Aaron M. Lindenberg Stanford University
David Reis PULSE Institute
Paul Fuoss Argonne National Laboratory
Thomas Tschentscher Deutsches Elektronen-Synchrotron DESY
Bradley Siwick McGill University
MM1: Short Wavelength Sources
Monday AM, November 30, 2009
Room 201 (Hynes)
9:00 AM - **MM1.1
Ultrafast X-ray Imaging with Free-Electron Lasers.
Henry Chapman 1 2 Show Abstract
1 Centre for Free-Electron Laser Science, DESY, Hamburg Germany, 2 , Universität Hamburg, Hamburg Germany
The ultrafast pulses from future X-ray free-electron lasers may enable imaging of non-periodic objects at near-atomic resolution. These objects could include single macromolecules, protein complexes, or virus particles, and the method will be particularly valuable to determine the structures of proteins that cannot be crystallized. The specimen would be completely destroyed by the pulse, but that destruction will only happen after the termination of the pulse. To address the many challenges that we face in attempting molecular diffraction, we have been developing experimental methods at the FLASH free-electron laser at DESY in Hamburg. We have reconstructed images from single-pulse ultrafast diffraction patterns. We also performed quantitative measurements of the explosion of test particles in the focused FEL pulse, by recording their diffraction patterns. No motion occurred during the pulse and we followed the evolution of the explosion with a novel holographic time-resolved technique. Our results confirm the basic principles of flash imaging and lend great confidence to achieving molecular imaging at future short-wavelength X-ray FELs.
9:30 AM - MM1.2
Collinear Generation of Few-cycle UV and XUV Laser Pulses for Probing and Controlling Ultrafast Electron Dynamics at Solid Interfaces.
Agustin Schiffrin 1 , Elisabeth Botschafter 1 , Ralph Ernstorfer 2 1 , Adrian Cavalieri 1 , Markus Fiess 1 , Ulrich Graf 1 , Eleftherios Goulielmakis 1 , Giulio Alighieri 1 , Reinhard Kienberger 2 1 , Ferenc Krausz 1 3 Show Abstract
1 , Max Planck Institute of Quantum Optics, Garching Germany, 2 Physics, Technische Universität München, Garching Germany, 3 Physics, Ludwig-Maximilians-Universität, Garching Germany
The generation of isolated attosecond extreme ultraviolet (XUV) coherent light pulses by means of high-harmonic generation in noble gases with few-cycle near-infrared (NIR) laser fields has been established in recent years . Lately, the generation of few-cycle low-order harmonics has been demonstrated: sub-4 fs ultraviolet (UV) pulses were produced by third and fifth harmonic generation of few-cycle NIR laser pulses in a noble gas target . Here, we present the experimental implementation of the simultaneous generation of low-order harmonics and high-harmonics by two subsequent gas targets in a collinear geometry. Such achievement will enable attosecond pump-probe spectroscopy with any combination of XUV, UV, visible and NIR few-cycle pulses. Future experiments employing this experimental configuration with such unique laser pulses include probing ultrafast intraband electron dynamics in semiconductors, time-resolving ultrafast electron transfer in organic/condensed matter interfaces, and controlling electronic motion in metal and semiconductor nanostructures with coherent optical fields. E. Goulielmakis, et al., Science 320, 1614 (2008).  U. Graf, et al., Optics Express 16, 18956 (2008).
9:45 AM - **MM1.3
Ultracold Electron Source for Single-shot, Ultrafast Electron Diffraction.
Jom Luiten 1 Show Abstract
1 , Eindhoven University of Technology, Eindhoven Netherlands
The combination of femtosecond laser photoemission and recently developed radiofrequency (RF) bunch compression techniques enables single-shot electron diffraction studies of structural dynamics at atomic length and time scales, i.e. 0.1 nm and 0.1 ps. At present ultrafast electron diffraction (UED) experiments are based on photoemission from solid state cathodes. These photoemission sources perform excellently, but are not sufficiently bright for single-shot studies of crystals with large (> few nm) lattice constants, e.g., biomolecular samples. Recently we have developed a new type of electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas. The electron temperature of this source can be as low as 10 K, implying an increase in brightness by orders of magnitude. Currently we are developing a setup which combines the ultracold electron source with RF acceleration and bunch compression techniques. In this talk I will discuss particle tracking simulations which show that in this way electron bunches can be produced of sufficient quality for sub-100 fs, single-shot studies of biomolecular samples. I will report on recent RF bunch compression experiments and the first single-shot diffraction measurements. I will present new results on the generation and characterization of high-charge, ultracold electron bunches.
10:15 AM - **MM1.4
RF Photoinjector Based Ultrafast Electron Diffraction.
Pietro Musumeci 1 Show Abstract
1 Physics and Astronomy, UCLA, Los Angeles, California, United States
Electron diffraction holds the promise to yield real time resolution of atomic motion in a easily accessible environment like a university laboratory at a fraction of the cost than 4th generation x-ray sources. Currently the limit in time-resolution for conventional electron diffraction is set by how short an electron pulse can be made. A possible solution to maintain the highest possible beam intensity without excessive pulse broadening from space charge effects is to increase the electron energy to the MeV level where relativistic effects significantly reduce the space charge forces. Rf photoinjectors can in principle deliver up to 10^7 -10^8 electrons packed in bunches of ~100 fs length allowing an unprecedented time resolution and enabling the study of irreversible phenomena by single shot diffraction patterns. The UCLA Pegasus laboratory has recently demonstrated time resolved single shot electron diffraction using a 100 fs long relativistic beam from an rf photoinjector. The results of this experiment and the future directions of this technique will be discussed.
10:45 AM - MM1.5
Electron Bunch Compression for Single Shot 100 fs Electron Diffraction.
Thijs van Oudheusden 1 , Brad Siwick 2 , Jom Luiten 1 Show Abstract
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands, 2 Departments of Physics and Chemistry, McGill University, Montreal, Quebec, Canada
We present the status of our set-up for producing sub-100 fs electron bunches that are suitable for single shot ultrafast electron diffraction experiments in the 100 keV energy range. Contrary to other approaches that aim for avoiding the space charge dominated regime our idea is to make use of a space charge driven expansion to produce electron bunches that have linear velocity-position-correlations. Such bunches can be realized by femtosecond photoemission with a transversely shaped laser pulse. Because of the linear phase space distributions the expansion can be reversed completely. Transverse compression is accomplished by regular solenoid lenses. Longitudinal compression can be accomplished by using a radio-frequency (RF) field to invert the longitudinal velocity-position correlation in such a way that electrons at the back of the bunch have higher velocities than those at the head. This leads to ballistic compression of the bunch in the subsequent drift space. Therefore we have developed a 3 GHz RF cavity oscillating in the TM010 mode. In this mode there is an on-axis longitudinal electric field. The phase of the RF field is synchronized to the photoemission laser pulse with an accuracy better than 50 fs. We will show our first results of tenfold longitudinal compression of a 100 keV electron bunch that contains ~106 electrons.Besides the bunch length we are examining other important bunch properties as well, like the transverse coherence length. This quantity is fundamentally limited by the photoemission process that gives rise to a thermal velocity spread. Bunches with a charge of 0.1 – 1 pC are routinely realized with our source and their coherence length is close to the thermal limit.Our goal is to create 100 keV, 0.1 pC, sub-100 fs electron bunches with a spotsize smaller than 500 micron and a transverse coherence length of several nanometers . The expected relative energy spread is smaller than 1%. We will report on the experimental progress of electron diffraction experiments on gold nanoparticles test samples to demonstrate the applicability of our compressed bunches for single shot ultrafast electron diffraction. T. van Oudheusden, B.J. Siwick, E.F. de Jong, S.B. van der Geer, W.P.E.M. Op ’t Root, and O.J. Luiten, J. Appl. Phys. 102, 093501 (2007).
MM2: Dynamics of Nanomaterials
Monday AM, November 30, 2009
Room 201 (Hynes)
11:30 AM - MM2.1
Multipath Generation of Coherent Phonons in Quantum Dots.
Patanjali Kambhampati 1 Show Abstract
1 Chemistry, McGill University, Montreal, Quebec, Canada
We report on coherent optical and acoustic phonons in colloidal CdSe quantum dots. The role of quantum confinement upon exciton-phonon coupling is of primary importance to our understanding of their electronic structure. Yet most experiments and theories on this subject are widely conflicting. We recently reported on the first simultaneous observation of coherent optical and acoustic phonons in quantum dots, thereby offering the first quantitative test of theory in nearly two decades. These experiments reconcile prior disagreements by virtue of showing the importance of excitonic eigenstate upon the strength of exciton-phonon coupling. We find that the Frohlich coupling to optical phonons is strongly dependent upon the excitonic eigenstate in contrast to the deformation potential coupling to acoustic phonons. Finally, we show how the femtosecond time domain measurements relate to frequency domain measurements of coupling via resonance Raman and single dot photoluminescence. The excitonic state-dependence of exciton-phonon coupling is described in terms of the standard displaced harmonic oscillator picture which is commonly used in quantized systems, e.g. molecules. An important contrast between quantum dots and molecules is that quantum dots can easily support multiple excitons. At very high exciton density one may expect to create an excitonic plasma which is more akin to the dynamics of metals, metal nanoparticles, and bulk semiconductors. In this regime, the displaced harmonic oscillator approach is generally not used. We explore the interpolation between quantized systems and continuum systems by creating high densities of excitons in these quantum dots. Upon creation of high densities of excitons, we show for the first time quantization of piezoelectricity in quantum dots. One may anticipate that rapid cooling of the excitonic plasma may impulsively heat the lattice thereby launching coherent acoustic phonons. We show that this mechanism is minor. In contrast, we show that the high exciton density creates rapid polarization which yields an impulsive piezoelectric response thereby launching large amplitude coherent acoustic phonons. This piezoelectric response is unique to quantum dots in that it is an extrinsic effect and furthermore it is quantized.  “State-resolved exciton-phonon couplings in CdSe semiconductor quantum dots”, D.M. Sagar, R.R. Cooney, S.L. Sewall,, and P. Kambhampati, J. Phys. Chem. C., 112, 9124 (2008) - Letter. “Size dependent, state-resolved studies of exciton-phonon couplings in strongly confined semiconductor quantum dots”, D. M. Sagar, Ryan R. Cooney, Samuel L. Sewall, Eva A. Dias, Mirela M. Barsan, Ian S. Butler, and Patanjali Kambhampati, Phys. Rev. B., 77, 235321 (2008). “Quantizing piezoelectricity in semiconductor quantum dots”, P. Tyagi, R.R. Cooney, S.L. Sewall, D.M. Sagar, and P. Kambhampati, To be submitted (2009).
11:45 AM - MM2.2
Magnetization Dynamics in Superparamagnetic CoxFe3-xO4 Nanocrystals.
Dong Hee Son 1 , Tai-Yen Chen 1 , Chih-Hao Hsia 1 Show Abstract
1 Chemistry, Texas A&M University, College Station, Texas, United States
We investigated spin-lattice relaxation dynamics in photoexcited colloidal superparamagnetic CoxFe3-xO4 nanocrystals as a function of particle size (5-15nm), stoichiometry (0< x < 0.9) and interparticle distance using pump-probe Faraday rotation technique. The time scale of the magnetization recovery following optically induced demagnetization was correlated with the variation of the above parameters that change average spin-orbit coupling strength of the nanocrystal or magnetic dipole interaction between the particles. As the particle size increased, the rate of magnetization recovery became slower. Fitting of the size-dependent relaxation rate to a simple model, where the spin-lattice relaxation has the bulk and surface contributions, suggested that the surface of Fe3O4 nanocrystal is 3 times more efficient than the bulk for spin-lattice relaxation. Spin-lattice relaxation rate in CoxFe3-xO4 nanocrystals became faster with the increase of Co content, reflecting the larger average spin-orbit coupling of the nanocrystals. However, varying Co content in CoxFe3-xO4 nanocrystals of the same particle size had a larger effect on bulk contribution than the surface contribution to spin-lattice relaxation of the nanocrystals. Weaker dependence of surface spin-orbit coupling on Co content in CoxFe3-xO4 nanocrystals is probably due to the lower ligand symmetry of the surface compared to the bulk resulting in quenching of residual orbital moment of Co2+ responsible for the stronger spin-orbit coupling than Fe2+ ions. By modifying the binding affinity of the surface-passivating organic groups, the degree of aggregation among the nanocrystals was also varied. Unlike in the well-separated nanocrystals without dipole coupling between the nanocrystals, aggregated nanocrystals exhibit slower and non-exponential recovery of the magnetization.
12:00 PM - MM2.3
Ultrafast Phase Change Kinetics in Nanocrystalline Superionic Copper (I) Sulfide.
Timothy Miller 1 2 , Steven Connor 1 , Yi Cui 1 , Aaron Lindenberg 1 2 Show Abstract
1 Materials Science, Stanford University, Stanford, California, United States, 2 PULSE Institute, SLAC National Accelerator Center, Menlo Park, California, United States
Superionic materials are of interest in the energy storage field due to their remarkably high ionic conduction rates. Above a critical temperature, copper (I) sulfide exhibits superionic behavior in which copper ions become distributed in a liquid-like manner over vacant sites in a rigid sulfur lattice. Consequently, copper displays liquid diffusion rates, despite remaining solid. In this work we present novel results in which the kinetics of the superionic phase transition are resolved using pump-probe techniques. It is possible to drive the superionic phase transition in an repeatable manner using the fundamental or second harmonic output of an ultra-short pulsed Ti:Sapphire laser. Optical and THz radiation, as well as soft and hard x-rays, allow probing of electronic and structural changes of a material undergoing a superionic phase transition on ultrafast timescales. Hard x-ray diffraction measurements show a clear laser-induced structural rearrangement, while ultrafast x-ray absorption measurements at the copper L-edge show a change in the local copper bonding environment immediately following laser excitation. From these results a picture of the first steps of the superionic transition in copper (I) sulfide can be constructed.
12:15 PM - MM2.4
Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes.
