A ttosecond electron wavepackets are produced when an intense laser field ionizes an atom or a molecule 1 . When the laser field drives the wavepackets back to the parent ion, they interfere with the bound wavefunction, producing coherent subfemtosecond extreme-ultraviolet light bursts. When only a single return is possible 2,3 , an isolated attosecond pulse is generated. Here we demonstrate that by modulating the polarization of a carrier-envelope phase-stabilized short laser pulse 4 , we can finely control the electron-wavepacket dynamics. We use high-order harmonic generation to probe these dynamics. Under optimized conditions, we observe the signature of a single return of the electron wavepacket over a large range of energies. This temporally confines the extreme-ultraviolet emission to an isolated attosecond pulse with a broad and tunable bandwidth. Our approach is very general, and extends the bandwidth of attosecond isolated pulses in such a way that pulses of a few attoseconds seem achievable. Similar temporal resolution could also be achieved by directly using the broadband electron wavepacket. This opens up a new regime for timeresolved tomography of atomic or molecular wavefunctions 5,6 and ultrafast dynamics.During high-order harmonic generation (HHG) in gas 7 , short electron wavepackets (EWPs) are periodically released by high-field ionization. Their subsequent coherent interaction with the remaining bound wavefunction leads to coherent extremeultraviolet (XUV) emission. The T 0 /2 periodicity of this process (T 0 being the laser optical period) ensures that only odd harmonics of the fundamental radiation are emitted. Temporally, the XUV pulses are emitted as a train of chirped attosecond pulses [8][9][10] (1 attosecond = 10 −18 s). For both plateau (low energy) and cut-off (high energy) harmonics, specific focusing conditions ensure that only a single attosecond pulse is emitted every half cycle 11,12 . Extracting an isolated attosecond pulse from this train requires breaking the periodicity of the process, so that XUV emission is only possible within a single half cycle of the fundamental pulse. In this way, isolated 250-attosecond-long Figure 1 Spectra generated in argon. Spectra emitted from an argon medium irradiated with a polarization-modulated pulse (τ = 5 fs, δ = 6.2 fs, β = 0 • ) as a function of the CEP shift. For some CEPs, harmonic peaks appear, whereas for other CEPs, they broaden up to a continuum.pulses were recently obtained 13 by selecting the (highly intensity dependent) cut-off harmonics generated in neon by a 5-fs linearly polarized, fundamental pulse with stabilized carrierenvelope phase (CEP). With this technique, the minimum pulse duration achievable is limited by the (∼10 eV) bandwidth of the selected cut-off harmonics, which prevents us reaching the sub-100-attosecond domain.To isolate a broadband attosecond pulse, we used a different approach 2 . Our approach relies on the strong HHG sensitivity on the ellipticity, ε, of the fundamental field, which is largely nature phy...
