Search Results (44 total)

Xenon multiphoton double ionization pathways are studied in a reaction microscope using a pump-probe arrangement of extreme ultraviolet high harmonic and infrared laser radiation. The momentum of photoelectrons is recorded in coincidence with singly or doubly charged ions. Among all possible routes to multiphoton double ionization, sequential processes using ionic excited states as intermediate steps are clearly identified.

A theoretical investigation is presented that examines the wavelength scaling from near-visible (0.8  μm) to midinfrared (2  μm) of the photoelectron distribution and high harmonics generated by a “single” atom in an intense electromagnetic field. The calculations use a numerical solution of the time-dependent Schrödinger equation (TDSE) in argon and the strong-field approximation in helium. The scaling of electron energies (λ²), harmonic cutoff (λ²), and attochirp (λ^-1) agree with classical mechanics, but it is found that, surprisingly, the harmonic yield follows a λ^-(5–6) scaling at constant intensity. In addition, the TDSE results reveal an unexpected contribution from higher-order returns of the rescattering electron wave packet.

We propose a new method to reconstruct the electric field of attosecond pulse trains. The phase of the high-order harmonic emission electric field is Taylor expanded around the maximum of the laser pulse envelope in the time domain and around the central harmonic in the frequency domain. Experimental measurements allow us to determine the coefficients of this expansion and to characterize the radiation with attosecond accuracy over a femtosecond time scale. The method gives access to pulse-to-pulse variations along the train, including the timing, the chirp, and the attosecond carrier envelope phase.

Toluene fragmentation by intense femtosecond laser pulses is experimentally investigated. A strong increase of the toluene fragmentation appears to correlate with an increase of the focal area due to changes in the focal geometry and nonlinear small-scale self-focusing. The scenario of Raman modes excitation proposed in an earlier publication [A.M. Müller , Phys. Rev. Lett. 88, 023001 (2002)] is ruled out as the dominant effect for the enhancement.

We investigate high harmonics generated from rubidium atoms irradiated simultaneously by an intense 3.5   μm fundamental field and a weak cw diode laser. When 5p, 5d, and 4d excited states are populated through cascade excitation or deexcitation, orders-of-magnitude increases in harmonic yield as compared with the ground state are observed. It appears that, quite unexpectedly, the population accumulated in the 4d state alone is responsible for the observed enhancement.

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains.

The double ionization of xenon in the multiphoton regime has been studied at two wavelengths (0.77 and 0.79  μm) using an electron-ion coincidence technique and an intensity binned ion ratio method. Sharp resonant structures in the electron energy distribution correlated with the doubly charged ion, as well as a wavelength dependence of the Xe²⁺/Xe⁺ ratio provides new insights. A mechanism involving the shelving of population in Rydberg states followed by excitation of a core electron is proposed.

Cesium vapor interacting with a tightly focused, intense midinfrared (3.4  μm) source produces high harmonic radiation in the visible∕UV spectral range. The measured yields of harmonic orders 9–17 are found to yield good quantitative agreement with numerical simulations that include both the single-atom and the macroscopic response. The 5th–9th harmonic orders are generated with sufficient pulse energies (∼100  pJ) for direct temporal measurements using an autocorrelation method and when correlated with bandwidth measurements are found not to be transform limited. A blueshift connected to a strong time-dependent ionization appears to be the cause of this spectral broadening.

The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur 190±20   as after each pump field maxima (twice per optical cycle), in agreement with the “short” quantum path of the quasiclassical model of harmonic generation.

The double ionization of argon and xenon in an intense laser field has been studied in detail using an electron-ion coincidence technique. The observed double ionization electron spectra in xenon show resonancelike structures here resolved for the first time. In argon, the featureless spectra are consistent with rescattering. This represents a clear transition in the dynamics of strong-field double ionization, analogous to the well-known transition between the tunneling and multiphoton regimes in single ionization.

We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation.

The case is considered of a weak Gaussian harmonic pulse of frquency (2s+1)ω passed through an atomic medium collinearly with an intense Gaussian laser pulse of frequency ω. The harmonic beam may be amplified or deamplified according to the balance between stimulated emission and absorption. A simple analytic expression for the gain is derived in the tunneling limit of ionization. It predicts a positive (negative) gain for positive (negative) delay of the harmonic pulse with respect to the laser pump pulse. The gain may be significant and proportional to the delay for delays of a few laser periods, much shorter than the pulse envelopes. Possibilities for experimental observations are briefly discussed.

The double ionization of helium in the strong-field limit has been studied using an electron-ion coincidence technique. The observed double ionization electron energy spectra differ significantly from the single ionization distributions. This gives new support to the rescattering model of double ionization and explicitly reveals the role of backward electron emission following the e-2e ionizing collision.

