Welcome to the Horn-von Hoegen Research Group
07.08.2019 Editor´s Pick in Structural Dynamics
Structural Dynamics 6 (2019)
The velocity of a 1D propagating recrystallisation front was determined for the atomic wire system of indium on Si(111) by ultrafast electron diffraction. At a speed of 100 m/s, the system falls back from an excited metastable state - generated by an fs-laser pulse - to the ground state: like a chain of falling dominoes. This recrystallization front is triggered by the few atomic step edges of the almost perfectly smooth surface of the silicon sample.
17.07.2019 Graphene likes the Twist
Nano Lett. 19 (2019)
Graphene does not like to be compressed – instead it forms twisted domains during epitaxial growth on an iridium substrate. This surprising result of small-angle rotations were found in the team of Michael Horn von Hoegen through unique moiré pattern in high-resolution low energy electron diffraction. The thermal expansion of the substrate acts as an effective compressive biaxial pressure on the graphene, which is more easily accommodated by small rotations rather than by compression. This effect is lately referred to as rotational epitaxy. The results have now been published in “ACS NanoLetters”.
26.02.2019 Editors Pick in Applied Physics Letters
The publication "Rapid Onset of Strain Relief by Mass Generation of Misfit Dislocations in Bi(111)/Si(001) Heteroepitaxy" was honored as "Editors Pick" at Applied Physics Letters. Dennis Meyer describes the relaxation of a heteroepitaxial Bi(111) film on a Si(001) substrate. The anisotropic lattice mismatch of 2.4% between bismuth and silicon at a layer thickness of 4 nm is compensated by suddenly generated dislocations at the interface between Bi and Si. From the relative displacement of two LEED diffraction spots, the formation of these dislocations was observed with high accuracy during growth.
26.02.2019 Plasmons in ACS Photonics
In the 19th century, Augustin Jean Fresnel invented his zone-plate; basically a hologram, which - by diffraction - forms a defined focal point if illuminated with a plane light wave. Frank Meyer zu Heringdorf's TR-PEEM Team has now applied the concept of Fresnel-focusing to surface-plasmon-polaritons, i.e., to electron-density waves that can propagate at metal surfaces with almost the speed of light in vacuum. Instead of focusing traveling plasmon waves by a Fresnel type zone plate, however, the team structured particular Fresnel-type grating couplers that were illuminated by femtosecond laser pulses, and that very effectively excited plasmon waves forming a focus at a desired location. Using their time-resolved photoemission microscope, the team could characterize this "Fresnel-optics for surface plasmon waves" in detail, and they could follow the formation of the focus in a super-slow motion movie on the femtosecond time-scale. The results were now published in ACS Photonics.
08.11.2017 New Publication in Nano Letters
It is a well-established fact that the illumination of a surface with light of sufficient photon energy results in the emission of electrons. But is the same also true if surface plasmon polaritons are used instead of the light? Surface plasmon polaritons are electron density modulations that can optically be exited at noble metal surfaces and that can propagate across the surface as a wave. The local variation in electron density of the plasmon is coupled to an electric field, so it sounds reasonable to assume that electron emission might be possible from a surface plasmon polariton. To experimentally verify this hypotheses, the TR-PEEM team around Prof. Frank Meyer zu Heringdorf used custom-tailored grating couplers to focus surface plasmon polaritons into a small spot. By employing femtosecond time-resolved microscopy, the team was able to separate light-induced effects from plasmon-induced effects in time and space and were able to unambiguously distinguish between the well-known photoemission and what the team coined “plasmoemission”. As the plasmon intensity is high in the focus spot, the team observed highly nonlinear emission behavior, i.e. processes, where five plasmon quanta were absorbed by one electron prior to emission from the solid.
12.07.2017 New Publication in „Science Advances“
It is not possible to focus light into an arbitrarily small spot. Even if one used enormously large lenses, a parallel beam of light would be focused into a “beam waist” instead of a point-like focal spot. The diameter of the beam waist depends on the wavelength of the light to be focused. In a collaboration with colleagues from Stuttgart and Haifa, the team of Prof. Dr. Frank-J. Meyer zu Heringdorf has now used a trick to nevertheless indirectly focus light into a small sport. The team used 800nm light to excite surface plasmon polaritons at the interface between an atomically smooth Gold platelet and a Silicon substrate. Such “short range” plasmons exhibit a wavelength of only 180nm. By forming a plasmonic focus and by using a transient standing wave field, the authors were able to create an electron-emission spot with a minimal dimension of only 60 nm.
30.03.2017 Read our Article in Nature
Using ultrafast electron diffraction, the dynamics of a structural phase transition driven by femtosecond laser pulses could be observed with atomic spatial and femtosecond-temporal resolution. With a time resolution of less than 350 fs (10-15 s), Dr. Tim Frigge succeeded in tracing the movement of the atoms for the first time. These ground-breaking results have been published in Nature, supplemented by the work of the Bovensiepen group at the University of Duisburg-Essen and the work of the Schmidt group at the University of Paderborn.
