We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T-TaS(2). This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

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22 23 24 Photons have fixed spin and unbounded orbital angular momentum (OAM). While the 25 former is manifested in the polarization of light, the latter corresponds to the spatial 26 phase distribution of its wave front [1]. The distinctive way in which the photon spin dictates the electron motion upon light-matter interaction is the basis for numerous well-established spectroscopies that reveal the electronic, magnetic and structural 29 properties of matter. In contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable [2]. Indeed, this amounts to transferring the phase of a classical electromagnetic wave, defined within several hundreds of nanometres, to a quantum particle localized within the few angstroms of an atom. In addition, the centre of symmetry of irradiated atoms does not in general coincide with the axis of the photon beam. In [3], the authors provided evidence of OAM-dependent absorption of light by a cold trapped atom, located in the centre of the light beam. Off-centre excitation was studied in [4]. Here we seek to observe an OAM-dependent dichroic photoelectric effect, using an extended sample of He atoms. Surprisingly, we find experimentally, and confirm theoretically, that the OAM of an optical field can be imprinted coherently onto a propagating electron wave, and that this phase information survives ensemble averaging out to macroscopic distances, where the electron is detected. We also show that electronic transitions, which are otherwise optically inaccessible due to selection rules, are essential for this process to occur. Our results reveal new aspects of light-matter interaction and point to a new kind of single-photon electron spectroscopy for accessing electronic optical transitions that are usually forbidden by symmetry. In our experiment, He atoms are ionized by XUV radiation, generated by a freeelectron laser (FEL) [5], in the presence of an intense infrared (IR) laser field, see Fig.

The extension of nonlinear optical techniques to the extreme-ultraviolet (EUV), soft and hard x-ray regime represents one of the open challenges of modern science since it would combine chemical specificity with background-free detection and ultrafast time resolution. We report on the first observation of a four-wave-mixing (FWM) response from solid-state samples stimulated exclusively by EUV pulses. The all-EUV FWM signal was generated by the diffraction of high-order harmonics of the FERMI free-electron laser (FEL) from the standing wave resulting from the interference of two crossed FEL pulses at the fundamental wavelength. From the intensity of the FWM signal, we are able to extract the first-ever estimate of an effective value of ∼6×10^{-24}   m^{2} V^{-2} for the third-order nonlinear susceptibility in the EUV regime. This proof of principle experiment represents a significant advance in the field of nonlinear optics and sets the starting point for a manifold of techniques, including frequency and phase-resolved FWM methods, that are unprecedented in this photon-energy regime.