Microwavephysics and Atmospheric Physics
Biomedizinische Photonik
Ultrafast Science and Technology
HS 2017  ·  FS 2017
HS 2016  ·  FS 2016
HS 2015  ·  FS 2015
HS 2014  ·  FS 2014
HS 2013  ·  FS 2013
HS 2012  ·  FS 2012
HS 2011  ·  FS 2011
Last update: 13.09.2017
HS 2015: Seminare über Ultrafast Science and Technology
Thursday 11:15am
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Donnerstag, 10.09.2015

Revealing hidden symmetry breaking in strongly correlated matter

Zeit: 11:15 Uhr
Hörsaal: B116
 
Prof. Darius H. Torchinsky
Temple University
Philadelphia, PA, USA

Essential to a microscopic understanding of strongly correlated materials is a clear picture of the relationship between their myriad quantum ground states. However, in phenomena ranging from unconventional magnetism to high temperature superconductivity, this picture is often obscured by the presence of broken symmetries hidden from view of existing experimental techniques. This may include hidden structural symmetries or tensor order parameters representing complex spatial arrangements of multipolar electric and magnetic moments. It may even include electronic forms of order which come in and out of existence on ultrashort timescales, invisible to static probes. I will demonstrate how ultrafast time resolved and nonlinear optical methods can reveal hidden symmetry breaking in some of the most intensely researched strongly correlated materials of the past decade, including high-temperature superconductors, spin-orbit coupled transition metal oxides and heavy fermion materials, and I will discuss how the newly uncovered symmetries play a fundamental role in their physics.

 
Donnerstag, 17.09.2015

Self-Assembly of Phenanthrene-Oligomers into Nanotubes with Light-Harvesting Properties

Zeit: 11:15 Uhr
Hörsaal: B116
 
Caroline Bösch
Department of Chemistry and Biochemistry
University of Bern

Abstract see announcement

 
Donnerstag, 15.10.2015

Exploring the long-wavelength limit of the electric dipole approximation

Zeit: 11:15 Uhr
Hörsaal: B116
 
PD Dr. Lukas Gallmann
Institute of Applied Physics
University of Bern, and ETH Zurich

The electric dipole approximation is heavily used throughout strong-field and attosecond science. This approximation assumes that the object exposed to the laser field is much smaller than the wavelength of this radiation. While its breakdown towards short wavelengths follows directly form this definition, it has a lesser-known limit towards long laser wavelengths that manifests itself in the onset of magnetic field effects. Using a few-cycle mid-infrared optical parametric chirped-pulse amplifier developed in-house, we were able to experimentally demonstrate this long-wavelength limit in above-threshold ionization at non-relativistic intensities. In my presentation I will review the laser technology that allowed us to reach this sparsely explored regime of light-matter interaction and I will present our recent results on strong-field ionization in the non-dipole limit.

 
Donnerstag, 22.10.2015

This presentation is divided in two parts:
i) Intrinsically stable all in-fiber broadband light-source
ii) Two dimensional refractive index mapping of optical fibers

Zeit: 11:15 Uhr
Hörsaal: B116
 
Philippe Raisin
Institute of Applied Physics
University of Bern

Part i): Many experiments rely on the measurement of an optical power. The precision of these experiments increases with the stability of its optical components, including the light source. In the course of a Master thesis, an intrinsically stable, broadband all in-fiber source is being developed. The approach relies on reaching full inversion in a short piece of rare-earth doped active optical fiber by applying high pump power. The resulting purely spontaneous emission can theoretically reach a temporal stability two orders of magnitude higher than the stability of the pump source. The talk will focus on the influence of different rare-earth active media on the achievable stability enhancement as well as constraints for the fiber parameters when designing the source. In order to achieve the optimal stability enhancement, dedicated detector circuitry as well as temperature and mechanical stabilization of the setup are needed.
Part ii): One of the possible applications of this source is to use it in a fiber-optical reflection based refractive index mapping instrument. The instrument is needed for the fiber research and development activities in our group. The fiber-coupled setup allows the point-wise high precision measurement of the Fresnel back-reflection. The index steps of commercially available fibers can be measured accurately down to < 10-3. The precision limit of the instrument is currently on the order of 10-4. An overview of recent developments in improving stability and reliability of the instrument will be given.

