Microwavephysics and Atmospheric Physics
Biomedizinische Photonik
Ultrafast Science and Technology
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Last update: 13.09.2017
FS 2011: Seminare über Ultrafast Science and Technology
Thursday 11:15am
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Mittwoch, 19.01.2011

Highly Efficient Nonlineaer Optical Organic Single Crystals for THz Wave Applications

Zeit: 10:15 Uhr
Hörsaal: B116
Prof. O-Pil Kwon
Department of Molecular Science and Technology
Ajou University

Organic nonlinear optical crystals are very promising for numerous applications such as integrated photonic devices and THz wave generation and detection. Here new series of nonlinear optical crystals based on configurationally locked polyene (CLP), nitrophenylhydrazone (NPH) and stilbazolium derivatives are presented.

Donnerstag, 10.03.2011

Microscopic description of the melting of a charge density wave

Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Fabrizio Carbone
Laboratory for ultrafast electron scattering
École Polytechnique Fédérale de Lausanne (EPFL)

During a phase transition, a thermodynamic potential or one of its derivatives presents a discontinuity as a function of temperature, pressure, magnetic field or chemical doping. In anisotropic solids, a transition towards an electronically ordered state termed charge density wave (CDW) can occur. Such state of matter can be perturbed by an external electric field causing the sliding of the density wave , or it can be molten providing a unique microscopic playground for the study of phase transition mechanisms in complex systems, through their evolution via multiple intermediate states. CDWs, Josephson arrays, or sandpile automata have been investigated in relation to the scale invariance of self-organized critical phenomena, of which avalanches are dramatic manifestations. Here, we melt the charge ordering in a complex quasi-molecular solid with laser light and monitor the consequent charge redistribution by probing the optical response over a broad energy range with polarized fs supercontinuum pulses; the electronic structure is modeled ab-initio and we show that abruptly perturbing the system leaves the charge order intact until the lattice has crossed the phase transition temperature.

Donnerstag, 24.03.2011


Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Marc-A. Nicolet
Professor of Electrical Engineering and Applied Physics, Emeritus, Caltech, Pasadena

Im letzten Band der wissenschaftlichen Zeitschrift Science von 2004, Seite 666, haben zwei russische Emigranten von der Universität Manchester, Andre K. Geim (geb.1958) und Konstantin S. Novoselov (geb. 1974) und sechs Mitarbeitende einen kurzen Rapport publiziert ("Electric Field Effect in Atomically Thin Carbon Films"). Die dort beschriebenen experimentellen Resultate über Graphene erzeugten grosse Aufregung und lösten eine enorme Forschungstätigkeit mit Graphene aus. Zwei Stossrichtungen entwickelten sich sobald. Eine Richtung untersucht spezifisch die Möglichkeiten elektrischer Anwendungen. Die anderen Studien trachten ganz allgemein nach der wissenschaftlichen Charakterisierung von Graphene. Der Vortrag wird Beispiele beider Arten präsentieren, um einen kleinen Einblick in dieses neue Gebiet zu geben.

Donnerstag, 07.04.2011

Ultrafast Spin Dynamics and Spin Manipulation

Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Mathias Kläui
Laboratory of Nanomagnetism and Spin Dynamics
SwissFEL, Paul Scherrer Institut & Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne

So far conventional magnetic fields have been used to excite spin dynamics and manipulate magnetization. While this approach is now reasonably well understood and widely employed in devices, it entails limitations for the speed of magnetic switching as intrinsically the spin dynamics is limited by the precession frequency corresponding to the magnetic field.

To overcome this limitation and explore the rich physics of the interaction between spin currents, photons and the magnetization, we have used spin-polarized charger carriers and photons to excite spin dynamics and manipulate the magnetization on ultrafast timescales.

Spin-polarized currents can directly transfer spin angular momentum to the magnetization, leading to fast switching of multilayer nanopillars (reciprocal effect to the Giant Magnetoresistance effect for which the Nobel prize was awarded 2007) and the generation of GHz microwave oscillations. We have shown how the dynamics of inhomogeneous spin textures, such as domain walls can be efficiently controlled using such large spin transfer torques [1] and we have explained how pure diffusive spin currents can induce even more efficient switching [2].

Using photons allows us to overcome the time limitations imposed by the charge carrier drift and diffusion dynamics and to studyon the femtosecond timescale the fundamental ultrafast energy and angular momentum transfer between the electron, spin and lattice systems. We have used optical laser pulses to study the angular momentum pathways leading to ultra-fast demagnetization in advanced materials [3]. Recent measurements at Free Electron Laser sources (where potentially sub-femtosecond time resolution can be achieved) reveal the dependence on the nanoscale spin structure.

