Thomas Fordell

Probing Single-Photon Ionization on the Attosecond Time Scale

We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the 3s(2) and from the 3p(6) shell, at different excitation energies ranging from 32 to 42 eV. The determination of photoemission time delays requires taking into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using a universal formula and is found to account for a substantial fraction of the measured delay.

Simultaneous compression and characterization of ultrashort laser pulses using chirped mirrors and glass wedges

We present a simple and robust technique to retrieve the phase of ultrashort laser pulses, based on a chirped mirror and glass wedges compressor. It uses the compression system itself as a diagnostic tool, thereby making unnecessary the use of complementary diagnostic tools. We used this technique to compress and characterize 7.1 fs laser pulses from an ultrafast laser oscillator.

Characterization of broadband few-cycle laser pulses with the d-scan technique

We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.

Photoemission electron microscopy using extreme ultraviolet attosecond pulse trains

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.

Phase Measurement of Resonant Two-Photon Ionization in Helium

We study resonant two-color two-photon ionization of helium via the 1s3p (1)P(1) state. The first color is the 15th harmonic of a tunable Ti:sapphire laser, while the second color is the fundamental laser radiation. Our method uses phase-locked high-order harmonics to determine the phase of the two-photon process by interferometry. The measurement of the two-photon ionization phase variation as a function of detuning from the resonance and intensity of the dressing field allows us to determine the intensity dependence of the transition energy.

Measurements of relative photoemission time delays in noble gas atoms

We determine relative photoemission time delays between valence electrons in different noble gas atoms (Ar, Ne and He) in an energy range between 31 and 37 eV. The atoms are ionized by an attosecond pulse train synchronized with an infrared laser field and the delays are measured using an interferometric technique. We compare our results with calculations using the random phase approximation with exchange and multi-configurational Hartree-Fock. We also investigate the influence of the different ionization angular channels.

Carrier-envelope phase stabilization of a multi-millijoule, regenerative-amplifier-based chirped-pulse amplifier system

This article reports on the successful stabilization of the carrier-envelope phase of a 1-kHz laser system that includes a large grating stretcher, a regenerative amplifier, a multipass amplifier and a grating compressor. Phase stability for pulse energies up to 6 mJ is demonstrated using electronic feedback to the oscillator locking electronics as well as feedback via an acousto-optic programmable dispersive filter.