Attosecond transient absorption instrumentation for thin film materials: Phase transitions, heat dissipation, signal stabilization, timing correction, and rapid sample rotation

Toward ultrafast soft x-ray spectroscopy of organic photovoltaic devices

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

X-FAST: A versatile, high-throughput, and user-friendly XUV femtosecond absorption spectroscopy tabletop instrument

We present the X-FAST (XUV Femtosecond Absorption Spectroscopy Tabletop) instrument at the University of Wisconsin-Madison. The instrument produces femtosecond extreme ultraviolet photon pulses via high-harmonic generation in the range of 40-72 eV, as well as optical pump pulses for transient-absorption experiments. The system implements a gas-cooled sample cell that enables studying the dynamics of thermally sensitive thin-film samples. This paper provides potential users with specifications of the optical, vacuum, data acquisition, and sample cooling systems of the X-FAST instrument, along with performance metrics and data of an ultrafast laser-induced phase transition in a Ni2MnGa Heusler thin film.

Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide

Excitation of ionic solids with extreme ultraviolet pulses creates localized core-level excitons, which in some cases couple strongly to the lattice. Here, core-level-exciton states of magnesium oxide are studied in the time domain at the Mg L_{2,3} edge with attosecond transient reflectivity spectroscopy. Attosecond pulses trigger the excitation of these short-lived quasiparticles, whose decay is perturbed by time-delayed near-infrared pulses. Combined with a few-state theoretical model, this reveals that the infrared pulse shifts the energy of bright (dipole-allowed) core-level-exciton states as well as induces features arising from dark core-level excitons. We report coherence lifetimes for the two lowest core-level excitons of 2.3±0.2 and 1.6±0.5  fs and show that these are primarily a consequence of strong exciton-phonon coupling, disclosing the drastic influence of structural effects in this ultrafast relaxation process.

Novel beamline for attosecond transient reflection spectroscopy in a sequential two-foci geometry

We present an innovative beamline for extreme ultraviolet (XUV)-infrared (IR) pump-probe reflection spectroscopy in solids with attosecond temporal resolution. The setup uses an actively stabilized interferometer, where attosecond pulse trains or isolated attosecond pulses are produced by high-order harmonic generation in gases. After collinear recombination, the attosecond XUV pulses and the femtosecond IR pulses are focused twice in sequence by toroidal mirrors, giving two spatially separated interaction regions. In the first region, the combination of a gas target with a time-of-flight spectrometer allows for attosecond photoelectron spectroscopy experiments. In the second focal region, an XUV reflectometer is used for attosecond transient reflection spectroscopy (ATRS) experiments. Since the two measurements can be performed simultaneously, precise pump-probe delay calibration can be achieved, thus opening the possibility for a new class of attosecond experiments on solids. Successful operation of the beamline is demonstrated by the generation and characterization of isolated attosecond pulses, the measurement of the absolute reflectivity of SiO₂, and by performing simultaneous photoemission/ATRS in Ge.

Reduction of laser-intensity-correlated noise in high-harmonic generation

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation. We show that the fluctuations of the extreme-ultraviolet (XUV) spectral power density can be predicted solely by monitoring the generating laser pulses; this method is in contrast with traditional balanced detection used in optical spectroscopy, where a replica of the signal is monitored. Such possibility emerges from a detailed investigation of high-harmonic generation (HHG) noise. We find that in a wide parameter range of the HHG process, the XUV fluctuations are dominated by a spectral blueshift, which is correlated to the near-infrared (NIR) driving laser intensity variation. Numerical simulations support our findings and suggest that non-adiabatic blueshift is the main source of XUV fluctuations. A straightforward post-processing of the XUV spectra allows for noise reduction and improved precision of attosecond transient absorption experiments. The technique is readily transferable to attosecond transient reflectivity and potentially to attosecond photoelectron spectroscopy.