Search

X-ray absorption spectroscopy (XAS) is a versatile technique for studying the electronic structure and/or the local geometry of matter, and time-resolved XAS has been developed into a powerful tool for studying the dynamics of materials [1, 2]. The temporal resolution of this technique is usually determined by the X-ray pulse duration in pump and probe geometries. Although this technique has been developed in the synchrotron environment and takes advantage of existing X-ray beamlines, the study of ultrafast dynamics on time scales on the order of a picosecond or faster is limited because typical pulse durations from third-generation light sources is on the order of tens of picoseconds. In addition, XAS typically requires the integration of a large number of X-ray pulses to get quality data. These features make it difficult to apply this technique to research areas in which samples undergo irreversible processes.

Ultrafast X-ray science is an exciting frontier that promises the visualization of electronic, atomic and molecular dynamics on atomic time and length scales. A largely unexplored area of ultrafast X-ray science is the use of light to control how X-rays interact with matter. To extend control concepts established for long-wavelength probes to the X-ray regime, the optical control field must drive a coherent electronic response on a timescale comparable to femtosecond core-hole lifetimes. An intense field is required to achieve this rapid response. Here, an intense optical control pulse is observed to efficiently modulate photoelectric absorption for X-rays and to create an ultrafast transparency window. We demonstrate an application of X-ray transparency relevant to ultrafast X-ray sources: an all-photonic temporal cross-correlation measurement of a femtosecond X-ray pulse. The ability to control X-ray–matter interactions with light will create new opportunities for present and next-generation X-ray light sources. Intense optical beams can alter the way that a material interacts with X-ray radiation. This is now demonstrated by experiments that use femtosecond laser pulses to affect inner-shell processes in neon atoms, increasing the transmission of X-rays. This could allow imprinting of optical pulse trains onto much longer X-ray pulses.

Laser pump and X-ray probe core-level photoemission experiments probe surface photovoltage transients on p-type Si(111) surfaces. The data are consistent with a picture where the dynamics of mobile surface charge dominate the photovoltage shift, with changes in the surface-states charge density of only secondary importance. A value for the equilibrium band bending is determined, which suggests that a residual oxide layer reduces the density of surface states.

The impulsive superheating of matter by an intense, ultrashort laser pulse drives material expansion into vacuum (ablation) and an associated formation of nanoparticles. The underlying dynamics of particle formation are complex and direct experimental probes of the rapid material evolution are essential. Femtosecond lasers coupled to modern synchrotrons offer an important new opportunity to probe ejecta dynamics on an atomic lengthscale. Here, the impulsive heating of a semiconductor (silicon) by an intense femtosecond laser pulse leads to material ejection and time-resolved photoemission spectroscopy probes rapid solidification kinetics occurring within the ejecta. Transient photoemission peak-shifts indicate that material is ejected predominantly as liquid droplets and that solidification occurs rapidly (

The formation of particles in a laser ablation plume is a complex process requiring direct local probes. Here, core-level photoemission spectroscopy is extended to the picosecond time domain to probe local expansion dynamics and associated transient chemical properties as a highly pressurized, metallic fluid expands into vacuum following impulsive heating of a semiconductor by an intense, ultrashort laser pulse. These experiments probe constituents species and solidification kinetics occurring in the early moments of material ejection and provide insight into how particles arise in the current laser ablation regime. Photoemission peak shifts reveal rapid metal–insulator phase transitions and support a view of fragmentation over vapor-condensation as the relevant mechanism determining the initial size-distribution of particles.

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

The first use of pseudo-single-bunch pulse picking in time-resolved experiments at the Advanced Light Source is reported. The pseudo-single-bunch technique improves signal to noise and drastically reduces dose-induced sample damage., Laser pump–X-ray probe experiments require control over the X-ray pulse pattern and timing. Here, the first use of pseudo-single-bunch mode at the Advanced Light Source in picosecond time-resolved X-ray absorption experiments on solutions and solids is reported. In this mode the X-ray repetition rate is fully adjustable from single shot to 500 kHz, allowing it to be matched to typical laser excitation pulse rates. Suppressing undesired X-ray pulses considerably reduces detector noise and improves signal to noise in time-resolved experiments. In addition, dose-induced sample damage is considerably reduced, easing experimental setup and allowing the investigation of less robust samples. Single-shot X-ray exposures of a streak camera detector using a conventional non-gated charge-coupled device (CCD) camera are also demonstrated.

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 (∼100 fs). This new research area depends critically on the development of suitable femtosecond X-ray sources with the appropriate flux (ph/(s·0.1% BW)), brightness (ph/(s·mm 2 ·mrad 2 ·0.1% BW)), 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.