Jigang Wang 1 2 , M. Graham 3 , Y. Ma 3 , G. Fleming 3 , R. Kaindl 2 Show Abstract
1 Physics, Iowa State University and Ames Lab, Ames, Iowa, United States, 2 Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Chemistry, University of California at Berkeley, Berkeley, California, United States
Optical excitations in single-walled carbon nanotubes are predicated to be correlated due to one dimensional confinement, reduced screening and distinctive symmetries. Photoexcited electron/hole quasiparticles form strongly bound excitons (e-h pairs) with large binding energies of several 100 meV and complex internal structures. The recent success in the preparation of “individualized” nanotube ensembles, along with chirality enriched samples, has rendered intrinsic optical properties and electronic correlations of specific SWCNs experimentally accessible. Femtosecond mid-IR spectroscopy can provide unique access to low-energy excitations and dynamics of photoexcited, Coulomb interaction bound e-h pairs in SWCNs. This provides a new avenue to elucidate many still elusive issues, e.g. the exciton internal structure, population densities of dark excitons, exciton formation dynamics, or exciton-exciton interaction at high momentum states. Photoexcited e-h pairs by visible pulses relax to the E11 subband and populate the lowest exciton levels. This implies new dipole-allowed intra-excitonic transitions from intra-exciton levels of opposite parity, which should exhibit a resonant shape with an absorption gap at low frequency. This will allow one to access genuine exciton distributions in a large range of momentum space, even including dipole-forbidden dark states, and help clarify otherwise hidden relaxation processes in inter-band spectroscopy. Ultrafast studies in the relevant spectral range, however, are scarce and prior mid-IR and THz studies on bundled nanotubes and/or mixtures of semiconducting and metallic tubes, which are limited to underpin key spectral features arising from intra-excitonic transitions. We present ultrafast visible pump and mid-IR probe experiments on individualized, chirality-enhanced (6,5) and (7,5) SWCNs, providing resonant access to exciton fine structure and dynamics of quasi-1D Coulomb-correlated e-h pairs. Transient spectra evidence a “gapped” asymmetric photoinduced absorption around 200 meV, associated with intra excitonic transitions with an absorption cross section of σeff ≈ 0.7*10-15 cm2 at the peak wavelength. The quasi-instantaneous establishment and bi-molecular decay of intra-excitonic resonance reflect the critical roles of Coulomb correlations at 1D and exciton-exciton interactions. We will discuss the features of this excitonic mid-IR absorption of photoexcited Coulomb-correlated e-h pairs, including the temporal dynamics and power dependence of the transient signals in these semiconducting carbon nanotubes, comparing with theoretical simulations. Our work was supported by the Office of Science of the U.S. Department of Energy. JW thanks for financial support from Ames Laboratory, operated by Iowa State University for the U.S. Department of Energy-Basic Energy Sciences.
12:30 PM - MM2.5
Optical Phonon Lifetimes in Graphene and Graphite by Time-resolved Incoherent Anti-Stokes Raman Scattering.
Kwangu Kang 1 , Daner Abdula 2 3 , David Cahill 2 3 , Moonsub Shim 2 3 Show Abstract
1 Mechanical Engineering, Yonsei University, Seoul Korea (the Republic of), 2 Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Optical phonons play a critical role in the exchange of energy between a non-equilibrium distribution of electronic excitations and the thermal bath of acoustic phonons that typically dominate the heat capacity and thermal transport properties of a material. In this work, we directly measure the lifetimes of optical phonon in graphene and graphite by time-resolved Raman scattering using a subpicosecond pump-probe method. Non-equilibrium populations of phonons are observed by incoherent anti-Stokes Raman scattering from the G mode. To separate Raman scattering created by the probe beam from scattering created by the pump beam, we used a two-tint pump-probe method based on the broad bandwidth of the Ti:sapphire laser oscillator and narrow bandpass optical filters. Graphene samples are prepared by mechanical exfoliation in air and thermally annealed under N2 to remove oxygen-containing species that shift the Fermi level. Lifetimes of graphite and single-layer graphene at room temperature are 2.3 ps and 1.2 ps, respectively; and lifetimes of multi-layer graphene increase monotonically with increasing number of layers. The optical phonon lifetimes in this work are an order-of-magnitude larger than what is expected based on the spectral width of the G mode observed by conventional Raman scattering. For >5-layer graphene and for graphite, the optical phonon lifetime decreases with increasing temperature as ~1/T, suggesting that anharmonic coupling to thermally excited acoustic phonons is the dominant channel for the decay of the optical phonon population.
12:45 PM - MM2.6
Femtosecond Multi-Particle Auger Recombination of Photoexcited Dirac Fermions in Graphene.
Tianqi Li 1 , John Solomon 1 , Myron Hupalo 1 , Michael Tringides 1 , Jigang Wang 1 Show Abstract
1 Physics, Iowa State University and Ames Lab, Ames, Iowa, United States
Graphene – a single layer of carbon atoms –has been a topic of strong current interest due to its basic scientific properties and application potential arising from two-dimensional (2D) quantum confinement and unique massless Dirac Fermion quasiparticles. The recent success in preparation of single- and few-layer epitaxial graphenes has rendered intrinsic optical properties and ultrafast electronic relaxation experimentally accessible in a well-controlled manner. Ultrafast visible and terahertz (THz) spectroscopy were used, revealing various decay pathways of photo-excited, highly non-equilibrium carriers in graphene [1-2]. To date, the majority of prior ultrafast studies conclude semiconductor-like carrier dynamics in this 2D semi-metal, which are characterized by phonon-mediated intraband carrier cooling and interband electron-hole recombination processes. It has been established that semiconductor materials under relatively high excitation exhibit a new nonradiative decay channel characterized by multi-particle scattering, called Auger recombination, based on long-range Coulomb interactions. Although the understanding of electronic transport and carrier dynamics are progressing, no experiments in graphene have shown evidence of such multi-particle Auger recombination. Furthermore, the Auger recombination is expected to strongly depend on dimensionality and electronic correlation. A “cubic” process involving three uncorrelated quasi-particles dominates in zero-dimensional (0D) semiconductor quantum dots , while a bi-molecule decay process of excitons (strongly bound e-h pairs) is seen in one-dimensional (1D) carbon nanotubes . The Auger decay times can vary from hundreds of ps to a few ps in these low-dimensional nanostructures, due to competing trends of energy and momentum conservation associated with dimensionality. Thus far these issues remain largely unexplored in 2D graphene. Here we report the observation of multi-particle Auger recombination in monolayer and double-layer epitaxial graphene using ultrafast differential reflection spectroscopy. A nonlinear dependence of the amplitude of the near-IR reflection on the pump fluence Ipump is clearly observed, while a plot of the amplitude versus Ipump1/3 shows a linear dependence. This behavior can be described by a cubic, three-particle decay of transient carrier population, which is indicative of the occurrence of Auger recombination. Temporal profiles of the differential reflectivity sets an upper bound of Auger decay time of 40 fs, reflecting the strong Coulomb correlations between Dirac Fermions in Graphene. We will discuss the features of this three-particle Auger scattering, including the temperature and probe wavelength dependence of the transient signals.  D. Sun, et al., PRL 101, 157402 (2008);  J. M. Dawlaty, et al., APL. 92, 042116 (2008).  H. Htoon, et al., PRL. 91, 227401 (2003).  L. Huang et al., PRL. 96, 057407 (2006).
MM3: Phase Transitions and Structural Dynamics
Monday PM, November 30, 2009
Room 201 (Hynes)
2:30 PM - **MM3.1
XAFS of the Ge2Sb2Te5 Memory Alloy - From Static Studies to Time-resolved Sub-nanosecond in-situ Experiments.
Alexander Kolobov 1 2 , Paul Fons 1 2 , Hitoshi Ohsawa 2 , Toshio Fukaya 1 2 , Motohiro Suzuki 2 , Tomoya Uruga 2 , Robert Simpson 1 , Naomi Kawamura 2 , Masafumi Takagaki 2 , Hajime Tanida 2 , Milos Krbal 1 , Junji Tominaga 1 Show Abstract
1 CANFOR, AIST, Tsukuba Japan, 2 SPring8, JASRI, Sayo Japan
Ge2Sb2Te5 (GST) is a prototypical phase-change alloy commercially used in re-writable optical disks and is also a leading candidate for future non-volatile electronic memories due to the large property contrast between the crystalline and amorphous phases combined with high switching rate, long-term stability, and large cyclability. The switching between the two phases is achieved by exposure to optical or electronic pulses. A short intense pulse melts the material that is subsequently quenched to the amorphous state. On the other hand, exposure to a less intense pulse that heats the material above the crystallization threshold reverts the structure to the crystalline phase. Our XAFS experiments  have demonstrated that the amorphous-to-crystalline transition involves a flip of Ge atoms within the Te fcc sublattice of the rocksalt structure of GST. Interestingly, the material can also be rendered amorphous by hydrostatic compression . In order to further investigate the phase-change process we have recently developed and successfully applied in-situ time resolved sub-nanosecond x-ray absorption fine structure (XAFS) experiments . We demonstrate, in particular, that the structural transformation around the Ge atoms following exposure by a 500 ps pulse is complete within less than 2 nanosecondsReferences A.V. Kolobov, P. Fons, et al., Nature Materials, 3 (2004) 703 A.V. Kolobov, et al., Phys. Rev. Lett. 97 (2006) 035701 P. Fons, A.V. Kolobov, et al., Jpn. J. Appl. Phys, 46 (2007) 3711
3:00 PM - MM3.2
Sub-picosecond Non-thermal Structure Change in a GeSbTe Film Induced by Femtosecond Single Pulse Excitation.
Toshiharu Saiki 1 , Mitsutaka Konishi 1 , Hisashi Santoh 1 , Yuki Hongo 1 , Kazuyuki Tajima 1 Show Abstract
1 Electronics and Electrical Engineering, Keio University, Yokohama, Kanagawa, Japan
A recent study has clarified that the phase change of the crystalline GeSbTe (GST) into amorphous is due to a slight displacement of Ge atoms from their position in the crystalline lattice. It suggests that ultrafast phase change may occur under irradiation of a single femtosecond laser pulse, which excites a large fraction of valence electrons to the conduction band at a time. In this study femtosecond pump-probe measurements were performed to discuss amorphization dynamics of a GST thin film. We found that the reflectivity dropped abruptly within 500 fs after a single pulse excitation and then remained almost constant. The observation using a scanning electron microscope revealed that the transition occurred without melting. The ultrafast non-melting structure change is anticipated to be applied to photonic switches and buffer memories in optical telecommunications.We measured temporal reflectivity evolutions for various pulse fluences. For all fluences, the reflectivity dropped abruptly within 500 fs after a single pulse excitation. After that, the reflectivity continued to slowly decrease or recovered within 10 ps, depending on the irradiation pulse intensity. According to the model by Kolovov et al., the following scenario is possible to explain the temporal evolutions. The initial drop of reflectivity was attributed to break-up of relatively weak Ge-Te bonds and displacement of Ge atoms due to the electron excitation. The additional change is governed by the occupancy of the complex defects related to the Ge displacement. If the excitation density is sufficient to occupy the complex defects and to maintain the Ge atoms at the displaced position, the reflectivity remains constant or exhibits an additional reduction. If the excitation density is insufficient, the GST recovers to the crystalline structure and the reflectivity regains its original value.To make clear whether ultrafast photoexcitation plays an essential role in the phase change mechanism, or not, we performed double-pulse excitation measurements, where we control the temporal photoexcited carrier density by tuning the delay of two pump pulses of equal fluence. We compared amorphization threshold for double pulse excitation with a delay of 0 fs and 300 fs. The total fluences irradiated on the sample are same in both cases, however, the amorphization did not occur for the delay of 300 fs. The result indicates that high density carrier excitation at a time, within a femtosecond time scale, makes an important contribution to the amorphization. To occupy the complex defects and maintain the Ge atoms at the displaced position, more than two electrons should be excited per unit cell at the same time (one for the break-up of Ge-Te bond and the other for the occupancy of defect). If such amorphization mechanism is valid, the photoexcitation by a femtosecond pulse is crucially advantageous.
3:15 PM - MM3.3
Time Resolved Optical Studies of Nano-structured Phase Change Materials.
Robert Simpson 1 , Toshio Fukaya 1 , Paul Fons 1 , Xiaomin Wang 1 , Milos Krbal 1 , Alex Kolobov 1 , Reiko Kondou 1 , Junji Tominaga 1 Show Abstract
1 CAN-FOR, AIST, Tsukuba, IBARAKI, Japan
Two optical methods have been used to measure the switching dynamics of composite Ge2Sb2Te5 (GST) thin films and nano-structured GeTe-Sb2Te3 meta-materials. The phase change data storage material GST has two meta-stable states- amorphous and a distorted rock-salt crystalline phase. The amorphous state has higher optical transmission and electrical resistivity in comparison to that of the crystalline state hence allowing the possibility to encode data. The transition from the crystalline to the amorphous state can occur on the sub-nano second time scale whilst the crystallization typically requires more than 70 ns. In 2002, this lab reported that the transition between the states is dependent on the change in co-ordination of the Ge atom from a position in which it is tetrahedrally coordinated with Te in the amorphous states to octahedrally Te coordination in the crystalline state. Extensive modeling of this transition has allowed us to develop new design principles for nano-structured GeTe-Sb2Te3 phase change materials. These materials provide the perfect environment for the GeTe-Sb2Te3 transition. To investigate these new materials, two optical methods have been employed. A dual laser, optical pump (650 nm)-probe (518 nm), time resolved, static tester and a more conventional, digital versatile disc (DVD) tester. For the static tester, perfect alignment was ensured by passing both lasers through a single mode optical fiber. The pump can provide up to 60 mW into a 1 um diameter spot with a minimum pulse width of 5 ns. The shorter wavelength of the probe laser allows the centre of the heated spot to be sampled with a 1 ns time resolution. Both transmission of the light through the sample and reflection from the sample surface are measured simultaneously. These measurements allow analysis of the crystallization kinetics. In contrast, the DVD tester uses a single 650 nm laser to write (amorphous) marks, erase (crystallize) the marks and, at lower powers, read the reflectivity of the material. Marks of length 500 nm were written into a sample disk, the maximum disc linear velocity at which erasure could be realized was then measured to give an estimation of crystallization rate. These two complimentary techniques have allowed us to measure a two fold increase in the crystallization rate, reducing the crystallization time to 23 ns for nano-structured films versus 52 ns for conventional, composite, GST films. This presentation will report our latest time resolved, optical measurements of the crystallization and amorphization process in nano-structured phase change materials. These measurements will be supported by Finite Element Models (FEM) to fully model laser-induced heating effects in an effort to describe the crystallization time dependence on atomic structure.