17.03.2017 Read our Article in Science
In cooperation with scientists from the universities of Haifa (Israel), Kaiserslautern and Stuttgart, Professor Frank Meyer zu Heringdorf’s team published an article in Science magazine. Using time-resolved non-linear photoemission microscopy, surface plasmon polariton waves were observed with sub-femtosecond time-resolution to spiral to the center of an archimedean spiral. The spiral plasmon waves show an orbital angular momentum that was imprinted into the plasmon wave by using circularly polarized femtosecond laser pulses.
You can read an interview about it here (in German).
28.10.2016 SPA-LEED Workshop 2016: A Great Success!
For almost two days, 50 participants presented and discussed their research results in the "Wolfsburg" in Mülheim an der Ruhr. Four high-ranking speakers (Claus Ropers (University of Göttingen), Stefan Kowarik (HU Berlin), Michael Tringides (Ames Lab, University of Iowa) and Michael Ramsey (University of Graz)). The workshop was financially supported by CENIDE, the SFB 1242, the FOR 1700 and the SPA-LEED manufacturer ScientaOmicron. The next SPA-LEED workshop will be organized by Professor Christian Kumpf of the Forschungszentrum Jülich in 2018.
14.10.2016 DFG Information Meeting
At the next DFG information meeting, organized by the Science Support Center of the University of Duisburg Essen, Professor Michael Horn-von Hoegen will be giving advice for writing successful poposals. The meeting will be held at the Fraunhofer Haus 2 on November 15, 2016. After the general meeting, individual coaching will take place in workshops until 6 pm.
You can find more information here (in German).
08.09.2016 Project in FOR1700 Is Funded
The DFG will support the project "Time-resolved diffraction of photo-induced phase transitions in 1D metal wires on semiconductor surfaces" within the FOR1700 research group, for 3 years.
20.05.2016 The Collaborative Research Centre 1242 Is Established
The DFG has established the SFB 1242, titled "Non-equilibrium dynamics of condensed matter in the time domain" in the Department of Physics. Starting on July 1, three projects in our workgroup will be funded with a total of four doctoral students and one postdoc, and these projects will examine the ultrafast structural and electronic dynamics of surfaces and thin films. In project C3 "Driven phase transitions on surfaces", a new apparatus for ultrafast electron diffraction will be built to break the current world record for the time resolution of 350 femtoseconds. With an improved time resolution of 200 femtoseconds the movement of atoms could be directly followed! In project B4, "Non-equilibrium dynamics of the phonon system", the generation, transformation and non-equilibrium of phonons in adsorbate systems and thin metal films is monitored by means of inelastic ultrafast electron diffraction after impulsive excitation. In project B6, the spatial-temporal separation of laser and plasmon pulses on the femtosecond time scale achieved in preliminary work is to be used in order to allow the targeted interaction of plasmon waves with one-particle electron excitations in nanostructures.
01.05.2016 Humboldt Foundation Scholarship Awarded to Dr. Marin Petrovic
After completing his dissertation in Zagreb on the topic of graphene, Dr. Petrovic successfully competed for a coveted Alexander von Humboldt Foundation fellowship. In cooperation with his host, Professor Frank Meyer zu Heringdorf, Dr. Petrovic will use low-energy electron microscopy (LEEM) to examine the growth of hexabornite nitride (hBN) Layers on iridium as well as the formation of lateral graphene / hBN heterostructures (in-situ).
15.04.2016 SPA-LEED Workshop 2016
Spot Profile Analysis Low Energy Electron Diffraction, i.e., SPA-LEED, was developed at the University of Hannover in the 1980s, and has since then acquired a dedicated group of researchers who have applied the technique to a variety of surface science problems. Researchers who work actively with SPA-LEED meet every 1-2 years for an informal exchange of their research and to discuss the future of the method.
The last meeting was in Hannover on the occasion of Professor Martin Henzler’s 80th birthday; the next SPA-LEED workshop will be hosted by Professor Michael Horn-von Hoegen and Professor Frank Meyer zu Heringdorf from the University of Duisburg-Essen, Germany.
12.08.2015 Frank Meyer zu Heringdorf Appointed Associate Professor
Dr. Frank Meyer zu Heringdorf was appointed associated professor on August 12, 2015, with the three external experts on the recommendation panel praising his stellar performance in research and teaching. Professor Meyer zu Heringdorf will hold an introductary lecture addressing plasmon propagation in slow-motion as part of the physics colloquium on April 27th, 2016.
29.5.2015 Tim Frigge Wins Award at Japanese Symposium
At the "9th International Symposium on Ultrafast Surface Dynamics" in Lake Biwa, Japan, Tim Frigge won the "Best Presentation Award for Young Scientists" award for his lecture "Ultrafast Structural Dynamics of Photo Induced Phase Transitions on Surfaces Observed by fs-RHEED."
19.03.2015 DPG Spring Meeting
The following invited talk will be presented at the Spring Meeting of German Physical Society in Berlin:
"1D Metal Wires at Surfaces: Preparation, Phase Transitions, and Ultrafast non Equilibrium Dynamics"
You can download the PDF-version here.