Prof. Thomas Weiss
Resonant phenomena have been extensively used in micro- and nanophotonics. These phenomena originate in a discrete set of basis functions known as resonant states or quasi-normal modes that are eigensolutions of Maxwell’s equations. I will introduce the fundamental principles and challenges of describing light-matter interaction in terms of these resonant states. Such a description is very intuitive and provides deep insight about the underlying physical mechanisms. I will demonstrate this for various applications in fields such as chiral and nonreciprocal nanophotonics.
Polymorphism describes the ability of compounds to crystallize in different crystal structures and has a substantial impact on fundamental properties like thermal and chemical stability, solubility and electronic properties. Over the last decades, research on polymorphism played an essential role in various scientific fields like materials science, pharmaceutics, etc. One special topic in polymorph research is the influence of surfaces on the crystallization process and the thin film formation since specific polymorphs can be exclusively found in the vicinity of surfaces. Phenoxazine, a well-known heterocycle compound, is mainly used as a parent molecule for different organic electronics and pharmaceutics applications.  Since no crystal structure is known so far, one crystal structure is solved by single-crystal X-ray diffraction from extended crystals.  The phenoxazine crystallization in thin films revealed an additional 2nd polymorph. Selection between the two phases results from the crystallization process within the thin films. Fast methods solely give the kinetically driven 2nd phase, and slow crystallization ends in the formation of the 1st phase.  Facilitating various substrates revealed a surface needed for the formation, but those had no strong influence on the molecule orientation. The second example is the highly symmetric but non-planar molecule lead phthalocyanine(PbPc). In-situ scanning tunnelling microscopy together with computational simulations predicted the formation of a surface-induced phase (SIP) on a graphite/solution interface.  X-ray diffraction techniques confirmed the formation of a SIP on highly orientated pyrolytic graphite (HOPG) comparable to the prediction. Thus, the face on conformation (110) of the PbPc molecules could be revealed.  The same procedure on the PbPc deposited on graphene gave a different crystal orientation, whereas the face on conformation seemed to be the same. Finally, using pristine silica as substrate concluded that the SIP is not necessarily surface-induced. As a third example, the chiral drug naproxen is a common non-steroidal anti-inflammatory drug used to treat rheumatic issues. Previous investigations revealed that the (S)-naproxen could build four different crystal structures. [5,6] X-ray diffraction techniques on multiple thin films revealed an unknown polymorph's formation using chlorobenzene, while other solvents only gave Form 1. Furthermore, naproxen thin films on HOPG showed a change in the orientation of the molecules. Instead of standing, end-on, lying, or face-on, molecules were observed. In the presented examples, the combination of established X-ray techniques with an innovative crystal solution strategy allowed the solution of crystal structures within thin films.
 Katsamakas, Set al., CMC 2016, 23 (26), 2972–2999
 M. Kaltenegger, L. Delaive, et al., Cryst. Growth Des. 2022, 22, 3, 1548–1553
 M. Kaltenegger, et al., CrystEngComm, 2022, accepted
 Y. Hao, G. Velpula, M. Kaltenegger, et al., Chem. Mater. 2022, 34, 5, 2238–2248
 J.S. Song, Y.T. Sohn, Arch. Pharm. Res. 2011, 34, 87-90
 D. Braun, et al., Cryst. Growth Des. 2011, 11, 12, 5659–5669
Organic electronics play a subtle, yet integral role in our lives. An example are the organic LEDs in the screens of our phones. Their performance greatly depends on the properties of the organic/inorganic interfaces in these devices, which, in turn, strongly depend on the geometric structure of the interface.
For example, some organic molecules can adopt both flat lying or upright standing adlayers on metal substrates. The work function of such an interface can change by multiple eV between these “standing” and “lying” geometries. However, predicting which geometries interfaces can assume is nontrivial due to the infinite number of possibilities and the high cost for ab-initio calculations.
In this talk, I present my work towards understanding under which conditions some molecules exhibit standing/lying phases. I use symbolic regression on large ab-initio datasets to generate analytical formulas for interface properties such as the work function. I discuss the potential for extracting physical insight (e.g. for lying/standing transitions) from these formulas as well as possible pitfalls. Finally, I present my findings for forcing standing/lying transitions with external electric fields.
