Physikalische Kolloquien im Wintersemester 2022/23

Physikalisches Geburtstagskolloquium zu Ehren von Herrn Prof. Dr. Bernd Brügmann

Nach einer Einleitung und Grußworten des Dekans, Prof. Dr. Christian Spielmann folgt der Festvortrag:

Prof. Dr. Abhay Ashtekar
The Pennsylvania State University (USA)

The Enigma of Black Hole Horizons

Gravitational wave observations have established that black holes are ubiquitous
in our universe. But what exactly is a black hole? What exactly is it that forms as
a result of a gravitational collapse and evaporates due to quantum radiation? The
common answer is of course „event horizons“. But they are teleological and have
unphysical properties. An event horizon may be contained in the room you are
sitting in – and growing – in anticipation of a gravitational collapse that may take
place a billion years from now. Similarly, much of the confusion about the
„information loss issue“ stems from over-reliance on event horizons. Is there a
better notion of black hole horizons? We will see that the answer is in the
affirmative and leads to new and interesting insights. For example, they have
opened up the possibility of gravitational wave tomography: although horizons are
invisible to outside observers, one can reconstruct an image of their dynamics
using gravitational waveforms at infinity.
This account of the fascinating and vexing properties of black hole horizons will
be presented at a level that is accessible to non-experts.

live streamExterner Link

Gastgeber/Verantwortlicher: Prof. Dr. Thomas Pertsch

Habilitationsvorstellung

Dr. Sina Saravi
Institut für Angewandte Physik

Nanoscale nonlinear quantum photonics

Nonlinear nanostructured and nanoscale photonic systems offer new opportunities for optical quantum technologies. Aside from the obvious advantage of miniaturization, they also offer new capabilities in engineering the properties of the quantum states generated through nonlinear interactions, such as photon pairs and squeezed light. More generally, in such nanoscale systems, one gains access to new degrees of freedom in tailoring nonlinear quantum optical interactions. This could be done through strong control over modal dispersion and optical density of states, having access to near-field interactions and evanescent modes, and possibility of having new types of phase-matching conditions or lack of them. At the same time, commonly in such nanoscale photonic systems, there is an unavoidable presence of loss, either through material absorption or scattering, or inherent leakage mechanisms. When the loss or decay mechanisms are directly combined into the nonlinear quantum process, it can create different behaviors compared to when the nonlinear interaction is separated from the loss mechanism, creating new challenges and at the same time new opportunities.

In this talk we review the state of nonlinear quantum photonics in nanostructured and nanoscale systems, with a focus on engineered generation of quantum light. We then focus on how to describe nonlinear quantum optical interactions in nanoscale photonic systems in the presence of loss, leakage, evanescent fields, and in non-paraxial regimes, and how such effects and regimes of operation can be utilized for novel applications in optical quantum technologies, such as quantum spectroscopy, quantum imaging, entangled photon-pair generation, and creating hybrid systems.

Gastgeberin/Verantwortliche: PD Dr. Cormelia Jäger

Habilitationsvorstellung

Dr. Sergiy Krasnokutskiy
Institut für Festkörperphysik, AG Laborastrophysik

Role of Low-Temperature Chemistry of Carbon in our Astrochemical History

The chemistry that takes place between stars determines the molecular  composition of the solids from which all solid objects in new  planetary systems are formed. A large fraction of this material  survives during the star formation process maintaining its chemical  composition. It is expected to be delivered to planets at their  earlier stages.

Laboratory studies of the formation of carbonaceous solids under  conditions similar to those of the interstellar medium have proved  their predominantly organic nature. Therefore, the massive delivery of  such organics to Earth and exoplanets should have a significant impact  on the chemistry occurring on their surfaces and may also trigger the  emergence of life. Recent discoveries of the presence of peptides in  meteorites and the way how these molecules may be formed in the  interstellar medium give strong support to the hypothesis of molecular  panspermia.

Gastgeberin/Verantwortliche: Prof. Dr. Silvana Botti

Habilitationsvorstellung

Ab-Initio Approaches to Electronic and Optical Excitations: From Correlated Oxides to Novel Light Emitters

Dr. Claudia Rödl
Institut für Festkörpertheorie und -optik

Modern ab-initio approaches to the quantum many-body problem open unprecedented possibilities to compute, analyze, and predict materials properties of complex systems ranging from bulk semiconductors over correlated systems to nanomaterials. Whereas density-functional theory with state-of-the-art functionals allows for efficient calculations of larger systems, first-principles many-body perturbation theory provides a deeper physical understanding of many-body effects, such as electronic quasiparticle excitations, dynamical screening, or excitons. The increasing complexity of materials systems for technological applications, for instance in optoelectro-nics or photovoltaics, demands for continuous development of novel efficient and accurate theoretical methods along with improved computational schemes to predict electronic and optical properties.

In this presentation, I will focus on two examples: First, it will be demonstrated how ab-initio many-body calculations and state-of-the-art synchrotron experiments can go hand in hand to unravel the electronic screening mechanisms in strongly correlated CuO. The dynamical screening of the electron-electron interaction is a key quantity in many-body perturbation theory. We elucidate the crucial role played by d-d excitations in renormalizing the band gap and dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.

Second, it will be shown how theoretical modeling of material properties can drive technological innovation. Silicon is known as a notoriously bad light emitter, due to its indirect band gap. Making silicon a direct-gap efficient ligth-emitter would enable the integration of microelectronics and optoelectronics, which is expected to revolutio-nize various fields of technology, such as communication, sensing, and imaging. We predict hexagonal silicon-germanium alloys to be direct-gap materials with a dipole-active absorption edge and optical oscillator strengths comparable to III-V semiconductors. The band gap, and therefore the emission frequency, can be tuned with alloy composition. Our findings are supported by recent data from our experimental collaborators, which are promising towards the development of silicon-based nanolasers.