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
HS 2017: Seminare über Ultrafast Science and Technology
Thursday 11:15am
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Freitag, 04.08.2017

Ultrafast and ultrasensitive coupling to molecular vibrations

Zeit: 10:15 Uhr
Hörsaal: B116
 
Dr. Johannes Haase
Paul Scherrer Institute PSI
Villigen Switzerland

The unification of ultrafast laser physics and chemistry opens new insights to the fundamentals of chemical reactions. The ultimate femto-chemistry probe is the X-ray free electron laser providing ultrashort pulses in the significant wavelength region for inter-atom investigations. Time resolved studies of ensemble reactions however rely on a synchronization and ultrafast initialization. Chemical processes that are triggered by heat are especially tricky since heat is defined as an equilibrium state that is not reached on ultrafast time scales. In my talk, I will present ultrafast heating mechanisms and coupling to molecular vibrations. These studies also require ultrasensitive detection of molecular monolayers that is reached by plasmonic field enhancement of nanoscopic antennas.

 
Donnerstag, 21.09.2017

Advancing Ultrashort Laser Pulse Technology: Controlling Ultrafast Electron Motion

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Yunpei Deng
Paul Scherrer Institute PSI
Villigen Switzerland

State-of-the-art laser technology is on the verge of providing synthesized waveforms at optical frequencies. Such flexibility in shaping arbitrary light waveforms permits sculpting an electric force for steering electrons in any desired fashion within atomic, molecular and solid systems, on the electronic time scale. This has promising potential in pushing the frontiers of attosecond physics and of coherent control schemes to completely unexplored regions.

Optical parametric chirped-pulse amplifier (OPCPA) systems can produce nearly single-cycle pulses at a mJ energy level, or few-cycle pulses at a hundred mJ level. In my talk I will present the details of a mid-IR OPCPA laser system that I developed at MPQ. As I will show, this system has already demonstrated an output of 1.2-mJ, 1.5-cycle (10.3fs) pulses at 2.1µm central wavelength and at 3 kHz repetition rate. Such source is the key to achieving HHG X-ray photons in the keV domain, and is also important to a wide range of highly interesting applications, such as the investigation of ultrafast structural dynamics and conformational changes of relevant molecules in biology. An additional benefit of the IR carrier wavelength is the increased duration of its optical cycle (e.g. 7 fs for 2.1 µm) compared with a NIR pulse (e.g. 2.5 fs for 750 nm). The increased spacing between successive half-cycles of the laser pulse provides a sufficient time window to capture the full dynamics of an arbitrary sub-femtosecond relaxation process, before the identical process is re-triggered by the next half-cycle of the laser pulse. It is a powerful and unique source for coherent control, ionization and dissociation experiments. I will also show how an OPCPA system, which can be developed to deliver more than two-octave bandwidth pulses, is capable of producing sinusoidal-like, square, sawtooth or any arbitrary waveforms. Such laser pulses will open the door to many ultrafast coherent control applications.

 
Donnerstag, 28.09.2017

No seminar due to EAAC

Zeit: 11:15 Uhr
Hörsaal: B116
 

 
Donnerstag, 05.10.2017

THz-driven graphene plasmon-based undulator

Zeit: 11:15 Uhr
Hörsaal: B116
 
David Rohrbach
Institute of Mathematics
University of Bern

By using a classical undulator with an alternating magnetic field one can produce X-ray with a high energetic electron beam. Numerical simulations of the new type of graphene plasmon-based undulator, which is driven by a THz source, indicate narrow-band emissions in the X-ray even for a low energetic beam and a total undulator length of some centimeters.

 
Donnerstag, 12.10.2017

No seminar due to UFO

Zeit: 11:15 Uhr
Hörsaal: B116
 

 
Donnerstag, 19.10.2017

Nonlinear THz spectroscopy and NIR pump THz ellipsometry

Zeit: 11:15 Uhr
Hörsaal: B116
 
Gregory Gäumann
Institute of Applied Physics
University of Bern

In the first part of this talk measurements on graphene and vanadium dioxide in our high field THz setup will be presented and compared with simulations. For graphene an increase in transmission (saturable absorption) as high as 12% for strong fields is observed and it’s dependence on the Fermi level was measured for the first time. The obtained results are compared to a simulation that is based on a simple thermodynamic energy balance. For VO2 the onset of a THz field induced metal to insulator transition is observed. In the second part recent progress in building up a NIR pump THz ellipsometer is reported and first measurements on silicon and VO2 are presented.

 
Donnerstag, 26.10.2017

Digital Silicon Photo-Multipliers for biomedical, particle physics and quantum physics applications

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Leonardo Gasparini
Fondazione Bruno Kessler (FBK), Trento (IT)

Digital Silicon Photo-Multipliers (DSiPM) in Complimentary Metal-Oxide Semiconductor (CMOS) technology represent a relatively recent innovation in time-resolved single-photon detection for scientific applications. Arrays of Single-Photon Avalanche Diodes (SPAD) can be combined with custom-designed electronics implemented on the same substrate to create 100k-channel detectors with timestamping circuits having less than 100ps timing resolution. High efficiency is achieved using on-chip smart mechanisms to discriminate the signal to monitor from noise, minimizing dead times. The seminar will give the attendees an overview of the technology and provide few examples of sensors applied to different fields of application.