3:30 PM - MM3.4
Sub Nano-second Time Resolved XAFS Measurements of the Optical Recording Process in Ge2Sb2Te5 at the Te K-edge.
Paul Fons 1 2 , Hiroshi Osawa 2 , Alexander Kolobov 1 2 , Tomoya Uruga 2 , Hajime Tanida 2 , Robert Simpson 1 , Milos Krbal 1 , Junji Tominaga 1 Show Abstract
1 Center for Applied Near-Field Optics Research, Nat. Inst. of Adv. Ind. Sci. & Tech., Tsukuba, Ibaraki, Japan, 2 SPring-8, Japan Synchrotron Radiation, 1-1-1, Kouto, Sayo-cho, Hyogo-ken, Japan
In non-volatile optical recording, information is encoded in a phase-change medium as re-amorphized marks on a crystalline background and the corresponding differences in optical reflectivity between the phases are used to read out information. These re-writable phase change materials have surprising characteristics with switching speeds on the nanosecond time scale, but at the same time offer decades of archival storage once switched. This class of materials, typified by the phase-change alloy Ge2Sb2Te5 (GST), exhibits large electronic and structural property changes as a consequence of the significant changes in local order between the phases. The sizable changes in optical constants that occur during the recording process have been described as a change from resonant bonding in the crystalline state to a N-8 coordinated glass like state with a concomitant coordination change of Ge atoms from octahedral Te coordination to tetrahedral coordination. To understand the details of the switching process on an atomistic level, we have carried out in-situ optical pump/x-ray probe measurements of the local changes in Te coordination using x-ray absorption near-edge measurements with sub-nanosecond resolution during the switching process. Here we report on the experimental configuration of the measurements as well as preliminary results on the changes observed in the local Te environment during the switching process. Measurements were carried out at the Te K-edge (32 keV) using a 2 micron x-ray beam focused by a K-B mirror. The x-ray beam was directed at the optical disk stack grown on an optical flat quartz disk substrate. A 50 nm diameter, concentric, 600 ps, 532 nm optical pump beam was directed through the transparent substrate to eliminate jitter from the rotating sample which was mounted on an ultra-low wobble optical disk spindle. These observations offer information with an even wider impact than optical recording alone in that the same material GST is currently the material of choice for phase-change based electrical memory (PC-RAM) which utilizes the three order of magnitude increase in resistivity between crystalline and amorphous phases to achieve non-volatile electrical memory at speeds comparable to capacitive dynamic like memory (DRAM). We talk about the implications of these measurements towards a complete atomistic description of the recording process and resultant possibilities for further optimization of the switching process.
3:45 PM - MM3.5
Extreme Phonon Softening in Laser-excited Bismuth – Towards an Inverse Peierls-transition.
Klaus Sokolowski-Tinten 1 , Wei Lu 1 , Uladzimir Shymanovich 1 , Matthieu Nicoul 2 1 , Alexander Tarasevitch 1 , Martin Kammler 1 , Michael Horn von Hoegen 1 , Dietrich von der Linde 1 Show Abstract
1 , University of Duisburg-Essen, Duisburg Germany, 2 , University of Cologne, Cologne Germany
Irradiation of a solid material with intense ultrashort laser pulses can lead to significant changes of the interatomic forces. Upon photoexcitation electrons are usually promoted from bonding states to less bonding or even anti-bonding states, thereby setting off atomic motion in the system. A prominent example is the so-called displacive excitation of coherent phonons (DECP) . It has been found that DECP occurs only in materials with phonon modes of A1-symmetry which do not lower the symmetry of the material, and that only A1-modes are excited. The equilibrium structure of these materials can be derived by a Peierls-type transition from a state of higher symmetry. Bismuth is a prominent example in which this type of coherent vibrational excitation has been studied in great detail. The majority of published results are based on time-resolved all-optical studies which cannot provide direct structural information. More recently time-resolved X-ray diffraction has also been used to directly follow the atomic motion associated with the laser-excited coherent phonon [2-4]. In particular the work performed at the Sub-Picosecond Pulse Source  has allowed, for the first time, to quantitatively measure the transient changes of the potential energy surface which underlie DECP and the softening of the phonon modes. In the present work we have used time-resolved X-ray diffraction to extend our studies of coherent optical phonons in laser-excited Bismuth to a higher fluence range that has not been studied previously. Femtosecond X-ray pulses at 8 keV (Cu Kα) from a laser-produced plasma served as probe pulses in an optical pump – X-ray probe experiment. The transient changes of the (111)- and the (222)-diffraction peaks of a crystalline, 50 nm thick Bismuth film have been measured in a symmetric Bragg-configuration. For absorbed laser fluences above 2 mJ/cm2 our experimental data reveal an extreme softening of the A1g-mode down to frequencies of about 1 THz, only 1/3 of the unperturbed A1g-frequency. The observed softening follows qualitatively the predictions of density functional calculations . For even higher fluences (above 3 mJ/cm2) the measured diffraction signals no longer exhibit an oscillatory behaviour. Our experimental observations present strong indication that upon intense laser-excitation the Peierls-transition which determines the equilibrium structure of Bismuth can be reversed and that the material is transformed into a transient ordered state of higher symmetry.  H. J. Zeiger et al., Phys. Rev. B. 45, 768 (1992). K. Sokolowski-Tinten et al., Nature 422, 287 (2003). D. M. Fritz et al., Science 315, 633 (2007). S. L. Johnson et al., Phys. Rev. Lett. 100, 155501 (2008). E. D. Murray et al., Phys. Rev. B 72, 060301 (2005).
4:30 PM - **MM3.6
Ultrafast Structural Dynamics Observed with Atomic Scale Resolution.
Nuh Gedik 1 Show Abstract
1 Physics, MIT, Cambridge, Massachusetts, United States
Ultrafast optical spectroscopy has long been used with great success to generate and probe non-equilibrium electronic excitations with femtosecond time resolution. The spatial resolution in these techniques, however, is limited to micron scales and structural dynamics can only be inferred indirectly. I will report direct measurements of structural dynamics with atomic scale spatial resolution by using ultrafast electron diffraction (UED). In UED, a femtosecond laser pulse is split into two, the first part is used to induce structural change and the second part is used to generate ultrafast high energy electron packets via photoelectric effect. Recording the diffraction pattern of these electron packets at different times after the photo-excitation of the sample provides a movie of the laser induced structural change with sub-picosecond temporal and sub-Angstrom spatial resolution. I will discuss recent experiments where we used UED to observe lattice dynamics in correlated electron materials in response to photo-excitation of the charge carriers.
5:00 PM - **MM3.7
Direct Measurement of Transient Electric Fields Induced By Ultrafast Pulsed Laser Irradiation.
Jian-Min Zuo 1 2 , Hyuk Park 1 2 Show Abstract
1 Materials Science, Univ. Illinois, Urbana, Urbana, Illinois, United States, 2 Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, Illinois, United States
Understanding the interaction of ultrafast pulsed laser with matter is critical for probing ultrafast processes in materials science, understanding the physics of laser ablation and the laser induced non-equilibrium carrier dynamics in metals and semiconductors, including plasmonics. When an intense laser pulse of femtoseconds (fs) in duration hits the surface of a targeted matter, it excites a hot electron gas. Part of the hot electrons is emitted from the surface in a way similar to thermionic emission. Electrons can also be emitted through multiphoton photoemission (MPPE) or thermally assisted MPPE. The emitted electrons travel at speeds that create transient electric fields (TEFs). To detect TEFs and study the dynamics of emitted electrons, we have developed a time resolved electron beam imaging technique that allows us to measure TEFs above a sample surface at picoseconds time resolution. We have also developed a model of the TEFs based on the propagation of emitted electrons and the percentage of electrons escaping from the surface. The results will be reported for silicon. The measured field strength and direction change with time; at the pump laser fluence of 67.7mJ/cm2, the maximum field reaches 34 kV/m at 0.29 mm away from the sample surface. The significance of TEFs for ultrafast structural studies and applications will be discussed in the talk.
5:30 PM - MM3.8
Ultrafast Lattice Heating in Thin Metal Films Observed by Time Resolved Electron Diffraction.
Manuel Ligges 1 , Ping Zhou 1 , Ivan Rajkovic 1 , Oliver Posth 1 , Christoph Hassel 1 , Guenter Dumpich 1 , Dietrich von der Linde 1 Show Abstract
1 Institut für Experimentelle Physik, Universität Duisburg-Essen, Duisburg, NRW, Germany
Ultrafast lattice heating in thin metal films observed by time resolved electron diffractionThe interaction of electrons and phonons is an important fundamental issue in solid state physics. Ultrafast optical spectroscopy has previously been used to study the relaxation of electrons following short pulse optical excitation. The electrons lose their energy primarily byexcitation of lattice vibrations, ultimately causing the lattice temperature to rise. In the past, detection of lattice excitations resulting from the electronic relaxation was rarely achieved. Recently, however, time-resolved electron diffraction has successfully been used for latter purpose.We present results of sub-ps time-resolved electron diffraction experiments in which we studied lattice heating of thin metal films following relatively weak (non-destructive) femtosecond laser excitation. The built-up of lattice vibrations reveals itself by a decrease in the diffraction efficiency due to the Debye-Waller effect. The experiments were performed on 20 nm thick, polycrystalline films of Au, Cu and Ag, which were excited by laser pulses of 1 to 10 mJ/cm2 (below the damage threshold). The electron probe pulses had a kinetic energy of 30 keV. The number of electrons per pulse was kept at a level of a few thousand per pulse in order to keep the pulse duration well below one picosecond.We observed Debye-Scherrer diffraction rings and measured the decrease of the diffraction intensity in the various Bragg orders as a function of the delay time between the laser pump- and the electron probe-pulses. The temporal evolution of the diffraction intensity clearly indicated anapproximately exponential built-up of the lattice temperature with time constants of 4.7 ps and 1.1 ps for gold and copper, respectively. These results are in good agreement with the temporal evolution predicted by the two-temperature-approximation, where we used the coupling constants obtained from all-optical measurements [2,3] of the electronic temperature to model the rise in lattice temperature.We observed that the asymptotic value (long delay times) of the diffraction intensity decreased with increasing diffraction order as expected from the Debye-Waller law. However, an offset was observed when the diffraction efficiency was extrapolated to zero. In fact, a decrease of intensity was also measured in the directly transmitted beam. The temporal evolution of the zero order transmitted intensity followed that of the higher Debye-Scherrer diffraction rings. A similar decrease of the diffraction intensity was observed earlier , but a convincing explanation of this effect is still lacking. Anisimov et al., Sov. Phys. JETP 39, 375 (1974) Hohlfeld et al., Chem. Phys. 251, 237 (2000) Elsayed-Ali et al., Phys. Rev. Lett. 58, 1212 (1987) Boersch et al., Z. Phys. A 180, 407 (1964)
5:45 PM - MM3.9
Ultra Fast Time Resolved Electron Diffraction Study of Strongly Driven Phase Transitions on Silicon Surfaces.
Simone Moellenbeck 1 , Anja Hanisch-Blicharski 1 , Friedrich Klasing 1 , Paul Schneider 1 , Boris Krenzer 1 , Martin Kammler 1 , Michael Horn-von Hoegen 1 Show Abstract
1 Department of Physics, University Duisburg-Essen, Duisburg Germany
We set up an experiment for ultra fast electron diffraction at surfaces [1-4]. The surface sensitivity in a RHEED (reflection high energy electron diffraction)-geometry was utilized to analyze the structural dynamics of monolayer adsorbate systems on a ps-timescale upon excitation by fs-laserpulses in a pump probe measurement. Here we will present time resolved measurements of the dynamics of strongly driven surface phase transitions.
The first example is the famous order-disorder transition of the c(4×2) to (2×1) reconstruction on the Si(100) surface. This phase transition occurs under stationary conditions at a transition temperature of 200K. After preparing a well-ordered c(4×2), we cool down to 90K and excite the surface with a fs laser pulse with a wavelength of 800nm, an energy of 1.55eV and a maximum fluence of 30mJ/cm2. After laser excitation, the intensity of the c(4×2) spots decreases and recovers on a time scale of a few hundred picoseconds.
As a second example we studied the dynamics of the Peierls-like phase transition from the (8×"2") to (4×1) reconstruction on Si(111) covered with 1ML Indium. This phase transition is observed by preparing the (8×"2") well below 90K and excitation of the surface with a fs-laserpulse. During this strongly driven transition the (8×"2")-diffraction spots disappear, while the intensity of the (4×1)-spots increases. The increase of the (4×1) spot intensity excludes an explanation by the Debye-Waller-Effect. Thus, we will present first time measurements of the time resolved structural evolution of true surface phase transitions.
 A. Janzen, B. Krenzer, O. Heinz, P. Zhou, D. Thien, A. Hanisch, F.-J. Meyer zu Heringdorf, D. von derLinde, and M. Horn-von Hoegen, Review of Scientific Instruments 78, 013906 (2007).
 A. Janzen, B. Krenzer, P. Zhou, D. von der Linde, and M. Horn-von Hoegen, Surface Science 600, 4094(2006).
 B. Krenzer, A. Janzen, P. Zhou, D. von der Linde, and M. Horn-von Hoegen, New Journal of Physics8, 190 (2006).
 A. Hanisch, B. Krenzer, T. Pelka, S. Möllenbeck, and M. Horn-von Hoegen, Physical Review B 77,125410 (2008).
Aaron M. Lindenberg Stanford University
David Reis PULSE Institute
Paul Fuoss Argonne National Laboratory
Thomas Tschentscher Deutsches Elektronen-Synchrotron DESY
Bradley Siwick McGill University
MM4: Ferroelectric and Ferromagnetic Dynamics
Tuesday AM, December 01, 2009
Room 201 (Hynes)
9:00 AM - **MM4.1
X-ray Diffraction Probes for Electrically Driven Dynamics in Complex Oxides.
