Institute of Applied Physics

The IAP was founded on June 2nd, 1961 as part of the faculty of natural sciences. Today about 56 employees research and teach in four divisions focusing on Biomedical Photonics, Laser Physics, Nonlinear Optics and Microwave Physics.

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News

Wideband dispersion‑free THz waveguide platform

We present a versatile THz waveguide platform for frequencies between 0.1 THz and 1.5 THz, designed to exhibit vacuum-like dispersion and electric as well as magnetic field enhancement. While linear THz spectroscopy benefits from the extended interaction length in combination with moderate losses, nonlinear THz spectroscopy profits from the field enhancement and zero dispersion, with the associated reshaping-free propagation of broadband single- to few-cycle THz pulses. Moreover, the vacuum-like dispersion allows for velocity matching in mixed THz and visible to infrared pumpprobe experiments. The platform is based on the motif of a metallic double ridged waveguide. We experimentally characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate a junction and an interferometer, essentially because those elements are prerequisites for THz waveform synthesis, and hence, for coherently controlled linear and nonlinear THz interactions.

Noncollinear, inelastic four-wave mixing in the extreme ultraviolet

Driving four-wave mixing (FWM) processes with extreme ultraviolet (EUV) pulses could enable experimental approaches that have the potential to provide unique information on dynamics and correlations. In this work, we demonstrate inelasticFWMobtained by noncollinear mixing of two EUV pulses with different photon energies and an optical pulse in a diamond sample. This three-pulse interaction leads to the emission of an optical signal, propagating in the phase-matching direction and blue shifted by the photon energy difference of the two EUV pulses. The presented results demonstrate the feasibility of experiments such as the soft X-ray analogue of coherent anti-StokesRaman scattering, so far only theoretically conceived [Phys. Rev. Lett. 89, 043001 (2002)], which can be further extended for studying vibrational and electronics dephasing in solid, liquid, or gaseous samples.

Merging of Azulene and Perylene Diimide for Optical pH Sensors

Polycyclic aromatic hydrocarbons (PAHs) have emerged as promising materials for organic electronics, including organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Particularly, non-hexagonal ring-fused PAHs are highly desirable due to their unique optoelectronic properties. Herein, a new redox-active azulene-perylene
diimide triad 1 and its ring-fused counterpart, diazulenocoronene diimide 2, were synthesized and fully characterized by a combination of NMR, cyclic voltammetry, and UV-visible absorption spectroscopy. Direct comparison of their electronic properties leads us to the conclusion that a significant change in the localization of HOMO and LUMO occurs upon the fusion of azulene and
perylene diimide in 2, leading to the lack of intramolecular charge-transfer character for transitions in the visible spectral region. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to gain further insight into various electronic transitions. Moreover, we found that the adaptive response to acids and bases manifests itself in a reversible two-color change that can be attributed to changes in the chemical structures. Our findings pave the way for manipulating the relative HOMO and LUMO energy levels of organic chromophores by fusing non-alternant azulenes to an otherwise flat PAH, which could possibly lead to applications in organic electronics and optical sensors.

Thomas Feurer wird Vorsitzender der Geschäftsführung des Röntgenlasers European XFEL

Der European XFEL Council hat Thomas Feurer, Leiter des Instituts für Angewandte Physik an der Universität Bern, zum Vorsitzenden der Geschäftsführung ernannt. Feurer wird sein Amt am 1. Januar 2024 antreten. Der internationale Röntgenlaser European XFEL wurde 2017 in Schenefeld bei Hamburg in Betrieb genommen. Er erzeugt extrem intensive Röntgenlaserblitze und ermöglicht damit Spitzenforschung auf unterschiedlichen Gebieten.
Thomas Feurer übernimmt ab dem 1. Januar 2024 für eine Amtszeit von zunächst fünf Jahren den Vorsitz der Geschäftsführung von European XFEL. Er folgt auf den Dänen Professor Robert Feidenhans’l, der die Organisation seit 2017 erfolgreich führt und 2024 in den Ruhestand geht. Thomas Feurer wurde 2004 zum Professor für Physik an der Universität Bern ernannt und leitet dort das Institut für Angewandte Physik. Thomas Feurer bleibt weiterhin Professor an der Universität Bern.
Die Schweiz ist – zusammen mit elf anderen Staaten – Gründungsmitglied des European XFEL. Am Paul Scherrer Institut in Villigen (AG) hat die Schweiz einen eigenen Röntgenlaser (SwissFEL), der den European XFEL ergänzt. Durch den parallelen Bau dieser Infrastrukturen konnten Synergien genutzt und Wissen ausgetauscht werden.
Die Nomination von Thomas Feurer stärkt die bereits engen Beziehungen zwischen der Forschungsgemeinschaft in der Schweiz und dem European XFEL sowie dessen Partnerstaaten. Sie ist auch Zeichen für das partnerschaftliche Engagement der Schweiz innerhalb des Europäischen Forschungsraums. Der gemeinsame Bau und Betrieb von Forschungsinfrastrukturen durch die europäischen Staaten bildet eine wesentliche Grundlage für die langfristige Zusammenarbeit, Integration und Vernetzung von Forschenden über Ländergrenzen hinweg.

Excluding Echo Shift Noise in Real-Time Pulse-Echo Speed-of-Sound Imaging

Computed ultrasound tomography in echo mode (CUTE) allows real-time imaging of the tissue speed of sound (SoS) using handheld ultrasound. The SoS is retrieved by inverting a forward model that relates the spatial distribution of the tissue SoS to echo shift maps detected between varying transmit and receive angles. Despite promising results, in vivo SoS maps often show artifacts due to elevated noise in echo shift maps. To minimize artifacts, we propose a technique where an individual SoS map is reconstructed for each echo shift map separately, as opposed to a single SoS map from all echo shift maps simultaneously. The final SoS map is then obtained as a weighted average over all SoS maps. Due to the partial redundancy between different angle combinations, artifacts that appear only in a subset of the individual maps can be excluded via the averaging weights. We investigate this real-time capable technique in simulations using two numerical phantoms, one with a circular inclusion and one with two layers. Our results demonstrate that the SoS maps reconstructed using the proposed technique are equivalent to the ones using simultaneous reconstruction when considering uncorrupted data but show significantly reduced artifact level for data that are corrupted by noise.

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