David A. Reis

Femtosecond electronic response of atoms to ultra-intense X-rays

An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 1018 W cm−2, 1.5–0.6 nm, ∼105 X-ray photons per Å2). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse—by sequentially ejecting electrons—to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces ‘hollow’ atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

POSITRON PRODUCTION IN MULTIPHOTON LIGHT-BY-LIGHT SCATTERING

A signal of 106 14 positrons above background has been observed in collisions of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the rst laboratory evidence for inelastic light-by-light scattering involving only real photons. Submitted to Physical Review Letters Work supported by Department of Energy contract DE{AC03{76SF00515 and grants DE{FG02{ 91ER40671, DE{FG02{91ER40685 and DE{FG05{91ER40627. Present address: Hughes Leitz Optical Technologies Ltd., Midland, Ontario, Canada L4R 2H2. Present address: Lawrence Livermore National Laboratory, Livermore, CA 94551. also Department of Mechanical Engineering Present address: Panoramastrasse 8, 78589 Durbheim, Germany The production of an electron-positron pair in the collision of two real photons was rst considered by Breit and Wheeler [1] who calculated the cross section for the reaction !1 + !2 ! e e (1) to be of order r e , where re is the classical electron radius. While pair creation by real photons is believed to occur in astrophysical processes [2] it has not been observed in the laboratory up to the present. After the invention of the laser the prospect of intense laser beams led to reconsideration of the Breit-Wheeler process by Reiss [3] and others [4, 5]. Of course, for production of an electron-positron pair the center-of-mass (CM) energy of the scattering photons must be at least 2mc 1 MeV. While this precludes pair creation by a single electromagnetic wave, the necessary CM energy can be achieved by colliding a laser beam against a highenergy photon beam created, for example, by backscattering the laser beam o a high-energy electron beam. With laser light of wavelength 527 nm (energy 2.35 eV), a photon of energy 111 GeV would be required for reaction (1) to proceed. However, with an electron beam of energy 46.6 GeV as available at the Stanford Linear Accelerator Center (SLAC) the maximum Compton-backscattered photon energy from a 527-nm laser is only 29.2 GeV. In strong electromagnetic elds the interaction need not be limited to initial states with two photons [3], but rather the number of interacting photons becomes large as the dimensionless, invariant parameter = e q hA A i=mc 2 = eErms=m!0c = eErms 0=mc approaches or exceeds unity. Here the laser beam has laboratory frequency !0, reduced wavelength 0, root-mean-square electric eld Erms, and four-vector potential A ; e and m are the charge and mass of the electron, respectively, and c is the speed of light. For photons of wavelength 527 nm a value of = 1 corresponds to laboratory eld strength of Elab = 6 10 V/cm and intensity I = 10 W/cm. Such intensities are now practical in tabletop laser systems based on chirped-pulse ampli cation [6]. Then the multiphoton Breit-Wheeler reaction

X-ray and optical wave mixing

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.

Probing Impulsive Strain Propagation with X-Ray Pulses

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.

Time-resolved pump-probe experiments at the LCLS

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.

Solid-state harmonics beyond the atomic limit

Strong-field laser excitation of solids can produce extremely nonlinear electronic and optical behaviour. As recently demonstrated, this includes the generation of high harmonics extending into the vacuum-ultraviolet and extreme-ultraviolet regions of the electromagnetic spectrum. High harmonic generation is shown to occur fundamentally differently in solids and in dilute atomic gases. How the microscopic mechanisms in the solid and the gas differ remains a topic of intense debate. Here we report a direct comparison of high harmonic generation in the solid and gas phases of argon and krypton. Owing to the weak van der Waals interaction, rare (noble)-gas solids are a near-ideal medium in which to study the role of high density and periodicity in the generation process. We find that the high harmonic generation spectra from the rare-gas solids exhibit multiple plateaus extending well beyond the atomic limit of the corresponding gas-phase harmonics measured under similar conditions. The appearance of multiple plateaus indicates strong interband couplings involving multiple single-particle bands. We also compare the dependence of the solid and gas harmonic yield on laser ellipticity and find that they are similar, suggesting the importance of electron–hole recollision in these solids. This implies that gas-phase methods such as polarization gating for attosecond pulse generation and orbital tomography could be realized in solids.

Coherent control of pulsed X-ray beams

Synchrotrons produce continuous trains of closely spaced X-ray pulses. Application of such sources to the study of atomic-scale motion requires efficient modulation of these beams on timescales ranging from nanoseconds to femtoseconds. However, ultrafast X-ray modulators are not generally available. Here we report efficient subnanosecond coherent switching of synchrotron beams by using acoustic pulses in a crystal to modulate the anomalous low-loss transmission of X-ray pulses. The acoustic excitation transfers energy between two X-ray beams in a time shorter than the synchrotron pulse width of about 100 ps. Gigahertz modulation of the diffracted X-rays is also observed. We report different geometric arrangements, such as a switch based on the collision of two counter-propagating acoustic pulses: this doubles the X-ray modulation frequency, and also provides a means of observing a localized transient strain inside an opaque material. We expect that these techniques could be scaled to produce subpicosecond pulses, through laser-generated coherent optical phonon modulation of X-ray diffraction in crystals. Such ultrafast capabilities have been demonstrated thus far only in laser-generated X-ray sources, or through the use of X-ray streak cameras.

Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity.

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

Single-cycle Terahertz Pulses with >0.2 V/A Field Amplitudes via Coherent Transition Radiation

We demonstrate terahertz pulses with field amplitudes exceeding 0.2 V/A generated by coherent transition radiation. Femtosecond, relativistic electron bunches generated at the Linac Coherent Light Source are passed through a beryllium foil, and the emitted radiation is characterized as a function of the bunch duration and charge. Broadband pulses centered at a frequency of 10 THz with energies of 140 μJ are measured. These far-below-bandgap pulses drive a nonlinear optical response in a silicon photodiode, with which we perform nonlinear autocorrelations that yield information regarding the terahertz temporal profile. Simulations of the spatiotemporal profile agree well with experimental results.

Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate

The dynamics of an order parameters amplitude and phase determines the collective behaviour of novel states emerging in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of measuring material properties at atomic and electronic time scales out of equilibrium, can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here we combine time-resolved femotosecond optical and resonant X-ray diffraction measurements on charge ordered La(1.75)Sr(0.25)NiO(4) to reveal unforeseen photoinduced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, distinct from thermal phase fluctuations near the critical temperature in equilibrium. Importantly, relaxation of the phase fluctuations is found to be an order of magnitude slower than that of the order parameters amplitude fluctuations, and thus limits charge order recovery. This new aspect of phase fluctuations provides a more holistic view of the phases importance in ordering phenomena of quantum matter.