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: 18.10.2017
FS 2017: Seminare über Ultrafast Science and Technology
Thursday 11:15am
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Donnerstag, 23.02.2017

Mechanical contact of skin and textiles: THz imaging of the interface

Zeit: 11:15 Uhr
Hörsaal: B116
 
Lorenzo Valzania
EMPA Dübendorf

In medical applications, contact of patients’ skin with body monitoring devices and bed sheets cannot be avoided. Mechanical properties at the interface, namely the surface texture and the moisture con-tent, are responsible for reactions causing skin irritations. Theoretical models for friction and contact behavior can estimate the real contact area and the intrinsic shear strength as a function of the contact pressure. Nevertheless, experimental in situ observations which can validate such models are missing. Peculiar properties of matter at THz frequencies make THz radiation a suitable probe for its non-ionizing nature. THz waves are transmitted by many non-conducting materials like textiles, reflected by skin and absorbed by interfacial water, thus allowing the investigation of hidden biomechanical inter-faces in a safe way for the human body. Therefore, we are developing an experimental setup based on THz digital holography [1, 2], which is a full-field, continuous-wave imaging technique preserving phase sensitivity and delivering images with a single-shot acquisition. Topographic reconstructions can be ob-tained with a resolution below the radiation wavelength. Reconstructions of fingertip replicas in transmission and in reflection will be shown. A lateral resolution of 200 ?m and a depth resolution of 20 ?m were achieved [3], which are enough to resolve the micro-scopic ripple structure of skin. Finally, the capability of our technique to retrieve profiles of objects hid-den behind THz-transparent samples will be discussed. We present a method for the separation of multiple interfering signals in the framework of THz digital holography [4]. A metallic resolution target behind a Teflon plate was successfully reconstructed. This study is essential since it prepares future ex-periments where a textile patch is inserted between a skin replica and a Teflon plate, mimicking a real life contact situation.

1. P. Zolliker, and E. Hack, "THz holography in reflection using a high resolution microbolometer array," Optics express 23, 10957-10967 (2015).

2. E. Hack, L. Valzania, G. Gäumann, M. Shalaby, C. P. Hauri, and P. Zolliker, "Comparison of thermal detector arrays for off-axis THz holography and real-time THz imaging," Sensors 16, 221 (2016).

3. E. Hack, and P. Zolliker, "High-resolution terahertz holography for profilometry in transmission," in PhotoMechanics Conf(2015), pp. 116-118.

4. L. Valzania, P. Zolliker, and E. Hack, " Topography of hidden objects by analysis of multiple-beam interference patterns with THz digital holography" (to be submitted)

 
Donnerstag, 02.03.2017

Administrative News

Zeit: 11:15 Uhr
Hörsaal: B116
 
Prof. Thomas Feurer
Institute of Applied Physics
University of Bern

 
Donnerstag, 09.03.2017

Fiber optical Gyroscope

Zeit: 11:15 Uhr
Hörsaal: B116
 
Raphael Blümli
Institute of Applied Physics
University of Bern

With the discovery of the Sagnac effect in the 20th century optical rotation rate sensors become possible. It took more than 50 years and the invention of Lasers to build such a sensor in the 1960’s, namely the Ring Laser (RLG). These devices are extremely accurate, sensitiveness down to 0.001°/hr, but also extremely costly. The invention of fibers in the late seventies provided a good alternative, the interferometric fiber optical gyroscope, also IFOG. Fibers are used to enhance the Sagnac effect and can be packed in a very small way. This Master thesis considers a new fiber winding technology to improve the linearity and the long-term stability of the IFOG. The set-up, goals and the timeline of the Master thesis will be presented. The second part of the presentation is dedicated to the multifunctional device, called RedPitaya. This credit card sized open source data acquisition platform can be implemented in various forms into experiments. For example: Oscilloscope, frequency analyser, function generator, Bode-plotter, logic analyser etc. Remote control through SCPIE-server with MatLab, SciLab, LabView is also possible.

