Attosecond metrology has so far largely remained limited to titanium:sapphire lasers combined with an active stabilization of the carrier-envelope phase (CEP). These sources limit the achievable photon energy to ∼100 eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. Therefore, intense efforts are underway to extend attosecond metrology to the soft-X-ray (SXR) domain using mid-infrared (mid-IR) drivers. Here, we introduce and experimentally demonstrate a method that solves the long-standing problem of the complete temporal characterization of ultra-broadband (≫10 eV) attosecond pulses. We generalize the recently proposed Volkov-transform generalized projection algorithm (VTGPA) to the case of multiple overlapping photoelectron spectra and demonstrate its application to isolated attosecond pulses. This new approach overcomes all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation (CMA), and it incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. These properties make our approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth. We apply this method to attosecond SXR pulses generated from a two-cycle mid-IR driver, covering a bandwidth of ∼100 eV and reaching photon energies up to 180 eV. We extract an SXR pulse duration of (43±1) as from our streaking measurements, defining a new world record. Our results prove that the popular and broadly available scheme of post-compressing the output of white-light-seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. Our new reconstruction method and experimental results open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering X-ray absorption edges of most light elements.

No abstract

Conical intersections allow electronically excited molecules to return to their electronic ground state. Here, we observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. We selectively launch wave packets in the two lowest electronic states (D0 and D1) of C2H4+. The electronic D1 → D0 relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D1 and D0 bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. Our results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period.

Recent advances in high-harmonic generation gave rise to soft X-ray pulses with higher intensity, shorter duration and higher photon energy. One of the remaining shortages of this source is its restriction to linear polarization, since the yield of generation of elliptically polarized high harmonics has been low so far. We here show how this limitation is overcome by using a cross-polarized two-colour laser field. With this simple technique, we reach high degrees of ellipticity (up to 75%) with efficiencies similar to classically generated linearly polarized harmonics. To demonstrate these features and to prove the capacity of our source for applications, we measure the X-ray magnetic circular dichroism (XMCD) effect of nickel at the M 2,3 absorption edge around 67 eV. There results open up the way towards femtosecond time-resolved experiments using high harmonics exploiting the powerful element-sensitive XMCD effect and resolving the ultrafast magnetization dynamics of individual components in complex materials.

Abstract.We report on the global temporal pulse characteristics of individual harmonics in an attosecond pulse train by means of photo-electron streaking in a strong low-frequency transient. The scheme allows direct retrieval of pulse durations and first order chirp of individual harmonics without the need of temporal scanning. The measurements were performed using an intense THz field generated by tilted phase front technique in

The recent entry of X-ray free electron lasers (FELs) to all fields of physics has created an enormous need, both from scientists and operators, for better characterization of the beam created by these facilities. Of particular interest is the measurement of the arrival time of the FEL pulse relative to a laser pump, for pump-probe experiments, and the measurement of the FEL pulse length. This article describes a scheme that corrects one of the major sources of uncertainty in these types of measurements, namely the jitter in the arrival time of the FEL relative to an experimental laser beam. The setup presented here uses a combination of THz streak cameras and a spectral encoding setup to reduce the effect of an FEL's jitter, leaving the pulse length as the only variable that can affect the accuracy of the pulse length and arrival time measurement. A discussion of underlying principles is also provided.

Quantum efficiency studies for various wavelength and various technical metal surfaces were carried out in a dedicated unbaked vacuum chamber. Copper, magnesium, aluminium and aluminium-lithium photocathodes were irradiated by two different high power, high repetition rate, laser systems. We have observed an emission of electrons for photon energy below the work function of the material. This is explained by multiple photon absorption at the photocathode. We have not observed any degradation of the QE for those materials, but an improvement when irradiating them over a long period of time. This is contrary to observations made in RF photoguns.

The current density limit for photoemission from metals was measured in an rf photogun to be below 10(9)  A/m2. We have achieved 1.6×10(11)  A/m2 by photofield emission from a new type of photocathode made from a metallic-composite, multifilamentary Nb3Sn wire driven by a 266 nm picosecond laser pulse and a 2 ns, 50 kV accelerating voltage. This cathode has a micrometer arrayed structure with tens of thousands of Nb/Nb3Sn filaments embedded in a bronze matrix. Our measurements revealed the existence of a new electron emission regime at high laser fluence (100  mJ/cm2). We have extracted stably, and without any surface ablation, up to 4800 pC of charge. This corresponds to 0.9% quantum efficiency, 100 times larger than what is measured from conventional metallic photocathodes. The unexpected large and stable charge extraction cannot be explained by the 3-step model. Thanks to the small emitting area, the measured emittance (0.6  mm·mrad) is low in spite of the high current density and space charge effects. This cathode will be of benefit for many applications based on short and bright electron bunches.