Anne L'Huillier

Multiple-harmonic conversion of 1064 nm radiation in rare gases

The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 1013 W cm-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

Super-intense laser-atom physics

Preface. SILAP 2000: an overview. Part I: Double Ionization of Complex Atoms. Energy distribution of two-electron ionization of helium in an intense field R. Lafon, et al. Double ionization in strong fields: ion momenta and correlated electron momenta A. Staudte, et al. Mechanism of the non sequential double ionization of helium D.G. Lappas, et al. Electron momentum distributions for double ionization in the strong field limit S.P. Goreslavski, S.V. Popruzhenko. S-matrix theory of recoil-ion momentum distribution for double ionization in femtosecond laser fields F.H.M. Faisal, A. Becker. Non-sequential double ionization: a minimal correlation approach R. Kopold, W. Becker. Non-sequential double ionization of atoms in strong fields K. Sacha, B. Eckhardt. Double-electron ionization of two-electron system in strong laser field D.V. Tikhonova. Calculation of double ionization of helium H.G. Muller. The two-electron response in laser driven helium L.R. Moore, et al. The helium atom in strong and short laser pulses: multielectron H. Bachau, R. Hasbani. Dynamics of a two-electron system driven by an ultrashort and intense laser pulse B. Piraux, G. Lagmago-Kamta. Angular distributions for double ionization by an ultrashort, intense laser pulse: the case of Li- G. Lagmago-Kamta, A.F. Starace. Two-and-three electron atoms in strong laser fields P. Lambropoulos, et al. Part II: Ionization and Dissociation of Molecules. Tunnelling ionization and the Franck-Condon principle J.H. Posthumus, et al. Dissociative ionization of few-electron molecules in intense laser fields D. Dundas, et al. One-photon breakup of H+2 in a strong DC field Z. Mulyukov, R. Shakeshaft. Part III: Interaction of Clusters with Very Intense Femtosecond Laser Pulses. Nuclear fusion in gases of deuterium clusters and hot electron generation in droplet sprays under irradiation with an intense femtosecond laser T. Ditmire, et al. The absorption of energy by large atomic clusters from superintense laser pulses V.P. Krainov, M.B. Smirnov. Part IV: Production of Very Intense Femtosecond Laser Pulses. A semi-classical model for high-harmonic generation D.B. Milosevic. Beyond the simple maris model for high harmonic generation M. Brewezyk, K. Rzazewski. Anisotropy induced polarization effects in harmonic generation by an absorptive medium B. Borca, et al. About a new method of high harmonic amplification E.A. Nersesov, et al. X-ray generation via stimulated recombination of electrons and Bohrs correspondence principle A. Jaron, et al. Part V: Stabilization and Relativistic Effects in Super Strong Fields. Interference stabilization: LAMBDA- and V-schemes, dynamics of ionization, initial coherent population of Rydberg levels and quantum phase control of the ionization yield M.V. Fedorov, N.P. Poluektov. Computer experiments on atomic stabilization in a strong laser field A.M. Popov, et al. Quasistationary stabilization of the decay of a weakly-bound level and its breakdown in a strong laser field N.L. Manakov, et al. The strong field limit of atomic stabilization in ultrashort pulses M. Dorr, et al. 3-D numerical calculations of laser atom interactions -- subrelativistic and weakly relativistic regime A. Scrinzi, et al. Momentum space description of hydrogen atom interacting with a low frequency strong laser field A. de Bohan, et al. Relativistic effects in the atomic res

Theoretical aspects of intense field harmonic generation

The authors present theoretical studies of high-order harmonic generation in a rare-gas medium. The experimental results obtained at Saclay with a 1064 nm Nd-YAG laser in the 1013 W cm-2 intensity range are summarized. The harmonic emission strengths, first decrease rather steeply for the first orders, then form a long plateau up to the 21st harmonic in xenon, or up to the 33rd harmonic in argon, before decreasing again rather abruptly. The theoretical description of these experiments consists first in the calculation of the photoemission spectra emitted by a single atom. The spectra are obtained by numerically integrating a time dependent Schrodinger equation for the laser-excited rare-gas atom. Second, one must account for collective effects in the medium, described by Maxwells equations. A theoretical framework for describing the generation and propagation of harmonics in strong laser fields is developed. An numerical solution of the propagation equations for the harmonic fields in xenon at 1064 nm provides results which agree well with experimental data.