Paul Evans 1 Show Abstract
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Complex oxides including ferroelectrics, magnetoelectric multiferroics, and heterostructures with ferroelectric components, have a range of dynamical phenomena with characteristic times ranging from femtoseconds to seconds or even longer. These phenomena can be driven by applied electric fields and are coupled to structural distortions that allow them to be detected using the structural sensitivity of x-ray diffraction. Synchrotron x-ray microdiffraction techniques presently have time resolutions below 1 ns and simultaneously have spot sizes on the order of 100 nm. Experiments with these parameters allow dynamics to be studied in complex oxide device structures with short rise-times and well-defined electrical and structural properties. At the longest timescales, the dynamics of ferroelectric domains occurs in times from nanoseconds to microseconds or more and includes the nucleation of domains and the lateral propagation of domain walls. Images assembled from measurements of the diffracted x-ray intensity from ferroelectric Pb(Zr,Ti)O 3 thin films as a function of time allow the velocity of domain walls to be determined, and compared to previous area-averaged measurements of domain dynamics. Electric field pulses shorter than the microsecond timescale characteristic of domain wall motion allow the electric field to reach far higher values before switching because the pulses are not sufficiently long for low-field processes to occur. In the regime after switching to the state where the polarization and electric field are parallel, electric fields of 5 MV/cm or higher before electrical breakdown can be applied using nanosecond-duration pulses, allowing the structures of complex oxides to be studied under conditions that have previously been inaccessible to structural probes. Under these conditions elastic strains in Pb(Zr,Ti)O 3 and BiFeO 3 reach 2% or more, extremely high strains for inorganic materials. The structural response under these extreme conditions can be compared to newly available predictions derived from density functional theory. The structural response of ferroelectric/dielectric superlattices can be used as a local probe of the properties of these materials. The emergence of new ultrafast x-ray sources will has the potential to allow new phenomena to be probed, including the possibility of reaching far higher electric fields approaching the intrinsic coercive field.
9:30 AM - MM4.2
Comparison of Ultrafast X-ray and Optical Techniques for Functional Multilayers.
Matias Bargheer 1 2 , Marc Herzog 1 , Mareike Kiel 2 , Wolfram Leitenberger 1 Show Abstract
1 Institute of Physics and Astronomy, University of Potsdam, Potsdam Germany, 2 , Max-Planck-Institute of colloids and interfaces, Potsdam Germany
We present experimental results of ultrafast x-ray scattering and ultrafast optical experiments that simultaneously measure optical reflection and transmission changes on various multilayer systems. From x-ray techniques (both syncrotron and laser based sources) we assess the exact structural properties and structural changes. Visible and infrared measurements are used to measure inter- and intraband contributions to the changes in the dielectric constant.The investigated multilayer samples include expitaxially grown oxides (STO, SRO, LSMO, PZT, BTO) with dielectric, metallic, ferromagnetic and ferroelectric character as well as polyelectrolyte multilayers with embedded gold nano-paticles.
9:45 AM - MM4.3
Ferroelastic Domain Switching Observed by Time-resolved X-ray Diffraction.
Hengameh Allaf Navirian 1 Show Abstract
1 Atomic physics, Lund University, Lund Sweden
Domain polarization switching in potassium dihydrogen phosphate (KH2PO4, KDP) induced by a propagating strain wave has been observed with time-resolved X-ray diffraction. At room temperature KDP has tetragonal symmetry and belongs to the space group . When the crystal is cooled below TC, it undergoes a phase transition to the ferroelectric phase, which has an orthorhombic symmetry and belongs to the Fdd2 space group. The single paraelectric peak splits into four separate peaks in the ferroelectric phase. This is due to the 4-fold rotation-inversion symmetry axis of the paraelectric phase. Thus, there are four different ways in which the structure can be transformed, and hence four possible domain types with a permanent dipole moment can exist below TC. These domains are termed A+, A-, B+ and B-. A pulsed electric field with amplitude of 6 kV/cm and duration of 1 µs was applied along the crystallographic c-axis. Close to TC, the piezoelectric modulus is orders of magnitude higher than at room temperature, resulting in large-amplitude strain waves emanating from the surfaces when an electric pulse is applied due to the converse piezoelectric effect. In the center of the probed surface two waves interfered constructively inducing ferroelastic domain switching, in the absence of an external electric field, at a delay of 3 µs, corresponding to acoustic propagation at a velocity found to be 1500 m/s. In ferroelastic material, strain is made up from two contributions. These contributions are the linear strain and the ferroelastic strain respectively. The amplitude of the linear strain can be evaluated from the shift in the energy of each peak and the amplitude of the ferroelastic strain can be evaluated by the intensity changes in the diffracted X-ray signal originating from the two domain types. The stress-strain relation for ferroelastic materials is described by a hysteresis loop. Knowing the elastic constants, the stress can be calculated from the linear strain. The ferroelastic strain can be derived from the volume fractions derived from the relative intensities of the two domain types. The hysteresis loop of KDP has been measured in this experiment.
10:00 AM - MM4.4
Inducing Coherent Spin Excitations in Metallic Multilayers: The Role of Thermal Transport.
Vladimir Stoica 1 , Lynn Endicott 2 , Donald Walko 3 , Yuelin Li 3 , Eric Landahl 4 , Roy Clarke 1 2 Show Abstract
1 Applied Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Physics, University of Michigan, Ann Arbor, Michigan, United States, 3 Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois, United States, 4 Physics and Astronomy, DePaul University, Chicago, Illinois, United States
Ultrafast magnetic switching was studied for many years due the increased demand for speed in non-volatile memory devices. The use of either magnetic field or current pulses, with tens of picoseconds in duration, have consistently demonstrated the possibility of rapid control of spin orientation at room temperature in metal-based materials that include ferromagnetic thin films and related multilayers. The application of much shorter laser pulses, on the femtosecond time scale, is additionally available for the realization of even faster magnetic switches, which is less exploited in practice at the present. One difficulty, which arises when optical pulses are employed for spin manipulation in metallic ferromagnets, is the simultaneous presence of strong electronic and phonon excitations under transient non-equilibrium conditions. Finding new experimental techniques for investigating the role of individual quasiparticle excitations and elucidating their interaction relationship is an important step in reaching record rates during the manipulation of magnetism.In the present talk, we will present our latest results in studies of spin, charge and phonon dynamics excited with femtosecond optical pulses in fully epitaxial metallic multilayers that are prepared using molecular beam epitaxy. Optical probes are used to study spin wave excitations and their relation with uniaxial strain pulses that propagate across metallic heterostructures. A complimentary X-ray diffraction probe is used to quantify the role of ballistic electron propagation and the phonon transport across metallic interfaces. We will show that suitably designed multilayers permit spatio-temporal separation of different excitations as well as their interactions, which opens new possibilities for coherent control of spin dynamics.
10:15 AM - MM4.5
Tracking Photoswitching Dynamics of Bistable Spin-crossover Molecules in the Solid State by Time-resolved Optical and X-ray Diffraction.
Eric Collet 1 2 , Maciej Lorenc 1 , Marina Servol 1 , Marylise Buron 1 , Herve Cailleau 1 Show Abstract
1 Institut de Physique Rennes, University Rennes 1, Rennes France, 2 , Institut Universitaire de France, Paris France
Controlling with an ultrashort laser pulse molecular states in a solid material represents a next step in ultrafast science, ensuing the now established field of femtochemistry. Molecular materials offer the possibility to be directed between different macroscopic states by using appropriate electronic excitations as, contrary to dilute solutions, all the constituent molecules in solids can be photoactive. This opens new avenues for light-control of various photoswitchable functions (magnetic, optical, conduction...), with some direct consequences for future developments of information technologies. Spin-crossover compounds are acclaimed prototypes of molecular bistability, here between low spin (LS) and high spin (HS) states, which promise new route for molecular memory devices. Until now, ultrafast investigations of such systems have been limited to solution phase, whereas in solids only photosteady states have been addressed with recent extension to nanosecond laser excitation. Here we report, by coupling time-resolved optical and X-ray diffraction techniques, the spin state switching triggered by a femtosecond laser flash of a new Fe(III) solid and demonstrate that different processes and time-scales are involved [1,2,3]. The dynamics span from picosecond non-thermal molecular transformation to slower diffusive heating processes through the lattice, so far largely neglected and unveiled here by delayed enhancement of molecular switching. The new generation of ultrafast structural and optical experiment set the scene for imminent exploits in light-driven transforming of matter and crossing the boarder between femtochemistry and femtoswitching of materials. N. Moisan et al., C.R. Chimie 11 (2008) 1235. M. Lorenc et al., Phys. Rev. Lett. (2009), in press. E. Collet et al., Z. Krystallogr. 223 (2008) 272.
10:30 AM - MM4.6
Combined Nanosecond and Nanometer Scale Dynamics of Ferroelectric Domains to Map Nucleation Activation Energies.
Nicholas Polomoff 1 , Vincent Palumbo 1 , James Bosse 1 , Sungjun Lee 1 2 , Bryan Huey 1 Show Abstract
1 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Division of Physical Metrology, Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
High Speed Piezo Force Microscopy (HSPFM) is employed to investigate ferroelectric domain nucleation and growth of an exposed PZT film. 20 nanometer spatial and 10 nsececond temporal resolution is achieved using a pump:probe methodology, allowing area switching and individual domain dynamics to be monitored. Two complementary investigations are performed, mapping switching with durations ranging from 20 to 60 nanoseconds, or for amplitudes varying from 4 to 4.7 Volts. In this manner, nascent domains, as well as long term growth, are efficiently quantified with substantial statistical significance due to the hundreds of images that can reasonably be acquired in a practical experimental session. The switching mechanism, areal switching rate, domain nucleation time, and domain wall velocity are each clearly independent of pulse width. In contrast, these parameters are strongly influenced by increasing pulse heights, including a faster switching rate, shorter nucleation times, and additional nucleation sites. This supports a spatially and energetically heterogeneous landscape of activation energies for domain nucleation, and notably separately for growth. An important outcome is that only some nucleation sites can practically participate in switching when weak pulses are applied, essentially growing past other higher-energy nucleation sites that negligibly participate in low voltage switching, while many domains are activated for stronger pulses. This yields disproportionately faster area switching than predicted based on single bias measurements and standard exponential relationships between switching rates and bias. The additional insight provided through such multi-voltage measurements is therefore crucial for predicting and optimizing ultimate switching speeds of ferroelectric devices, since these characteristics are fundamentally linked to the spatial and energetic distributions of nucleation and growth activation energies.
11:15 AM - **MM4.7
Real-time Structural Dynamics of Polar Solids Studied by Femtosecond X-ray Diffraction.
Thomas Elsaesser 1 Show Abstract
1 , Max-Born-Institute, Berlin Germany
The study of structural dynamics on their intrinsic femto- to picosecond time scales has developed into an important area of solid state research. X-ray diffraction with a femtosecond time resolution allows for probing such processes most directly by determining transient atomic and/or molecular positions. Presently, ultrafast x-ray diffraction undergoes a rapid development based on novel approaches for generating ultrashort hard x-ray pulses in laser-driven or accelerator based sources. In this paper, results of recent prototype experiments addressing photoinduced structural dynamics of polar solids will be presented. In a first series of measurements, Bragg scattering of femtosecond x-ray pulses is applied to elucidate lattice dynamics in superlattice structures containing sequences of ferroelectric or ferromagnetic nanolayers. The coupling of different types of lattice motions and their role for the ferroelectric and –magnetic properties are determined. As a second example, the first powder diffraction study in the ultrafast time domain will be presented. Transient Debye-Scherrer ring patterns of ammonium sulfate [(NH4)2SO4] demonstrate an ultrafast phase transition of electronic structure, resulting in a dislocation of hydrogen atoms in this hydrogen-bonded material and the formation of conducting channels of high electron density.
11:45 AM - MM4.8
Terahertz-induced Kerr-effect in Relaxor Ferroelectrics.
Matthias Hoffmann 1 2 , Ka-Lo Yeh 1 , Harold Hwang 1 , Nathaniel Brandt 1 , Keith Nelson 1 Show Abstract
1 Chemistry, MIT, Cambridge, Massachusetts, United States, 2 Max Planck Research Group for Structural Dynamics, Center for Free Electron Laser Science, Hamburg Germany
We demonstrate the Kerr-effect induced by the electric field of single-cycle THz pulses in the relaxor ferroelectrics potassium-tantalum niobate KTa1-xNbxO3 (KTN) and K1-yLiyTa1-xNbxO3 (KLTN). This class of relaxor ferroelectrics may enter ferroelectric phases with true long-range order or dipole glass states with localized nanopolar domains, depending on the dopant concentration of the impurity introduced. The ferroelectric soft mode in these materials is in the terahertz (THz) frequency range and is strongly temperature dependent.In traditional optical Kerr effect (OKE) experiments an ultrashort laser pulse creates a birefringence via a Raman process that depends on the square of the pump field amplitude. In the newly developed THz-Kerr/optical-probe analog, a strong (>100kV/cm) single-cycle THz electric field orients and perturbs the polar clusters in KTN, causing the centrosymmetric paraelectric phase material to experience local ionic distortions that lasts for 1-10 ps. These ionic distortions induce a change in the refractive index of the material, which is then detected via an optical probe. Because the electric field strength and phase of the applied pulse can be determined absolutely, this technique enables us to study the subpicosecond material response to an electric field directly instead of the intensity.Two KTN samples with different Nb5+ dopant concentrations were subjected to high-field THz pulses and an optical probe with polarization oriented at 45 degrees with respect to the THz polarization. Both samples were <100> cut, one piece was 1.93-mm thick, KTN with x=0.345 and an estimated 1% per mol of Li (KLTN) while the other piece was a 50 µm thick KTN with x=0.018 (KTN1.8). The Curie temperature of KLTN and KTN1.8 was 264 K and 30 K, respectively. We observe a slow response in KTN1.8 which persists even after the THz pump pulse has left the sample. The signal from KLTN shows a much faster decay and seems to be following the square of the THz field and not persisting beyond the THz pulse duration.This experiment serves to demonstrate a first step toward collective coherent control in ferroelectric materials using THz fields.