 
Donnerstag, 23.03.2017

High energy THz pulse generation by tilted pulse front pumping

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Zoltan Ollmann
Institute of Applied Physics
University of Bern

High-intensity and high-field-strength THz pulses are required for nonlinear THz spectroscopy, particle manipulation, and many other (medical, security) applications. Optical rectification of femtosecond laser pulses in nonlinear crystals is an efficient method for the generation of picosecond THz pulses. THz generation by optical rectification in collinear geometry is possible in ZnTe when pumped at 800 nm. However, two-photon absorption limits the useful pump intensity, and thereby the pump-to-THz conversion efficiency. An alternative material for optical rectification is LiNbO3, since its nonlinear coefficient is very high and at 800 nm pumping 2 photon absorption is not possible for this material. The refractive index of LiNbO3 is much higher in the THz range than in the near-IR, so collinear velocity matching is not possible. However, velocity matching condition can be fulfilled with tilted-pulse-front-excitation. This talk will give an overview the principles of tilted pulse front excitation, some numerical and experimental results and some other useful THz pulse generation techniques.

 
Donnerstag, 30.03.2017

Radiofrequent High Voltage Gas Discharges in Hollow Core Photonic Crystal Fibers

Zeit: 11:15 Uhr
Hörsaal: B116
 
Christa Biberstein
Institute of Applied Physics
University of Bern

Gas lasers reach wavelengths that are difficult to generate with alternative solid-state systems and operate mostly with relatively inexpensive gain media, which are almost impossible to damage, but they remain very bulky. We started the development of compact, fiber-integrated gas lasers based on hollow core photonic crystal fiber filled with noble gases. We successfully achieved stable gas discharges in a capillary as well as into the core of the fiber using high voltage at RF frequencies applied via external electrodes. Therefore we used xenon, krypton, helium, neon and air to get the discharges and had encouraging results for further research. In the future this setup can be modified to a fully fiber-integrated gas-discharge laser.

 
Donnerstag, 06.04.2017

Towards plasma-based techniques for high-field FEL e- beam characterization

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Roxana Tarkeshian
Institute of Applied Physics
University of Bern

Ultrashort, ultra-intense X-ray pulses produced by free-electron lasers (FELs) driven by conventional or future plasma-based accelerators are crucial elements in experiments ranging from single-molecule imaging to few femtosecond down to atto-second X-ray science. The longitudinal charge distribution within the electron beam and the peak current are crucial factors for determining the lasing process of the FEL. In order to obtain a short gain-length the electron bunch needs to be compressed to achieve high peak current. A method for characterizing ultra-short high brightness electron beams is proposed that relies on tunnel-ionizing neutral gas through which they propagate. Experiments with conventional and advanced accelerators are proposed and it is shown via simulations and analytic theory that, by measuring the properties of field-induced ions and secondary electrons, single-shot beam charge density can be obtained. This novel concept will aid with the development of state-of-the-art accelerators that power free electron lasers and future colliders that operate at the frontiers of performance. Here 3D particle-in-cell simulation results, experimental setup, potential implementations of this technique for bunch characterisation at LCLS, and transverse characterisation of sub-micrometer beams from the laser-driven wakefield accelerator BELLA will be presented.

 
Donnerstag, 13.04.2017

The physics of skiing

Zeit: 11:15 Uhr
Hörsaal: B116
 
Prof. Thomas Feurer
Institute of Applied Physics
University of Bern

 
Donnerstag, 20.04.2017

no seminar (Easter holiday)

Zeit: 00:00 Uhr
Hörsaal:
 

 
Donnerstag, 27.04.2017

Deep Eutectic Solvents: Underlying Dynamics and Uses

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Anuradha Das
Institute of Applied Physics
University of Bern

Deep eutectic solvents (DESs) are fast emerging as important and economically viable alternative media for large-scale use in chemical industry. DESs are multi-component molten mixtures at or near room temperature and made of components with high individual melting points. Extensive intra-species hydrogen-bonding (H-bonding) and entropic gain supports the liquid phase of such multi-component melts. The key question that has been investigated is whether deep depression of freezing points via inter-species H-bonding renders spatio-temporal heterogeneity in such systems. Steady state absorption, fluorescence and time-resolved fluorescence measurements have been performed for a variety of DESs for this purpose. Both ionic and non-ionic DESs have been studied. Some systems, particularly those made of acetamide and electrolyte, have shown strong dependence, indicating presence of pronounced spatial heterogeneity. Solute-centred relaxation rates, such as solute solvation and rotation rates in these media, have been found to show fractional viscosity dependence. This has been interpreted as due dynamic (temporal) heterogeneity. Interestingly, addition of urea in such DESs have been found to remove signatures of both spatial and temporal heterogeneity. Effects of solution structure and dynamics on a chemical reaction in such a medium have also been studied.

