Gustavo Moriena

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron–lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS2. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron–phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Terahertz-driven linear electron acceleration

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeV m−1 gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Full characterization of RF compressed femtosecond electron pulses using ponderomotive scattering

High bunch charge, femtosecond, electron pulses were generated using a 95 kV electron gun with an S-band RF rebunching cavity. Laser ponderomotive scattering in a counter-propagating beam geometry is shown to provide high sensitivity with the prerequisite spatial and temporal resolution to fully characterize, in situ, both the temporal profile of the electron pulses and RF time timing jitter. With the current beam parameters, we determined a temporal Instrument Response Function (IRF) of 430 fs FWHM. The overall performance of our system is illustrated through the high-quality diffraction data obtained for the measurement of the electron-phonon relaxation dynamics for Si (001).

Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ~1 J cm(-2) in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play.

Direct longitudinal laser acceleration of electrons in free space

Compact laser-driven accelerators are versatile and powerful tools of unarguable relevance on societal grounds for the diverse purposes of science, health, security, and technology because they bring enormous practicality to state-of-the-art achievements of conventional radio-frequency accelerators. Current benchmarking laser-based technologies rely on a medium to assist the light-matter interaction, which impose material limitations or strongly inhomogeneous fields. The advent of few cycle ultra-intense radially polarized lasers has materialized an extensively studied novel accelerator that adopts the simplest form of laser acceleration and is unique in requiring no medium to achieve strong longitudinal energy transfer directly from laser to particle. Here we present the first observation of direct longitudinal laser acceleration of non-relativistic electrons that undergo highly-directional multi-GeV/m accelerating gradients. This demonstration opens a new frontier for direct laser-driven particle acceleration capable of creating well collimated and relativistic attosecond electron bunches and x-ray pulses.

Femtosecond electron diffraction: Preparation and characterization of (110)-oriented bismuth films

Here, we present a new approach to synthesize (110)-oriented ultrathin membranes of bismuth (Bi). This rather exotic orientation was achieved by directing the growth through rationale control of lattice matching. Bi films were hetero-epitaxially grown on the (100)-surface of freshly cleaved potassium chloride crystals. The sample orientation was characterized by x-ray and electron diffraction. In addition, high quality free-standing films were obtained after dissolution of the substrate in water and controlled evaporation. Femtosecond electron diffraction (FED) was, therefore, used to monitor the coherent shear acoustic phonons in (110)-oriented free-standing Bi films produced by impulsive femtosecond optical excitation. The small de Broglie wavelength (flat Ewald sphere) of keV-electrons combined with an off-Bragg detection scheme provided a magnified view of shear atomic motions, i.e., lattice distortions in the transverse direction. All-optical pump-probe experiments are usually insensitive to shear di...

Structural Dynamics upon Photoexcitation in a Spin Crossover Crystal Probed with Femtosecond Electron Diffraction

Photoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single-crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time-dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high-spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal-ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.

REGAE: New Source for Atomically Resolved Dynamics

In this paper, we show the design and theoretical calculation of our new femtosecond electron source based on rf-accelerator generating 2-5 MeV electron bunches with high electron density and high coherence length.

Ultrabright Femtosecond Electron Sources: Ultrafast Structural Dynamics in Labile Organic Crystals

1. Max Planck Institute for the Structure and Dynamics of Matter and, Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany. 2. Departments of Chemistry and Physics, 80 St. George Street, University of Toronto, Toronto, Ontario, M5S 3H6, Canada. 3. Interactive Research Center of Science, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 4. PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012, Japan. 5. Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ōokayama, Meguroku, Tokyo 152-8551, Japan. 6. CREST, Japan Science and Technology Agency (JST), 5-3, Yonbancho, Chiyoda-ku, Tokyo 1028666, Japan. 7. Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. 8. Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501 Tempaku-ku, Nagoya 468-8502, Japan. *. Present address: Department of Chemistry, 200 University Ave. W, University of Waterloo, Ontario, N2L 3G1, Canada.