Optical metasurfaces are ultra-thin, precisely engineered surfaces that can produce optical responses not found in naturally occurring materials. They are composed of regular arrangements on resonant sub-wavelength building blocks (often called meta-units) that allow them to concentrate light into small hot-spots at nanometer length scales. These extraordinary properties have allowed metasurfaces to deliver breakthroughs in a wide variety of fields ranging from biosensing (Adv. Mater. 2022) to energy conversion (Chem. Rev. 2022).

In our research, we develop new metasurface concepts for pushing the limits of light-matter coupling. Towards this goal, we utilize precise numerical modeling to to discover and optimize new metasurface designs, and harness state-of-the-art nanofabriation protocols to demonstrate their advanced functionalities in experiments.

A focus of our work are metasurfaces based on the physics of photonic bound states in the continuum (BICs, see e.g. Nat. Rev. Phys. 2023). These states allow precise control over the resonance position and the resonance quality factor (defined as the resonance wavelength divided by the linewidth) by carefully breaking the symmetry of the meta-unit geometry, making them ideal for tailored light-matter interactions. Below are some recent examples of advanced metasurface concepts realized in the group.

BlueMat's mission is to create interactive, sustainable, and intelligent material systems that either unfold their unique functionality in aqueous environments or derive their properties directly from nature’s most powerful resource: water.

Research Area C: Photonic Materials seeks to develop water-driven structured materials that can alter their electromagnetic properties in the optical and microwave ranges through water interaction. Depending on the wavelength of the electromagnetic radiation, we will change the effective permittivity of or scattering from structured media. This research aims to create adaptive surfaces, windows, and thermal insulation for sustainable architecture and garments, as well as customizable optical and microwave elements for adaptive optics, radars, and sensors. The focus is on understanding water’s electromagnetic properties at the interface of nanoporous media and controlling water distribution at nm- and μm-scales to achieve mechanically stable photonic materials activated by water.

Resolving and understanding the dynamic interactions of biomolecules in complex biochemical samples is one of the major challenges of medical diagnostics, biophysics, and biology in general. For many applications, compact sensor systems are highly desirable, because they enable the retrieval of actionable biochemical information directly at the point-of-need, for example in the field or in a patient's home.

Our BIC-driven metasurfaces feature extremely narrow resonances and strong signal enhancements, making them ideal for detecting biomolecules, analyzing biomimetic membrane processes, and monitoring energy conversion dynamics with exceptional sensitivity and in compact footprints (Adv. Mater. 2023). Furthermore, the ultra-sharp resonances enable spectrally selective and label-free detection by measuring the characteristic molecular fingerprints of different analytes, enabling effective molecular differentiation and quantification. By integrating these metasurfaces with microfluidic systems, we enhance their capability for in-situ and real-time measurements of dynamic biochemical reactions (Adv. Opt. Mater. 2022). Below are some of our recent results on biochemical sensing, spectroscopy, and energy conversion.

Van der Waals (vdW) materials are crystalline solids composed of atomically thick layers that are tighly bound in-plane, but experience only weak vdW forces in the out-of-plane stacking direction. This unique structure gives them extraordinary mechanical, electronic, and optical properties even when used as single layers, launching breaktroughs in a variety of fields (Science 2016).

Their atomically thin nature, combined with their diverse range of optical properties, makes them ideal for the creation of novel photonic devices with enhanced functionality and performance (Laser Photonics Rev. 2023). In our research, we merge vdW materials with metasurfaces to improve their coupling efficiency with light and realize advances in polaritonic coupling and quantum light generation.

One of the most remarkable features of vdW materials is their ability to be stacked and combined in various configurations, creating artificial heterostructures with tailored properties. This deterministic assembly approach offers unprecedented flexibility in designing and fabricating complex nanophotonic devices with functionalities that are beyond the reach of conventional materials. Below are some of our recent results on nanophotonic systems based on vdW materials.

In this project on time-varying media we investigate different index front-induced optical transitions close to the band edge of a periodical waveguide. Namely, light stopping, time lens, and ultrafast arbitrary waveform generation are considered. For that we utilise silicon waveguides with Bragg gratings and operate them close to the band edge. The moving refractive index front is generated in the same waveguide via free carriers and/or instantaneous Kerr effect from the copropagating switching/pump pulse. The proposed effects are utilising the theoretical concepts developed by our group in recent year which we now demonstrate experimentally. In case of light stopping, like in a mechanical collision, the light pulse hits a propagating front and stops. Here, the light energy can be transferred to the zero group velocity mode and later released by a second front with an opposite slope. In case of a so-called optical push broom, the signal pulse is trapped by the index front with a simultaneous frequency change and compression. As we have recently shown theoretically the trapping in the front is equivalent to Fourier optics observed at the focal plane of conventional lenses. This effect allows not only pulse compression but also the transfer of information from real to reciprocal space and vice versa. The proposed system provides signal temporal compression, light stopping, and pulse structuring in integrated optics technology.