Study of the Spatial and Temporal Coherence of High-Order Harmonics

We apply the theory of high-order harmonic generation by low-frequency laser fields in the strong field approximation to the study of the spatial and temporal coherence properties of the harmonics. We discuss the role of dynamically induced phases of the atomic polarization in determining the optimal phase matching conditions and angular distributions of harmonics. We demonstrate that the phase matching and the spatial coherence can be controlled by changing the focusing parameters of the fundamental laser beam. Then we present a detailed study of the temporal and spectral properties of harmonics. We discuss how the focusing conditions influence the individual harmonic spectra and time profiles, and how the intensity dependence of the dynamically induced phase leads to a chirp of the harmonic frequency. This phase modulation can be used to control the temporal and spectral properties of the harmonic radiation. Temporally, the harmonic chirped pulse can be recompressed to very small durations. Spectrally, chirping of the fundamental beam may be employed to compensate for the dynamically induced chirp and to control the individual harmonic spectrum. Finally, we discuss the short pulse effects, in particular nonadiabatic phenomena and the possibility of generating attosecond pulses. Comment: Latex file with 37 pages, 25 postscript figures. to appear in Advances in Atomic, Molecular and Optical Physics

Influence of atomic density in high-order harmonic generation

We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique. The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3–80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau.

Optimizing high-order harmonic generation in strong fields

The authors present high-order harmonic generation results obtained with different laser systems, a 1 ps 1053 nm Nd-glass laser, a 2 ps 616 nm synchronously pumped dye laser, a 36 ps 1064 nm mode-locked Nd-YAG laser and the second harmonics (308 nm and 532 nm) of the latter two systems. They investigate the influence of the laser pulse width, the excitation wavelength (from the near infrared to the ultraviolet) and the atomic medium on the number of photons produced and on the maximum energy attained. Harmonic generation also depends strongly on the focusing conditions. By using simple arguments and results of numerical calculations in xenon, they show that the conversion efficiency in general follows a simple b3 power law, b denoting the laser confocal parameter, up to a transition regime where the coherence length of the process becomes equal to the medium length. By applying the b3 scaling as a normalization factor, they can then compare experimental results obtained in different focusing geometries. Their experimental data show that the optimization of the photon energy produced is favoured by using long incident wavelengths and light atomic systems with a high ionization energy.

Angular distributions of high-order harmonics generated by a femtosecond laser

We present a systematic study of the angular distributions of high-order harmonics generated with a femtosecond Cr:LiSrAlF6 laser. We investigate the influence of different parameters, namely laser intensity, nonlinear order, nature of the gas and position of the laser focus relative to the generating medium. We show that when the laser is focused before the atomic medium, harmonics with regular spatial profiles can be generated with reasonable conversion efficiency. Their divergence does not depend directly on the nonlinear order, the intensity or even the nature of the generating gas, but rather on the region of the spectrum the considered harmonic belongs to, which is determined by the combination of the three preceding elements. When the focus is drawn closer to the medium, the distributions get increasingly distorted, becoming annular with a significant divergence for a focus right into-or after-the jet. We perform numerical simulations of the angular distributions. The simulated profiles reproduce remarkably well the experimental trends and are thus used to interpret them.

High-order Harmonics - A Coherent Source In the Xuv Range

We review the main results concerning high-order generation processes from the point of view of a potential user of this new source of XUV radiation. The perspectives for optimizing the source, both in efficiency and in spectral range, its characteristics and in particular, its coherence properties, are discussed. Finally, we describe two experiments, which demonstrate the usefulness of the harmonics as a short-pulse, coherent source in the XUV domain.

High-order harmonic generation in rare gases: a new source in photoionization spectroscopy

We demonstrate that we can use the extreme ultraviolet radiation produced by high order harmonic generation to perform photoionization experiments. With harmonics from the 11th to the 69th of a 140 fs Cr:LiSAF laser operating at 825 nm, we measure the relative photoionization cross sections of xenon, krypton, argon and neon over the range 10 to 110 eV. With narrow bandwidth harmonics produced by a tunable, 1 ps dye laser, we observe the autoionizing states between the 4p5 ionization thresholds in krypton.

Spatial profiles of high-order harmonics generated by a femtosecond Cr:LiSAF laser

Reports the angular distributions of high-order harmonic radiation generated in neon and in argon using a 140 fs Cr:LiSAF laser system. The profiles show an evolution from Gaussian to near flat-top. No substructure is observed except at the highest intensity in a completely ionized gas. The divergence of the harmonics is found to depend essentially on whether they belong to the plateau or to the cutoff. It is approximately constant in the plateau, of the order of 15 mrad and decreases from 15 mrad to 6 mrad in the cutoff region.