In attosecond metrology, an attosecond extreme-ultraviolet (XUV) pulse is combined with a phase-locked few-cycle infrared (IR) pulse to study ultrafast electron dynamics on the sub-to few-femtosecond time scale. Due to the strong absorption of materials in the XUV regime, the beam recombination is commonly done geometrically, for example with a center-hole mirror (Fig. 1 (a)). In this case, the XUV beam is transmitted through the hole in the center. The larger IR beam is reflected on the outer part of the mirror. This results in an annular beam profile with propagation parameters deviating considerably from those of an ideal Gaussian, which also affects the Gouy phase behavior across the focus [1, 2]. Here, we present a detailed study of the Gouy phase of a truncated beam using our two-foci setup (Fig. 1 (a)) in combination with the RABBITT [3] (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) technique. Furthermore, we discuss the non-negligible influence of the Gouy phase shift on attosecond pump-probe measurements with spatially separated targets.

Attosecond pump-probe measurements are typically performed by combining attosecond pulses with more intense femtosecond, phase-locked infrared (IR) pulses because of the low average photon flux of attosecond light sources based on high-harmonic generation (HHG). Furthermore, the strong absorption of materials at the extreme ultraviolet (XUV) wavelengths of the attosecond pulses typically prevents the use of transmissive optics. As a result, pump and probe beams are typically recombined geometrically with a center-hole mirror that reflects the larger IR beam and transmits the smaller XUV, which leads to an annular beam profile of the IR. This modification of the IR beam can affect the pump-probe measurements because the propagation that follows the reflection on the center-hole mirror can strongly deviate from that of an ideal Gaussian beam. Here we present a detailed experimental study of the Gouy phase of an annular IR beam across the focus using a two-foci attosecond beamline and the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique. Our measurements show a Gouy phase shift of the truncated beam as large as 2π and a corresponding rate of 50 as/mm time delay change across the focus in a RABBITT measurement. These results are essential for attosecond pump-probe experiments that compare measurements of spatially separated targets.

We performed a detailed measurement of the Gouy phase of an infrared beam reflected on a drilled mirror and demonstrated its influence on attosecond measurements with two spatially separated targets.

We studied reversible sub-cycle dynamics in the diamond conduction band by attosecond transient absorption spectroscopy. We show that absorption bleaching or enhancement can be controlled with few-femtosecond infrared pulses. Ab initio calculations support our interpretation.

Photoemission delays from copper (111) are measured in an attosecond pump - infrared probe experiment. The influence of the probe pulse and the validity of the Fresnel equations on atomic length and time scales are investigated.

Combining attosecond transient absorption spectroscopy in polycrystalline diamond with theoretical models we were able to identify dominant signatures from the dynamical Franz- Keldysh effect for excitations high into the conduction band and on few-femtosecond time scales.

Short, intense laser pulses can be used to access the transition regime between classical and quantum optical responses in dielectrics. In this regime, the relative roles of inter- and intraband light-driven electronic transitions remain uncertain. We applied attosecond transient absorption spectroscopy to investigate the interaction between polycrystalline diamond and a few-femtosecond infrared pulse with intensity below the critical intensity of optical breakdown. Ab initio time-dependent density functional theory calculations, in tandem with a two-band parabolic model, accounted for the experimental results in the framework of the dynamical Franz-Keldysh effect and identified infrared induction of intraband currents as the main physical mechanism responsible for the observations.

Ultrafast relaxation dynamics of the ethylene cation are investigated with unprecedented temporal resolution. This enabled us to redefine the isomerization time to 30 fs and identify relaxation channels evolving on time scales <50 fs.

We present a combined experimental and computational study of the relaxation dynamics of the ethylene cation. In the experiment, we apply an extreme-ultraviolet-pump/infrared-probe scheme that permits us to resolve time scales on the order of 10 fs. The photoionization of ethylene followed by an infrared (IR) probe pulse leads to a rich structure in the fragment ion yields reflecting the fast response of the molecule and its nuclei. The temporal resolution of our setup enables us to pinpoint an upper bound of the previously defined ethylene-ethylidene isomerization time to 30 ± 3 fs. Time-dependent density functional based trajectory surface hopping simulations show that internal relaxation between the first excited states and the ground state occurs via three different conical intersections. This relaxation unfolds on femtosecond time scales and can be probed by ultrashort IR pulses. Through this probe mechanism, we demonstrate a route to optical control of the important dissociation pathways leading to separation of H or H2.

We present a semi-classical model to calculate RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) traces in the presence of a reference infrared field with a complex two-dimensional (2D) spatial distribution. The evolution of the electron spectra as a function of the pump-probe delay is evaluated starting from the solution of the classical equation of motion and incorporating the quantum phase acquired by the electron during the interaction with the infrared field. The total response to an attosecond pulse train is then evaluated by a coherent sum of the contributions generated by each individual attosecond pulse in the train. The flexibility of this model makes it possible to calculate spectrograms from non-trivial 2D field distributions. After confirming the validity of the model in a simple 1D case, we extend the discussion to describe the probe-induced phase in photo-emission experiments on an ideal metallic surface.

We present quantum-beat spectroscopy of excited states of helium atoms populated selectively with high-orderharmonic emission below the atomic ionization potential by means of low-pass filtering of the pump radiation. The created electron wave packet is ionized by few-cycle infrared (IR) pulses leading to characteristic peaks in the photoelectron yield, which beat with a frequency proportional to the energy gap between the states involved in the two-color photoionization process. Minimizing the direct ionization by the extreme ultraviolet (XUV) radiation, we can follow the evolution of the electron wave packet also in the region of temporal pump-probe overlap. A detailed time-frequency analysis of the quantum beats and direct comparison with the solution of the time-dependent Schrodinger equation reveal the existence of quantum beats characterized by a final state of mixed parity. Finally, we show that by varying the carrier-envelope offset phase of the probe pulse, one can optically control the preferred direction of photoelectron emission and the contrast of such beats.

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.