12:00 PM - MM4.9
Ultrafast All-optical Switching and Laser Action in Rotaxane.
Marta Mroz 1 , Stefano Perissinotto 1 2 , Tersilla Virgili 1 , Marco Salerno 2 , Giuseppe Sforazzini 3 , Harry Anderson 3 , Guglielmo Lanzani 1 2 Show Abstract
1 Physics, Politecnico di Milano, Milano Italy, 2 , IIT, Italian Institute of Technology, Genova Italy, 3 Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Oxford United Kingdom
Linear and non-linear optical properties of conjugated polymers (CP) are often masked by the inter-chain network in solid state. The formation of aggregates may trap excitons, reduce oscillator strength and modify relaxation processes. The control of the inter-chain interaction is the main reason for developing “threaded” polymers, where supra-molecular encapsulation should reduce aggregation. Here, we investigate the influence of the encapsulation with b-cyclodextrin (b-CD) macrocycles on the photophysics of the poly(4,4’-diphenylenevinylene) (PDV) using femtosecond non-linear spectroscopy. Upon threading we observe enhancement of the stimulated emission (SE) in the visible range and reduction of the charge absorption. These phenomena are ascribed to the reduced inter-chain interaction. In more isolated chains the dynamics of intra and inter-chain charge states are distinguished. In addition, we performed three-beam experiments in which a first pulse (pump) creates singlet excited states; a second (push) pulse re-excite the singlet state and a broadband probe pulse detects the induced changes in transmission . This technique shows: 1) Charges are generated from higher lying singlet states also in isolated chains. 2) Ultrafast optical gain switching is possible in threaded chain. Finally, we demonstrate that ASE occurs in films of threaded polymers and lasing can be achieved with much lower threshold than the neat polymer chain in the DFB configuration . All our findings point out the potential role of rotaxane in photonics, as amplifiers and reopen the route to the electrically pumped organic lasers and all-optical logic devices.  Mroz M. M. et al. Phys. Rev. B, accepted.  Mroz M. M. et al., Appl.Phys. Lett., accepted
12:15 PM - MM4.10
Ultrafast Tuning of Magnetization Precession and Magnetic Anisotropy in Thin Iron Films.
Ettore Carpene 1 , Eduardo Mancini 1 , Daniela Dazzi 1 , Claudia Dallera 1 , Ezio Puppin 2 , Sandro De Silvestri 1 Show Abstract
1 Dipartimento di Fisica, CNR-INFM, Politecnico di Milano, Milano Italy, 2 Dipartimento di Fisica, CNISM, Politecnico di Milano, Milano Italy
We have developed an experimental set-up based on time-resolved Magneto-Optical Kerr Effect (MOKE) that allows to retrieve the vectorial magnetization dynamics in thin films with sub-picosecond resolution. This method has been exploited to measure the variations of the magnetization (modulus and orientation) induced by an ultrashort laser pulse. The initial demagnetization is established at the electronic level within a few hundreds of femtoseconds through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 picosecond time-scale, around an effective, time-dependent field. Following the full dynamics of the magnetization, we have unambiguously determined the temporal evolution of the magneto-crystalline anisotropy, providing the clear experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective field. This method provides a simple and widely applicable way to study both magnetization and anisotropy in the sub-picosecond regime and therefore to unravel the mechanisms underlying the ultrafast evolution of the spin order in magnetic media.
MM5: Materials in Extreme Conditions
Tuesday PM, December 01, 2009
Room 201 (Hynes)
2:30 PM - **MM5.1
The Generation of Transient Transparent Aluminum Alloys with Intense Femtosecond XUV Light.
Justin Wark 1 Show Abstract
1 Department of Physics, University of Oxford, Oxford United Kingdom
Fourth generation light sources are set to revolutionize several areas of science. These novel radiation sources have spectral brightnesses ten orders of magnitude brighter than any synchrotron, and when their output is focussed down to small spots, they can create solid density, 'crystalline' plasmas on femtosecond timescales, long before the atoms have had time to move. As such, we infer that we can produce a new form of plasma, where - at least for a short time - the ions are still on their crystallographic positions, yet each ion is further ionized beyond its natural state in the metal. We report specifically on the first experiments performed using the FLASH XUV laser in Hamburg. This laser can produce 15 fsec pulses of XUV radiation containing tens of microjoules per pulse. Using a multi-layer-coated off-axis parabola, we have focused 13.5nm radiation to spots of order a few microns in diameter, corresponding to peak intensities in excess of 10^16Wcm^-2 – a region of intensity that had, until recently, been the domain of optical high power lasers. We discuss here some of the interesting absorption physics that takes place at this new frontier, where the intensity is so great that we can eject L-shell electrons from every atom in an aluminum target placed in the focal region of the laser, and how the observed saturable absorption may impact on the creation and diagnosis of warm dense matter. We further discuss the XUV emission spectra recorded while the heated material was at solid density, before the ions had time to move, and how it may provide unique information on the electronic structure of warm dense matter. We show that as many atoms are photo-ionized, from the point of view of the electronic structure as revealed by fluorescence emission, the system is acting very much like a photo-generated alloy. Bob Nagler et al, Nature Physics, to be published* This work was performed by a large collaborative team - 'The Peak Brightness Consortium'. Reference  shows the full list of contributors.
3:00 PM - MM5.2
Investigating Atomic Bonding and Electronic Structure of Warm Dense Matter using Time Resolved X-ray Absorption Spectroscopy.
Byoung-ick Cho 1 , Phillip Heimann 1 , Jun Feng 1 , Kyle Engelhorn 1 , Christopher Weber 2 , Hae Ja Lee 2 , Roger Falcone 1 3 Show Abstract
1 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Physics, Santa Clara University, Santa Clara, California, United States, 3 Physics, University of Calfornia, Berkeley, Berkeley, California, United States
Warm dense matter (WDM) refers to states characterized by comparable thermal and Fermi energies, and ion-ion coupling parameters that exceed unity . These states are in-between solid and plasmas in the region of the density temperature phase space, where the standard theories of condensed matter and/or plasma physics are invalid. Research on the properties of WDM become active and growing because understanding and characterizing matters under extreme conditions are important to model many phenomena in astrophysics , geophysics  and inertial confinement fusion . Among the variety of interests, non-equilibrium states of WDM over the time scale relevant to atomic and molecular motion is drawing special attentions. Not only it is of interest relevant to fundamental significance in the phase transitions and energy relaxation processes, but also it provides a key understanding of radiation induced damaging processes, which impact a broad range of technologies from optics, electronics and biological imaging. X-ray absorption spectroscopy has been a powerful technique for mapping unoccupied electronic density of states (DOSs) and atomic bonding properties. However the static method is not compatible with highly transient state of WDM. Therefore we use time resolved x-ray absorption spectroscopy to mapping unoccupied DOS of carbon at a few eV temperatures and near solid density. The experiment was performed at beam line 6.0.2 at Advanced Light Source (ALS). An intense 150fs, 800nm laser pulse isochorically heated amorphous carbon and diamond foil with energy density of 10^6~10^7 J/kg, creating warm dense carbon with different densities. Unoccupied DOS of heated carbon were probed using broadband 70ps soft x-rays from ALS. Laser and x-ray pulses were both spatially and temporally overlapped on the target. Transmitted x-rays were dispersed with a grating spectrometer and a spectrum near carbon K-edge was detected with an ultrafast x-ray streak camera with 2ps resolution. Near edge X-ray absorption fine structure (NEXAFS) spectra of carbon K-edge region show band structures around Fermi level, which indicating π* and σ* anti-bonding features of carbon. The time resolved NEXAFS spectra show significant modification of band structure and we obtain data on the DOS of highly transient and exotic state of matter. Evolutions of both resonances, the increase of π* and the decrease of σ* resonances, indicate the changes in coordination number and strength of interaction between atoms. Both resonances have two different time scales which may be related to the time scales of energy relaxation and expansion of super heated solid. Detailed analysis of the spectra and the associated changes in electronic properties of warm dense carbon will be presented. Y. Ping et al., PRL 96, 255003 (2006). D. Saumon et al., High Press. Res. 16, 331 (2000). G. Huser et al., Phys. Plasmas 12, 060701 (2005). M. Koenig et al., APL 72, 1033 (1998).
3:15 PM - MM5.3
Short-pulse Laser Induced Transient Structure Formation and Ablation Studied with Time-resolved Coherent XUV-scattering.
Klaus Sokolowski-Tinten 1 , Anton Barty 2 , Sebastien Boutet 3 , Uladzimir Shymanovich 1 , Mike Bogan 3 , Stefano Marchesini 8 , Stefan Hau-Riege 2 , Nikola Stojanovic 5 , Joern Bonse 9 , Raanan Tobey 6 , Henri Ehrke 6 , Andrea Cavalleri 4 6 , Stefan Duesterer 5 , Matthias Frank 2 , Sasa Bajt 4 , Joachim Schulz 4 , Marvin Seibert 7 , Janos Hajdu 7 , Rolf Treusch 5 , Henry Chapman 4 Show Abstract
1 , University of Duisburg-Essen, Duisburg Germany, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States, 3 , Stanford Linear Accelerator Laboratory, Menlo Park, California, United States, 8 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 , HASYLAB, DESY, Hamburg Germany, 9 , Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin Germany, 6 , University of Oxford, Oxford United Kingdom, 4 , Centre for Free-Electron Laser Science, Hamburg Germany, 7 , Uppsala University, Uppsala Sweden
XUV- and X-ray free-electron-lasers (FEL) combine short wavelength, ultrashort pulse duration, spatial coherence and high intensity. This unique combination of properties opens up new possibilities to study the dynamics of non-reversible phenomena with ultrafast temporal and nano- to atomic-scale spatial resolution.In this contribution we wish to present results of time-resolved experiments performed at the XUV-FEL FLASH (HASYLAB/Hamburg) aimed to investigate the nano-scale structural dynamics of laser-irradiated materials. Thin films and fabricated nano-structures, deposited on Si3N4-membranes, have been excited with ultrashort optical laser pulses. The dynamics of the non-reversible structural evolution of the irradiated samples during laser-induced melting and ablation has been studied in an optical pump – XUV-probe configuration by means of single-shot coherent scattering techniques (i.e. diffraction imaging ). In a first set of experiments we investigated the formation of laser induced periodic surface structures (LIPSS) on the surface of thin Si-films (thickness 100 nm). In a simplified view LIPPS are generated as a result of interference between the incident laser pulse and surface scattered waves which leads to a periodically modulated energy deposition. Time-resolved scattering using femtosecond XUV-pulses (with a wavelength of 13.5 nm and 7 nm) allowed us to directly follow LIPSS evolution on an ultrafast time-scale and with better than 40 nm spatial resolution. The observed scattering patterns show almost quantitative agreement with theoretical predictions  and reveal that the LIPSS start to form already during the 12 ps pump pulse. In the second set of measurements we studied picosecond and femtosecond laser induced ablation and disintegration of fabricated nano-structures. Correlations of coherent diffraction patterns measured at various time delays to the pattern of the undisturbed object show that order in the structure is progressively lost starting from short length scales. This structural rearrangement progresses at close to the speed of sound in the material. Under certain circumstances (e.g. adequate sampling) it became also possible to reconstruct real-space images of the object as it evolves over time . The possibility of femtosecond single-shot imaging of ultrafast dynamic processes with nanoscale resolution provides yet more details of the physical processes involved.  H. N. Chapman et al. Nature Phys. 2, 839 (2006). J. F. Young et al., Phys. Rev. B 27, 1155 (1983). A. Barty et al. Nature Phot. 2, 415 (2008).
3:30 PM - MM5.4
Time-Resolved X-Ray Diffraction Study of Laser-Induced Shock and Acoustic Waves in Single Crystalline Silicon.
Klaus-Dieter Liss 1 , Thierry d' Almeida 2 , Maik Kaiser 3 4 , Rainer Hock 5 , Andreas Magerl 5 , Jean-Francois Eloy 6 Show Abstract
1 The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia, 2 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge United Kingdom, 3 Swiss Light Source, Paul-Scherrer Institut, Villigen Switzerland, 4 Laboratoire de Spectroscopie Ultrarapide, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 5 Lehrstuhl für Kristrallographie und Strukturphysik, Erlangen-Nürnberg, Erlangen Germany, 6 , European Synchrotron Radiation Facility, Grenoble France
A rod of single crystalline silicon has been subjected to high-power nanosecond laser pulses inducing ultrasonic and shock waves travelling into the bulk of the material. Stroboscopic time resolved high-energy X-ray diffraction measurements were carried out in-situ to probe for strain states in the bulk of the sample. First, a supersonic shock front is observed which moves faster than the longitudinal acoustic phonons. Following the shock front, a much slower bunch of waves travels along the crystal. The X-ray diffraction records obtained in different configurations reflect a strong dependence of the wave propagation on the sample geometry. These results offer an experimental approach for the investigation of coherent phonons, structural phase transformations, plastic deformations induced during shock peening, and for the development of X-ray free-electron-laser optics.
3:45 PM - MM5.5
The Potential For Investigating Sub-Picosecond Atomic Displacement Cascade Dynamics Using The LCLS.