[1] M. Eltschka et al., Phys. Rev. Lett. *105*, 56601 (2010); L. Heyne et al., Phys. Rev. Lett. *105*, 187203 (2010) - see Viewpoint article: Physics *3*, 91 (2010). [2] D. Ilgaz et al., Phys. Rev. Lett. *105*, 76601 (2010). [3] J. Walowski et al., Phys. Rev. Lett. *101*, 237401 (2008)

Donnerstag, 14.04.2011

Multi-modal CARS Microscopy Using a Simple Femtosecond Source

Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Albert Stolow
Steacie Institute for Molecular Sciences, National Research Council
Ottawa, Canada

We discuss a simplified CARS microscopy using a simple femtosecond Ti:Sapphire laser source combined with a photonic crystal fibre (PCF). By optimally chirping the fs pump and Stokes laser pulses, we achieve high quality multi-modal imaging (simultaneous CARS, two-photon fluorescence, and second harmonic generation) of live cells and tissues. The tunable Ti:sapphire output provides the pump beam directly, while part of this is converted to the red-shifted Stokes pulse using a PCF having two close-lying zero dispersion wavelengths. This type of PCF gives good power and stability over Stokes shifts ranging from below 1700 cm-1 to over 4000 cm-1. The noise equivalent power of the PCF output at frequencies relevant to live cell microscopy is not a significant issue, as is seen in the high quality images. Chirp as a control parameter permits simultaneous optimization of the spectral resolution and signal levels of these imaging modalities. By matching the chirp rates to create a constant frequency difference between the pump and Stokes pulses, we enhance the CARS spectral resolution (spectral focusing). By simply controlling the time delay between the input pulses, we achieve fast and continuous computer-controlled tuning of the Stokes shift over a broad range, without involving any adjustment of either the fs laser or the PCF. The simultaneous optimization of CARS, two-photon fluorescence and second harmonic generation is achieved by controlling the degree of chirp and involves a trade-off between spectral resolution and signal strength. This trade-off can be optimized under user control to suit the problem at hand. This simplified optical arrangement for multi-modal CARS microscopy [1,2] was recently commercialized by Olympus Corp [3].

In order to develop CARS microscopy applications outside the laboratory (e.g. in hospitals and clinics), the sensitive free space femtosecond laser and optics must be replaced with telecom-stability all-fibre sources. Using an IMRA fibre femtosecond laser as the pump and an Ultra Highly Nonlinear Fibre (suspended core) for Stokes generation, we recently demonstrated that this is feasible [4] and will likely be the route to further commercialization of CARS microscopes. With this very simple source, we demonstrate additional functionalities such as Spectral Scanning FM CARS. This is based on the fact that, in our arrangement, Stokes time delay corresponds to a tuning of the second order (CARS) resonance. Therefore, rapid modulation of this time delay leads to a rapid frequency sweep (FM) which can maintained while simultaneously scanning the pump-Stokes time offset (spectral scan). We also demonstrate time-correlated photon counting Fluorescence Lifetime Imaging (FLIM)-CARS microscopy, again based on this same simple femtosecond source. Live cell viability issues limit light exposure to signal levels corresponding typically to less than one anti-Stokes photon per laser pulse. In such cases, single photon counting becomes profitable, removing the detector electronics amplitude noise and permits the use of the time-correlated lifetime as an additional contrast mechanism.

REFERENCES 1. A.F. Pegoraro, A. Ridsdale, D.J. Moffatt, Y. Jia, J.P. Pezacki & A. Stolow, Optics Express 17, 2984 (2009) 2. A.F. Pegoraro, A. Stolow, A. Ridsdale, D.J. Moffatt, J.P. Pezacki, Y. Jia, Biophotonics 18(8), 36 (2009) 3. http://www.olympusamerica.com/ FV1000MPE femtoCARS Add-On 4. A.F. Pegoraro, A. Ridsdale, D.J. Moffatt, J.P. Pezacki, A. Stolow, B.K. Thomas, L. Fu, L. Dong, M.E. Fermann, Optics Express 17, 20700 (2009)

Donnerstag, 21.04.2011

Trapping of metallic Mie particles using a radially polarized laser beam

Zeit: 10:15 Uhr
Hörsaal: B116
Jan Locher
Institute of Applied Physics
University of Bern

In the talk, an optical tweezer will be presented which I have built for my master's thesis. After a short historical introduction, I will describe the physical basics behind optical trapping and point out the difference in trapping dielectric or metallic particles. A simple mathematical model, which helped to design the optical tweezer, will be presented. The second part will focus on the experimental set-up, with emphasis on the generation of a radially polarized laser beam. In the last part, the results are presented and discussed.

