Colloquia of the Physics, LiMat, and Frimat departments

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April 2018

Wednesday, 18.

Colloquium Physics

Magneto-optical spectroscopy of light fermions

Prof. Ana Akrap- Physics Departement of the University of Fribourg

Time: 17:15h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 0.51, Chemin du Musee 3, 1700 Fribourg Switzerland

 

Imagine a gas of free electrons. When we apply a magnetic field, the electrons begin to orbit. Their energy states become quantized in the direction perpendicular to the field. These quantized levels are called Landau levels. Each of the Landau levels has a large degeneracy, propotional to the value of the magnetic field. For a sufficiently large magnetic field, the degeneracy becomes so high that all the free electrons fall into the lowest few Landau levels. If we can measure the transitions between Landau levels in a given material, we may in this way understand the band structure. 

 

In this colloquium, I will illustrate how to apply magneto-optical spectroscopy to understand the low energy behavior of systems with small carrier densities, small effective masses, and small or vanishing band gaps, whose properties are in some conditions expected to be topologically nontrivial.

 

 

Wednesday, 25.

Colloquium Physics

Fundamentals of Pulsed Laser Deposition and Applications for Renewable Energy Applications

Prof. Dr. Thomas Lippert - Paul Scherrer Institut, Villigen

Time: 17:15h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 0.51, Chemin du Musee 3, 1700 Fribourg Switzerland

 

Thin films of organic, polymeric, biological, and inorganic thin films are utilized in many applications, e.g. catalysis, microelectronics, sensors, food industry, tools, optics, decorative coatings, and renewable energy applications, etc.. The preparation of these thin films can be achieved with a variety of tools, ranging e.g. from chemical to physical vapor deposition methods (PVD). One PVD method which is used extensively in research, especially for oxides, but recently also in industry in pulsed laser deposition (PLD). PLD can be divided into 3 steps, i.e. laser ablation and plasma formation, plasma expansion, and film growth. Different processes associated to these steps can have a pronounced influence on the film growth and film composition, i.e. properties. It is noteworthy, that it is often assumed, without further analysis, that the films will have automatically the same composition as the target, which is not really the case.

I will show, that all steps and the associated parameter, such as background gas type and pressure, target composition, and substrate type and temperature have a pronounced effect on the PLD process, and therefore on the film composition and properties. The sum of all these effects suggests, that in many cases it can be difficult and time-consuming to find conditions to achieve the desired film composition (properties).

The application of thin films as model systems for renewable energy applications will be shown for two topics, i.e. photocatalysis and ion conduction. Pulsed reactive crossed beam laser ablation (PRCLA), a modification of PLD, is used to obtain oxynitride films. Oxynitrides have gained a lot of attention over the last decade due to their photocatalytic properties using visible light. We utilize photoelectrocatalytic measurements (PEC) to study the oxynitride thin films, mainly LaTiOxNy, where we could show that the crystalline orientation has a pronounced influence on the activity. For ion conductors, we are looking mainly at the influence of strain on oxygen and proton conductivity. We could show, that tensile strain will improve conductivity but not by orders of magnitude as reported elsewhere.

 

 

May 2018

Wednesday, 23.

Colloquium Physics

How SLM-based wavefront shaping impacts Biophotonics, from holographic optical micro-manipulation to programmable microscopy

Prof. Dr. Monika Ritsch-Marte - Department for Physiology and Medical Physics, Innsbruck, Austria

Time: 16:00h

Location: Adolphe Merkle institut - salle de Seminaires - Chemin des Verdiers 4 - 1700 Fribourg - Switzerland

 

Wavefront shaping with liquid-crystal spatial light modulators (SLMs) has become a powerful tool in Biophotonics. It is well-known that SLM-controlled holographic optical tweezers can be used to steer optical traps in real time. But an SLM can also be integrated into optical imaging systems, to use it as a programmable Fourier-filter which emulates classic microscopy techniques. This only requires sending a pre-calculated voltage pattern to the pixels of the SLM placed in the Fourier plane, which imprints a spatially-varying phase shift to the optical wavefront. Going from dark-field microscopy, to Zernike phase contrast or to spiral phase contrast, and toggling between these modalities, is as easy as replacing the pattern on the SLM. Thus microscopy becomes programmable and customizable with respect to a given sample.  A particular strength of the SLM-approach with programmable phase masks is the possibility to “pack several things into one hologram”, i.e. for multiplexed imaging. It is thus possible to take microscopic images that contain sub-images belonging to different imaging modalities, to different depths inside the sample, or to different parameter settings. This opens a “treasure box” of advanced possibilities in optical imaging and trapping.

 

 

Wednesday, 30.

Colloquium Physics

Time resolved photoelectron spectroscopy of quantum materials

Prof. Luca Perfetti - Ecole Polytechnique, Paris

Time: 17:15h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 0.51, Chemin du Musee 3, 1700 Fribourg Switzerland

 

The recent developments of femtosecond photon sources from the THz to ultraviolet spectral range have largely enriched the methods to probe condensed matter on ultrafast timescales. Among the possible approaches, time and angle resolved photoelectron spectroscopy is the only one capable of visualizing the temporal evolution of electronic excitations in reciprocal space. I will review the basic principles of this experimental method and dis- cuss some of our recent results in quantum materials. First I will discuss about semiconducting materials with interesting optoelectronic applications. In the case of hybrid perovskites, we explicitly discriminate the initial thermalization of the electrons from slower dynamical processes. The Anderson localization of the excited electrons in degraded samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. In the second part of the talk I will move to the carriers dynamics in high correlated materials. Our analysis show that fluctuations are essential in order to model the photoinduced melting of the superconducting gap in the cuprates.

 

Monday, 02.04.2018 - Sunday, 22.07.2018

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