Bennett Larson 1 , Jon Tischler 1 Show Abstract
1 Materials Science & Technology Division, ORNL, Oak Ridge, Tennessee, United States
Atomic displacement cascades, generated by the direct collision of energetic ions or fast neutrons with lattice atoms, represent the primary source of retained damage for materials in radiation environments. While the structure and dynamics of cascades has been studied for over 50 years by theory and simulations, the sub-picosecond time-scale generation of high densities of vacancies and interstitials within the nanometer scale volumes of displacement cascades and lifetimes of only a few picoseconds has precluded direct experimental observations of the local structure and dynamics. The ultra-high brilliance of hard x-ray pulses to be produced by the Linac Coherent Light Source (LCLS) will provide ~1 e12 photons within ~100 femtoseconds at a repetition rate of 100 Hz; thus, the conditions necessary for sub-picoseond structural measurements will be present in the near future. Experimental possibilities related to the use of diffuse scattering near Bragg reflections to investigate the structure and dynamics of energetic ion-induced displacement cascades in metals will be discussed. Issues such as synchronizing the arrival of energetic ions with the arrival of LCLS pulses will be considered in addition to discussing the information that can be extracted from the diffuse scattering measurements and how this information will relate to theory and simulations of cascades. Research supported by the Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering.
4:15 PM - **MM5.6
``Making the Molecular Movie": From Warm Dense Matter to the Structure-Function Correlation in Biology.
R.J. Dwayne Miller 1 Show Abstract
1 Chemistry/Physics, University of Toronto, Toronto, Ontario, Canada
Femtosecond Electron Diffraction harbours great potential for providing atomic resolution to structural changes as they occur, essentially watching atoms move in real time --- directly observe transition states. This experiment has been referred to as "making the molecular movie" and has been previously discussed in the context of a gedanken experiment. The challenge is to develop a source with sufficient spatial/time resolution and effective brightness to capture these motions under generally nonreversible conditions. With the recent development of femtosecond electron pulses with sufficient number density to execute nearly single shot structure determinations, this experiment has been finally realized. A new concept in electron pulse generation was developed based on a solution to the N-body electron propagation problem involving up to 10,000 interacting electrons that has led to a new generation of extremely bright electron pulsed sources that minimizes space charge broadening effects. Previously thought intractable problems of determining t=0 and fully characterizing electron pulses on the femtosecond time scale have now been solved through the use of the laser pondermotive potential to provide a time dependent scattering source. Synchronization of electron probe and laser excitation pulses is now possible with an accuracy of 10 femtoseconds to follow even the fastest nuclear motions. The camera for the “molecular movie” is now in hand. Through the Debye-Waller factors, it is now possible to directly observe lattice heating and discern thermal from electronic effects on lattice potentials. It has been possible to study rarified states of matter up to conditions corresponding to warm dense matter as well as optical manipulation of electron distribution and associated bonding on ultrafast timescales. These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules – the fundamental molecular basis of biological systems.
4:45 PM - MM5.7
Studying Nanoscale Material Processes under Extreme Conditions with High Time Resolution Electron Microscopy.
Thomas LaGrange 1 , Geoffery Campbell 1 , David Grummon 2 , Nigel Browning 1 3 , Bryan Reed 1 , Wayne King 1 Show Abstract
1 Physical and Life Sciences, Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Chemical Engineering and Materials Science, Michigan State University, E. Lansing, Michigan, United States, 3 Chemical Engineering and Materials Science, University of California, Davis, Davis, California, United States
Often a material’s macroscopic properties and behavior under external stimuli can be described through the observation of its microstructural features and dynamical behavior. Materials models and computer simulations that are used to predict material behavior in different environments, e.g., phase transformation kinetics under high pressures and temperatures, typically require experimental data for validation or interpretation of simulated quantities. However, most materials dynamics are extremely rapid, making it difficult to capture transient, fine-scale features of the material process, especially on the length and time scale relevant for most mesoscale models. In effort to meet the need for studying fast dynamics and transient states in material processes, we have constructed a nanosecond dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory to improve the temporal resolution of in-situ TEM observations. The DTEM consists of a modified JEOL 2000FX transmission electron microscope that provides access for two pulsed laser beams. One laser drives the photocathode (which replaces the standard thermionic cathode) to produce the brief electron pulse and nanosecond exposure times. The other strikes the sample, initiating the process to be studied. A series of pump-probe experiments with varying time delays enable, for example, the reconstruction of the typical sequence of events occurring during rapid phase transformations. This presentation will discuss the core aspects of the DTEM instrument with particular focus on how it has been used to study the “superheated” crystallization processes in amorphous NiTi films. In particular, we will show how the high time resolution capabilities of the DTEM have been used to quantify the nucleation rates and crystallization kinetics at high temperatures far above the canonical crystallization temperatures and measurements made with conventional techniques. Work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory and supported by the Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under contract No. DE-AC52-07NA27344.
5:00 PM - **MM5.8
Atomic-Scale Simulations of Femtosecond Laser Ablation.
George Gilmer 1 3 , Alexander Rubenchik 1 , Steven Yalisove 2 , Ben Torralva 2 Show Abstract
1 , lawrence livermore national laboratory, Livermore, California, United States, 3 Mechanical Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
The deposition of laser energy into a thin near-surface layer creates a highly non-equilibrium state of matter, resulting in a rich diversity of surface morphologies and ejecta. We have simulated short-pulse ablation of metals, using a continuum model coupled with large molecular dynamics simulations. We will present data from models of aluminum, copper, and silica. Our purpose is to investigate the mechanisms leading to the ejection of liquid droplets and slabs, and surface roughening. Three distinct regimes are exhibited in the simulations: (i) low pulse energy densities where only a minute flux results from the evaporation of atoms from the hot metal surface; (ii) an intermediate energy regime, where void nucleation and coalescence causes the ejection of a liquid layer several tens of nanometers thick, and (iii) high energy pulses that eject liquid droplets with a wide range of sizes. The dynamics of short-pulse ablation in the simulations are illustrated in computer-generated movies. We will touch on a number of applications, including a variety of morphologies that can be formed, including nano-porous films nano-particles, and structures for nano-fluidics. Other applications include the imparting of momentum to freely moving objects such as satellites, and a pulsed-laser deposition procedure to lay down a uniform film on a non-wetting substrate.This work performed by LLNL under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344.
5:30 PM - MM5.9
Collateral Damage Induced by a Femtosecond Laser Pulse on Single-Crystal Superalloy CMSX-4.
Ben Torralva 1 , S. Ma 1 , M. Das 1 , J. McDonald 2 , S. Yalisove 1 , T. Pollock 1 , K. Thornton 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
The interaction of ultrashort laser pulses (ULP) with materials holds fundamental importance, as it allows exploration of processes on time scales shorter than those of typical electron-phonon interactions. One of the important applications of high-irradiance ULP is laser machining where a significant reduction of collateral damage is observed as compared to nanosecond lasers. To understand the damage and the dynamic deformation behavior under the unique conditions of ultrafast laser ablation, we combine computational modeling and experimental characterization of the collateral damage in the single-crystal superalloy CMSX-4. Hydrodynamics-based continuum modeling is employed to simulate the interaction of ULP with metals to gain understanding of the state of the material during deformation; a combination of experimental techniques including scanning electron microscopy, atomic force microscopy, and cross-sectional transmission electron microscopy of the plastic deformation zone in the superalloy are used to characterize the damage. We show the dislocation distribution is confined to the region beneath the laser-irradiated region, unlike what is expected from nanosecond laser machining. Moreover, the amount of damage injected can be dramatically enhanced or suppressed by setting the fluence above or below a critical value. Through simulations, we elucidate the origin of the sharp transition in ablation efficiency and damage injected into the bulk. Particular attention will be given to the role of the shock wave that is launched into the material.
5:45 PM - MM5.10
Large-scale Molecular Dynamics Studies of the Solid-solid Phase Transformation Pathways and Kinetics in Shocked Fe and Cs.
Timothy Germann 1 Show Abstract
1 T-1, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Molecular dynamics (MD) simulations have provided unique insight into the atomic-scale mechanisms of shock-induced plasticity and phase transformations, revealing detailed information about pathways and their dependence on crystallographic loading direction [1,2]. For the bcc-hcp transformation in iron single crystals, in situ dynamic x-ray diffraction studies have shown excellent agreement with these simulations, for thin foils subjected to laser shock. However, the rapid kinetics demonstrated by both these simulations and experiments is several orders of magnitude faster than that observed in traditional macroscopic shock experiments (e.g. gas gun), including recent experiments with single crystal targets as thin as 100 μm. Moreover, these latter experiments indicate a three-wave structure, with bcc plasticity evident ahead of the phase transformation wave, unlike the smaller-scale MD simulations and laser-shock experiments. After reviewing this earlier work, we will present the results of a series of very-large-scale MD simulations of single crystal and polycrystal iron samples (as long as 10 μm), subject to both shock and ramp wave loading, to explore these discrepancies.We have also utilized large-scale classical molecular dynamics simulations to study the isomorphic fcc-fcc transformation in shocked cesium perfect crystals. Ackland and Reed  developed an interatomic potential to describe the similar volume collapse transition in Cs, adding an internal variable for the relative occupation of two (s and d) electronic bands on each atom to an embedded atom method (EAM)-like model. Using an orientation imaging map (OIM) algorithm, we find a significant dependence upon initial crystallographic orientation: shock compression in the  direction leads to a product with a predominantly -like texture, while  loading accomplishes the volume collapse transition without any crystallographic rotation. A three-wave (elastic-plastic-product phase) structure is also observed for shock pressures around 5 GPa in the  case, while the  plastic wave is overdriven prior to the onset of transformation.  T. C. Germann, B. L. Holian, P. S. Lomdahl, and R. Ravelo, Orientation Dependence in Molecular Dynamics Simulations of Shocked Single Crystals, Phys. Rev. Lett. 84, 5351 (2000). K. Kadau, T. C. Germann, P. S. Lomdahl, and B. L. Holian, Atomistic simulations of shock-induced transformations and their orientation dependence in bcc Fe single crystals, Phys. Rev. B 72, 064120 (2005). G. J. Ackland and S. K. Reed, Two-band second moment model and an interatomic potential for caesium, Phys. Rev. B 67, 174108 (2003).
MM6: Poster Session
Tuesday PM, December 01, 2009
Exhibit Hall D (Hynes)
9:00 PM - MM6.1
Silicide Film Formation in the Ta/Ti/Si System by RF Induction Heating.
Joshua Pelleg 1 , Shmuel Rosenberg 2 , Michael Sinder 3 Show Abstract
1 Materials Engineering, Ben Gurion University of the Negev, Beer Sheva, Negev, Israel, 2 materials Engineering, Ben Gurion University of the Negev, Beer Sheva Israel, 3 Materials Eng., Ben Gurion University of the Negev, Beer Sheva, Negev, Israel
Silicidation of Ta-Ti-Si film on Si (111) and Si (100) substrates was investigated by a new induction heating (IH) technique in order to evaluate the progress of reaction and establish whether the substrate orientation influence on the rate of reaction prevails. Substrate orientation was observed notwithstanding the high temperatures applied and the very short duration of IH. It was observed that while the reaction on Si (111) goes to completion, on Si (100) substrates under the same conditions intermediate phases remained.A qualitative analysis of the IH treatment of a conductor film on the silicon substrate is presented, according to which the specimens might experience either heating at constant temperature or by a sudden temperature increase. Measurable quantities such as sheet resistance and the magnetic field applied determine the stage of the process. The value of the resulting sheet resistance indicates whether the progress of the IH occurred by heating in the slow growth temperature regime or in the heat explosion stage where reactions of a conductor film occur almost instantaneously within a fraction of a second.
9:00 PM - MM6.10
Temporal and Spatial Evolution of Transient Electric Fields Induced by Ultrafast Pulsed Laser Irradiation of Graphite.
Hyuk Park 1 , Jian-Min Zuo 1 Show Abstract
1 Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, Illinois, United States
Ultrafast electron diffraction studies suggest that the transfer of energy from hot electrons excited by femtoseconds pulsed laser to the lattice in graphite[1,2] induces unusually large structural changes, including the formation of transient sp3 like bonding. The same hot electrons can be emitted from the surface and generate transient electric fields (TEFs). Here, we report a direct measurement of temporal and spatial behavior of TEFs under pulsed laser irradiation of fluences up to 108 mJ/cm2. We use electron beam deflection above the surface to measure TEFs. By changing the distance between probe electron and the surface, we can separate the electric fields generated by emitted, traveling, electrons and the surface charges at a fixed position. The maximum electric field detected is on the order of tens of kV/m at times ranging from few tens to hundreds of picoseconds for the probe beam at 92 μm to 930 μm from the surface. The strength of TEFs and its time dependence changes with the pulse width of the laser beam. The implications of TEFs for the interpretation of prior experimental data and the mechanism of electron emission will be discussed in this talk.  R.K. Raman, Y. Murooka, C.Y. Ruan, T. Yang, S. Berber, and D. TománekDirect observation of optically induced transient structures in graphite using ultrafast electron crystallographyPHYSICAL REVIEW LETTERS 101, 077401 (2008) F. Carbone, P. Baum, P. Rudolf, and A.H. ZewailStructural preablation dynamics of graphite observed by ultrafast electron crystallographyPHYSICAL REVIEW LETTERS 100, 035501 (2008)
9:00 PM - MM6.11
Fast Speed Camera Imaging and Synchrotron X-ray Studies of Dynamic Structuring from Nano-layered Clay Particles.
Zbigniew Rozynek 1 , Jon Fossum 1 , Baoxiang Wang 1 , Rene Castberg 2 , Knut Maloy 2 Show Abstract
1 Physics, NTNU, Trondheim Norway, 2 Physics, UiO, Oslo Norway
The magnetic and electric field induced structuring of nano-layered clay particles are studied by means of fast speed camera imaging and both wide and small angle X-ray synchrotron scattering. This dynamic structuring results from the interaction between induced electric and magnetic dipoles. Application of an electric (and/or magnetic) field induces polarization of the suspended dielectric particles (that can also possess magnetic properties) and a chain-like structure can be formed along the electric field direction . In order to predict the response and behaviour of these nano-layered clay particles under an applied electric (or/and magnetic) field, it is important to gain an understandable knowledge of the physical mechanism of the structuring (phase transition) and its dynamics. In the present study, synthetic Na-fluorohectorite and cation exchanged Ni-fluorohectorite clay particles suspended in insulating, non-polar silicone oil form with time chain-like structures when subjected to an external electric and magnetic fields. Wide Angle X-Ray Scattering (WAXS) diffraction patterns together with Fast Speed Camera Imaging reveal changes of the direction of the electric and magnetic dipolar moments induced in clay particles when the fields are applied. For the first time in such a system the orientational switching behaviour was achieved and is reported here.
