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...

We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.

Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

Calculations are presented for the generation of an isolated attosecond pulse in a multicycle two-color strong-field regime. We show that the recollision of the electron wave packet can be confined to half an optical cycle using pulses of up to 40 fs in duration. The scheme is proven to be efficient using two intense beams, one producing a strong field at and the other a strong field detuned from 2. The slight detuning ␦ of the second harmonic is used to break the symmetry of the electric field over many optical cycles and provides a coherent control for the formation of an isolated attosecond pulse.High-order harmonics generation (HHG) of intense laser pulses in gases is attracting much attention due to both fundamental and applied interests. One of the most attractive properties of the HHG source is the generation of attosecond pulses. This field has been growing spectacularly (for reviews see [1,2]). Trains of attosecond pulses are naturally produced in the HHG process, whereas the generation of a single or isolated attosecond pulse is more challenging. Isolated attosecond pulses have been, however, achieved using absolute phase-stabilized few-optical-cycle state-of-the art laser systems. The methods rely either on the laser intensity [3] or ellipticity (polarization state) [4,5] dependencies of the harmonic emission. Another approach involves control of the wave packet recollision using a two-color field. Proposals based on a mixing scheme of a strong fundamental field and a weak [6] or strong [7] second-harmonic field have been investigated computationally. Experimental indications of an isolated attosecond pulse have been observed in the latter scheme explored experimentally. These schemes are very promising but require very short pulses ͑Ͻ12 fs͒ except if harmonic heterodyning with a weak lower frequency field is used [8], which is, however, more difficult to produce experimentally than the second harmonic.In this Letter, we propose to generate single attosecond pulses in the long optical cycle regime by mixing the fundamental field to its detuned second harmonic. To illustrate the principle, we perform onedimension quantum mechanical calculations using an argon atom with a smoothed Coulomb potential. Our approach is based on the numerical solution of the time-dependent Schrödinger equation. The calculated attosecond time structure is shown in Fig. 1a when only the fundamental ͑ 1 ͒ is present. The laser intensity is constant. We see that an attosecond pulse is emitted every 1 / 2 optical cycle. This reflects the symmetry of the HHG process. In Fig. 1b we perform the same calculations but add an intense spectrally detuned second-harmonic field. The detuning is chosen so that 2 = 1 ͑2+1/N͒, with N = 5. Due to the new periodicity of the electric field we show that an attosecond pulse is now emitted every 5 / 2 optical cycle. This property of HHG can be used to isolate a single attosecond pulse. Indeed, if the laser envelope duration is of the same order as (or a few times) the delay between two consecutive ...

3D terahertz computed tomography has been performed using a monochromatic millimeter wave imaging system coupled with an infrared temperature sensor. Three different reconstruction methods (standard back-projection algorithm and two iterative analysis) have been compared in order to reconstruct large size 3D objects. The quality (intensity, contrast and geometric preservation) of reconstructed cross-sectional images has been discussed together with the optimization of the number of projections. Final demonstration to real-life 3D objects has been processed to illustrate the potential of the reconstruction methods for applied terahertz tomography.

Soft-x-ray digital in-line microscopic holography is achieved using a fully coherent high-order harmonic source emitting at 32 nm. Combination of commercial-grade soft-x-ray optics and a back-illuminated CCD detector allows a compact and versatile holographic setup. Different experimental geometries have been tested by imaging calibrated 50 nm tips and 1 microm wires. Spatial resolution of 800 nm is measured with magnifications ranging from 30 to 110 and a numerical aperture around 0.01. Finally, the potentiality of three-dimensional numerical reconstruction from a single hologram acquisition is shown experimentally.

Three-dimensional terahertz computed tomography has been used to investigate dried human bones such as a lumbar vertebra, a coxal bone, and a skull, with a direct comparison with standard radiography. In spite of lower spatial resolution compared with x-ray, terahertz imaging clearly discerns a compact bone from a spongy one, with strong terahertz absorption as shown by additional terahertz time-domain transmission spectroscopy.