 
Dienstag, 09.05.2017

Nonlinear Plasmonics of Germanium Nanoantennas in the Midinfrared Range

Zeit: 14:15 Uhr
Hörsaal: B5
 
Aaron Christopher Riede, University of Konstanz, DE

Large biomolecules show a specific footprint in the MIR range, which is detectable with MIR absorption spectroscopy. In order to increase the absorption cross section, we investigate the resonance behavior of different Germanium antennas. This is done by measuring the augmented third harmonic generation and the increased ecxtinction of resonant antennas. The main focus of the work will be on the generation of the broad band MIR light in the range of 10...20um and, of course, the presentation of the results.

 
Donnerstag, 11.05.2017

Yellow light-generation by frequency doubling the output of a fiber Bragg grating (FBG) based cavity

Zeit: 11:15 Uhr
Hörsaal: B116
 
Christoph Bacher
Institute of Applied Physics
University of Bern

Laser sources with light-emission in the yellow spectral range around 580 nm are very favorable for a variety of applications. These include applications in astronomy, in ophthalmology or in quantum optics. An application in astronomy is the artificial laser guide star where D2 sodium is excited at the 589 nm absorption line and the resulting spontaneous emission is then used as an artificial laser guide star. A medical application is in ophthalmology where photocoagulation of vessels is performed. Moreover, yellow light at 580 nm is used in quantum networks, where the quantum state of light is stored in a 153Eu3+:Y2SiO5 crystal. These examples show the demand for Watt-level yellow lasers with a beam quality close to diffraction limit. However, reliable fiber-laser sources at the required optical power levels of several Watts, hardly exist. The generation and amplification of 1154 nm light is not straight forward when using Yb-doped optical fibers, since lasing occurs preferentially around the gain-maximum of 1030 nm. We succeeded to build a prototype with yellow light emission at the power level of 600 mW. Our approach uses a linear fiber Bragg grating (FBG) based cavity at 1154 nm and a frequency doubling crystal. To force the cavity to operate at 1154 nm FBGs and heating of the Yb-doped fiber are crucial for suppressing the unwanted process of amplified spontaneous emission (ASE). Finally the output of the amplifier is frequency doubled to 577 nm by using a second harmonic crystal.

Nevertheless, further optimization of the system, for example different pumping schemes, fine-tuning of the pump wavelength or filtering, must be undertaken to achieve Watt-level light emission.

 
Donnerstag, 18.05.2017

Promotionsvortrag; Terahertz field enhancement in sub-nanometer sized slit arrays

Zeit: 14:15 Uhr
Hörsaal: B006
 
Yannik Waeber
Institute of Applied Physics
University of Bern

 
Donnerstag, 25.05.2017

no seminar (Ascension Day)

Zeit: 00:00 Uhr
Hörsaal:
 

 
Donnerstag, 01.06.2017

Super-Resolution Quantum Imaging and the Supertwin Project

Zeit: 11:15 Uhr
Hörsaal: B116
 
Manuel Unternährer
Institute of Applied Physics
University of Bern

The spatial resolution of microscopic imaging is limited by the numerical aperture of the used objective lens, due to the Rayleigh/Abbe criterion, and can maximally achieve half a wavelength of the light used for illumination. Quantum states of light can exhibit intensity correlation features which beat this classical resolution limit by using multi-photon interferences. In the project Supertwin, funded through the Horizon 2020 program of the European Union, these quantum states will be harnessed in order to improve resolution in microscopy beyond the classical limit. In this seminar, the concepts of quantum super-resolution imaging are introduced and illustrated intuitively. Supertwin and our work within the project, ranging from multi-photon correlation measurements with a novel type of sensor array to proof-of-principle imaging experiments, is presented.

 
Donnerstag, 22.06.2017

Promotionsvortrag; Design and fabrication of specialty optical fibers by the sol-gel granulated silica method

Zeit: 14:15 Uhr
Hörsaal: B7
 
Jonas Scheuner
Institute of Applied Physics
University of Bern