We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7-300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

Development of x-ray phase contrast imaging applications with a laboratory scale source have been limited by the long exposure time needed to obtain one image. We demonstrate, using the Betatron x-ray radiation produced when electrons are accelerated and wiggled in the laser-wakefield cavity, that a high quality phase contrast image of a complex object (here, a bee), located in air, can be obtained with a single laser shot. The Betatron x-ray source used in this proof of principle experiment has a source diameter of 1.7 µm and produces a synchrotron spectrum with critical energy Ec = 12.3 ± 2.5 keV and 10 9 photons per shot in the whole spectrum.

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance R(c) is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO(2), by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry ≈ 2.1 Å and <θ(OCO)> ≈ 163° (equilibrium CO(2) geometry is ≈ 1.16 Å and <θ(OCO)> ≈ 172°). The CO(2)(3+) molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO(2) and identify the electronic states responsible for CREI.

Betatron X-ray radiation in laser-plasma accelerators is produced when electrons are accelerated and wiggled in the laser-wakefield cavity. This femtosecond source, producing intense X-ray beams in the multi kiloelectronvolt range has been observed at different interaction regime using high power laser from 10 to 100 TW. However, none of the spectral measurement performed were at sufficient resolution, bandwidth and signal to noise ratio to precisely determine the shape of spectra with a single laser shot in order to avoid shot to shot fluctuations. In this letter, the Betatron radiation produced using a 80 TW laser is characterized by using a single photon counting method. We measure in single shot spectra from 8 to 21 keV with a resolution better than 350 eV. The results obtained are in excellent agreement with theoretical predictions and demonstrate the synchrotron type nature of this radiation mechanism. The critical energy is found to be Ec = 5.6 ± 1 keV for our experimental conditions. In addition, the features of the source at this energy range open novel perspectives for applications in time-resolved X-ray science.A femtosecond X-ray beam, called Betatron, can be produced by focusing an intense femtosecond laser pulse at relativistic intensities, on the order of 10 18 − 10 19 W.cm −2 , onto a gas jet target. Interacting with the quasi-instantaneously created under-dense plasma, the laser pulse excites a wakefield in which electrons can be trapped and accelerated to high energies in short distances [1][2][3][4][5]. These electrons perform Betatron oscillations across the propagation axis, and emit Xray photons [6-10] (radiation from accelerating chargedparticles). The Betatron radiation consists on a broadband X-ray beam, collimated within 10's mrad, with a femtosecond duration [11].During the past few years, several experiments have been dedicated, at different laser facilities, to the characterization of Betatron radiation. Even if the origin of the radiation was clearly identified, its spectrum has never been precisely determined. This information is however crucial to improve our knowledge of the physical mechanisms driving the source, identify the electrons participating to the emission, and determine the most appropriate routes for its development. In addition, for any potential application the precise shape of the spectrum must be known.So far, spectra estimations were either based on the measurement of the transmission through an array of filters or by using the diffraction from crystals. The use of filters is the most elementary method and it allows a single shot measurement. The results obtained using this method are generally fitted with the synchrotron distribution theoretically predicted [12][13][14][15]. However, this rely on the assumption that the spectrum is synchrotron-like and can not give any deviation from such distribution, or details in the structure of the spectrum. When the Bragg diffraction from a crystal is used, the resolution is important but the characterization range is limit...

Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (H2CO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel–resolved measurements, but not in real-time observations of the roaming fragment itself. Using time-resolved Coulomb explosion imaging (CEI), we directly imaged individual “roamers” on ultrafast time scales in the prototypical formaldehyde dissociation reaction. Using high-level first-principles simulations of all critical experimental steps, distinctive roaming signatures were identified. These were rendered observable by extracting rare stochastic events out of an overwhelming background using the highly sensitive CEI method.

We demonstrate that laser beam shaping can be used to precisely control an electric discharge trail, avoiding or bypassing obstacles in the line of sight.

International audienceSurface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7-450 fs) and applied fluence (F (th)< F < 10F (th)). We show that varying the pulse duration gives access to high selectivity (with resolution similar to 10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F (th). The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (a parts per thousand currency sign30 fs). The deduced optimal fluence F (opt) corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F (opt), the removed material depth can be accurately adjusted in a large range (similar to 40-200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F (opt), the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter

By using the novel approach for pulse compression that combines spectral broadening in hollow-core fiber (HCF) with linear propagation in fused silica (FS), we generate 1.6 cycle 0.24 mJ laser pulses at 1.8 m wavelength with a repetition rate of 1 kHz. These pulses are obtained with a white light seeded optical parametric amplifier (OPA) and shown to be passively carrier envelope phase (CEP) stable. Krausz, "X-ray pulses approaching the attosecond frontier," Science 291(5510), 1923-1927 (2001). ©2011 Optical Society of AmericaTaïeb, B. Carré, H. G. Muller, P. Agostini, and P. Salières, "Attosecond synchronization of high-harmonic soft xrays," Science 302(5650), 1540-1543 (2003).Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, "Isolated single-cycle attosecond pulses," Science 314(5798), 443-446 (2006). 10. E. Goulielmakis, M. Schultze, M. Hofstetter, V. S. Yakovlev, J. Gagnon, M. Uiberacker, A. L. Aquila, E. M.Gullikson, D. T. Attwood, R. Kienberger, F. Krausz, and U. Kleineberg, "Single-cycle nonlinear optics," Science 320(5883), 1614-1617 (2008). 11. P. B. Corkum, "Plasma perspective on strong field multiphoton ionization," Phys. Rev. Lett. 71(13), 1994-1997 (1993). 12. G. Tempea, M. Geissler, M. Schnürer, and T. Brabec, "Self-phase-matched high harmonic generation," Phys.Rev. Lett. 84(19), 4329-4332 (2000). 13. V. S. Yakovlev, M. Ivanov, and F. Krausz, "Enhanced phase-matching for generation of soft X-ray harmonics and attosecond pulses in atomic gases," Opt. "Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating," Rev.