Events: Interesting happenings in the next 4 weeks.

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November 2017

Wed., 29.

Colloquium Physics

How microwaves shed light on topological physics

Prof. Fabrice MORTESSAGNE, Unstitut de Physique de Nice (INPHYNI), Universite Nice Sophia Antipolis &CNRS

Time: 17:15 - 18:15h

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

The group Waves in complex systems in Nice (France) is interested in controlling the wave transport properties in various systems whose mastered designs range from homogeneous systems with complex geometries to either (quasi-)periodic or disordered structured materials. Thanks to a versatile experimental platform in the microwave domain, we addressed in the last years the vivid field of topological physics/photonics, and obtained different results ranging from the concept of topological reflective limiter, to a physical interpretation of the gap-labelling in a Penrose tilling, not forgetting the observation of a topological phase transition in strained artificial graphene. During the colloquium, I will give a simple introduction of topological physics, describe the experimental platform, and present a selection of results.

Thu., 30.

Master thesis presentation

A magnetic particle imaging scanner based on atomic magnetometry

Simone PENGUE, Physics Department of the University of Fribourg

Time: 10:00 - 11:00h

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

Magnetic Particle Imaging (MPI) is a promising tomographic imaging technique based on the detection of the oscillating magnetization of superparamagnetic iron-oxide nanoparticles (SPIONs) exposed to an inhomogeneous oscillating magnetic excitation field. When injected into the bloodstream, SPIONs prepared in non-toxic water suspensions will enable biomedical imaging and/or drug delivery applications. Standard MPI devices use inductive pick-up coils to detect the signal, and the high excitation frequency required for efficient signal detection impedes so far applications to humans. The reasons are the high specific absorption rate, SAR, inducing tissue heating, and the possible peripheral nerve stimulation, PNS, by the deployed high-power, high-frequency oscillating magnetic fields. The Fribourg Atomic Physics (FRAP) team has proposed a novel variant of MPI that is free from the SAR- and PNS-related limitations. The present thesis reports on proof-of-principle demonstrations of an MPI method, in which the conventionally used pick-up coils are replaced by an optically-pumped atomic magnetometer (AM). For this we have developed (in the frame of the Ph.D. thesis of S. Colombo) a dedicated apparatus comprising coil systems for generating mT fields and T/m field gradients at the SPION sample, which, together with specifically-designed compensation coils and a suitable choice of geometry, allows an atomic magnetometer with pT sensitivity to be operated in close (few cm) vicinity of the SPION sample. I have joined the team at the time when first tomographic images in one and two dimensions were being recorded. The thesis gives a detailed account on the apparatus and the underlying principles and theoretical aspects of the deployed methods. It culminates by demonstrating two variants of 1 D scans as well as a 2D scan of structures SPION samples. Inverse problem solving methods are deployed to infer the shape and iron content of the samples from the (blurred) recorded images.

Thu., 30.

Doctoral thesis presentation

Propagation of time-energy entangled photons through optical fibers

Jos KOHN, Physics Department of the University of Fribourg

Time: 16:15 - 17:15h

Location: UNI-Perolles, Physics Department, building 8, auditorium 2.73, Chemin du Musee 3, 1700 Fribourg, Switzerland

Entangled photons are a promising tool for outperforming classical measurement methods. In this thesis, we will study the propagation of entangled photons through single mode and polarization maintaining fibers of short length. The invention of optical glass fibers has revolutionized the field of telecommunications. The signal transmission rate is increased by adapting the fiber properties such as dispersion, leading to a temporal broadening. Two entangled photons are created in our case during the spontaneous parametric down-conversion process arising in a nonlinear crystal. Their generation occurs within a time window of a few tens of fs. Delay and dispersion are applied on the two-photon state with a prism compressor being combined with a spatial light modulator. These characteristics are applied to sense the fiber properties, such as the dispersion, based on the ultrafast coincidence detection in a second non-linear crystal. The advantage for using entangled photons is their low peak energy, which avoids the appearance of non-linear effects in the fiber. The propagation through polarization maintaining fibers is as well analyzed and the difference in refractive index between the fiber modes is determined with an uncertainty of 1%. Additionally, the propagation of classical light is compared to the transmission of time-energy entangled photons through fibers. We determine the dispersion of various fiber types using a white light Mach-Zehnder interferometer, where we superpose different optical path lengths in the reference arm by introducing delay loops. This improvement speeds up the measurement time of high dispersion fiber Bragg gratings by a factor of 5. Furthermore, we implement a calibration based on the determination of the deviations between the fitting model and experimental data of the relative phase change perceived by the passage of the light through the single mode fiber. Our enhanced interferometer can be used to determine the dispersion of optical fibers with less than 1% uncertainty.

