Flexible attosecond beamline for high harmonic spectroscopy and XUV/near-IR pump probe experiments requiring long acquisition times

We describe the versatile features of the attosecond beamline recently installed at CEA-Saclay on the PLFA kHz laser. It combines a fine and very complete set of diagnostics enabling high harmonic spectroscopy (HHS) through the advanced characterization of the amplitude, phase, and polarization of the harmonic emission. It also allows a variety of photo-ionization experiments using magnetic bottle and COLTRIMS (COLd Target Recoil Ion Momentum Microscopy) electron spectrometers that may be used simultaneously, thanks to a two-foci configuration. Using both passive and active stabilization, special care was paid to the long term stability of the system to allow, using both experimental approaches, time resolved studies with attosecond precision, typically over several hours of acquisition times. As an illustration, applications to multi-orbital HHS and electron-ion coincidence time resolved spectroscopy are presented.

Reconstruction of attosecond pulse trains using an adiabatic phase expansion

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.

Extreme ultraviolet Fourier-transform spectroscopy with high order harmonics

We demonstrate a new scheme for extreme ultraviolet (xuv) Fourier-transform spectroscopy based on the generation of two phase-locked high-harmonic beams. It allows us to measure for the first time interferograms at wavelengths as short as 90 nm, and open the perspective of performing high-resolution Fourier-transform absorption spectroscopy in the xuv. Our measurements also demonstrate that a precise control of the relative phase of harmonic pulses can be obtained with an accuracy on an attosecond time scale, of importance for future xuv pump-xuv probe attosecond spectroscopy.

High harmonic XUV spectral phase interferometry for direct electric-field reconstruction

We demonstrate the first experimental complete temporal characterization of high-harmonic XUV pulses by spectral phase interferometry, with an all-optical setup. This method allows us to perform single-shot measurements of the harmonic temporal profile and phase, revealing a remarkable shot-to-shot stability. We characterize harmonics generated in argon by a 50 fs 800 nm laser pulse. The 11th harmonic is found to be 22 fs long with a negative chirp rate of -4.8 x 10(27) s(-2). This duration can be reduced to 13 fs by modulating the polarization of the generating laser. The technique is easy to implement and could be routinely used in femtosecond XUV pump-probe experiments with harmonics.

Optimization of attosecond pulse generation

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains.

Attosecond Synchronization of High-Harmonic Soft X-rays

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.

Measurement of the subcycle timing of attosecond XUV bursts in high-harmonic generation

The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur 190+/-20 as after each pump field maxima (twice per optical cycle), in agreement with the "short" quantum path of the quasiclassical model of harmonic generation.

Observation of a train of attosecond pulses from high harmonic generation

In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.