Coherent acoustic phonons have been observed in the X-ray diffraction of a laser-excited InSb crystal. Modeling based on time-dependent dynamical diffraction theory has allowed the extraction of fundamental constants, such as the electron-acoustic phonon coupling time. A dedicated beamline for time-resolved studies has been developed at the Advanced Light Source with special considerations toward high transmission, low scattering and a wide photon energy range. The facility combines a bend magnet beamline, time-resolved detectors and a femtosecond laser system.

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 300 fs 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 structural dynamics in InSb are studied using time-resolved x-ray diffraction. A delayed onset of significant lattice expansion and an indication of subpicosecond disordering are observed.

The interaction of relativistic electrons with terawatt femtosecond laser pulses is reviewed with emphasis on how the scattering process can be used as a source of ultrashort X-ray pulses and as a time-resolved diagnostic for measuring transverse and longitudinal phase space distributions of an electron beam with microns spatial and subpicosecond temporal resolution. Recent experimental results are presented in which 90/spl deg/ Thomson scattering was utilized to generate 300 fs duration pulses of X-rays with peak energy of 30 keV. It is shown that, through analysis of the scattered X-ray beam properties, transverse and longitudinal distributions of the electron beam can be obtained.

We report the first observation of laser-induced free-free transitions in the primary photoelectron spectra of gaseous helium ionized by ultrashort soft x-ray pulses. Measured transition amplitudes are well described by projecting the initial electronic state onto a Volkov wavefunction. Additionally, we report the first direct measurements on the temporal duration of femtosecond high order harmonic radiation. The harmonic pulse duration is observed to exhibit strong dependencies upon both laser intensity and position of the generating medium relative to the laser focus.

We report observation of x-ray and optical sum frequency generation. An ultrafast optical pulse drives charge oscillations to the chemical bonds in diamond. A co-propagating x-ray pulse probes the accompanying atomic-scale chemical bond distortion.

International audience; Light-matter interactions are ubiquitous, and underpin a wide range of basic research fields and applied technologies. Although optical interactions have been intensively studied, their microscopic details are often poorly understood and have so far not been directly measurable. X-ray and optical wave mixing was proposed nearly half a century ago as an atomic-scale probe of optical interactions but has not yet been observed owing to a lack of sufficiently intense X-ray sources. Here we use an X-ray laser to demonstrate X-ray and optical sum-frequency generation. The underlying nonlinearity is a reciprocal-space probe of the optically induced charges and associated microscopic fields that arise in an illuminated material. To within the experimental errors, the measured efficiency is consistent with first-principles calculations of microscopic optical polarization in diamond. The ability to probe optical interactions on the atomic scale offers new opportunities in both basic and applied areas of science.

We report the first observation of x-ray/optical sum frequency generation. An ultrafast optical pulse drives charge oscillations to the chemical bonds in diamond. A co-propagating x-ray pulse micro-probes the accompanying chemical bond distortion.

We report results on experiments using combined laser and synchrotron radiation. Picosecond laser pulses at 800 nm are used to induce surface photovoltage transients in p-type Si samples. A two-component decay is observed. The fast component of decay provides a direct measure of synchrotron soft X-ray pulse durations.

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.

Pulses of x-rays 300 femtoseconds in duration at a wavelength of 0.4 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly relativistic electrons from an accelerator. In the right-angle scattering geometry, the duration of the x-ray burst is determined by the transit time of the laser pulse across the ∼ 90-micrometer waist of the focused electron beam. The x-rays are highly directed (∼ 0.6° divergence) and can be tuned in energy. This source of femtosecond x-rays will make it possible to combine x-ray techniques with ultrafast time resolution to investigate structural dynamics in condensed matter.

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 similar to1.

The photo-induced insulator-to-metal transition in strongly-correlated VO2 is studied with picosecond near edge X-ray absorption spectroscopy. Progress toward femtosecond measurements using sliced pulses of synchrotron radiation will be reported

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.

Core-level photoemission spectroscopy provides a local probe of expansion dynamics and associated transient chemical properties as a highly pressurized, metallic fluid expands into vacuum following impulsive heating of a semiconductor by an intense, ultrashort laser pulse. Transient photoemission peak shifts reveal that metal-insulator transitions occur rapidly following laser heating. These experiments probe constituents species and solidification kinetics occurring in the early moments of material ejection and provide insight into how particles arise in the current laser ablation regime.

The advent of femtosecond lasers has permitted the study of ultrafast dynamics in condensed matter, mainly by observing changes in optical properties. To study ultrafast structural dynamics in InSb under high-intensity laser excitation, we performed femtosecond time-resolved X-ray diffraction using femtosecond X-ray pulses generated via right-angle Thomson scattering.

