Pierre Agostini

The physics of attosecond light pulses

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Imaging ultrafast molecular dynamics with laser-induced electron diffraction

Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-ångström spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-ångström precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3 μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1 Å displacement in the oxygen bond length occurring in a time interval of ∼5 fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution.

Multiphoton above-threshold ionisation of xenon at 0.53 and 1.06?m

Standard techniques of photoelectron spectrometry have been used to study multiphoton ionisation of xenon. At 0.53 mu m, up to ten-photon ionisation of the 5p shell is detected above the six-photon ionisation threshold for both the Xe+ 2P1/2 and 2P3/2 states. The intensity dependence is compared with the predictions of perturbation theory. The branching ratio 2P3/2:2P1/2 has been measured as a function of the ionisation order. At 1.06 mu m up to 21-photon ionisation has been detected above the eleven-photonionisation threshold.

Broadband femtosecond infrared parametric amplification in β-BaB 2 O 4

Infrared femtosecond pulses of 100 μJ of energy are generated in a two-stage, triple-pass, type I β-barium borate parametric amplifier. A focusable intensity of 5 × 1012 W/cm2 with an f/20 optic is demonstrated.

Strong-field and attosecond physics in solids

We review the status of strong-field and attosecond processes in bulk transparent solids near the Keldysh tunneling limit. For high enough fields and low-frequency excitations, the optical and electronic properties of dielectrics can be transiently and reversibly modified within the applied pulse. In Ghimire et al (2011 Phys. Rev. Lett. 107 167407) non-parabolic band effects were seen in photon-assisted tunneling experiments in ZnO crystals in a strong mid-infrared field. Using the same ZnO crystals, Ghimire et al (2011 Nat. Phys. 7 138–41) reported the first observation of non-pertubative high harmonics, extending well above the bandgap into the vacuum ultraviolet. Recent experiments by Schubert et al (2014 Nat. Photonics 8 119–23) showed a carrier envelope phase dependence in the harmonic spectrum in strong-field 30 THz driven GaSe crystals which is the most direct evidence yet of the role of sub-cycle electron dynamics in solid-state harmonic generation. The harmonic generation mechanism is different from the gas phase owing to the high density and periodicity of the crystal. For example, this results in a linear dependence of the high-energy cutoff with the applied field in contrast to the quadratic dependence in the gas phase. Sub-100 attosecond pulses could become possible if the harmonic spectrum can be extended into the extreme ultraviolet (XUV). Here we report harmonics generated in bulk MgO crystals, extending to ∼26 eV when driven by ∼35 fs, 800 nm pulses focused to a ∼1VA −1 peak field. The fundamental strong-field and attosecond response also leads to Wannier–Stark localization and reversible semimetallization as seen in the sub-optical cycle behavior of XUV absorption and photocurrent experiments on fused silica by Schiffrin et al (2013 Nature 493 70–4) and Schultze et al (2013 Nature 493 75–8). These studies are advancing our understanding of fundamental strong-field and attosecond physics in solids with potential applications for compact coherent short-wavelength sources and ultra-high speed optoelectronics.

Resonant multiphoton ionisation of xenon with high-intensity femtosecond pulses

The authors report on the intensity and polarisation dependences of electron energy spectra from multiphoton ionisation of xenon with 120 fs pulses at 615 nm. The spectra show a large number of narrow structures assigned to resonances induced by the AC Stark shift. The structures are suppressed with circular polarisation. The maximum shift is observed for the 6s state which is up-shifted by 1.6 eV. The assignments are based on a lowest-order perturbative calculation of the AC Stark shifts.

Above-threshold ionisation without space charge

The multiphoton ionisation of xenon has been studied using a picosecond Nd:YAG laser and a time-of-flight electron spectrometer with a collection angle of 2 pi sr. In the absence of space charge effects, suppression of the first electron peak is observed at an intensity of 5*1012 W cm-2 in linear polarisation, while saturation occurs at 1.3*1013 W cm-2. In circular polarisation, the same effect is observed at a slightly lower intensity. The intensity dependences of the first three peaks have been measured.

Time character of phase-conjugate reconstructed waves

The time dependence of phase-conjugate wave generation in liquids has been studied using four-wave mixing of picosecond laser pulses and different polarization configurations. Response and decay times of some possible physical processes contributing to the phase-conjugate wave generation have been measured.

Multiphoton ionisation involving multiphoton continuum-continuum transitions

The energy spectrum of electrons resulting from the multiphoton ionisation of xenon by a frequency-doubled Nd-YAG laser is analysed. It is shown that for intensities of 1011 W cm-2, processes involving up to four continuum-continuum transitions coexist with the lowest-order ionisation process, with comparable probabilities.

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.