S. Fourmaux

Band-Selective Measurements of Electron Dynamics in VO2 Using Femtosecond Near-Edge X-Ray Absorption

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from laser-sliced electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.

Giga-electronvolt electrons due to a transition from laser wakefield acceleration to plasma wakefield acceleration

We show through experiments that a transition from laser wakefield acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can drive electrons up to energies close to the GeV level. Initially, the acceleration mechanism is dominated by the bubble created by the laser in the nonlinear regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, leading to a depletion of the laser pulse, whose transverse ponderomotive force is not able to sustain the bubble anymore, the high energy dense bunch of electrons propagating inside bubble will drive its own wakefield by a PWFA regime. This wakefield will be able to trap and accelerate a population of electrons up to the GeV level during this second stage. Three dimensional particle-in-cell simulations support this analysis and confirm the scenario.

Erratum: Enhanced Photosusceptibility nearTcfor the Light-Induced Insulator-to-Metal Phase Transition in Vanadium Dioxide [Phys. Rev. Lett.99, 226401 (2007)]

D. J. Hilton, ∗ R. P. Prasankumar, S. Fourmaux, A. Cavalleri, D. Brassard, M. A. El Khakani, J. C. Kieffer, A. J. Taylor, and R. D. Averitt † Center for Integrated Nanotechnologies, MS K771, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Université du Québec, INRS-Énergie et Matériaux et Télécommunications, Varennes, Québec, Canada, J3X 1S2 Department of Physics, Clarendon Laboratory, University of Oxford, Parks Rd. Oxford, OX1 3PU, United Kingdom (Dated: February 1, 2008)

Hard x-ray emission from intense femtosecond laser-solid target interactions

Hard x-ray (8-100 keV) spectrum emission from plasma produced by femtosecond laser solid target interactions and Kα x-ray conversion efficiency have been studied as a function of laser intensity (1017 W/cm2 ~ 1019 W/cm2), pulse duration (70 fs ~ 400 fs), laser pulse fluence and laser wavelength (800 nm and 400 nm). The Ag Kα x-ray conversion efficiency produced by a laser pulse at 800 nm with an intensity I = 4x1018 W/cm2 can reach 2x10-5. We discuss the behavior of Kα conversion efficiency scaling laws as a function of the laser parameters. We found that the Kα x-ray conversion efficiency is more dependent on laser fluence than on pulse duration or laser pulse intensity. The conversion efficiency exhibits a similar value at I ~ 1x1018 W/cm2 when we work with a high contrast laser pulse at 400 nm or with a low contrast laser pulse at 800 nm, but in the first case it presents a higher scaling law. Consequently, the use of 400 nm laser pulses could be an effective method to optimize the Kα x-ray emission via vacuum heating mechanisms.

Generation of GeV energy electrons from laser wakefield acceleration via ionization induced injection

Summary form only given. Laser wakefield acceleration (LWFA) is a promising approach to realize table-top accelerators. The injection process into the wakefield bubble to some extent determines the charge, divergence, energy gain as well as the energy distribution of the accelerated electrons. Traditionally, self injection using pure helium or hydrogen gas as the interaction medium was employed to accelerate the electrons. However, very high laser powers are required to achieve self injection at the low densities which are compatible with acceleration to GeV energies. Recently, a new technique, ionization induced injection, which takes advantage of the large ionization potential difference between the inner and outer shell electrons of trace atoms in the plasma, has been demonstrated to generate electron beams beyond 1 GeV at lower threshold laser powers than self injection would require [1].

Time resolved conductivity dynamics in vanadium dioxide

Optical-pump terahertz-probe spectroscopy is used to investigate the photoinduced insulator-to-metal phase transition in vanadium dioxide. The observed dynamics are consistent with a spatially inhomogeneous insulating state.