Ph. Balcou

Imaging and quality assessment of high-harmonic focal spots

We present a direct method of studying the focusability of an intense, short-pulse extreme-ultraviolet (XUV) beam obtained by high-harmonic generation. We perform near-field imaging of the focal spot of five high-harmonic orders strongly focused by a broadband toroidal mirror. To visualize the focal spot directly, we image the fluorescence induced by an XUV beam on a cerium-doped YAG crystal on a visible CCD camera. We can thus measure the harmonic spot size on a single image, together with the Strehl ratio, to evaluate the quality of focusing. Such techniques should become instrumental in optimizing the focusing conditions and reaching intensities required for exploring attosecond nonlinear optics in the XUV range.

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.

Temporal superresolution of ultrashort laser pulses

We introduce the notion of temporal superresolution for ultrashort pulses, by analogy with the well-known method of optical superresolution. Without spectral width modification we shape a pulse into a central peak of duration well below the Fourier limited.

Investigation of ultraintense femtosecond laser–plasma interactions through [omega] and 2[omega] imaging and spectroscopy

A laser‐plasma interaction experiment was performed in order to match effective conditions for wake-field electron acceleration. A 30-fs pulse was made to interact with a preformed plasma generated via the exploding foil technique from a1 - mm-thick plastic film. The irradiance of the femtosecond pulse in the plasma was 10 20 Wcm 22 . The interaction conditions were investigated via imaging and spectroscopy at the fundamental and the second harmonic of the laser frequency, both forward and backward. Our data clearly show that conditions suitable for electron acceleration are achieved close to the propagation axis and can be easily reproduced from shot to shot. In contrast, significant growth of instabilities occurs at the boundaries of the interaction region. These observations are consistent with a preliminary evidence of forward acceleration of high-energy electrons. Optical, X-ray, and g-ray data obtained for different positions of the foil target with respect to the laser focal plane further support this promising scenario.

Frustrated total internal reflection of laser eigenstates

The role of frustrated total internal reflection in the dynamics of laser eigenstates is investigated theoretically and experimentally. We first derive the Jones matrix of a frustrating element for a realistic Gaussian beam in a single-pass geometry. We point out the existence of three different angular regimes, namely, a pure frustration regime, an intermediate regime, and a quasi-Fabry–Perot regime. We then explore in each case the nature and the competition between the laser polarization eigenstates. A novel spiraling behavior of the laser parameters is demonstrated when the frustrating gap is varied, along with puzzling polarization-flipping effects, that may modify strongly the conditions that optimize the laser-output power. Experimental results agree with a theoretical model.

Anomalous high-order harmonic generation

We present experimental harmonic spectra showing anomalously high orders generated at low laser intensities (around 2 × 1013 W cm−2) in argon, krypton and xenon. These high orders cannot be explained with the usual three step model that predicts the existence of a sharp cut-off after the region of constant efficiency called the plateau: the observed orders are far above the cut-off limit inferred from the laser intensity in the medium. The systematic study of these spectra are presented, and some possible theoretical models are explored.

Double-helicoidal eigenstates in lasers

Longitudinal segmented helicoidal eigenstates in laser resonators, with different field distributions of either identical or opposite handedness, are studied theoretically and experimentally. The eigenmodes and frequencies are derived in the framework of the Jones-matrix formalism. Double helices can be applied to birefringence compensation for helicoidal waves. A novel Fabry–Perot laser cavity is realized in which both spatial hole-burning suppression and birefringence compensation are simultaneously achieved.

Neutron generation by laser irradiation of CD4 clusters

It was shown in 1999 that D2 cluster explosion under ultrashort intense laser irradiation can lead to ion energies sufficient to drive nuclear fusion reactions [Ditmire et al. Nature 398, 491]. We show how the use of molecular clusters allows to further enhance the ionic acceleration, allowing one to reach optimal fusion cross sections. A new low density high energy regime is described, in which fusion occurs via a spallation-like process. The process increases with laser intensity up to the relativistic threshold, at which the neutron production is overwhelmed by gamma production resulting from electron acceleration.

Negative Ion Production by fs, High‐Intensity Laser Beam Interactions with Clusters

We present experimental results concerning the observation and acceleration of negative and positive ions produced from high‐intensity laser beam interactions with deuterated and hydrogenated clusters. The 30‐fs, 10‐Hz rep. rate, 30 TW Ti‐Sa laser of LOA was focused on clusters produced from a pulsed gas nozzle. The novelty of our experimental work was the gas composition (CD4 and CH4), which was used for the cluster formation. Both kinds of ions, negative and positive, were observed with approximately equal energies. The maximum energy of both kinds of ions as well their energy distribution was measured using a Thomson parabola mass spectrometer. Negative and positive ions of H, D, and C were observed with a maximum energy up to 70 keV for a maximum laser beam intensity of 1018 W/cm2. The mass spectrometer allowed us to correlate both the positive and negative ion energy spectrums with the laser beam intensity in order to study the process of cluster explosions, ion acceleration, and negative ion formati...

Optical-activity measurements with bihelicoidal laser eigenstates

A novel, doubly differential method for the measurement of optical activity that uses the so-called bihelicoidal eigenstates of a laser is demonstrated. An experimental realization is presented, and prospects are discussed.