We demonstrate a novel scheme for generating ultrashort pulses of synchrotron radiation. Our approach is to create femtosecond time-structure on a long electron bunch by using a femtosecond laser pulse to modulate the energy of an ultrashort slice of the bunch Femtosecond synchrotron pulses are directly measured from a bend-magnet beamline at the Advanced Light Source (ALS). The measured pulse duration is determined by the storage ring dispersion integrated from the wiggler to the radiation bend-magnet. A new beamline, dedicated to ultrafast X-ray spectroscopy is currently under construction at the ALS and will provide X-ray pulses of

Progress in ultrafast xray science demands a new generation of high time-resolution x-ray detectors. We discuss an xray and laser cross-correlation technique which could serve as a basis for a femtosecond xray detector. The cross- correlation technique is based on visible-laser-induced modifications to x-ray photoelectron spectra.

We report, to our knowledge, the first use of pump-probe photoelectron spectroscopy to study core-level dynamics during a laser-driven phase transition. Synchrotron xrays probe the phase transition dynamics during the initial moments of femtosecond laser ablation.

Femtosecond synchrotron pulses were generated directly from an electron storage ring. An ultrashort laser pulse was used to modulate the energy of electrons within a 100-femtosecond slice of the stored 30-picosecond electron bunch. The energy-modulated electrons were spatially separated from the long bunch and used to generate approximately 300-femtosecond synchrotron pulses at a bend-magnet beamline, with a spectral range from infrared to x-ray wavelengths. The same technique can be used to generate approximately 100-femtosecond x-ray pulses of substantially higher flux and brightness with an undulator. Such synchrotron-based femtosecond x-ray sources offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.

Time-resolved x-ray diffraction with picosecond time resolution is used to observe scattering from coherent acoustic phonons in laser-excited InSb crystals. The observed oscillations in the crystal reflectivity are in agreement with a model based on dynamical diffraction theory. Synchrotron radiation pulses of ∼300 fs in duration have been generated by femtosecond laser pulses modulating the electron beam in the Advanced Light Source.

Measurement of femtosecond high harmonic radiation indicates that significant pulse shortening can be accomplished by limiting the transverse intensity variation of the generating laser pulse.

We report the generation of femtosecond synchrotron radiation pulses from the Advanced Light Source by using a femtosecond laser to accelerate an ultrashort slice of the electron beam. This is a new high-brightness source for ultrafast X-ray spectroscopy.

Ultrashort x-ray pulses are powerful diagnostic tools, combining the temporal resolution typically reserved for optical (laser) pulses with atomic scale spatial resolution which, in principle, can be achieved using high energy electromagnetic radiation. Progress in the development of femtosecond x-ray sources has been hindered by an inability to measure x-ray pulses of femtosecond duration. Streak cameras are typically used to measure ultrashort x-ray pulses, however the ~1 ps resolution limit of these instruments abates their usefulness in the femtosecond regime. We report use of a visible/x-ray cross correlation technique, based on laser assisted x-ray photoionization, to measure 50 fs soft x-ray pulses. The technique permits direct measurements on the duration of femtosecond high order harmonic radiation. We find that harmonic pulse durations are determined by the spatial and temporal profile of the generating laser pulse.

Ablation driven by intense, femtosecond laser pulses offers a novel route to fabrication of nanometer-sized particles. I model particle formation by considering the hydrodynamics of material expansion into vacuum. Modeling reveals rapid material dilution and cooling. Vacuum expansion is found to quench the ejected material 1–3 orders of magnitude more efficiently than thermal conduction quenches the residual bulk surface. Efficient quenching implies that solid-phase particles are produced rapidly (in ≪1 ns) following laser excitation; this may allow unique material states to be frozen within the ejected particles. Finally, the mean particle size is estimated to range from ∼1 to ∼10 nm for initial lattice temperatures ranging from 0.3 to 10 eV.

One of the primary purposes of this study was to examine possible differences between students identified as gifted and other students after they experienced an educational program focusing on alternative futures. A pressing—if not overriding—concern of education at this time of rapid change and mounting stress must be how education can be of most value to individuals who will live in a world which they, and we, are largely unable to imagine. The increasing strain on individuals, institutions, and societies is unsettling evidence that we may indeed be headed for what John Platt has termed “a crisis of crises all at one time” (1974). Even without such a pessimistic premise, it is not unreasonable to consider that technological advances and socio-economic-political changes will continue to shape our world in numerous and varied ways. With this hypothesis, educators must, if they are to serve their clientele, re-examine their specific goals with regard to developing in individuals skills and personal qualities most appropriate for coping with uncertainty and taking action. In this respect, this project sought, not to identify and develop all those abilities and qualities, but instead to focus on a selected few which the authors assumed to be missing in traditional educational endeavors. This study, then, documents the authors' efforts to begin to develop in a group of adolescent youth a certain set of abilities/qualities, identified as a futures perspective, self-actualization, and futuring competencies. As such, the study serves as both a quasi-research study and a rigorous and comprehensive curriculum evaluation. The authors believe that the project results have important implications in both areas.