 
Donnerstag, 12.11.2015

Plenoptic cameras (student project)

Zeit: 11:15 Uhr
Hörsaal: B116
 
Armin Fehr
Nils Schnabel
University of Bern

 
Donnerstag, 19.11.2015

Optimization of the plasma active-medium for short-wavelength laser-probes (PhD Defense)

Zeit: 11:00 Uhr
Hörsaal: B116
 
Leili Masoudnia
EMPA, Dübendorf, and Institute of Applied Physics
University of Bern

 
Donnerstag, 26.11.2015

Towards application of nonlinear time resolved X-ray spectroscopy

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Gregor Knopp
Paul Scherrer Institute PSI
Villigen

Extending the methodologies of nonlinear and quantum optics to X-ray wavelengths is a promising and exciting avenue, in the light of recent successful experiments at X-ray synchrotrons and X-ray Free FELs. With the development of free electron laser (FEL) radiation sources a new area of X-ray spectroscopy commenced which can have a ‘comparable’ impact as lasers had in nonlinear optics and spectroscopy. In nonlinear ultrafast time-resolved techniques, state specific information is often provided through multiphoton resonances with combinations of sequential photons. Theoretically, also combinations of X-ray photons resonant with high frequency core transitions, characterize different excitation processes due to specific sequences of light-matter interaction. In the recent years several estimations have been given but no clear general picture could be drawn even for the ‘simplest’ nonlinear X-ray processes like two-photon absorption (TPA). The dependency of TPA on the third or der nonlinear susceptibility enables to determine the strength of other nonlinear X-ray spectroscopies, including X-ray stimulated Raman (X-SR), transient gratings (X-TG) or four wave mixing (X-FWM). Recently an EUV-TG FEL experiment has been realized by Bencivenga et al. [1]. An objective goal of FEL driven ultrafast X-TG experiments is to access wave-vectors at the nm scale. I will discuss strategies and preliminary experimental results towards future nonlinear time-resolved X-ray approaches.

1. F. Bencivenga et al., Nature 520 (2015).

 
Donnerstag, 26.11.2015

Table-top nano-imaging using plasma-photon source (PhD Defense)

Zeit: 14:00 Uhr
Hörsaal: B7
 
Maria Isabel (Mabel) Ruiz Lopez
EMPA, Dübendorf, and Institute of Applied Physics
University of Bern

 
Donnerstag, 10.12.2015

High Field Physics and Laser Filamentation

Zeit: 11:15 Uhr
Hörsaal: B116
 
Prof. Jean-Pierre Wolf
Group of Applied Physics
University of Geneva

Laser filaments are self-sustained light structures of typically 100 um diameter and up to tens of meters in length, widely extending the traditional linear diffraction limit1. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or negative higher-order Kerr terms2. In the recent years, this Kerr polarization saturation appeared as a signature of electrons experiencing a combined ionic-laser field potential3, closely related to the one involved in high harmonics generation (HHG). The relative contributions of free, Drude type, electrons and electrons in this mixed potential depend on the driving laser wavelength, intensity and pulse duration. This dependence has been recently emphasized by filamentation experiments4 (and associated simulations) in the mid-IR. Applications of these high intensity structures include laser machining, broadening and shortening of laser pulses down to few cycles, as well as atmospheric monitoring and control. In particular, we show that laser filaments can induce water condensation and fast droplet growth up to several µm in diameter in the atmosphere5,6 as soon as the relative humidity (RH) exceeds 70%. We also demonstrate that the radiative forcing properties of high altitude ice crystal clouds could be modulated by their interaction with high intensity laser filaments.

1 J. Kasparian et al, Science 301, 61-64 (2003) 2 P. Bejot et al, Phys.Rev.Lett. 104, 103903 (2011) 3 M. Richter et al, New J. of Phys. 15, 083012 (2013) 4 D. Kartashov et al, Opt.Lett. 37(16), 3456 (2012) 5 P. Rohwetter et al, Nature Photonics 4, 451 - 456 (2010) 6 S.Henin et al, Nature Comm. 2, 456 (2011)

 
Montag, 21.12.2015

Soft-X-ray laser pumped by electron collisions in laser-produced plasma (PhD Defense)

Zeit: 10:15 Uhr
Hörsaal: B116
 
Fei Jia
Institute of Applied Physics
University of Bern