual framework to merge the field localization and phase-transition response, enabling precise, nanoscale optical memory functionalities