9:00 PM - MM6.2
Process Coatings with Sol-gel as ``Glue.
Yuhong Huang 1 , Qiang Wei 1 , Qifa Zhou 2 , Kirk Shung 2 , Ichiro Nishimura 3 Show Abstract
1 , Chemat Technology Inc., Northridge, California, United States, 2 , NIH Transducer Resource Center, Department of Biomedical Engineering, University of Southern California, Los Angeles CA, USA, Los Angeles, California, United States, 3 , UCLA School of Dentistry, The Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, Biomaterials and Hospital Dentistry, Los Angeles, CA, USA, Los Angeles, California, United States
Processing coating products often request 1) low processing temperature, and 2) strong bonding to substrate. Sol gel chemistry can provide a strong bonding between particles, particle to surface, and interface layers which results in durable products. The sol gel process uses inorganic or metal organic precursors. The precursors subject hydrolysis and condensation. Condensation reactions involving the hydroxyl ligands produce polymers composed of covalent bonds. The reaction can take place at room temperature. By using sol gel as “glue”, we have designed and developed products which can be fabricated at low temperature with desired adhesion and cohesion. In this presentation, we will discuss how to bond bone conductive hydroxyapatite (HA) nanoparticles onto titanium implant surface to create a unique nanostructure. This discrete nano-HA layer is chemically bonded to the Ti substrate. The ultra-thin nano-HA layer did not adversely reduce the roughness of the surface topography of the Ti substrate at micron levels. This unique topography resulted in accelerating tissue implant integration. The presentation will also give several other examples of how we developed technologies based on this concept. The work is supported by NIH SBIR Phase II funding with grant numbers of 2R44DE014927 and 5R44RR014127.
9:00 PM - MM6.3
The Role of Thermal and Electronic Forces in the Picosecond Acoustic Response of Femtosecond Laser-excited Solids.
Uladzimir Shymanovich 1 , Matthieu Nicoul 3 1 , Stefan Kaehle 1 , Wei Lu 1 , Alexander Tarasevitch 1 , Ping Zhou 1 , Tobias Wietler 2 , Michael Horn von Hoegen 1 , Dietrich von der Linde 1 , Klaus Sokolowski-Tinten 1 Show Abstract
1 , University of Duisburg-Essen, Duisburg Germany, 3 , University of Cologne, Cologne Germany, 2 , University of Hannover, Hannover Germany
Upon excitation of a solid with ultrashort laser-pulses the optical energy is initially stored in the electronic subsystem and subsequently transferred to the lattice in a few picoseconds. Both electronic excitation and the quasi isochoric heating of the material lead to a nearly instantaneous increase in pressure. Relaxation of the pressure triggers strain waves which can be regarded as a coherent superposition of acoustic phonons. Laser generated coherent acoustic phonons (CAPs) have been extensively studied in the past using time-resolved optical spectroscopy and, more recently, time-resolved diffraction techniques. The general behaviour of CAPs can be qualitatively understood within the phenomenological model of Thomson et al. . However, there are significant quantitative discrepancies among different studies, in particular with respect to the role of electronic and thermal contributions to the driving pressure. In this work we apply ultrafast time-resolved X-ray diffraction to study the dynamics of CAPs in Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal driving forces. Femtosecond X-ray pulses at 4.51 keV (Ti Kα) from a laser-produced plasma served as probe pulses in an optical pump – X-ray probe experiment. We measured the transient changes of the angular distribution of Bragg-diffracted X-rays (rocking curves – RCs) as a function of pump-probe time delay. Using thin, crystalline films as samples excited with 100 fs laser pulses at 800 nm we were able to generate well-defined excitation conditions and to exclude the influence of carrier diffusion and heat conduction. This greatly simplifies interpretation of the experimental data based on a numerical modelling of the transient RCs. The model includes calculations of the acoustic response (with time-dependent electronic and thermal pressure contributions) coupled to dynamical X-ray diffraction theory. For Ge our measurements reveal that the relative strength of the electronic pressure decreases with increasing laser fluence. For larger laser fluences the thermal pressure exceeds the electronic one, and only at low excitation strength the electronic pressure becomes the dominant driving force, as predicted by theory . We attribute this behaviour to the fact that only for low fluences the excited electrons and holes are concentrated near the band extrema. Only in this situation the effective deformation potential can be related to the pressure dependence of the fundamental energy gap.For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Grüneisen parameters.  C. Thomsen et al., Phys. Rev. B 34, 4129 (1986).
9:00 PM - MM6.5
Monte Carlo Modelisation of Photoexcited Carriers Relaxation including Auger Effect in Narrow Band Gap InGaAs.
Eric Tea 1 , Frederic Aniel 1 Show Abstract
1 , Institut d'Electronique Fondamentale, UMR 8622 CNRS, Université Paris Sud, 91405 Orsay Cedex France
The Auger effect is one of the fastest recombination mechanisms in narrow band gap semiconductors at high carrier concentration. This regime is of great interest for high efficiency hot carrier solar cells application and is also involed in many optical devices. Thus, the knowledge of this limitting process is required for the determination of carrier lifetime useful to accurate solar cell efficiency calculations. We present a carrier lifetime study with carrier concentration in InGaAs by Monte Carlo modelling.
9:00 PM - MM6.6
Minoritary Transport in Heavily Doped p-type InGaAs.
Eric Tea 1 , Frederic Aniel 1 Show Abstract
1 , Institut d'Electronique Fondamentale, UMR 8622 CNRS, Université Paris Sud, 91405 Orsay Cedex France
Minoritary carrier’s transport properties are of fundamental importance in the HTB’s physics. The base transit time is a key parameter to improve microwave figure of merit. Some recent minoritary electron mobilities measurements versus acceptor doping level using magneto transport method exhibit a dramatic increase at very high majority carrier concentration. This effect has been attributed to the coupling of polar optical phonons with hole plasmons (LOPC) which controls the balance between enegy gain by electric field acceleration and energy loss by polar optical phonon emission. We present minoritary mobilities as a function of majotity carrier doping calculated in the frame of electrons and holes Monte Carlo modelling including LOPC.
9:00 PM - MM6.7
Chemical Treatment Influence on the Glass Substrate to the Growth of V2O5/PANI Thin Film.
Elidia Guerra 1 , Mirela Santos 1 , Rodrigo Bianchi 1 Show Abstract
1 ICEB, UFOP, Ouro Preto Brazil
Vanadium pentoxide xerogel (matrix) has a two-dimensional structure which is suitable for intercalation of a variety of ions, organic compounds, and even polymeric species . Intercalation studies with V2O5 xerogel have been performed with the purpose to improve the properties, such as, electrical, as well as the possibility of making new materials that cannot withstand the temperatures used in traditional synthesis of oxides (e.g. polymers) . In this context, the main goal of this work is investigate the chemical treatment influence on the glass substrate to the growth of V2O5/PANI thin film using dip-coating deposition methodology. Further, applications using this V2O5/PANI thin film sensor will focus on ammonia detection in poultry shed. The V2O5/PANI was prepared by reacting 0.5 ml of 0.2 M aqueous solution of aniline with 5.0 ml of the vanadium pentoxide gel. The reaction were carried out at room temperature (24°C), stirred during 48 h in air, resulting in a dark green suspension. The glass substrates were, previously, immersed in RCA clean and UV/ozone treatment. Afterwards, each substrate was immersed for five times into the suspension by dip-coating and dried at room temperature and in air.Contact angle measurements indicate a value of 12° with RCA and 6° with UV/ozone treatment. Theses values show an enhancement of the adhesion with UV/ozone treatment indicating that the hydrophobic character of the film/surface is stronger compared with the presence of RCA. The electronic spectrum of V2O5 gel has absorption bands at 269 nm and at 382 nm, which we assign to VV oxide charge transfer transition. However, one can observed that V2O5/PANI has absorption bands at 385 nm and at 272 nm, which this shift can be indicating that the intercalation reaction was successful. In addition, UV-Vis absorption spectroscopy was also used for monitoring the deposition of V2O5/PANI layers. It was verified the presence of multilayer formation due to the increase in absorption in function of the number of layers. The growth of V2O5/PANI layers in RCA treatment showed to be nonlinear with R = 0.77. However, using UV/ozone treatment, it was observed a growth curve in function of layers more linear compared to RCA treatment with R = 0.99. This difference in linearity is probably due to the influence of treated surfaces and it can interact more strongly with V2O5/PANI, as observed by contact angle measurement with UV/ozone, resulting in a stronger adhesion when compared with RCA treatment. Therefore, the V2O5/PANI thin film will be tested as possible candidate for further use as disposable ammonia sensorReferences C.-G. Wu, D.C. DeGroot, H.O. Marcy, J.L. Schindler, C.R. Kannewurf, Y.-J. Liu, W. Hirpo, M.G. Kanatzidis, Chem. Mater. 8 (1996) 1992. H.P. Oliveira, C.F.O. Graeff, C.A. Brunello, E.M. Guerra, Journal of Non-Crystalline Solids 273 (2000) 193.This work was supported by FAPEMIG from Brazil.
9:00 PM - MM6.8
Synthesis of LNF Nano Particles for Fuel Cell Applications.
Reza Bateni 1 , Maryam Nasirpour 1 , Olivera Kesler 1 Show Abstract
1 Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada
LaNi0.6Fe0.4O3 (LNF) is a promising cathode material for solid oxide fuel cells (SOFCs) due to its resistance to chromium poisoning and moderately high ionic conductivity. Although conventional solid-state chemistry methods can be used for the preparation of LNF, high reaction temperatures (above 1000°C) are required for the formation of the target phase. However, combustion synthesis techniques are capable of producing nanocrystalline powders of oxide ceramics at lower calcination temperatures and in a shorter time. The powder obtained by such techniques generally has a high degree of phase purity and useful powder characteristics such as a narrow particle size distribution.The aim of the present work is to understand the effect of different parameters such as the molar ratio of metallic ions to fuel in the sol-gel solution, calcining temperature, and pH of the solution on the efficiency of the combustion synthesis technique in preparing submicron lanthanum nickel ferrite using metal nitrate-citrate/glycine mixtures. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was used to evaluate the powder morphology and elemental composition. The crystal structure of the calcined powders was evaluated by X-ray diffraction and the particle size distribution of the powders was determined by laser light scattering. The thermal characteristics of the LNF gels were examined by thermo-gravimetric analysis (TGA) in air, to identify suitable processing conditions.It was found that, by decreasing the molar ratio of metallic ions to fuel in the precursor solution from 6.1 to 1.5, calcining could take place at lower temperatures. However, at the lower end of the range of molar ratios of metallic ions to fuel in the precursor solution, the efficiency of the process was decreased, due to mass loss during the combustion reaction. Furthermore, the pH value of the precursor solution did not have any influence on the process efficiency. It was also found that decreasing the molar ratio of metallic ions to fuel in the precursor solution within the studied range resulted in finer particles of the calcined LNF powders. Overall, the results indicate the process conditions that can be utilized to obtain a set of resulting powder properties.
9:00 PM - MM6.9
Multiple Femtosecond Laser Pulse Delamination of Thin Oxide Films.
Kristen Tebo 1 , Ryan Murphy 2 , Michael Thouless 1 3 , Joel McDonald 4 , Steve Yalisove 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Applied Physics, University of Michigan, Ann Arbor, Michigan, United States, 3 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Sandia National Laboratories, Sandia National Laboratories, Albuquerque, New Mexico, United States
Previous work reported details of thin film silicon oxide modification on silicon via femtosecond irradiation at a range of fluences. A single femtosecond laser pulse is transmitted, with some absorption through the SiO2 film, to incident on the SiO2/Si interface. Above a film delamination fluence threshold (~0.49 J/cm2), a single pulse will induce delamination at the interface, and create a blister. Successive pulses will enlarge a single pulse blister in both height and width. New results demonstrate that below 0.49 J/cm2 a single pulse will only melt the Silicon at the interface (followed by a rapid quench into amorphous Si as confirmed by a change in reflectivity). However, successive laser pulses will induce delamination, and blister evolution. A new model will include energy absorption of the SiO2 allowing the glass to exceed the glass transition temperature (Tg) and cool inhomogeneously, thereby creating a local stress dipole sensitive to successive laser pulses. Emphasis will also be placed on the on the micron scale delamination mechanics and the effect of the initial stress state of the SiO2 film.
Aaron M. Lindenberg Stanford University
David Reis PULSE Institute
Paul Fuoss Argonne National Laboratory
Thomas Tschentscher Deutsches Elektronen-Synchrotron DESY
Bradley Siwick McGill University
MM7: Laser Processing of Materials
Wednesday AM, December 02, 2009
Room 201 (Hynes)
9:00 AM - **MM7.1
Optical Hyperdoping: Using Lasers to Tailor the Optoelectronic Properties of semiconductors.
Eric Mazur 1 Show Abstract
1 Department of Physics, Harvard University, Cambridge, Massachusetts, United States
Shining intense, ultrashort laser pulses on the surface of a crystalline silicon wafer drastically changes the optical, material and electronic properties of the wafer. The resulting textured surface is highly absorbing and looks black to the eye. The properties of this 'black silicon' make it useful for a wide range of commercial devices. In particular, we have been able to fabricate highly-sensitive PIN photodetectors using this material. The sensitivity extends to wavelengths of 1600 nm making them particularly useful for applications in communications and remote sensing.
9:30 AM - MM7.2
Anomalous Absorption of Multiple or Single Femtosecond Laser Pulse by Silica Thin Films.
