Generation of attosecond pulse trains

Attophysics group, SPAM, CEA-Saclay, France

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 limited to the study of processes occurring on a femtosecond (1fs = 10^-15 s) timescale due to the duration of their optical cycle (a few fs), High order Harmonic Generation (HHG) has recently opened a new regime by accessing the attosecond range (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 and whose duration decreases as the number of combined harmonics increases.

In collaboration with LOA and FOM, we performed in 2001 the first experimental demonstration of attosecond pulses with the measurement of a train of 250 as pulses, corresponding to the superposition of five consecutive harmonics [1]. In 2003, we pushed this study further 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.

Figure 2: 127 as pulse train obtained by superposing 11 harmonics generated in Ne

[1] "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)

[2] “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, and P. Salières, Science 302, 1540 (2003)