December 2017

Wed., 06.

Candidate review confererence, Professorship succession

DNA-Origami for nanophotonic applications

Dr. Guillermo ACUNA, Braunschweig Integrated Center of Systems Biology, TU Braunschweig, DE

Time: 08:45 - 09:45h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

This contribution will start with a brief introduction to nanophotonics and plasmonics. In this context, different nanofabrication techniques will be discussed with an emphasis on the DNAOrigami approach, which we have been using to build nanophotonic structures composed of metallic nanoparticles and single quantum emitters with nanometer precision in 3D. I will present single molecule spectroscopic studies on these hybrid structures, combined with super-resolution imaging, which provide unique information about nanoparticle-fluorophore interaction. As an example, I will show highly efficient optical nano-antennas that are able to strongly focus light into sub-wavelength regions and produce a fluorescence enhancement of more than 3 orders of magnitude, as well as an increment in fluorophore photostability. Finally, I will discuss future potential applications of this technology on smartphone-based point of care diagnostic platforms, antennas for optical communication and fundamental spectroscopic studies of strong coupling at the single molecule level.

Wed., 06.

Candidate review confererence, Professorship succession

Engineering nanomaterials to enhance nonlinear optical signal for compact photonic devices

Prof. Dr. Rachel GRANGE, Optical Nanomaterial Group, Institute for Quantum Electronics,Department of Physics, ETHZ, CH

Time: 09:45 - 10:45h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

Nonlinear optics started one year after the invention of laser in 1961 with an experiment of light conversion from one colour of frequency ω into another one of the exact double frequency 2ω, a phenomenon called second-harmonic generation. Many applications ranging from microsurgery light sources, green laser pointer, or boosting signals in telecommunication fibres have emerged in the last 50 years using nonlinearities of bulk materials. However, applying nonlinear effects at the nanoscale generates very tiny signals and finding materials with strong nonlinearities is still an open challenge to avoid using high power and large interaction length within the material. Here, we use perovskite ferroelectric materials, like barium titanate (BaTiO3) or lithium niobate (LiNbO3), as nanomaterials for nonlinear optics, because of their wide bandgap for a broad transparency, significant second-order optical nonlinearities, strong electro-optic effects, high damage threshold, and biocompatibility. We demonstrate several strategies to enhance nonlinear optical signals in spherical nanoparticles and nanowaveguides. These bright and robust nanostructures can serve for developing compact efficient optical devices for imaging applications and for harsh environment like space.

Wed., 06.

Candidate review confererence, Professorship succession

Surgical manipulation of collective modes at the nano-scale

Prof. Dr Fabrizio CARBONE, Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES),Institute of Physics, EPFL, CH

Time: 11:00 - 12:00h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

Inherent properties such as low-dimensionality, strong electron-electron correlations or topological protection are the fundamental ingredients of materials displaying novel emergent electronic, optical, and magnetic properties. Microscopically, such properties are often ruled by ordered textures of spins and charges and their concerted motions. Key to understand and manipulate these states of matter is the ability to act on specific degrees of freedom externally while being able to monitor at the atomic level in space and time the consequences. In this seminar, I will show that collective electronic modes in nanostructures (surface plasmon polaritons) can be imaged and controlled in an ultrafast Transmission Electron Microscope down to the nanometer and attosecond space and time scales. The implications and possibilities opened by such an ability will be also discussed, with a particular focus on the light-induced control of magnetic textures of interest for novel data storage applications.

Wed., 06.