We present an alternative method for measuring ultrashort extreme-ultraviolet pulses that can be synchronized with an intense infrared pulse. The method, based on photoionization of a target atom by the extreme-ultraviolet pulse in the presence of the infrared pulse, has a potential accuracy of close to 1 fs and is susceptible to single-shot operation. It is demonstrated on harmonic 15 of a titanium:sapphire laser. The minimum pulse duration that can be measured is limited only by the frequency of the radiation used for the ponderomotive shift of the ionization potential (3 fs in the case of the titanium:sapphire fundamental).

High harmonic radiation spectra up to 19th order in alkali metal vapors excited by an intense, picosecond mid-infrared ( 3–4 μm) laser are reported and compared to theory. The strong-field dynamics in the alkali metal atoms exhibit significant differences from all previously studied systems due to the strong coupling between their ground and first excited states.

We numerically study two-photon processes using a set of harmonics from a Ti:sapphire laser and, in particular, interference effects in the above-threshold ionization spectra. We compare the situation where the harmonic phases are assumed locked to the case where they have a random distribution. Suggestions for possible experiments, using realistic parameters, are discussed.

We study high-order harmonic generation in the general case of absorbing and dispersive atomic gas media. For ultrashort laser pulses, the harmonic conversion efficiency tends to a limit mainly imposed by the harmonic reabsorption in the gas. This limit, independent on the gas density, is the same for both the case of a loosely focused beam or a beam guided in a gas-filled hollow-core fiber. Under optimum conditions, we measured the highest conversion efficiency to date (4×10^-5) for the 15th harmonic generated in xenon using a 40 fs, 1.5 mJ, 800 nm pulse at a 1 kHz repetition rate.

Evolution of atomic ionization into the strong-field limit offers the opportunity to study the fundamentals of atom-laser interaction. In this study, we report on high precision measurements of the ion and electron distributions from laser-excited helium and neon atoms which reflect the changing continuum dynamics as the ionization process evolves into the pure tunneling regime. The experiments present evidence of both single- and two-electron ionization. These data provide a direct quantitative test of various theories of strong-field ionization. We show that a relatively simple semiclassical model which includes a description of a field-driven electron elastically rescattering from an accurate ion core potential reproduces the measured electron distributions for both atoms. However, using this model to calculate e-2e inelastic rescattering yields cross sections which are incompatible with the measured two-electron ionization.

Ultrashort self-guided laser pulses are employed to generate high-order harmonics in noble gases. Interference fringes between fundamental and harmonic waves over several cm reveal a long range longitudinal coherence of the generated harmonics. Quasiphase matching is demonstrated for the third harmonic wave.

The temporal behavior of the 19th harmonic of a Ti:sapphire laser generated in argon was investigated by a cross-correlation technique at intensities close to saturation. Pulses about half as long as predicted on the basis of the scaling law of the atomic dipole strength on intensity were found. Analysis shows that this is a consequence of both the intensity-dependent dipole phase and the depletion of the medium by ionization.

A Reply to the Comment by Lars G. Hanson, Jian Zhang, and P. Lambropoulos.

We investigate the effect of an intense optical field on the energies of electrons ejected in an Auger process. To this end, part of a subpicosecond, infrared laser beam is focused onto a metal target to produce subpicosecond, broadband x radiation. Electrons resulting from the x-ray-induced LMM Auger transition in argon are detected in a time-of-flight electron spectrometer. The other part of the infrared laser beam is directly focused onto the argon atoms. When the x-ray-induced Auger transition takes place in the presence of the intense dressing beam, sidebands appear around the Auger peaks in the electron spectrum, corresponding to absorption or emission of photons from the dressing field. The magnitude of the sidebands is studied as a function of dressing field intensity. Good quantitative agreement with quantum-mechanical theory is reported.

Photoelectron energy spectra resulting from two-photon ionization of calcium atoms by 180 fs pulses have been studied as a function of wavelength (380–405 nm) and intensity ( 9-900  GW/cm²). When the wavelength is tuned near the 4s→4p or 4s→5s ionic core transition, the photoelectron peaks display a characteristic splitting proportional to the field strength and assignable to a two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential-state model involving three coupled continua are compared to the experiment.

The interaction of intense ultrashort laser pulses from a chirped pulse amplification titanium sapphire laser system with dielectric and metallic surfaces was studied. With a p-polarized incident beam and a large angle of incidence we have observed odd and even harmonics of high order with relatively smooth roll-off in the harmonic spectra. The shortest wavelength was 55.3 nm, corresponding to the 15th harmonic order.

The effect of an intense laser field on the energies of Auger electrons has been investigated using electron spectroscopy. A laser-generated gallium plasma pumped by a subpicosecond laser yields pulsed broadband x radiation, used to induce LMM Auger transitions in argon. The intense dressing beam ((λ=800 nm) causes the appearance of up to four sidebands on both sides of the original Auger lines. Recording the magnitude of the sidebands versus the time delay between the dressing and x-ray pulses shows that our source delivers subpicosecond pulses in the 250-400 eV range.