Kristen Tebo 1 , Ben Torralva 1 , Steve Yalisove 1 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Recent work with femtosecond laser irradiated silica on Si has revealed a unique regime of fluence, film thickness, and initial silica residual stress where 780nm light is absorbed, non-linearly, in only a 1200nm thick oxide film. This anomalous absorption will be described and a range of phenomena presented for single and multiple irradiation of a Gaussian focused laser spot. At the lower end of the fluence range the underlying Si is melted, as evidenced by a high reflectivity region from the remaining amorphous Si. As the fluence is increased, the central region returns to the original reflectivity but is surrounded by a higher reflectivity ring. This is unusual because it suggests at the highest intensities in the peak of the Gaussian distribution there is no melting of the underlying Si. Yet, at lower fluences on the wings of the Gaussian, there is clear evidence of melting. At still higher fluences, the ring gets bigger and a "toroidal" or washer shaped blister is formed. A band bending model will be presented to explain the origin of this anomalous absorption of light by a very thin SiO2 film at the highest intensities. Discussion will also include the glass transition temperature (Tg), the creation of a local stress dipole, the micron scale delamination mechanics, and the effect of the initial stress state of the SiO2 film.
9:45 AM - MM7.3
Mass Migration Induced by Two-photon Absorption into a Polymer Containing Y-shape Azo-chromophores.
Antonio Ambrosio 1 , Pasqualino Maddalena 1 , Andrea Camposeo 2 , Marco Polo 2 , Antonio Neves 2 , Dario Pisignano 2 , Antonio Carella 3 , Fabio Borbone 3 , Antonio Roviello 3 Show Abstract
1 , CNR-INFM CRS-COHERENTIA and Dipartimento di Scienze Fisiche, Università degli Studi di Napoli Federico II, Napoli Italy, 2 , CNR-INFM National Nanotechnology Laboratory, Lecce Italy, 3 , Dipartimento di Chimica, Università degli Studi Federico II, Napoli Italy
We report about the possibility to drive mass migration into an azo-based polymer by means of two-photon absorption. A symmetric donor-acceptor-donor structured Y-shape azo-chromophore is specifically synthesized to enhance two-photon absorption in the polymer. Features resulting from two-photon absorption in the near-infrared show sub-diffraction limit linear dimensions (250nm). Azo-polymers have been shown to be suitable for TPL. These are polymers containing azo-based moieties in the main polymer chain or as side chains. Trans-cis-trans isomerization cycles of the azo-groups are activated by UV or visible light and are responsible for a mass migration (material displacement) on the free surface of the azo-polymer films. So far, only a few works have reported on the possibility of driving the mass migration by TPA.Our work is mainly about the possibility to drive mass migration by means of a two-photon absorption process on the free surface of a specifically synthesized Y-shape azo-polymer film. The azo-polymer we used (PU-AS-Y-DCV) is a polyurethane obtained containing, in the main chain, a symmetric azo-derivative of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)-styryl]-4H-pyran (DCM), a chromophore typically used as a very efficient red emitting dye. In accordance to what already reported in literature about the design of high-TPA molecules, our azo-chromophore, characterized by a symmetric donor-acceptor-donor π-conjugated structure, is expected to show good TPA features. The samples we used for TPL are high optical quality films, with thickness of 550 nm, prepared by spin coating a pyridine solution (polymer concentration 5% wt) on glass coverslips. The experimental setup is constituted by a near infrared femtosecond Ti:Sapphire laser (λ=800 nm, pulse width < 100 fs, repetition rate of 76 MHz) that is coupled to the high N.A. (1.3) oil immersion objective of an inverted microscope that focuses the laser beam into a diffraction limited spot on the sample surface. The latter is mounted on a piezo-scanner that allows to adjust the sample position with nominal resolution of the order of few nanometers. In our experiment, the sample is first placed in the focal plane, then the light is injected through the microscope objective. After several seconds exposure, the laser beam is blocked and the sample is moved into another position of the focal plane. The exposure can be repeated on an area a few tens of microns wide, depending on the piezo-scanner employed. These experiments have been performed by using laser peak intensities in the range 0.3-3.5 GW/cm^2 in order to prevent from other phenomena like laser ablation, occurring at a peak intensity as high as 50 GW/cm^2. By this way we have realized structures down to 250 nm wide (not shown here) employing the diffraction limited spot of the 800 nm wavelength pulsed laser, far below the half-wavelength diffraction limit of the focused laser beam.
10:00 AM - MM7.4
Formation of Elemental Distribution in Glass using Thermal Accumulation with Femtosecond Laser Irradiation.
Masahiro Shimizu 1 , Kiyotaka Miura 1 , Naomi Yasuda 1 , Masaaki Sakakura 1 , Shingo Kanehira 1 , Masayuki Nishi 1 , Yasuhiko Shimotsuma 1 , Kazuyuki Hirao 1 Show Abstract
1 , Kyoto University, Kyoto Japan
The femtosecond laser has been recognized as an effective tool for microscopically and three-dimensionally modifying transparent materials such as glass. In femtosecond laser processing, the heat generated by laser irradiation has often been thought of as a disadvantage that contributes to low accuracy in processing. However, a processing method that uses the heat accumulated during laser irradiation at a high repetition rate has attracted considerable attention because it is effective for the writing of low-loss optical waveguides and for space-selective precipitation of functional crystals inside the glass. In recent years, we confirmed that the elemental distribution of a ring-shaped pattern in homogeneous glass could be formed by focusing a lot of laser pulses at a high repetition rate (more than several hundred kHz) in the glass. This means the migration of specific ions on the order of micrometers. Based on experimental results obtained for various types of glass, we found that the tendency in the elemental distribution depended on the strength of the bond between cations and oxygen ions. Strongly bonded ions like Si or Al migrated to the center of the irradiated spot, whereas weakly bonded ions such as Na or Ca migrated to the outside. To clarify the phenomenon of ion migration, we investigated the temperature distribution around the focal spot. We verified that the temperature was over 1000 °C and the temperature gradient was over several tens of °C/µm in the ion migration area during laser irradiation. Therefore, this migration phenomenon might largely be due to the Soret effect. It is also possible that the iteration of thermal expansion and a pressure wave originating from pulse-to-pulse temperature elevation can be connected with this phenomenon. About ten years ago, our group discovered that refractive index changes on the order of 10-2 to 10-3 could be induced within various types of glass. However, in the present study, because the increase in refractive index is related to local densification in the glass, the maximum laser-induced refractive index difference was limited to ~10-2. The formation of the elemental distribution has the potential to induce a larger refractive index change compared with the densification. By controlling the glass compositions, we succeeded in increasing the concentration of Ga and Ge ions, which contributed to a high refractive index in the center of the focusing area. By using this result, we were able to successfully write optical waveguides in phosphate and borate glasses, where elemental distributions were continuously induced along a path traversed by the focal point. For the phosphate glass, the refractive index change Δn of the writing waveguide was estimated to be about 0.1, which is one order of magnitude larger than that in the conventional method using the density change.
10:15 AM - MM7.5
Ultrafast Defect Manipulation with Optical Anisotropy in Fused Silica.
Yasuhiko Shimotsuma 1 , Masaaki Sakakura 1 , Peter Kazansky 2 , Kiyotaka Miura 3 , Kazuyuki Hirao 3 Show Abstract
1 Innovative Collaboration Center, Kyoto University, Kyoto, 0, Japan, 2 Optoelectronics Research Centre, University of Southampton, Southampton United Kingdom, 3 Department of Material Chemistry, Kyoto University, Kyoto Japan
Oxygen vacancies affect fundamental properties of material ranging from ionic and electronic conductivities, light emitting, to form birefringence. Such oxygen defects are typically localized by intrinsic charge compensation or external electric field. However electron localization during the femtosecond time scale are widely studied, the dynamics of defect formation and evolution has remained poorly understood. Furthermore, until now alignment of molecules, which are required a time scale of nanosecond, along the light polarization direction in gas phase were demonstrated, there is few evidence of ultrafast defect alignment and evolution in solid phase under an intense light field. Here we demonstrate that directional control of self-organized defect structure with a time scale of pulse width inside material (~ 124 fs), which exhibits form birefringence created by sub-wavelength scale interfaces. Self-aligned nanostructure consists of oxygen defect, which indicates a remarkable non-reciprocity, has initially evolved from residual birefringence originated from internal stress distribution due to local heating with strongly anisotropy followed by structural change within at least 100 shots, regardless of interpulse time. Such anisotropic light-matter interaction could be interpreted as an asymmetric relation between light polarization and pulse front tilt. Apart from fundamental interest, the direction of encoded birefringence can introduce an entirely new concept for rewritable optical storage beyond the diffraction limit of light based on the liquid-crystal technology in read and write.
10:30 AM - MM7.6
Femtosecond Pulsed Laser Deposition of Metal and Metal Oxide Thin Films to Tune their Properties by Controlling their Particle Size and Surface Morphology.
Makoto Murakami 1 , Bing Liu 1 , Zhendong Hu 1 , Yong Che 1 Show Abstract
1 , IMRA America, Inc, Ann Arbor, Michigan, United States
Pulsed laser deposition using nanosecond pulsed laser (ns-PLD) such as excimer and Nd:YAG lasers has been proved that decent technique to grow high quality epitaxial thin films and various nano-structured thin films such as nanowire and nanorod. While, recent progress of ultrashort pulsed laser technologies provide alternative laser options for PLD. It has been demonstrated that PLD using femtosecond pulsed laser (fs-PLD) is able to avoid the generation of droplets, and nanoparticles are automatically produced during ultrafast ablation process. However, because of the unique feature of femtosecond laser ablation, it has been difficult to obtain atomically smooth thin films. Moreover, many varieties of high power pulsed lasers are available for PLD; however, tunability of the product material properties by only varying the laser parameters is limited with each single laser.In this presentation, to control the particle size and the surface morphology, we introduce a novel method of PLD using a femtosecond pulsed laser with burst-mode, which is a multi-pulse mode of selected number of pulses with very short time separation (20 ns) in between the pulses. This method enables controllability for thin-film morphologies, ranging from nanoparticle aggregates to epitaxial thin films with completely droplet-free and atomically smooth surfaces . TiO2, one of the wide band gap semiconductors, is used as a demonstration material in this work. A fiber based femtosecond laser (FCPA μJewel™ D-1000 laser developed by IMRA America, Inc.) is used for generating burst-mode laser pulses. The number of burst-mode pulses is selectively controlled from 1 to 20, and the repetition rate of the burst is also tunable in the range of 0.1 – 5 MHz. A TiO2 ceramic target placed in vacuum chamber is ablated with different number of pulses. When more number of pulses is used for the ablation (typically more than 10 pulses), atomically smooth thin films free of droplets and clusters are obtained. While by decreasing the number of bust-pulses and/or increasing pulse energy for the ablations, rough films of nanoparticle aggregates are obtained with increasing particle size. Plasma enhancement during burst-mode ablation is also confirmed by increasing the number of burst pulses. Tunability of the nanoparticle size, of the thin film surface morphology, and of the laser plume are achieved by controlling the number of burst-mode pulses and pulse energy. Morphology control of thin films is further demonstrated for many other materials such as metals (Au, Pt, Co etc.), metal oxides (SnO2, Fe3O4, HfO2, Cr2O3, SiO2, BaTiO3 etc.), and semiconductors (e.g. Si).  M. Murakami et al. Appl. Phys. Express, 2 042501 (2009)
10:45 AM - MM7.7
Resonance Picosecond IR Laser Decomposition of Molecular Precursors and Formation of Superhard Coatings of ReB2, RuB2, WB4 and B4C by Confined Plume Chemical Deposition.
Borislav Ivanov 1 , Matthew Wellons 2 , Charles Lukehart 2 Show Abstract
1 Physics, Vanderbilt University, Nashville, Tennessee, United States, 2 Chemistry, Vanderbilt University, Nashville, Tennessee, United States
Recent confirmation of superhard microcrystalline ReB2 (average hardness of 48 GPa) inspires a search for methods of preparing thin-film coatings of this and other transition metal borides. Thin-film ReB2 has been formed only by pulsed laser deposition onto a hot substrate (570 °C) under high vacuum at very low deposition rate. We report a general, one-step method of preparing ceramic coatings on inorganic and polymer substrates using chemical precursors and a confined-plume chemical deposition (CPCD) synthesis strategy. This method can be used to form microcrystalline ReB2 coatings on Kevlar® fabric, IR2® or Teflon® plates and Ultralene® films. Deposition of microcrystalline coatings of RuB2, WB4, or B4C has also been demonstrated using CPCD processing when different chemical precursors are used. When liquid chemical precursor, (B3H8)Re(CO) 1, is confined between two Si wafers is irradiated in ambient atmosphere with a pulsed Free Electron Laser (FEL) IR beam tuned to the most intense C–O stretching vibration of precursor 1 (5.1 μm), a vapor plume forms at the IR beam/precursor contact interface indicating rapid decomposition of precursor molecules. By rastering the FEL beam across the sample in linear scans (1 mm/s) separated spatially by 150 μm, a 0.75 cm x 0.75 cm sample area can be reacted within 6 minutes. Irradiation of precursor 1 specimens with FEL IR light tuned to 5.1 μm (resonant absorption) supported on unconfined, open-faced Si wafer or NaCl plate supports gives visible formation of a reaction plume but only Re metal or Re oxide as crystalline products. Physical confinement of the reaction plume is required for ReB2 formation. In addition, thermal decomposition of precursor 1 confined between two Si wafers in a tube furnace or by rapid microwave heating gives only Re metal with no detectable formation of ReB2 as crystalline phases.
11:00 AM - MM7.8
Rapid Solid-Liquid Phase-Change Oscillations in Nanocrystalline Silicon Wires.
Adam Cywar 1 , Gokhan Bakan 1 , Faruk Dirisaglik 1 , Kadir Cil 1 , Helena Silva 1 , Ali Gokirmak 1 Show Abstract
1 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States
High precision resistor trimming is used to fabricate resistors with precisions that cannot be achieved using conventional techniques due to process variations. Successive pulsed current anneals  of highly-doped polycrystalline Si resistors are used to increase or decrease wire resistance by controlling the magnitude and duration of the pulses. We have observed that it is possible to melt nanocrystalline Si (nc-Si) micro-wires when high current densities (> 20 MA/cm2) are forced through them. With the introduction of a series load resistor and a parallel capacitor, these wires are observed to melt and resolidify in repeated cycles  leading to strong current oscillations.The wires used in the experiments are formed by