Candidate review confererence, Professorship succession

Functional Materials and Bio-inspired Molecular Devises Investigated with Ultrafast Nonlinear Optical Spectroscopies

Prof. Dr. Andrea CANNIZZO, Institute of Applied Physics, University of Bern, CH

Time: 13:15 - 14:15h

Location: Functional Materials and Bio-inspired Molecular Devises Investigated with Ultrafast Nonlinear Optical Spectroscopies

My research is devoted to investigate with advanced ultrafast spectroscopies light-induced dynamics of molecular materials characterized by unique and unusual electronic and structural properties, potentially interesting for applications in the field of molecular and nano-electronics, photoresponsive materials, artificial photocatalysis, etc. The motivation is achieving a fundamental comprehension of the functional processes, crucial to design and optimize novel materials. I will present two examples of my recent activity: carbon dots and DNA-hosted multichromophoric systems. The formers are one of the most intriguing novelties in Nanoscience. With a rich and still not understood photo-physics, C dots are regarded as a bio-compatible, non-toxic alternative to semiconductor quantum dots. The latters are bio-inspired light harvesters with unique photo-physical properties, as energy transfers over 100s of nm. How this occurs, time scales and the origin of the process are still elusive and debated.

Wed., 13.

Candidate review confererence, Professorship succession

Quantum sensing in a new single-molecule regime

Dr. Peter MAURER, Dept of Physics, Stanford University, CA

Time: 08:45 - 09:45h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

Quantum optics has had a profound impact on precision measurements, and recently enabled the detection of various fields with nanoscale spatial resolution. Such advancements in ‘quantum sensing’ have brought the elusive dream of performing nuclear magnetic resonance spectroscopy (NMR) on individual biomolecules closer to reality. In my talk, I will discuss the development and application of novel quantum metrological technologies to study biological systems at a single-molecule level. I will start with a general introduction to quantum sensing, with a focus on the measurement of magnetic fields at a nanoscale. I will then show how we utilize such sensing techniques to control the temperature profile in living systems with subcellular resolution. In addition, I will provide an outlook on how quantum sensing and single-molecule biophysics can be utilized to perform NMR spectroscopy with unprecedented sensitivity, possibly down to the level of individual biomolecules.

Wed., 13.

Candidate review confererence, Professorship succession

Ultrafast spectroscopy: a powerful tool to shed light on the photophysics of hybrid organic/inorganic semiconductors

Dr Giulia GRANCINI, Institut des Sciences et Ingenierie Chimiques, EPFL-Valais, Sion, CH

Time: 09:45 - 10:45h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

Unveiling the light-induced phenomena and their dynamical evolution is crucial to advance in fundamental physical knowledge and drive technological innovation. Elementary photo-induced mechanisms such energy relaxation or energy/charge transfer, typically occur on ultrafast timescales ranging from 10-14 to 10-12 s. Their efficiency is intimately linked to their timescale, making ultrafast optical spectroscopy an invaluable tool for their investigation. In this talk I will review the principle of ultrafast spectroscopy and describe state of the art systems, with sub-20 fs temporal resolution over a broad spectral range, and sub-micrometer spatial resolution. These tools enable the rationalization of the main photophysical processes happening in a variety of systems. As an example, I will elucidate the complex photoexcited-state scenario and charge dynamics behind hybrid perovskites, an emerging class of semiconductors used in photovoltaics, whose understanding is essential to guide the future device development.

Wed., 13.

Candidate review confererence, Professorship succession

Optical quantum metrology

Prof. Dr. André STEFANOV, Institute of Applied Physics, University of Bern, CH

Time: 11:00 - 12:00h

Location: Grand auditoire de Physique (PER08, salle 2.73, 2eme etage) Faculte des Sciences, Universite de Fribourg

New measurement and sensing methods based on the principles of quantum mechanics hold the promise of surpassing the limitations inherent of classical schemes. Light being one of the main measurement tool, it is thus natural to apply quantum concepts to optical measurements. I will first present how quantum metrology relies on the two fundamental notions of coherence and entanglement. I will then present practical realizations with entangled photon pairs that are manipulated and detected, in both the time and spatial domains. Such photon pairs have correlation times in the femtosecond regime while simultaneously exhibiting narrowband spectral features. Light-matter interaction can thus be investigated at short time scales without relying on ultra-short laser pulses, for instance in scattering measurements or spectroscopy. Furthermore, photon pairs also show the spatial correlations required to demonstrate superresolution or super-sensitivity in quantum imaging protocols.

Mon., 20.11.2017 - Sun., 17.12.2017

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