Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X26: Chemical Physics at the Edges IIIFocus
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Sponsoring Units: DCP Chair: Libai Huang, Purdue University Room: 289 |
Friday, March 17, 2017 8:00AM - 8:36AM |
X26.00001: Attosecond electronic band gap dynamics. Invited Speaker: Stephen Leone High-order harmonics in the extreme ultraviolet (XUV) are used for attosecond transient absorption pump-probe measurements on solid-state materials. In crystalline silicon, an 800 nm, strong-field tunneling process is used to inject electrons into the conduction band, and the silicon 2p L edge absorption to the conduction band is measured at 100 eV photon energies. A 450 attosecond timescale broadening in the conduction band is observed, indicative of a rapid electronic response due to the injection of carriers into the conduction band. On longer timescales, participation of phonons and electron-phonon scattering differences within band valleys are observed. In germanium, both electrons and holes are detected through the 3d electrons of Ge, and hot carrier relaxation of both electrons and holes is measured. In vanadium dioxide, where an insulator to metal transition can take place, both the hot and cold phase are analyzed, revealing a short timescale insulator to metal transition of electronic origin. These will be discussed along with a general view of attosecond measurements in semiconductors and related electronic materials. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 9:12AM |
X26.00002: Isolated attosecond pulses in the water window Invited Speaker: Zenghu Chang Millijoule level, few-cycle, carrier-envelope phase (CEP) stable Ti:Sapphire lasers have been the workhorse for the first generation attosecond light sources in the last decade. The spectral range of isolated attosecond pulses with sufficient photon flux for time-resolved pump-probe experiments has been limited to extreme ultraviolet (10 to 150 eV). The shortest pulses achieved are 67 as. The center wavelength of Ti:Sapphire lasers is 800 nm. It was demonstrated in 2001 that the cutoff photon energy of the high harmonic spectrum can be extended by increasing the center wavelength of the driving lasers. In recent years, mJ level, two-cycle, carrier-envelope phase stabilized lasers at 1.6 to 2.1 micron have been developed by compressing pulses from Optical Parametric Amplifiers with gas-filled hollow-core fibers or by implementing Optical Parametric Chirped Pulse Amplification (OPCPA) techniques. Recently, when long wavelength driving was combined with polarization gating, isolated soft x-rays in the water window (280-530 eV) were generated in our laboratory. The number of x-ray photons in the 120--400 eV range is comparable to that generated with Ti:Sapphire lasers in the 50 to 150 eV range. The yield of harmonic generation depends strongly on the ellipticity of the driving fields, which is the foundation of polarization gating. When the width of the gate was set to less than one half of the laser cycle, a soft x-ray supercontinuum was generated. The intensity of the gated x-ray spectrum is sensitive to the carrier-envelope phase of the driving laser, which indicates that single isolated attosecond pulses were generated. The ultrabroadband isolated x-ray pulses with 53 as duration were characterized by attosecond streaking measurements. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:48AM |
X26.00003: Attosecond electron dynamics in molecules and liquids Invited Speaker: Hans Jakob Wörner The ultrafast motion of electrons and holes following light-matter interaction is fundamental to a broad range of chemical and biophysical processes. In this lecture, I will discuss some of our recent experiments that measure the atomic-scale motion of charge with attosecond temporal resolution (1 as = 10$^{-18}$s). The first experiment is carried out on isolated, spatially oriented molecules in the gas phase. Using high-harmonic spectroscopy, we resolve the migration of an electron hole across the molecule with a resolution of $\sim$100 as and simultaneously demonstrate extensive control over charge migration [1]. In the second class of experiments, we use an attosecond pulse train synchronized with a near-infrared laser pulse to temporally resolve the process of photoemission from molecules in the gas phase [2] and from a liquid-water microjet, resolving electron transport through liquid water on the attosecond time scale.\\ [1] P. M. Kraus, B. Mignolet, D. Baykusheva, A. Rupenyan, L. Horny, E. F. Penka, G. Grassi, O. I. Tolstikhin, J. Schneider, F. Jensen, L. B. Madsen, A. D. Bandrauk, F. Remacle, and H. J. W\"orner, {\it Science} {\bf 350}, 790 (2015).\\ [2] M. Huppert, I. Jordan, A. von Conta and H.J. W\"orner, {\it Phys. Rev. Lett.} {\bf 117}, 093001 (2016) [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X26.00004: Abstract Withdrawn
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Friday, March 17, 2017 10:00AM - 10:12AM |
X26.00005: Ultrafast Imaging of Chiral Surface Plasmon by Photoemission Electron Microscopy Yanan Dai, Maciej Dabrowski, Hrvoje Petek We employ Time-Resolved Photoemission Electron Microscopy (TR-PEEM) to study surface plasmon polariton (SPP) wave packet dynamics launched by tunable (VIS-UV) femtosecond pulses of various linear and circular polarizations. The plasmonic structures are micron size single-crystalline Ag islands grown in situ on Si surfaces and characterized by Low Energy Electron Microscopy (LEEM). The local fields of plasmonic modes enhance two and three photon photoemission (2PP and 3PP) at the regions of strong field enhancement. Imaging of the photoemission signal with PEEM electron optics thus images the plasmonic fields excited in the samples. The observed PEEM images with left and right circularly polarized light show chiral images, which is a consequence of the transverse spin momentum of surface plasmon. By changing incident light polarization, the plasmon interference pattern shifts with light ellipticity indicating a polarization dependent excitation phase of SPP. In addition, interferometric-time resolved measurements record the asymmetric SPP wave packet motion in order to characterize the dynamical properties of chiral SPP wave packets. