Microwavephysics and Atmospheric Physics
Biomedizinische Photonik
Ultrafast Science and Technology
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Last update: 13.09.2017
HS 2014: Seminare über Ultrafast Science and Technology
Thursday 11:15am
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Donnerstag, 11.09.2014

Simulation and production of reconfigurable opto-electronic traps on iron doped Lithium Niobate substrate

Zeit: 11:15 Uhr
Hörsaal: B116
Michela Gazzetto
Laboratory of Quantum Electronics and Nonlinear Optics
Electronics Department
University of Pavia, Italy

Opto-electronics tweezers are a promising technique for the realization of fast, highly-reconfigurable array of traps for particle immobilization and manipulation. Exploiting a photoconductive substrate, like Lithium Niobate, a static electric field induced by light can be used to trap a particle, either a cell or an inorganic neutral object by means of a dielectrophoretic force. LN is a very promising material thanks to the presence of the photorefractive effect, that is the insurgence of a semi-permanent yet erasable electric field, called space-charge field, when is illuminated with a light in the visible range. This work combines the known numerical results on photorefractive effect to calculate the dielectrophoretic force on particles suspended in a buffer solution, lying on top of the LN sample surface. An experimental validation of the simulation results was also performed using Poly-methyl-methacrylate beads suspended in paraffin oil on an iron doped LN sample (Fe: LiNbO3).

Donnerstag, 18.09.2014

Laser-induced field emission from a tungsten tip in weak and strong optical fields

Zeit: 11:15 Uhr
Hörsaal: B116
Dr. Hirofumi Yanagisawa
Institute for Quantum Electronics
ETH Zürich

Illuminating a sharp metallic tip with femtosecond laser pulses leads to pulsed field emission. In this presentation, we will present three topics on the laser-induced field emission. Firstly we go through our previous work on site-selective laser-induced field emission from a tungsten tip1) and its emission mechanism in weak optical fields2), and then we particularly focus on our recent results on emission mechanism in strong optical fields3).
1) H. Yanagisawa, et. al., Phys. Rev. Lett. 103, 257603 (2009).
2) H. Yanagisawa, et. al., Phys. Rev. Lett. 107, 087601 (2011).
3) H. Yanagisawa, et. al., arXiv: 1405.0609 (2014)

Donnerstag, 25.09.2014

Ultrafast electron transfer reactions of ZnO solar cells sensitized with indoline dyes

Zeit: 11:15 Uhr
Hörsaal: B116
Dr. Egmont Rohwer
Institute of Applied Physics
University of Bern

The initial charge transfer from dye molecules' excited states to the conduction band of a semiconductor, after absorption of visible light by the former, is critical to the performance of Dye sensitized Solar Cells (DSC). In a ZnO-based DSC sensitized by organic indoline dyes, the dynamics associated with charge transfer are investigated with femtosecond transient absorption spectroscopy. The time-resolved measurement of the photo-initiated processes reveal electron transfer rates corresponding to excited state lifetimes of 100s of fs, consistent with previously measured high absorbed photon to current conversion efficiencies and, on the molecular level, provide critical insight on the obstacles present in the pursuit of ever greater overall cell efficiency.

Donnerstag, 27.11.2014

The Light Field Camera: Extended Depth of Field, Aliasing and Superresolution

Zeit: 11:15 Uhr
Hörsaal: B116
Prof. Paolo Favaro
Institute of Computer Science and Applied Mathematics
University of Bern

Portable light field cameras have demonstrated capabilities beyond conventional cameras. In a single snapshot, they enable digital image refocusing, i.e., the ability to change the camera focus after taking the snapshot, and 3D reconstruction. We show that they also achieve a larger depth of field than conventional cameras while maintaining the ability to reconstruct detail at high resolution. More interestingly, we show that their depth of field is essentially inverted compared to regular cameras. Crucial to the success of the light field camera is the way it samples the light field, trading off spatial vs. angular resolution, and how aliasing affects the light field. We present a novel algorithm that estimates a full resolution sharp image and a full resolution depth map from a single input light field image. The algorithm is formulated in a variational framework and it is based on novel image priors designed for light field images. We demonstrate the algorithm on synthetic and real images captured with our own light field camera, and show that it can outperform other computational camera systems.

Mittwoch, 03.12.2014

Transmission of parametrically polarization shaped pulses through a hollow core photonic crystal fiber

Zeit: 16:15 Uhr
Hörsaal: B78
Dr. Georg Achazi
Institute for Experimental Physics
Freie Universität Berlin
Berlin, Germany

In this talk a method for ultrafast polarization pulse shaping through a micro structured hollow core photonic crystal fiber is presented. The pulses are shaped in pulse sequences in which the energy, distance, phases, and chirps as well as the state of polarization of each individual sub-pulse can be independently controlled. The application of these pulses for coherent control is demonstrated for feedback loop optimization of the multi-photon ionization of potassium dimers and the posiblity to reconstruct the pulse shape by reflecting the pulses back through the fiber is shown.

