Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

We exploit time-and angle-resolved photoemission spectroscopy to determine the evolution of the out-ofequilibrium electronic structure of the topological insulator Bi2Se3. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modelling the dynamics of the hot electrons after optical excitation. We disentangle a large increase of the effective temperature (T * ) from a shift of the chemical potential (µ * ), which is consequence of the ultrafast photodoping of the conduction band. The relaxation dynamics of T * and µ * are k-independent and these two quantities uniquely define the evolution of the excited charge population. We observe that the energy dependence of the non-equilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.The recent discovery of topological insulators (TIs) is renewing the attention on the effects of spin orbit interactions (SOI) in solids, paving the road for the emergence of new quantum states of matter [1][2][3][4][5][6][7][8][9]. The SOI acquires particular relevance in the case of systems containing high-Z elements, leading to the lifting of the Kramers spin-degeneracy in broken inversion symmetry systems, as described by Rashba [10][11][12], Dresselhaus [13] and 15]. Therefore, understanding the consequence of SOI is of primary importance also for future technological applications in spintronics.TIs are band insulators (semiconductors) where the conduction and the valence band states have opposite parities and their energy ordering is inverted by the SOI [8,16]. The most prominent feature of their electronic structure is the odd number of spin polarized Dirac cones at the surface, connecting the opposite sides of the bulk band gap, resulting in topological protection from backscattering [7]. [5,19]) have been discovered. Among these, Bi 2 Se 3 represents a paradigmatic case, owing to the simplicity of its band structure characterized by a single Dirac cone [2,9].In topological insulators, the spin helicity of the metallic surface state offers the unique possibility to support spincurrent. Hence, spin-polarized charge distributions can be generated by circularly polarized light [20,21]. The topological protection of the linearly dispersing surface state is expected to strongly affect the scattering mechanisms of the Dirac particle, with respect to normal metallic states. In particular, the different couplings to optical and acoustic phonon modes have been recently studied [22] and the optical excitation of long-lived electron population in the surface state might play an important role in forthcoming opto-spintronics devices [22,23].In this paper we report on the study of the out-ofequilibrium electronic properties of Bi 2 Se 3 , investigated by time-and angle-resolved photoemission spectroscopy (tr-ARPES). Although conventional ARPES, with its surface sensitivity, has proven to be effective and rich of information [1,3,6], the combined use of ultrashort laser pulses and angleresolved p...

The behavior of liquid Au–Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 °C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid–solid–vapor phase line. In addition, the behavior of Au–Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation.

International audienceThe two single-pass, externally seeded free-electron lasers (FELs) of the FERMI user facility are designed around Apple-II-type undulators that can operate at arbitrary polarization in the vacuum ultraviolet-to-soft x-ray spectral range. Furthermore, within each FEL tuning range, any output wavelength and polarization can be set in less than a minute of routine operations. We report the first demonstration of the full output polarization capabilities of FERMI FEL-1 in a campaign of experiments where the wavelength and nominal polarization are set to a series of representative values, and the polarization of the emitted intense pulses is thoroughly characterized by three independent instruments and methods, expressly developed for the task. The measured radiation polarization is consistently >90% and is not significantly spoiled by the transport optics; differing, relative transport losses for horizontal and vertical polarization become more prominent at longer wavelengths and lead to a non-negligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other, allow a cross-calibration of the instruments, and constitute a benchmark for user experiments

The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electronphonon scattering at the surface. By comparing time-resolved ARPES measurements of the TI Bi2Se3 with different probing depths we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.The scientific and technological interest on topological insulators (TIs) stems from the unusual properties of their topologically protected metallic surface states, which exhibit a linear dispersion and a characteristic spin helicity [1][2][3][4][5][6][7]. For the Dirac quasiparticles elastic backscattering is forbidden by time-reversal symmetry, and transport is controlled by scattering events mediated by phonons. Attempts to measure the strength of the electron-phonon coupling in the representative TI Bi 2 Se 3 by angle-resolved photoelectron spectroscopy (ARPES) have produced somewhat conflicting results. The estimated values of the dimensionless coupling constant λ vary from small (λ ∼ 0.08) [8] to moderate (λ ∼ 0.25) [9]. Time-resolved ARPES (tr-ARPES) can tackle the problem in the time domain, complementary to the energy domain of conventional ARPES at equilibrium [10][11][12][13]. In pumpprobe tr-ARPES experiments, the electrons excited by a light pulse are described by an effective Fermi-Dirac (FD) distribution. The relaxation of the electronic temperature (T e ), as well as the variation of the chemical potential (µ) that reflects photo-doping of the conduction band, provide fundamental information on the de-excitation mechanisms, namely between the conduction band (CB) and the topologically protected surface state [12,13]. In a recent experiment on Bi 2 Se 3 , the contribution of various phonon modes to the electronic cooling has been addressed by comparing the relaxation dynamics of the FD distribution at various sample temperatures and for different charge densities [12].In this work we present a tr-ARPES investigation of the conduction band dynamics in Bi 2 Se 3 , where two different photon energies are exploited to vary the surface sensitivity. Standard tr-ARPES experiments, performed with laser-based sources at 6.2 eV photon energy, are rather bulk sensitive, due to the very low kinetic energy of the photo-electrons [14]. The comparison between more bulk sensitive (hν = 6.2 eV; UV) and more surface sensitive (hν = 17.5 eV; extreme UV, EUV) measurements reveals two different relaxation dynamics for T e in the conduction band. Namely, we observe a freezing of T e to an elevated value (∼ 600 K) at the surface but not in the bulk, suggesting a reduced efficiency of the phonons in the electronic cooling at the surface.A quantitative estimation of the photo-electron mean free path, l, as a function of the photo-electron kinetic energy is challenging. In particular, it is well established that at low kinetic energies ...

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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.

Using variable-temperature scanning tunneling microscopy, the degree of short-range order of purely two- dimensional (2D) binary alloys on triangular lattices [Sn(1-x)-Si-x, Pb(1-x)-Si-x, and Pb(1-x)-Ge-x, all showing the root3x3 R30 degrees reconstruction onto Si(111) or Ge(111)] has been determined quantitatively via a statistical analysis of the atomic positions. The experimental data, also in comparison with Monte Carlo simulations, generally indicate that an effective nearest-neighbor repulsion between substitutional Si (Ge) adatoms explains with good accuracy the short-range-order features observed. This finding implies the occurrence of an ordered phase of the alloys for x = 0.33 and, in the case of a 1:1 ratio of Sn(Pb) and Si(Ge) adatoms on the surface (x = 0.5), demonstrates that the investigated alloys are very good practical realizations of a frustrated antiferromagnetic 2D Ising system on a triangular lattice. The implications of the observed short-range order for the electronic properties of these alloys (as a function of x) are discussed