C. Roedig

Ultra-fast and ultra-intense x-ray sciences: first results from the Linac Coherent Light Source free-electron laser

X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.

Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 μm from an optical filament

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 microJ 55 fs pulses at 2 microm produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 microm carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 microJ pulse energy. The self-locked phase offset of the 2 microm difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths (lambda>0.8 microm).

Femtosecond x-ray pulse length characterization at the Linac Coherent Light Source free-electron laser

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (>40?fs) x-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75?fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120?20?fs full-width at half-maximum (FWHM) and an upper limit of 40?20?fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results.

Strong field physics with long wavelength lasers

The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom–laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space.

Attosecond pulse shaping around a Cooper minimum.

High harmonic generation (HHG) is used to measure the spectral phase of the recombination dipole matrix element (RDM) in argon over a broad frequency range that includes the 3p Cooper minimum (CM). The measured RDM phase agrees well with predictions based on the scattering phases and amplitudes of the interfering s- and d-channel contributions to the complementary photoionization process. The reconstructed attosecond bursts that underlie the HHG process show that the derivative of the RDM spectral phase, the group delay, does not have a straightforward interpretation as an emission time, in contrast to the usual attochirp group delay. Instead, the rapid RDM phase variation caused by the CM reshapes the attosecond bursts.

Strong-field physics using mid-infrared lasers

This document reports recent theoretical and experimental investigations of strong field ionization and high harmonic generation from mid-infrared lasers at 2 and 4 microns. Numerical solution of the time-dependent Schrodinger equation as well as Strong Field approximation calculations are reported. Photoelectron and high harmonic spectra are discussed. Preliminary experimental results are compared to the theoretical predictions.

High harmonic generation from long wavelength drivers

The paper reviews recent advances in high harmonic generation from mid-infrared lasers. The results emphasize the advantages of long wavelength drivers for cut-off extension, GDD reduction, attosecond pulses and reports on below-threshold and liquid harmonics.

Strong-Field Atomic Physics in the X-Ray Regime

In 1999 the DOE Basic Energy Sciences Advisory Committee defined an innovative scientific direction for the new millennium. In the report the panel recognized the scientific opportunity posed by accelerator-based and tabletop sources of short wavelength radiation. The report emphasized that revolutionary science would be enabled by source parameters that include “high degree of coherence, pulse brevity and high pulsed energy” while pushing the frontiers towards the hard x-ray regime. The panel recommended strategic investment in a new light source, which marked the genesis of the world’s first x-ray free-electron laser project. This chapter reports on some of the initial atomic physics experiments conducted at the Linac Coherent Light Source (LCLS) at SLAC and addresses the question as to limits of validity of perturbation theory in describing an intense x-ray pulse interacting with matter.

High order harmonics from mid‐infrared drivers for attosecond physics

The generation of light pulses with attosecond (10−18 seconds) duration is studied using laser drivers operating in the mid‐infrared region. This paper first examines the fundamental principles of attosecond formation by Fourier synthesis of a high harmonic comb. Experimental demonstration of the extension of the harmonic cutoff is shown using a 2 micron driver. Then, the crucial spectral phase properties, responsible for the pulse structure on the attosecond time scale, are measured with an all‐optical technique using a mix of the fundamental pulse with its second harmonic. The expected 1/λ scaling is verified, which demonstrates a practical way towards pulses approaching the atomic unit of time (24 as).