H.H.W. Chong

Generation of femtosecond x-ray pulses via laser-electron beam interaction

Abstract.The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300xa0fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.

Ultrafast X-ray science at the advanced light source

Our scientific understanding of the static or time-averaged structure of condensed matter on the atomic scale has been dramatically advanced by direct structural measurements using x-ray techniques and modern synchrotron sources. Of course the structure of condensed matter is not static, and to understanding the behavior of condensed matter at the most fundamental level requires structural measurements on the time scale on which atoms move. The evolution of condensed-matter structure, via the making and breaking of chemical bonds and the rearrangement of atoms, occurs on the fundamental time scale of a vibrational period, ~;100 fs. Atomic motion and structural dynamics on this time scale ultimately determine the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes. The integration of x-ray measurement techniques, a high-brightness femtosecond x-ray source, femtosecond lasers, and stroboscopic pump-probe techniques will provide the unique capability to address fundamental scientific questions in solid-state physics, chemistry, AMO physics, and biology involving structural dynamics. In this paper, we review recent work in ultrafast x-ray science at the ALS including time-resolved diffraction measurements and efforts to develop dedicated beamlines for femtosecond x-ray experiments.

Femtosecond x-rays from relativistic electrons: New tools for probing structural dynamics

Abstract Femtosecond X-ray science is a new frontier in ultrafast research in which time-resolved measurement techniques are applied with X-ray pulses to investigate structural dynamics at the atomic scale on the fundamental time scale of an atomic vibrational period (∼100xa0fs). This new research area depends critically on the development of suitable femtosecond X-ray sources with the appropriate flux (ph/(s·0.1%xa0BW)), brightness (ph/(s·mm 2 ·mrad 2 ·0.1%xa0BW)), and tunability for demanding optical/X-ray pump probe experiments. In this paper we review recently demonstrated techniques for generating femtosecond X-rays via interaction between femtosecond laser pulses and relativistic electron beams. We give an overview of a novel femtosecond X-ray source that is proposed based on a linear accelerator combined with X-ray pulse compression.

Low signal FEL gain: measurement, simulation and analysis

We describe an experimental set-up and measurement technique for measurement of the FEL gain at the Advanced Light Source. Measurement results are compared with computer simulations and an analytical expression that includes effects of the laser beam diffraction and finite electron beam size.

Phase transition in strongly-correlated VO2: Time-domain assignment of cause and effect

We establish time-domain hierarchy between structural andelectronic effects in the strongly correlated system VO2. Theinsulator-to-metal transition is driven directly by structural changerather than by electron-electron correlations.

Femtosecond near edge X-ray absorption measurement of the VO2phase transition

Femtosecond Near Edge X-ray Absorption Measurement of the VO 2 Phase Transition. A. Cavalleri 1 , H.H.W. Chong (1) , S. Fourmaux (3) , T.E. Glover (2) , P.A.Heimannn (2) , J.C. Kieffer (3) , H.A. Padmore (2) , R.W. Schoenlein (1) . Abstract: We measure the insulator-to-metal transition in VO 2 using femtosecond Near-Edge X-ray Absorption. Sliced pulses of synchrotron radiation are used to detect the photo-induced dynamics at the 516-eV Vanadium L 3 edge. Email: acavalleri@lbl.gov Time-resolved spectroscopy can probe new physical pathways of phase transitions and reveal fundamental aspects that are hidden in time-integrated measurements. However, limited quantitative information about electronic structure can be extracted from measurements at visible wavelengths, motivating interest in short- pulse x-ray spectroscopy. V 2p 3/2 400 K 300 K O 1s 400 K 300 K V 2p 3/2 V 2p 1/2 O 1s Energy (eV) Figure 1. Static, VO 2 x-ray absorption spectra in the region around the V L 2,3 and the O K edge. Spectral changes at the V Ledge and the O K edge observed above 340 K Near-edge X-ray absorption spectroscopy techniques probe unoccupied valence states by measuring transitions from core levels, rather than from extended occupied valence states as in visible spectroscopy. Element specificity, symmetry selection rules and linear/circular dichroic effects are some of its most powerful aspects. Due to the stringent tunability requirements on the source, time-resolved spectroscopy with soft x-rays is ideally performed using synchrotron radiation. Novel laser e-electron beam interaction schemes provide schemes to access the sub-picosecond time domain. Here, we report on picosecond and femtosecond soft x-ray absorption measurements of the insulator-to-metal transition in photo-

Generation of Femtosecond Synchrotron Pulses: Performance and Characterization

Femtosecond synchrotron pulses of 13 MeV relative to the nominal 1.9 GeV beam energy. Femtosecond pulses are effectively isolated from the long-pulse background using the transverse dispersion of the storage ring in combination with an x-ray imaging optic and a pair of slits to achieve a signal/background ratio of {approx}1.

Generation of femtosecond synchrotron pulses from the Advanced Light Source

This postdeadline paper was presented at the Conference on Lasers and Electro-Optics (CLEO), May 23-28, 1999, Baltimore, Maryland.(c) 1999 Optical Society of America.