Donnerstag, 04.12.2014

Ultrafast biexcitonic signatures in single quantum dot pump-probe spectroscopy

Zeit: 11:15 Uhr
Hörsaal: B116
Dr. Johannes Haase
Paul Scherrer Institut

II-VI semiconductor quantum dots with high confinement potentials and large Coulomb correlation energies provide promising systems for quantum optical experiments. In my talk, I willreport on two-color femtosecond pump-probe measurements on a single self-assembled CdSe/ZnSe quantum dot. Recent developmentsin thesample design enable us to resonantly excite and spectro-temporally investigatesimultaneouslythe trion line and the biexcitonic emission lines of a singly charged quantum dot. After hot exciton creation by the pump, for positive time delays the trion resonance exhibits an initial bleaching followed by stimulated emission, which occurs after relaxation to the ground state [1]. Additionally, negative pump-probe features emerge on the low-energy side of the trion line. We attribute those to an induced absorption into the biexcitonic states [2]. This picture is corroborated by matching transient absorption and photoluminescence emission lines.

[1] F. Sotier, T. Thomay, T. Hanke, J. Korger,S. Mahapatra, A. Frey, K. Brunner, R. Bratschitsch, and A. Leitenstorfer, Femtosecond fewfermion dynamics and deterministic single-photon gain in a quantum dot, Nature Phys. 5, 352-356 (2009).

[2] J. Huneke, I. D'Amico, P. Machnikowski, T. Thomay, R. Bratschitsch, A. Leitenstorfer, and T. Kuhn, Role of Coulomb correlations for femtosecond pump-probe signals obtained from a single quantum dot, Phys. Rev. B 84,115320(2011).

Dienstag, 09.12.2014

Nonlinear spectroscopy with quantum light

Zeit: 10:15 Uhr
Hörsaal: B77
Frank Schlawin
Institute of Physics
Albert-Ludwigs University of Freiburg
Freiburg, Germany

We will present simulations of entangled-photon absorption in a photosynthetic complex. Our goal is to exploit the strong time-frequency correlations of entangled light to manipulate nonlinear spectra of complex quantum systems.

Exciton transport can be suppressed, and two-exciton states can be selectively excited.

Donnerstag, 18.12.2014

Ultrafast structural dynamics in solid matter studied by x-ray diffraction

Zeit: 11:15 Uhr
Hörsaal: B116
Dr. Paul Beaud
Paul Scherrer Institut

In many modern materials the correlation among outer shell electrons leads to interesting properties such as superconductivity, colossal magnetoresistance and multiferroicity. Often the interactions of the long range order of the electronic degrees of freedom such as charge, orbital and magnetic order couples to the underlying lattice. The study of these different types of orders using x-ray diffraction has become an essential tool for the characterization of strongly correlated electron systems. Recent developments in ultrashort x-ray sources offer new possiblities to extend diffraction techniques into the time domain to study the coupling between the lattice and different electronic subsystems as they develop in time. Using our low flux femtosecond hard x-ray source at the Swiss Light Source [1] we had the opportunity to make significant scientific and technical contributions in developing femtosecond x-ray diffraction to study laser induced structural dynamics in solid matter. Nevertheless the future in this area of research lies in free electron laser facilities that provide much higher x-ray flux and significantly better time resolution [2]. In my talk I will start with an overview over hard x-ray diffraction techniques mainly addressing the issues that arise when adding an intense laser pump to the experimental configuration.

In the second part I will focus in more detail on one of our ongoing research topics. Perovskite-type manganites are prototypical examples of strongly correlated electron systems which exhibit properties such as colossal magnetoresistance and insulator-to-metal transitions that are intrinsically related to symmetry changes of the atomic lattice and to intriguing ordering patterns of the spins, orbitals and charges [3]. Here the application of an intense optical pulse melts the electronic order and induces an ultrafast insulator-metal transition [4]. Initial diffraction experiments performed at the SLS slicing source demonstrated a concomitant change of structural symmetry occurring on a sub-picosecond time scale [5-7]. In a recent experiments at the LCLS free electron laser we were not only able to study in more detail the lattice dynamics, but also the changes in long-range order of the electronic subsystems by tuning the x-ray energy ‘to’ and ‘off’ an atomic resonance [8]. Despite the complex nature of this phase transition that involves symmetry changes of valence charge, orbital order and atomic structure, a fairly simple description relying on a single time-dependent order parameter is sufficient to capture the most essential aspects of the change in symmetry in the time domain.

[1] P. Beaud, S. L. Johnson, A. Streun, R. Abela et al., Phys. Rev. Lett. 99, 174801 (2007). [2] P. Emma, R. Akre, J. Arthur, R. Bionta, C. Bostedt, J. Bozek et al., Nature Phot. 4, 641 (2010). [3] Y. Tokura and N. Nagaosa, Science 288, 462–468 (2000). [4] D. Polli, M. Rini, S. Wall, R. W. Schoenlein, et al., Nature Mater. 6, 643 (2007). [5] P. Beaud, S. L. Johnson, E. Vorobeva, U. Staub et al, Phys. Rev. Lett. 103, 155702 (2009). [6] A. Caviezel, S. O. Mariager, S. L. Johnson, E. Möhr-Vorobeva et al., Phys. Rev. B 86, 174105 (2012) [7] A. Caviezel, U. Staub, S. L. Johnson, S. O. Mariager et al., Phys. Rev. B 87, 205104 (2013). [8] P. Beaud, A. Caviezel, S. O. Mariager, L. Rettig, G. Ingold et al., Nature Mater. 13, 923 (2014).