A complete quantum-mechanical description of matter and its interaction with the environment requires detailed knowledge of a number of complex parameters. In particular, information about the phase of wavefunctions is important for predicting the behaviour of atoms, molecules or larger systems. In optics, information about the evolution of the phase of light in time 1 and space 2 is obtained by interferometry. To obtain similar information for atoms and molecules, it is vital to develop analogous techniques. Here we present an interferometric method for determining the phase variation of electronic wave packets in momentum space, and demonstrate its applicability to the fundamental process of single-photon ionization. We use a sequence of extreme-ultraviolet attosecond pulses 3,4 to ionize argon atoms and an infrared laser field, which induces a momentum shear 5 between consecutive electron wave packets. The interferograms that result from the interaction of these wave packets provide useful information about their phase. This technique opens a promising new avenue for reconstructing the wavefunctions 6,7 of atoms and molecules and for following the ultrafast dynamics of electronic wave packets.

We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a pi phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime.

Design and characterization of extreme-ultraviolet broadband mirrors for attosecond scienceMorlens,

We use semiconductor (Si) and metallic (Al, Zr) transmission filters to shape, in amplitude and phase, high-order harmonics generated from the interaction of an intense titanium sapphire laser field with a pulsed neon gas target. Depending on the properties of the filter, the emitted attosecond pulses can be optimized in bandwidth and/or pulse length. We demonstrate the generation of attosecond pulses centered at energies from 50 to 80 eV, with bandwidths as large as 45 eV and with pulse durations compressed to 130 as.

We propose a new method to reconstruct the electric field of attosecond pulse trains. The phase of the high-order harmonic emission electric field is Taylor expanded around the maximum of the laser pulse envelope in the time domain and around the central harmonic in the frequency domain. Experimental measurements allow us to determine the coefficients of this expansion and to characterize the radiation with attosecond accuracy over a femtosecond time scale. The method gives access to pulse-to-pulse variations along the train, including the timing, the chirp, and the attosecond carrier envelope phase. DOI: 10.1103/PhysRevLett.95.243901 PACS numbers: 42.65.Ky, 32.80.Rm When an intense laser field interacts with a gas, highorder harmonics are emitted in subfemtosecond bursts of light [1,2]. The temporal characterization of this radiation is of great fundamental interest since it gives insight into the emission process. In addition, the ultrashort duration corresponding to a selected bandwidth makes this radiation a unique extreme ultraviolet (XUV) source of interest for a number of applications.Individual harmonic pulses can be characterized on the femtosecond time scale by techniques such as FROG (frequency-resolved optical gating) [3][4][5][6][7] and SPIDER (spectral phase interferometry for direct electric field reconstruction) [8,9]. Attosecond pulses are characterized by recently developed techniques [2,10] such as RABITT (reconstruction of attosecond beating by interference of two-photon transition). The latter shows the existence of a quadratic spectral phase, i.e., a frequency chirp [11,12] (hereafter called attochirp) [13], for the plateau harmonics.The RABITT technique, which assumes monochromatic harmonic components, gives access only to the average pulse shape in the train. New measurement techniques, based on extensions of the FROG method, have been proposed and numerically verified [14][15][16]. All these methods imply a scan of an optical delay, and the reconstruction of a complete attosecond pulse train (APT) demands a temporal accuracy of a few tens of attoseconds over several tens of femtoseconds, which is difficult to achieve experimentally.In this Letter, we propose a new technique that, by making physically reasonable assumptions, practically removes the high requirement on the experiment. The ultraprecise scan is replaced by a series of short scan RABITT measurements performed at different laser intensities. The reconstruction of the electric field uses a Taylor expansion of the harmonic phase with respect to frequency and time whose coefficients have simple physical interpretations. The method is illustrated by characterizing an APT generated in neon.We consider a coherent sum of consecutive odd harmonics (from q i to q f ) generated when a strong laser field interacts with a gas. We assume for simplicity that only one quantum path, selected by either phase matching or an aperture in the far field [17][18][19][20], contributes to the generation of these harmonics. The electric field resul...