Figure 1: Attosecond pulse train corresponding to the superposition of groups of 5 consecutive harmonics: 25 to 33 (red), 35 to 43 (green), 45 to 53 (blue), 55 to 63 (purple). The train corresponding to the full available spectral width (25 to 69) (Yellow-filled) is three times longer than the Fourier limit. Dots represent the fundamental field (absolute value).
Taking snapshots of the movements of molecules, atoms inside molecules, and even electrons inside atoms, is now possible thanks to ultrashort light pulses that act like ultrafast cameras. While infrared lasers are reaching the fundamental limitation imposed by the duration of the optical cycle (a few femtoseconds), High order Harmonics Generation has recently opened a new way by accessing the attosecond regime (1as = 10-18 s).
HHG spectra are made of lines corresponding to the odd multiples of the fundamental laser frequency, and can cover a very broad spectral range, from visible light to soft X-rays. If these harmonics are phase locked, then the corresponding temporal profile is a train of attosecond pulses separated by half the laser period, whose duration decreases as the number of combined harmonics increases. The first experimental demonstration of attosecond pulses was performed in 2001  with the measurement of a train of 250 as pulses, corresponding to the superposition of five consecutive harmonics. We recently extended this study by measuring the relative phases of the high harmonics over a broad spectral range. We found that harmonics were not synchronized on an attosecond timescale, their time of emission (within the optical cycle) increasing linearly with the order (Figure 1). The lowest harmonics are emitted before the highest ones, and the resulting attosecond pulses are thus longer than in the perfect phase locked case. This temporal drift in the emission is a direct signature of the dynamics of the electrons participating to the generation process, and sets an upper limit to the duration achievable by increasing the spectral range. By controlling the electron trajectories within the emission process, we managed to enhance the synchronization of high harmonics, and thus to measure pulses as short as 130 attoseconds (Figure 2)  . Such pulses could be used as a camera with an ultrafast shutter to resolve the dynamics of core electrons in atoms
 "Observation of a Train of Attosecond Pulses from High Harmonic Generation", P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, Ph. Balcou, H. G. Muller, and P. Agostini, Science 292, 1689 (2001)
 "Attosecond Synchronization of High-Harmonic Soft X-rays" , Y.Mairesse, A. de Bohan, L. J. Frasinski, H. Merdji, L. C. Dinu, P. Monchicourt, P. Breger, M. Kovacev, R. Taïeb, B. Carré, H. G. Muller, P. Agostini, et P. Salières Science 302, 1540 (2003)
Abstract Full text
Temporal gating for the generation of single attosecond pulses
The polarization gating technique has been developed to reduce the harmonic pulse duration obtained on high energy laser systems. Starting from 20fs (15fs) laser pulses should make it possible to produce sub-fs XUV pulses; this holds for high energy laser input and efficient HHG in the plateau region, which could result in much more intense sub-fs pulses than currently available. Prior reduction of laser pulse duration from typically 30fs to 15fs would allow for the generation of single attosecond pulses.
The idea to take advantage of the rapid dependence of high harmonics generation on the ellipticity of the pump light was proposed by Corkum et al. in 1994 . This dependence is intuitively clear since harmonic generation is forbidden in circular polarization by angular momentum conservation. Hence if the pump pulse polarization is changed as a function of time from circular to linear and back to circular the resulting harmonic pulse would last essentially the duration of the linear polarization.
The experiment has been carried out on the LUCA laser at Saclay. The initial linearly polarized 80 fs pulse (intensity, FWHM) is fed into a Michelson interferometer. The polarization direction of the pulses in each arm is controlled independently by two quarter-wave plates, equivalent to two half-wave plates due to the double pass. The two pulses leave the interferometer and pass through a quarter-wave plate whose optical axis orientation governs the resulting polarization. If the latter is set at 45° from two mutually orthogonal linearly polarized pulses, a time-dependent ellipticity is determined: the light polarization is first circular, becomes exactly linear during a short time and evolves to circular again ("narrow gate"). The width of the gate depends on the temporal profiles of the pulses and the delay introduced between them by the interferometer. The radiation is then focused inside a 3mm long argon jet where harmonics are generated. Using this technique, we have reduced the harmonic pulse duration by a factor 2.
"Temporal confinement of the harmonic emission through polarization gating" M. Kovacev, Y. Mairesse, E. Priori, H. Merdji, O. Tcherbakoff, P. Monchicourt, P. Breger, E. Mével, E. Constant, P. Salières, B. Carré, P. Agostini, European Physical Journal D 26, 79 (2003).