E. Papalazarou

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

The advent of Dirac materials has made it possible to realize two-dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity-the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses

We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate)--PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7 nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32 nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOHs tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface ardening making the beam profile measurement infeasible.

Full characterization and optimization of a femtosecond ultraviolet laser source for time and angle-resolved photoemission on solid surfaces

A novel experimental apparatus for time and angle-resolved photoemission on solid surfaces is presented. A 6.28 eV laser source operating at 250 kHz repetition rate is obtained by frequency mixing in nonlinear beta barium borate crystals. This UV light source has a high photon flux of 10(13) photons/s with relatively low number of photons/pulse so that Fermi surface mapping over a wide region of the Brillouin zone is possible while mitigating space charge effects. The UV source has been fully characterized spatially, spectrally, and temporally. Its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth. True femtosecond time resolution <65 fs is obtained while the energy resolution of 70 meV appears to be mainly limited by the laser bandwidth.

High-order harmonic wave fronts generated with controlled astigmatic infrared laser

We report an experimental study on the control of the high-order harmonic wave front through the adjustment of the wave front of the fundamental infrared laser. We have applied different levels of astigmatism to the IR beam via the use of a deformable mirror placed before the lens, and demonstrated that there exists a correlation between the IR and the high harmonic wave fronts.

Probing coherently excited optical phonons by extreme ultraviolet radiation with femtosecond time resolution

We present a pump-probe experimental approach to study time-resolve coherent optical phonons using light pulses in the extreme ultraviolet wavelength range with femtosecond time resolution. Using this technique, a Bi (111)-oriented single crystal was excited by near-infrared (1.55eV) pulses and probed by a high-order harmonic generation source, whose intrinsic flux instability was reduced by making use of a normalization procedure. This unconventional approach allowed us to perform measurements in a previously inaccessible range of phonon wavevectors within the Brillouin zone.

Electron-hole balanced dynamics in the type-II Weyl semimetal candidate WTe2

M. Caputo, ∗ L. Khalil, 2 E. Papalazarou, N. Nilforoushan, L. Perfetti, Q. D. Gibson, R. J. Cava, and M. Marsi Laboratoire de Physique des Solides, CNRS, Universitè Paris-Sud, Universitè Paris-Saclay, 91405 Orsay Cedex, France UR1-CNRS/Synchrotron SOLEIL, Saint Aubin BP 48, Gif-sur-Yvette F-91192, France Laboratoire des Solides Irradiés, Ecole Polytechnique, CNRS, CEA, Universitè Paris-Saclay, 91128 Palaiseau Cedex, France Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA (Dated: November 15, 2017)

Time-resolved photoemission ofSr2IrO4

We investigate the temporal evolution of electronic states in strontium iridate

Optimization of high harmonic generation by genetic algorithm

{\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}

Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control

. The time-resolved photoemission spectra of the intrinsic, electron-doped, and hole-doped samples are monitored in identical experimental conditions. Our data on the intrinsic and electron-doped samples, show that doublon-holon pairs relax near to the chemical potential on a time scale shorter than 70 fs. The subsequent cooling of low-energy excitations takes place in two steps: a rapid dynamics of

Dynamics of out-of-equilibrium electron and hole pockets in the type-II Weyl semimetal candidate WTe2

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