The international Research Training Group Meta-Active (GRK 2675), funded by the German Research Foundation, is dedicated to the investigation of active metasurfaces for the generation, manipulation, and detection of light. The focus lies on nanoscale, resonant structural elements whose collective arrangement opens up new degrees of freedom for light–matter interaction. In contrast to previously predominantly passive metasurfaces, these systems are designed to be dynamically controllable and to serve as a basis for novel light sources, programmable optical functionalities, and enhanced detector concepts, including applications in the field of quantum optics.

The structured training of doctoral researchers is at the core of the Research Training Group. A coordinated qualification program comprises joint courses, binational research activities, and a mandatory research stay at the respective partner institution. Doctoral projects are jointly supervised and lead to a Dual PhD degree awarded by both universities. In this way, harmonized training standards are ensured and international research experience is systematically conveyed.

Subproject C1, with the participation of Leibniz IPHT, aims at the development of multifunctional metasurfaces for highly sensitive chiroptical detection of chiral molecules. The objective is to convert incident far-field illumination into tailored optical near fields in order to enhance the chiroptical response of chiral matter and significantly improve the sensitivity of optical chiral sensing. To this end, both plasmonic and dielectric metasurfaces with chiral and achiral geometries are designed to provide broadband and tunable enhancement.

A particular focus is placed on the targeted engineering of the optical near field. Optimized structures are intended to maximize both near-field optical chirality for circular dichroism detection and field intensity for optical trapping. In this way, chiral target molecules can be actively guided into field-enhanced regions and analyzed there. In addition, a solid-state extraction approach is pursued, in which surface-functionalized micro- or nanoparticles act as carriers to pre-separate, preconcentrate, and selectively transport chiral molecules to the hotspots of the metasurfaces.

The work comprises theoretical modeling and numerical simulations to optimize structural geometries, as well as the experimental realization of the metasurfaces using established top-down and bottom-up nanofabrication techniques. These include, among others, three-dimensional metastructures, bilayer substrates, and stress-induced shaping. Chiroptical characterization is performed over a broad spectral window from the mid-infrared to the terahertz range, including the investigation of vibrational circular dichroism signals of selected model molecules such as α-pinene and limonene. In the longer term, active control of the metasurface morphology is also envisaged, for example through thermal effects or mechanical stretching, to dynamically tune the optical properties.

This project is funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under project number 437527638.