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X26.00006: Femtosecond time-resolved photoemission electron microscopy and directed launching of propagating surface plasmons Wayne Hess, Yu Gong, Alan Joly, Patrick El-Khoury We image propagating and localized surface plasmons (PSPs and LSPs) on noble metal surfaces using femtosecond time-resolved photoemission electron microscopy (tr-PEEM). Our experiments employ identical, spatially separated, and interferometrically-locked femtosecond laser pump-probe pulses with a time step resolution of 210 attoseconds. The recorded time-resolved movies of PSPs allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (785 nm) and group velocity (0.95c). In concert with finite-difference time domain simulations, tr-PEEM results indicate a lower limit of 75 $\mu $m for the decay length of the PSP on gold. In addition we discuss coupling and interferometric focusing of PSPs using nanohole arrays. Recorded photoemission patterns are attributed to constructive and destructive interference between propagating surface plasmons launched from individual nanoholes. We demonstrate how varying the array geometry (hole diameter, pitch, and number of rows/columns) ultimately yields focused photoemission patterns. Finally, we demonstrate polarization directed launching of surface plasmons, from simple symmetric trench nanostructures, and explain effects of laser polarization on coupling to PSP and LSP modes. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X26.00007: Probing Active Species in the Nanoscale by Combining XAFS and TEM in Operando Conditions Anatoly Frenkel Understanding mechanisms of work in nanoscale systems is often hindered by their inherent complexity and by our inability to identify and characterize their ``active'' sites. In the size range of 1-5nm, they feature a variety of structural motifs, sizes, shapes, compositions, degrees of crystalline order as well as multiple temporal scales. An additional challenge is that only a fraction of them are actors in the catalytic performance, while majority are spectators. Significant progress in developing such tools for studying nanomaterials can be achieved only when active species can be reliably isolated from spectators, and their role in mechanism of work is understood. In our approach the activity of nanomaterial is measured concurrently with other characteristics, obtained by advanced scattering, spectroscopy and imaging methods. In this talk I will demonstrate the application of a microreactor, compatible with electron microscopy and X-ray Absorption Fine Structure spectroscopy probes, for this purpose. I will illustrate its application by our observation of reaction-driven restructuring of Pt catalysts in the size range from single atoms to 3nm in diameter during catalytic hydrogenation of ethylene. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X26.00008: In-situ electrochemistry in photoemission microscopy Slavomir Nemsak, Johanna Hackl, Hongxuan Guo, Evgheni Strelcov, Alexander Yulaev, David Mueller, Claus Schneider, Andrei Kolmakov Until recently, photoemission electron microscopy (PEEM) could not be used in studies of solid/liquid interfaces due to major instrumental difficulties. The usual technique of differential pumping, which allows photoelectrons to reach the detection in ambient pressure photoemission spectroscopy, cannot be simply realized in PEEM, mostly due to the presence of high potential difference between a specimen and extractor lens. One of the ways to overcome this problem is to use a sample capped with electron transparent molecularly impermeable membrane, which would leave the vacuum conditions between the lens and the sample unaffected [Kolmakov et al., Nat. Nano. 6, 651 (2011)]. Application of different potentials at various points on the sample then allows a use of spectromicroscopy together with electrochemistry. We present a working concept of electrochemical cell inside a photoemission microscope. The cell uses a capping membrane made of a few-layer graphene. In this configuration, the graphene membrane acts also as a top electrode and the electrochemical cell is built vertically [Kolmakov et al., Topics in Cat. 59, 448 (2016)]. A liquid contained in the cell is then imaged with photoelectrons under operating conditions with high spatial resolution and chemical sensitivity. [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X26.00009: Photo-induced Subpicosecond Temperature Jump in MoSe$_{\mathrm{2}}$ bilayer Ming-Fu Lin, Clemens Weninger, Vidya Kochat, Amey Apte, Xiang Zhang, Pulickel Ajayan, Oleg Prezhdo, Kristin Persson, Aiichiro Nakano, Renkai Li, Xijie Wang, Priya Vashishta, David Singh, David Fritz, Uwe Bergmann We observed sub-picosecond dynamics of lattice vibration in MoSe$_{\mathrm{2}}$ bilayer initiated by optical excitations at 800 nm and 400 nm, respectively, at high electron-hole plasma density. These photoinduced dynamics are probed by a delayed mega-electronvolt ultrafast electron diffraction (MeV-UED) that allows us to measure the conversion of electronic energy of carriers to in-plane lattice motion in real time. Corresponding ultrafast dynamics of lattice vibration is different from the irreversible inertial atomic acceleration model. Efficient electronic energy conversion to lattice motion signifies the possibility of light-induced approach for controllable temperature jump for future application in phase patterning of few-layered devices using transition metal dichalcogenides. [Preview Abstract] |
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