Motivated by recent experiments and computational results on Pyrochlore Iridates, we compare the combined density functional and single-site dynamical mean field calculations (DFT+sDMFT) with combined density functional and triply-irreducible local expansion calculations (DFT+TRILEX) of the pyrochlore iridate Y2Ir2O7. Both calculations predict the transition from paramagnet to an all-in/all-out antiferromagnetic ordered system as function of the interaction strength. At the same time we see a transition from metal to insulator where the DFT+TRILEX approach indicates the additional presence of a Weyl-semimetal phase in between the metal and the insulating phases.
Despite the many successes of the Standard Model of particle physics, it is known to be incomplete. Many approaches to searching Beyond-the-Standard-Model (BSM) phenomenology rely on assumptions which may not actually be valid, due to nontrivial field-theory effects arising as a consequence of the principle of gauge invariance. We investigate the spectrum of a "GUT-like" toy model which displays a Higgs effect, via numerical simulations on the lattice, and show the presence of discrepancies with the standard approach which need to be taken into account when model-building.
Reconfigurable photonic integrated circuits enable on-chip processing of light and play an increasing role in many applications. Among others, they can be used to extract relative amplitudes and phases of on-chip light. This is realized by actively controlling interference across the chip by means of interconnected Mach-Zehnder interferometers. Additionally, the use of a carefully designed input interface to couple free-space light into the circuit enables it to resolve free-space phenomena. Here we introduce the building blocks, the layout and the measurement principle of such a novel detector. We discuss in detail how it retrieves lights fundamental properties: amplitude, phase and polarization.
Measuring al these properties can offer numerous advantages across many disciplines. This could make such detectors applicable to many applications and experiments.
Epsilon-near-zero (ENZ) materials have been shown to host exotic nonlinear optical effects which enable broadband optical responses , require smaller interaction lengths , and disobey conventional approaches to modeling nonlinear optical properties . In this work, we investigate the interactions between tightly focused vector beams and structured ENZ materials. Depending on the combination between the polarization of the incident field and the geometry of the ENZ environment, strong field enhancements can be produced that are unique to the ENZ regime. In order to first achieve a linear characterization of our system, a simple hole geometry is considered for the structured ENZ medium. Each individual hole is then probed with tightly focused vector beams. The transmission properties of these holes are measured via power-based scanning methods and are compared to finite-difference-time-domain simulations. As opposed to the expected transmission properties of such holes in metallic films , the ENZ regime suppresses the transmission of the holes where it’s traditionally found to be a maximum. This indeed indicates that a legitimate response from the ENZ regime can be observed depending on the polarization distribution of the incident field and the structure of the ENZ medium. Under appropriate configurations, this can possibly be exploited to achieve nonlinear optical responses without the traditionally high incident intensities and large length scales.
 Alam, M. Z., Schulz, S. A., Upham, J., De Leon, I., & Boyd, R. W. (2018). Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material. Nature Photonics, 12(2), 79-83.
 Reshef, O., De Leon, I., Alam, M. Z., & Boyd, R. W. (2019). Nonlinear optical effects in epsilon-near-zero media. Nature Reviews Materials, 4(8), 535-551.
 Reshef, O., Giese, E., Alam, M. Z., De Leon, I., Upham, J., & Boyd, R. W. (2017). Beyond the perturbative description of the nonlinear optical response of low-index materials. Optics Letters, 42(16), 3225-3228.
 Kindler, J., Banzer, P., Quabis, S., Peschel, U., & Leuchs, G. (2007). Waveguide properties of single subwavelength holes demonstrated with radially and azimuthally polarized light. Applied Physics B, 89(4), 517-520.
The predictions of a high breakdown field for vacuum have existed for a long time, however, for sub-micron distances this behavior is often impaired because of the geometry of the electrodes and the materials used. By employing silicon electrodes and symmetrical geometries it is possible to create vacuum gaps of few hundred nanometers that are able to sustain several hundred volts of applied voltage. The aim of our investigation is to obtain a complete, fundamental understanding of vacuum gaps within micro- and nanometer silicon structures at high electric fields. In this context, the application of vacuum gaps has promising prospects in both research and industry.
Marko Simic: Optofluidic Force Induction Scheme for the Characterization of Nanoparticle Ensembles
Tommaso Mazzocchi: Correlated Mott insulators in strong electric fields: role of phonons in heat dissipation
Anas Alatrash: Alternative HR-(S)TEM Sample Preparation of Semiconducting Materials
Robert Schwarzl: Femtosecond Transient Absorption Microscopy of Squaraines