Donnerstag, 05.05.2011


Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Thomas Wihler
Mathematical Institute
University of Bern

In this talk we will present an elementary yet very effective numerical method for the approximation of (possibly unknown) data. To this end, piecewise polynomial functions of possibly varying local degree will be employed. Here, the goal is to design a computationally inexpensive procedure that is able to produce high quality approximations by determining the length of the local intervals and the approximation orders automatically. Particular interest will be put on data that is not known explicitly as is the case, for example, in the numerical solution of differential equations. The talk is suitable for a broader audience of scientists.

Dienstag, 10.05.2011

Electro-Optical Bunch Length Measurements at the Swiss Light Source (Promotionsvortrag)

Zeit: 10:15 Uhr
Hörsaal: B78
Felix Müller
Institute of Applied Physics
University of Bern

Dienstag, 17.05.2011

Improvement of the Physical Parameters for Vascular and Gastric Laser Soldering (Promotionsvortrag)

Zeit: 16:15 Uhr
Hörsaal: B5
Serge Bogni
Institute of Applied Physics
University of Bern

Donnerstag, 19.05.2011

Non-linear vibrational microscopy

Zeit: 10:15 Uhr
Hörsaal: B116
Prof. Dr. Andreas Zumbusch
Department Chemie, Universität Konstanz, 78457 Konstanz

During the last decade, ultrasensitive microscopy has become one of the most important tools in biophysics. Most prominent among the various techniques is confocal fluorescence microscopy. It is a very widespread technique with which sensitivities down to the single molecule detection limit can be achieved. In applications of fluorescence microscopy, however, one faces two problems: i) In most cases, the samples need to be labeled with a fluorophore which can be difficult and can change the properties of the sample. ii) All fluorophores are prone to photobleaching which severely limits the maximum achievable observation times.

In order to solve these problems, recent years have seen the development of a broad variety of different techniques, which generate contrast without the need of external staining. In this talk, Coherent Anti-Stokes Raman Scattering (CARS) microscopy providing chemically selective imaging will be presented 1. CARS microscopy is a non-linear optical technique in which two laser frequencies are used to excite the sample simultaneously. If the frequencies are chosen such that their difference coincides with a vibrational transition of the sample, a strong signal which is higher in energy than the excitation beams is generated. Apart from its biological applications, CARS microscopy has recently attracted a lot of attention as an application field for modern approaches of ultrafast spectroscopy 2,3. As an outlook, I will discuss ways to detect time resolved vibrational spectra of single molecules.

1 M. Müller, A. Zumbusch "Coherent Anti-Stokes Raman Scattering (CARS) microscopy" ChemPhysChem, 8 (2007) 2156-2169

2 G. Krauss, T. Hanke, A. Sell, D. Träutlein, A. Leitenstorfer. R. Selm. M. Winterhalder, A. Zumbusch "A compact coherent anti-Stokes Raman scattering microscope based on a picosecond two-color Er:fiber laser system", Opt. Lett., 34 (2009) 2847-2849

3 R. Selm, M. Winterhalder, A. Zumbusch, G. Krauß, T. Hanke, A. Sell, A. Leitenstorfer” Ultrabroadband background-free Coherent anti-Stokes Raman Scattering microscopy based on a compact Er:fiber laser system”, Opt. Lett., 35 (2010) 3282-3284

4 M. Winterhalder, A. Zumbusch, M. Lippitz, M. Orrit, "Towards Far-Field Vibrational Spectroscopy of Single Molecules at Room Temperature" J. Phys. Chem. B, in press

Dienstag, 31.05.2011

Measuring tissue scattering by Optical Coherence Tomography

Zeit: 10:15 Uhr
Hörsaal: A97
Prof. Ton G. van Leeuwen
Biomedical Engineering and Physics, Academic Medical Center, University of Amsterdam
Biomedical Photonic Imaging Group, MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente

Optical Coherence Tomography allows quantitative measurements of the scattering coefficient µs . We show that µs is related to tissue structures and disease, e.g. due to increased proliferation (tumors) and cell death (apoptosis and necrosis). Still, measurements are influenced by multiple scattering effects and concentration dependent scattering effects, which both lead to an underestimation of µs . Their contribution to the OCT-measured µs was investigated at 600, 800, 1300 and 1600 nm on Intralipid dilutions. A non-linear increase of µs with concentration was found for all wavelengths. At the higher wavelengths the contribution of multiple scattering is expected to be low. We therefore conclude that concentration dependent scattering effects should be taken into account for quantitative µs measurements.

