Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.

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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. DOI: 10.1103/PhysRevLett.93.163901 PACS numbers: 42.65.Ky, 32.80.Wr, 42.65.Re The strongly nonlinear interaction taking place when an intense infrared (IR) laser pulse is focused into a rare gas jet results in the coherent emission of extreme ultraviolet (XUV) light [1] with a characteristic spectrum showing a plateau and a sharp cutoff at high energy [2]. This process offers the unique opportunity of generating attosecond pulses, as recently demonstrated by two techniques. First, starting from a few-cycle laser pulse, the highest energy photons are only emitted at the maximum of the laser envelope. The cutoff is then continuous and by spectrally selecting it, one can obtain a single pulse of 250 as duration [3,4]. Second, using a multicycle IR pulse provides a discrete spectrum containing only odd harmonics of the laser frequency. Selecting many harmonics in the plateau results in emission of XUV bursts every half laser period, forming an attosecond pulse train (APT) whose wagons can be as short as 130 as [5,6]. In both cases, the condition for the production of short pulses is a near-linear spectral phase of the XUV radiation. For a discrete spectrum, this means that harmonics must be phase-locked, i.e., synchronized. The phaselocking of harmonics is closely related to the electronic dynamics in the generation process. In the semiclassical ''three-step'' model [7,8], part of the electron wave packet first tunnels out of the atomic potential lowered by the laser field; it is then driven by the strong IR field in the continuum; finally, it can recombine with the parent ion, emitting a XUV photon whose energy is given by the electron return kinetic energy. The recombination times of the different electron trajectories determine the emission times of the different XUV frequencies, and thus their synchronization.Recent studies have stressed two main sources of asynchronism that raise important questions for the reliable generation of shorter pulses. First, each harmonic is associated to mainly two (''short'' and ''long'') electron trajectories that have the same return energy but very different return times. The single-atom response thus consists of at least two bursts per half laser cycle, which blurs the attosecond structure. Fortunately, the short trajectory contribution can be macroscopically selected by adjusting carefully the phase matching conditions [9]: when the generating laser is focused slightly before the gas jet, the macros...

In this paper we present some results of a high-harmonic generation experiment in argon, performed with a Ti:sapphire laser (787 nm, 140 fs) and a spatially shaped focus (flat top). The range of intensities is 3-100 TW cm −2 . The intensity dependence of the harmonic signal shows an interesting behaviour of the 13th harmonic, namely an initial suppression followed by a strong enhancement at 85 TW cm −2 , which exceeds the magnitude of the 11th harmonic. This behaviour is a consequence of a resonant atomic process. Numerical calculations were also performed. They partly reproduced the experiment.

We demonstrate that the main features of high harmonic generation by ultrashort laser pulses can be explained in terms of the intensity dependent phase of the atomic polarization. Focusing conditions and chirped driving fields may be used to control the harmonic spectrum, while temporal compression provides a unique way to tailor the time profile. [S0031-9007(98)07968-X]

With the help of a suitably chosen momentum-space analysis, we study some of the basic mechanisms governing the physics of the processes occurring when atoms are submitted to intense infrared laser pulses, with peak intensities 10(14) W cm(-2)

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