High Reflectivity of Focused Fields
The research project “High reflectivity of strongly focused fields” is a fundamental research project which establishes the theoretical understanding and the experimental implementation of the research on strongly-focused light fields. The underlying research has an impact on very small objects, such as single molecules, which are below the diffraction limit. Although our research originates from a very fundamental standpoint, the outcomes have the potential to influence present day research in high-resolution microscopy.
It is well established that a microscope allows to optically magnify objects from the nano-scale into the human accessible scale. Modern research microscopes have deviated from devices which are simple observation tools which allow a researcher to magnify certain objects for a view by eye. To date, microscopy has become quantitative, is recorded by camera or single pixel detectors and utilizes light fields which require more complex descriptions than just “a single point”.
In recent work by Ilja Gerhardt’s group, the researchers have calculated the emission pattern of a molecular dipole at a dielectric interface. This means they already have the knowledge of how a single molecule appears under a normal microscope. The next objective is to develop optical systems that can maximize the focusing of light onto the molecular emission pattern.
Focusing has several effects on the polarization of light: It creates a component of the polarization in the direction of the propagation and links the spatial structure of the light field to its polarization.
This creates an intricate polarization structure which is anchored around polarization singularities in the three-dimensional polarization of the highly focused light. And because the excitation of molecular dipoles depends on the relative orientation of the dipole and the polarization of the light. This causes the excitation of a molecular dipole to vary across the strongly focused illumination. In order to maximize the excitation of molecular dipoles it is therefore crucial to analyze the polarization pattern.
A new method of characterizing strongly focused fluids has emerged from applying concepts knows from solid state physics to the analysis of the topology of light fields: Skyrmions have originally been postulated to describe the topological structure of nucleons, Skyrmions have been predicted and observed in a wide range of contexts, including string theory, Bosonic condensates, spintronics, magnetic media, and more recently in plasmonics and optics.
A recent work by Joerg Goette’s group has shown that a description of the polarization in terms of a Skyrmionic field lends itself to trace field lines to lines where aspects of the polarization is fixed. Instead of anchoring the polarization structure along lines of circular or linear polarization, a description in terms of Skyrmionic structures would allow to describe the polarization in a holistic manner.
In this project, a fruitful collaboration between the two groups will be established from the PhD-level on. They will investigate the highly focused fields of a strong optical focus. This has a direct impact on microscopic methods, e.g., in microbiology.
“I am very grateful for this opportunity to cooperate with students and supervisors from Germany, I believe this will be an unprecedented experience for me. Thanks to the Lower Saxony – Scotland Tandem Fellowship Programme and the opportunity for this great collaboration.”
“I appreciate that the Lower Saxony – Scotland Tandem Fellowship Programme could provide us the opportunity to collaborate with Prof. Goette’s group. This project offers a valuable opportunity for me to expand my knowledge of optics theory. Moreover, I am happy to get to know my fellowship partner Zhujun through this project.”
Zhujun Ye is a PhD student at quantum theory group at University of Glasgow. Her research topic mainly lies on structured light, including strongly focussing of vector vortex beams and Skyrmions. She has done several relevant projects during the 3 years of PhD, one examined the (secondary) Faraday effect under strong focussing, which exhibits more subtlety in rotating the polarization structure after the beam has been focussed, the relevant paper is now being written. As for Skyrmions, she has found a generalization which allows measurements of Skyrmion numbers not to be limited on Schmidt bases; this has been proved to improve the precision in experimental measurements (McWilliam et al., 2022). Currently, she is looking into a further generalization of the current 2D Skyrmion model to a 3D one, and has found solid tools to characterize 3D polarization.
Yijun Wang obtained his bachelor’s degree in applied physics from Dalian University of Technology in China. Afterwards he started his master study in the University of Stuttgart in Germany, and completed his master’s degree in the field of quantum optics. Following his master’s degree, Yijun joined Prof. Dr. Ilja Gerhardt’s research group, and relocated to Hannover. He has been a PhD candidate at Leibniz University Hannover since 2021. His research interests include single-photon emitters, fluorescence spectroscopy and atom-photon interactions.