Metasurface Development
Understanding and shaping the flow of light below the optical diffraction limit with numerical simulations and high-resolution nanofabrication.
Understanding and shaping the flow of light below the optical diffraction limit with numerical simulations and high-resolution nanofabrication.
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.
Ultrasharp BIC-driven metasurfaces excel at probing the optical properties of a given material at given wavelengths. However, they usually only resolve a limited number of wavelength points and neglect the coupling dimension of light-matter interactions given by the resonance quality factor. We introduced a new type of dual-gradient metasurfaces, which simultaneously and continuously encode the spectral and quality-factor parameter space within a compact spatial area. This concept provides unprecedented resonance densities and opens up a new coupling-strength dimension for biochemical spectroscopy Nat. Nano. (2024).
The quality of metasurface resonances depends on the size of the structured area, the density of defects, and the influence of the edges. Using scanning near-field optical microscopy (SNOM), we quantified the crucial influence of these metasurface paramenters and were able to define design rules for optimal metasurface geometries. Adv. Mater. 2405978 (2024).
In most metasurface designs, all meta-atoms have the same thickness given by the deposition step of the resonator material. We showed that you can unlock the out-of-plane dimension for metasurface design by developing a new multi-step and multi-height nanofabrication approach. This method allowed us to realize maximally chiral metasurfaces, which selectively and optimally interact with specific circular polarizations of light. Light Sci. Appl. 12, 250 (2023)
In addition to modifying the geometry of the meta-atoms, changing the optical properties of individual resonators can offer new perspectives for metasurface design. We demonstrated precise control over the relative permittivities of the meta-atoms to produce BIC-driven resonances with unprecedented sharpness and light localization, showing great potential for realizing actively tunable metasurfaces leveraging Pockels or Kerr effects. Nano Lett. 23, 2651-2658 (2023).
A major constraint for many metasurface approaches are the large array sizes that are commonly required for sustaining high-quality resonances. We developed a new approach for realizing BIC-driven resonances in ultracompact footprints by arranging the individual meta-atoms in a semi-infinite circular geometry, leading to the excitation of so-called radial BICs. These structures retain the resonance control benefits of traditional extended BIC metasurfaces, but provide polarization independence and footprints down to square microns Nat. Comm. 13, 4992 (2022).
A recent direction in metasurface development is the creation of active metasuraces, where the optical response can be modified via an external tuning parameter (such as voltage, temperature, or magnetic field). This opens up interesting new avenues for creating optical modulators and other dynamic metadevices.
Vanadium dioxide (VO2) is a prominent phase change material that can be switched between metallic and dielectric states using temperature changes. Using the increased material loss associated with the dielectric to metal transition, we demonstrated precise loss-engineering and large modulation contrast in a hybrid silicon-VO2 metasurface Nano Lett. 2024.
By incorporating the electrochromic polymer polyaniline (PANI) directly into the indivdual meta-units, we can create environmental permittivity-asymmetric BIC metasurfaces with electrical reconfigurability. This approach provides dynamic resonance modulations down to milliseconds Nat. Comm. 15, 7050 (2024).
Can you create a metasurface without any nanostructuring? We demonstrated that by interfering ultrafast laser pulses, we could generate periodic modulations of the refractive index that acted as a virtual metasurface. Our concept provices ultrafast control over all important resonances parameters and time-selective near-field enhancement with picosecond precision arXiv:2403.05730 (2024).
Take a look at our full list of publications here.