 
Donnerstag, 02.11.2017

Wire Scanner on a Chip

Zeit: 11:15 Uhr
Hörsaal: B116
 
S. Borrelli, M. Bednarzik, C. David, E. Ferrari, V. A. Guzenko, G.L. Orlandi,
C. Ozkan-Loch, E. Pratt and R. Ischebeck
Paul Scherrer Institute PSI
Villigen Switzerland

Wire scanners are commonly used for high resolution beam profile and emittance measurements. However, their resolution is limited to few microns by the resolution of the position encoder, wire vibrations and the wire diameter. We propose to overcome the latter two limitations leveraging on nanofabrication techniques, to electroplate metallic strips on a thin membrane via e-beam lithography. This novel design opens up a pathway to sub-micrometer resolution beam profile monitors, which is a huge stride for future free electron lasers, as well as various advanced accelerator concepts, including dielectric laser accelerators. We show measurements performed with relativistic electron beams in the injector of the X-ray free electron laser SwissFEL.

 
Donnerstag, 09.11.2017

Cancelled!! - Charge and energy transfer processes in Photoactive Molecular Materials

Zeit: 11:15 Uhr
Hörsaal: B116
 
Michela Gazzetto
Institute of Applied Physics
University of Bern

I will present a study of ultrafast energy transfer and charge transfer processes in different photoactive macromolecular systems by means of femtosecond UV-Vis transient absorption spectroscopy. I will illustrate our progress in the comprehension of such processes and their key-role in the photo-response of different molecular systems with few examples from my research activity. As first example I will introduce a family of hyper-polarizable chromophores showing anti-Kasha behavior, capable to behave as multi-responsive optical switch. As second example I will present the photophysics of a novel family of carbon-based nanodots, fluorescent nanomaterials with an appealing combination of properties, such as a fluorescence tunability, bio-compatibility, photo- and chemical stability.

 
Donnerstag, 16.11.2017

Introduction to our Matlab software framework: FiberlabFramework

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

I present our Matlab software framework, FiberlabFramework (FLF). FLF is a framework in order to easily address and access our lab hardware such as optical spectrum analyzers, oscilloscopes, power meter and others. Gathering data from a lab device is as simple as it would be with LabView. However, people lacking of LabView skills could find FLF more intuitive than LV. The development of the framework is an ongoing process, input and feedback are welcome.

 
Donnerstag, 23.11.2017

Carbon Based Crystalline Photonic Devices

Zeit: 11:15 Uhr
Hörsaal: B116
 
Dr. Helena Alves
CICECO
University of Aveiro, Portugal

Carbon based materials have unique optical, mechanical and electronic properties that can be combined with chemical functionality. In the crystalline form, they reveal superior electronic performance. However, the use of organic single crystals in devices is still limited to a few applications, such as field-effect transistors. Very recently, we developed single-crystal interfaces presenting photoconductivity behaviour, [1] capable to extract excitons generated in acceptor materials. [2] By adequate material design and structural organisation it is possible to improve exciton diffusion efficiency, [3, 4] and use the concept of charge transfer interfaces in phototransistors. [5, 6] Inclusion of graphene as electrode, leads to high efficiency nanodevices, and high conduction, transparent textile fibers. [7] These results open the possibility of using carbon single-crystal in photonic and textile applications.

References: [1] Nature Commun., 2013, 4, 1842; [2] J. Mater. Chem. C, 2014, 2, 3639; [3] JACS, 2015, 137, 7104; [4] ACS Appl. Mater. Interfaces, 2015, 7, 27720; [5] Appl. Phys. Lett., 2015, 107, 223301; [6] Adv. Mater., 2017, 1702993; [7] Scientific Reports, 2015, 5, 9866.

 
Donnerstag, 30.11.2017

From Theory To Application: Role of Numerical Simulations in Laser-Material Interaction and Laser Amplifier

Zeit: 11:15 Uhr
Hörsaal: B116
 
Yiming Zhang
Institute of Applied Physics, University of Bern
BFH Bern University of Applied Sciences, Engineering and Information Technology, Burgdorf

This presentation contributes mainly to the theoretical investigation of spot size effect during the laser ablation on metals with picosecond and femtosecond laser pulses. In addition, the simulation of optimized design of laser amplifier in high power laser system is taken as an example, demonstrating extended functionality of simulation in practical application. With the help of beam expanders, short and ultrashort pulsed lasers with different spot sizes, ranging from several to tens of micrometers in radius, are applied to the laser ablation experiments on metals. Two kinds of femtosecond laser systems working at wavelength of 1026nm and 1030nm with pulse duration of 260fs and 350fs, respectively are used. Specific removal rates are derived from measuring the depth of the machined squares on the surface of copper and iron, and the ablation thresholds and energy penetration depths of both metals are deduced by measuring the depth of machined craters. Similar experiments are carried out by using a picosecond laser system working at wavelength of 1064nm with pulse duration of 10ps on copper. The beam spot size effect on ablation threshold and specific removal rate during the laser ablation process on noble metal were fund.

