Search Results (16 total)

It is demonstrated that three-dimensional propagation effects essentially influence attosecond-pulse generation by few-cycle, carrier-envelope-phase stabilized laser pulses used in a polarization-gating configuration. The rapidly changing polarization status gives rise to electron trajectories even longer than those observed with linearly polarized light, but the off-axis contributions and the propagation effects can efficiently act as a filter to produce a single attosecond pulse. It is also found that the attosecond beams can have a significant spatial divergence.

The effects of electronic structure and symmetry are observed in laser driven high-order harmonic generation for laser aligned conjugated polyatomic molecular systems. The dependence of the harmonic yield on the angle between the molecular axis and the polarization of the driving laser field is seen to contain the fingerprint of the highest occupied molecular orbitals in acetylene and allene, a good quantitative agreement with calculations employing the strong field approximation was found. These measurements support the extension of the recently proposed molecular orbital imaging techniques beyond simple diatomic molecules to larger molecular systems.

We have investigated the process of high-order harmonic generation in light alkanes by using femtosecond laser pulses. We show the experimental results cannot be matched by a model that assumes a single active electron only in a hydrogenic s orbital. Clear evidences are shown of the important role played by the p-like character originating from the covalent C-H bond. By constructing a suitable mixture of s-type and p-type atomic wave functions, an excellent agreement between measurements in methane and simulations is found, thus confirming the validity of the developed method as a general tool for the analysis of high-order harmonic generation in complex molecules.

We experimentally investigate the process of intramolecular quantum interference in high-order harmonic generation in impulsively aligned CO₂ molecules. The recombination interference effect is clearly seen through the order dependence of the harmonic yield in an aligned sample. The experimental results can be well modeled assuming that the effective de Broglie wavelength of the returning electron wave is not significantly altered by the Coulomb field of the molecular ion. We demonstrate that such interference effects can be effectively controlled by changing the ellipticity of the driving laser field.

High-order harmonic generation from molecules (O₂, N₂, H₂, and CO₂) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; 2×10¹⁴ and 6×10¹⁴  W  cm⁻², were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of O₂ and CO₂ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with N₂ and H₂ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein’s model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.

The influence of dynamical medium depletion in high-order above-threshold ionization (ATI) in left/right asymmetry of photoelectron energy spectra is analyzed. Based on a classical analysis of high-order ATI electrons produced by few-cycle laser pulses, calculated asymmetry maps of electron spectra reproduce very well the experimental results reported in Lindner [Phys. Rev. Lett. 92, 113001 (2004)], utilized for determining the Guoy phase shift of few-cycle laser pulses. The anomalous behavior of the high-energy part of the ATI electron spectra is, then, fully understood in terms of earlier medium depletion occurring in the leading edge of the laser pulse. In order to correctly reproduce the experimental findings a physical temporal envelope of the laser pulse, which only vanishes at the infinity, plays a crucial role.

We describe a very simple physical model that allows the analysis of high-order harmonic generation in gases when the pumping laser beam has an intensity profile that is not Gaussian but truncated Bessel. This is the typical experimental condition when sub-10-fs pump-laser pulses, generated by the hollow fiber compression technique, are used. This model is based on the analysis of the phase-matching conditions for the harmonic generation process revisited in view of the new spatial mode of the fundamental beam. In particular, the role of the atomic dipole phase and the geometric phase terms are evidenced both for harmonics generated in the plateau and in the cutoff spectral regions. The influence of dispersion introduced by free electrons produced by laser ionization has also been discussed in some detail. Spatial patterns of far-field harmonics are then obtained by means of a simplified algorithm which allows one to avoid the numerical integration of the harmonic beam propagation equation. Experimental spatial distributions and divergence angles of high-order harmonics generated in Ne with 7-fs titanium-sapphire pulses are compared with numerical simulations in various experimental conditions. The agreement between measurements and calculated results is found to be very satisfactory.

Low-divergence, high-brightness harmonic emission has been generated by using a fundamental beam with a truncated Bessel intensity profile. Such a beam is directly obtained by using the hollow-fiber compression technique, which indeed allows one to optimize both temporal and spatial characteristics of the high-order harmonic generation process. This is particularly important for the applications of radiation, where extreme temporal resolution and high brightness are required.

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.

A numerical model to calculate the high-order harmonics spectrum of a macroscopic gas target irradiated by a few-optical-cycle laser pulse is presented. The single-atom response, calculated within the nonadiabatic strong-field approximation, is the source term of a three-dimensional propagation code. The simulation results show remarkably good agreement with experiments performed in neon using laser pulses with durations of 30 and 7 fs. Both simulations and experiments show discrete and well-resolved harmonics even for the shortest driving pulses.

We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated by 30-fs Ti:sapphire laser pulses in a neon gas jet. Our observations clearly indicate the possibility of continuously tuning the harmonic wavelength in the region below 20 nm, by taking advantage of the blue shift of the harmonic wavelength induced by the presence of free electrons in the gas. We have experimentally demonstrated that this allows one to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift are imparted by simply changing the gas jet position relative to the laser beam waist, namely, by varying the effective laser intensity experienced by the gas jet when it is moved across the focal region. We have also interpreted the experimental results in terms of a simple model of the generation process based on the tunnel ionization of an atom exposed to an ultraintense laser field.

We report measurements of high-order harmonic spectra obtained with a 800-nm 150-fs laser pulse with a time-varying degree of ellipticity. The modulation of the polarization in time is achieved by using birefringent optics and self-phase modulation in a glass plate. We can create one or two temporal gates of a few femtoseconds width, during which the polarization is linear and harmonic emission is efficient. The harmonic spectra observed experimentally demonstrate that harmonics generated with linear polarization are frequency chirped. The values measured experimentally are consistent with theoretical predictions based on the strong field approximation.

We demonstrate that two harmonic sources generated independently in a xenon gas jet using the same picosecond Nd:YAG laser are locked in phase. The experiment is performed by separating a laser beam into two parallel beams focused at different locations under the nozzle of a gas jet, and therefore producing two independent sources of harmonic radiation, and studying the pattern obtained in the far field when the radiations from these sources interfere. A high and robust fringe visibility is obtained.

Multiphoton ionization (MPI) of a noble gas produces an electron plasma, acting as a defocusing lens for the laser beam. In this paper we show that some parameters of the MPI process, e.g, order of nonlinearity p, and of the plasma, e.g., size and density, can be estimated by examining the far-field intensity distribution of the laser fundamental. In the limit of small electron density, describing the scattering within the framework of the Born approximation, an analytic expression of the scattered field has been obtained. For the other cases, we have developed a numerical approach based on the expansion of the field in a series of Gaussian beams. On the other hand, the aberration of the fundamental beam interferes notably with the process of harmonic generation in the gas medium. Our theoretical findings agree well with experimental data relative to the fundamental and the third harmonic of a Nd:YAG (where YAG denotes ytirium aluminum garnet) laser radiation produced in the nonresonant 14-photon ionization of Ar. This agreement confirms that the distributions reported in literature describe quite well the plasma formed in our 30-ps experiments.

A kinetic description of inhomogeneous plasmas produced by irradiating an atomic gas by a well-focused ultrashort high-intensity laser pulse is presented. The Vlasov-Maxwell equation is integrated by representing the distribution function f(r,v;t) in the form f₀(Dr-Bv,-Cr+Av), with f₀ the initial distribution and the A,B,C,D functions depending on r, v, and t. On the basis of this representation the evolution of some charge distributions is examined, by focusing the attention on the transport of ion packets, the effect of the ponderomotive potential, and the capture of electrons by the ion potential well.