Donnerstag, 30.06.2011

Ultrafast Excited-State Processes of Re Carbonyl-Diimine Complexes: from Excitation to Photochemistry


Zeit: 10:15 Uhr
Hörsaal: B77
Prof. Antonín Vlcek, Jr.
School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom.
J. Heyrovský Inst. of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18

Rhenium (I) complexes of the type fac-[Re(L)(CO)3(N,N)]n show exceptionally rich excited-state behavior and redox chemistry. They are thermally and photo chemically robust and highly flexible synthetically. Broad structural variations of the N,N ligand as well as the axial ligand L are possible, allowing us to incorporate these chromophores into a range of media, attach them to proteins, intercalate into DNA or make them parts of supramolecules designed to perform specific functions. The same structural and medium variations affect the nature and energetic order of low-lying excited states and, thus, the spectroscopy, photophysics, photochemistry and electrochemistry of the Re chromophore. In this seminar, we will focus on dynamic aspects of the processes triggered by electronic excitation and follow the evolution of excited Re chromophores from the instant of the photon absorption until the ground state is recovered or a photochemical product formed. We will proceed from the intersystem crossing, which ensues the photon absorption, to the hot 3MLCT lowest excited state, its relaxation in various media (solvents, ionic liquids and proteins), nonradiative decay to the ground state and its electron-transfer or isomerization reactions. Alongside, we will discuss the structures and characters of the excited states involved, as documented by time-resolved IR spectra and DFT calculations.

Dienstag, 05.07.2011

Shaping the future of biophotonics: applying novel light fields

Zeit: 10:15 Uhr
Hörsaal: A97
Prof. Dr. Kishan Dholakia, FRSE, School of Physics and Astronomy
University of St Andrews, North Haugh, St Andrews, Scotland

Light is incredible. 2010 marked the fiftieth anniversary of the laser. Its impact has been immense across all of the Sciences with an array of ground-breaking studies. Laser light typically emanates in what is termed a standard Gaussian beam. As we explore light propagation we see this is a “basic” solution and indeed is the form of laser light most used today. However it is becoming critically apparent to a wide range of science that this basic form of light propagation is insufficient for numerous applications and indeed that our very understanding of the propagation and application of light needs to be re-addressed particularly for turbid or complex media. Very surprising and startling results are possible. By engineering and exploiting the phase and amplitude characteristics of light – optical sculpting – we may provide a major breakthrough of how light actually propagates in a variety of media. This will lead to us many exciting questions: how might we locally overcome the diffraction limit in the far field? How might we overcome the resolution criteria in focusing? How do we obtain an optimal beam for trapping in a turbid or complex media? How might we even explore ‘sub-diffractive’ nanosurgery or multiphoton imaging of cells and tissue?

In this talk, I will describe some recent work where we apply exquisite control over the phase and amplitude of light to yield some very surprising results. By performing in situ adaptive optics, optical trapping of nanoparticles and microparticles through highly turbulent media becomes a reality. This opens up new vistas in precision measurements in complex media and potentially studies in vivo.

Separately, cell transfection is an important area of ultrafast biophotonics. The application of such ultrashort pulses from a high repetition rate laser creates a low density free electron plasma that photochemically disrupts the cell membrane in a transient fashion.

Excitingly the method allows transfer of macromolecules and drugs as well as nanoscopic particles into various cells and is an emergent area in Nanobiophotonics. I will describe our latest work in this field emphasising new physics of using both ‘non-diffracting’ light beams and far-field ‘sub-diffraction’ beam shaping to instigate transfection.

Donnerstag, 07.07.2011

Disentanglement of electron dynamics and space-charge effects in time-resolved photoemission from h-BN/Ni(111)

Zeit: 10:15 Uhr
Hörsaal: B78
Dominik Leuenberger, Universität Zürich, Surface Physics Group

We present time-resolved photoemission spectroscopy data from Ni(111) surface capped with a monolayer of hexagonal boron nitride (h-BN/Ni(111)). These data were taken using high laser fluence in order to study hot carrier and magnetization dynamics in ferromagnetic nickel covered with an insulating capping layer.

In the past decade time-resolved photoemission has evolved to one of the most powerful instruments for the investigation of transient structural, electronic and magnetic responses of a condensed-matter system to the pertubation by intense femtosecond pulses. At high excitation densities a high photoelectron background severely distorts the spectra in time-resolved experiments. However after emission on their drift to the detector, all electrons interact strongly exchanging kinetic energy due to mutual Coulomb repulsion, leading to shifts and broadening of the spectral distribution, a phenomenon called space-charge effects.

Similar observations were made in experiments using undulator based 3rd generation synchrotron radiation or free-electron lasers, where in particular high-brilliance free-electron laser sources provide photons in the extreme ultraviolet up to the soft x-ray regime with extremely high pulse densities and high photoelectron yields.

We develop a simple but successful model which allows one to disentangle space-charge effects from the electron underlying dynamics probed in multi-photon transitions from a model system on a femtosecond timescale.

In addition we will discuss ultrafast demagnetization effects and an excitonic excitation in a single layer hexagonal boron nitride film on top of a ferromagnetic Ni(111) surface.