A set of axisymmetric 2-dimensional model are developed for both picosecond and femtosecond laser ablation on copper with lower and higher laser fluences, respectively. Some of the important thermal properties of electron and lattice for copper, such as thermal conductivity, heat capacity, and electron-phonon coupling coefficient, are deduced by employed the first-principles electron structure calculations of electron density of states and Debye model, respectively. Although the nonlinearity of the equations increases, these descriptions of thermal properties of subsystems are especially crucial for the thermal response of material to the ultrashort pulsed laser irradiation. The critical point model is applied to describe the dynamic response of optical properties, optical penetration depth and reflectivity, of material to the irradiation. The models are further developed on the basis of the principles of Two Temperature Model (TTM). For the laser fluence closed to the measured ablation threshold, the models are coupled with thermoplastic theory. As the laser fluence approaches to the melting threshold, thermal viscoplasticity is taken into consideration. For the cases where phase transition occurs hydrodynamics coupled with TTM becomes the corresponding model. The characterized features, which distinguish the models for femtosecond laser ablation and picosecond laser ablation are that, on one hand, the former one is derived from Boltzmann transport equations considering the dynamics of electrons under the ultrafast processes, and on the other hand the ballistic motion of the excited electrons during the ultrafast irradiation process.

The results from the simulations of picosecond laser ablation on copper with various spot sizes indicate that the deviations of thermomechanical responses to different spot sizes lead to the change of ablation threshold and in a certain range of laser fluence the specific removal rate predicted by the model has a good agreement with the experimental ones for all the tested beam radii. For the ones from simulation of femtosecond laser ablation on copper, however, it indicates that the ballistic movement of excited electrons plays a crucial role in the beam spot size effect.

The last part of the presentation presents the role of the simulation that plays in the practical application by showing the study of optimized design of Yb: YAG amplifier in high power solid state laser system. The validation of the model is confirmed by comparing the simulated and experimental results. For the study of optimization, the optimal ratio of the radii of pump beam to seed beam and the optical focal position of pump beam for the crystal in certain geometric scale and dopant concentration are obtained. By simulating the thermomechanical responses in different gluing conditions, the risk of the generation of cracks are analyzed.

 
Donnerstag, 30.11.2017

PhD Thesis: Propagation of time-energy entangled photons through optical fibers

Zeit: 15:30 Uhr
Hörsaal:
 
Jos Kohn
Institute of Applied Physics
University of Bern

The thesis will be held at University of Fribourg, Physics department, room 2.73 (2nd floor), Chemin du musée 3, 1700 Fribourg; http://physics.unifr.ch/de/page/216/

 
Freitag, 08.12.2017

Bachelor Talk: Quantum State Tomography with Regularized Finite Statistics

Zeit: 11:15 Uhr
Hörsaal: B116
 
Bruno Eckmann
Institute of Applied Physics
University of Bern

In Quantum State Tomography, the state of some quantum mechanical system is reconstructed by means of projective measurement results. For this, several reconstruction schemes exist, such as linear inversion and Maximum Likelihood estimation. While in linear inversion density matrix reconstructions are typically non-physical due to the consideration of finite statistics, schemes like Maximum Likelihood estimation are investigated. Unfortunately, the latter leads to intrinsically biased estimations. Therefore, in the context of entangled bipartite quantum states, an alternative approach has been studied. Through the regularization of measurement results, the statistical fluctuations are reduced to a valid subset of the probability space.

 
Donnerstag, 14.12.2017

PhD defense: THz Stark Spectroscopy of Molecules and Solids

Zeit: 11:15 Uhr
Hörsaal: B116
 
Maryam Akbarimoosavi
Institute of Applied Physics
University of Bern

 
Donnerstag, 21.12.2017

Laser cutting of X-mas gifts

Zeit: 11:15 Uhr
Hörsaal: B116
 
Prof. Dr. Samichlaus
Santa Claus Holiday Village, University of Rovaniemi, Finnland

We use different types of lasers to cut X-mas gifts from 3 mm thick cardboard. It turns out that kids love best the ones made by ultraviolet laser radiation, although the physical mechanism is not yet fully understood. In order to address the problem we cut different animals with different laser intensities, focal positions and cutting velocities. The kids' enthusiasm seems to be largely unaffected by variation of these parameters. What seems to matter most is the type of animal and its appearance in terms of color and fluffiness. To the present day these results puzzle the laser community but with an increased budget and new lasers we hope to solve that mystery in the near future.