A fundamental understanding of ultrafast electron dynamics in solids induced by light is of great interest for future high-speed electro-optical devices operating in the petahertz frequency regime [1]. In the last years, a number of publications demonstrated the possibility to resolve and control carrier dynamics in semiconductors [2,3] and dielectrics [4,5] on the few- to sub-femtosecond time scale using attosecond transient absorption spectroscopy (ATAS). These experiments were performed with a non-resonant pump pulse, i.e. pump photon energies smaller than the corresponding band gap. Here in contrast, we resolve for the first time the attosecond carrier dynamics induced by a resonant intense laser pulse. We study the attosecond electronic response in gallium arsenide (GaAs), a technologically important narrow band gap semiconductor [6].

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation by monitoring the generating near-infrared pulse energy. We apply this correction in an attosecond transient absorption experiment yielding an improved confidence interval on the mean. © 2019 The Author(s)

The ultrafast nuclear dynamics of the acetylene cation C2H2+ following photoionization of the neutral molecule is investigated using an extreme-ultraviolet pump/infrared probe setup. The observed modulation of the C2H+ fragment ion yield with pump-probe delay is related to structural changes induced by the extreme-ultraviolet pump pulse taking place on the femtosecond timescale. High-level simulations suggest that the trans-bending and C-C bond stretching motion of the C2H2+ cation govern the observed interaction with the infrared pulse. Depending on the molecular configuration at arrival of the infrared pulse, it either transfers population to higher-lying states or to the C2H2+ ground state, thereby enhancing or lowering the C2H+ yield. Our ultrafast pump-probe scheme can thus be used to track excited state nuclear dynamics with a resolution of a few femtoseconds, leading the way to studying fast dynamics also in larger hydrocarbon molecules.

A high degree of electron localization on the d-orbitals of transition metals and their compounds provides a lever to efficiently control their properties with light. For example, light absorption in VO2 may result in an ultrafast electronic phase transition from a dielectric into a metallic state [1]. The essential timescale of electronic phase transitions is connected to the screening dynamics, which typically belongs to the attosecond domain. It is followed by femtosecond electron-electron thermalization, which may blur the initial imprints of screening-induced charge re-distribution. Here we show that the properties of transition metals could in principle be manipulated much faster than the electron thermalization timescale and even faster than the optical cycle.

Experimental Methods The experiments were performed using the extreme-ultraviolet (XUV)-pump/infrared (IR)-probe setup1 shown in figure 1a of the main text. A 30 fs IR laser pulse at a central wavelength of 780 nm from a commercial Ti:sapphire system with a 1-kHz pulse repetition rate was spectrally broadened via filamentation2 in argon and subsequently compressed with a pair of chirped mirrors. Thereafter, the beam was split in a 80:20 ratio using a beam splitter (BS). The more energetic part of the beam was used for high-harmonic generation (HHG) in xenon, yielding an XUV attosecond pulse train (APT) spectrally extending from 20 to 35 eV. The residual IR radiation was subsequently blocked using a 100-nm thick aluminum (Al) filter. The spectrum of the XUV pulse could optionally be filtered to contain mainly one dominant harmonic around 23.5 eV using an additional 100-nm thick tin (Sn) foil. The second part of the IR beam was guided via a piezoelectric delay stage and could be varied in peak-intensity from 1 to 5

We theoretically investigate the role of intraband transitions in laser-induced carrier generation for different photon energy regimes: (i) strongly off resonant, (ii) multiphoton resonant, and (iii) resonant conditions. Based on the analysis for the strongly off resonant and multiphoton resonant cases, we find that intraband transitions strongly enhance photocarrier generation in both multiphoton absorption and tunneling excitation regimes, and thus, they are indispensable for describing the nonlinear photocarrier generation processes. Furthermore, we find that intraband transitions enhance photocarrier generation even in the resonant condition, opening additional multiphoton excitation channels once the laser irradiation becomes sufficiently strong. The above findings suggest a potential for efficient control of photocarrier generation via multicolor laser pulses through optimization of the contributions from intraband transitions.

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.