Elisabeth Bothschafter

Controlling dielectrics with the electric field of light

The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per ångström and significant modifications in their electronic system. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angström and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

Attosecond nonlinear polarization and light-matter energy transfer in solids

Electric-field-induced charge separation (polarization) is the most fundamental manifestation of the interaction of light with matter and a phenomenon of great technological relevance. Nonlinear optical polarization produces coherent radiation in spectral ranges inaccessible by lasers and constitutes the key to ultimate-speed signal manipulation. Terahertz techniques have provided experimental access to this important observable up to frequencies of several terahertz. Here we demonstrate that attosecond metrology extends the resolution to petahertz frequencies of visible light. Attosecond polarization spectroscopy allows measurement of the response of the electronic system of silica to strong (more than one volt per ångström) few-cycle optical (about 750 nanometres) fields. Our proof-of-concept study provides time-resolved insight into the attosecond nonlinear polarization and the light-matter energy transfer dynamics behind the optical Kerr effect and multi-photon absorption. Timing the nonlinear polarization relative to the driving laser electric field with sub-30-attosecond accuracy yields direct quantitative access to both the reversible and irreversible energy exchange between visible-infrared light and electrons. Quantitative determination of dissipation within a signal manipulation cycle of only a few femtoseconds duration (by measurement and ab initio calculation) reveals the feasibility of dielectric optical switching at clock rates above 100 terahertz. The observed sub-femtosecond rise of energy transfer from the field to the material (for a peak electric field strength exceeding 2.5 volts per ångström) in turn indicates the viability of petahertz-bandwidth metrology with a solid-state device.

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.

Waveform-controlled near-single-cycle milli-joule laser pulses generate sub-10 nm extreme ultraviolet continua.

We demonstrate the generation of waveform-controlled laser pulses with 1 mJ pulse energy and a full-width-half-maximum duration of ∼4  fs, therefore lasting less than two cycles of the electric field oscillating at their carrier frequency. The laser source is carrier-envelope-phase stabilized and used as the backbone of a kHz repetition rate source of high-harmonic continua with unprecedented flux at photon energies between 100 and 200 eV (corresponding to a wavelength range between 12-6 nm respectively). In combination we use these tools for the complete temporal characterization of the laser pulses via attosecond streaking spectroscopy.

Collinear generation of ultrashort UV and XUV pulses

We demonstrate the collinear generation of few-femtosecond ultraviolet and attosecond extreme ultraviolet pulses via a combination of third-harmonic and high harmonic generation in noble gases. The ultrashort coherent light bursts are produced by focusing a sub-1.5-cycle near-infrared/visible laser pulse in two subsequent quasi-static noble gas targets. This approach provides an inherently synchronized pair of UV and XUV pulses, where the UV radiation has a photon energy of approximately 5 eV and a pulse energy of up to 1 microJ and the XUV radiation contains up to 3.5 10(6) XUV photons per shot with a photon energy exceeding 100 eV. This source represents a novel tool for future UV pump/XUV probe experiments with unprecedented time-resolution.

Few-Femtosecond and Attosecond Electron Dynamics at Surfaces

Attosecond science recently celebrated its first decade which brought fascinating new insights into the dynamics of electrons in atoms and simple molecules. Most of these achievements are enabled by implementing attosecond extreme-ultraviolet (XUV) light bursts together with few-cycle near-infrared (NIR) and ultraviolet (UV) laser pulses in different pump-probe configurations. However, the application of these novel experimental tools for time-resolving attosecond and few-femtosecond electron dynamics in the solid state is still very limited. The study of electron dynamics in bulk materials, their surfaces and interfaces is crucial for both advancing our fundamental understanding of these processes and their application in future technological devices. In this chapter, electron dynamics in condensed matter will be reviewed in the light of open questions that can be addressed with state-of-the-art time-resolved spectroscopy. Experimental prerequisites particular to the study of condensed matter systems are discussed. The potential of using ultrashort light pulses to investigate electron dynamics at surfaces on ultrashort timescales is illustrated by few-femtosecond transient UV-reflectivity and attosecond time-resolved XUV-photoemission experiments on metal surfaces, semiconductors, and more complex adsorbate-substrate systems.

First Attosecond Pulse Control by Multilayer Mirrors above 100 eV Photon Energy

We report on our latest achievements in quantitatively controlling attosecond pulse parameters up to 180 eV by means of aperiodic, XUV multilayer mirrors focusing on a high signal to noise ratio and controlling spectral phase.

Attosecond nonlinear polarization and energy transfer in solids

Attosecond polarization spectroscopy is a new experimental technique resolving the nonlinear polarization and energy-transfer induced by visible few-cycle strong fields in solids. It reveals the intensity dependent response time of the system with attosecond resolution.