Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. In the presence of the chiral molecules, the SLRs become optically active, i.e. showing handedness-dependent excitation. Collaboration with the groups of Matthias Karg, Aitzol Garcia-Etxarri and Javier Aizpurua. Read our paper in ACS Photonics.
Strong electromagnetic fields emerge around resonant plasmonic nanostructures, focusing the light in tiny volumes, usually referred to as hotspots. These hotspots are the key regions governing plasmonic applications since they strongly enhance properties, signals or energies arising from the interaction with light. For maximum efficiency, target molecules or objects would be exclusively placed within hotspot regions. Here, we propose a reliable, universal and high-throughput method for the site-specific functionalization of hotspot regions over macroscopic areas. We demonstrate and visualize a successful targeting of the hotspot regions by binding small gold nanoparticles and show a targeting efficiency of 90%. Read our paper in RSC Nanoscale Advances.
Collective excitation of periodic arrays of metallic nanoparticles by coupling localized surface plasmon resonances to grazing diffraction orders leads to surface lattice resonances with narrow line widths. Here, a new degree of freedom of surface lattice resonances is experimentally investigated by demonstrating handedness-dependent excitation of surface lattice resonances in arrays of chiral plasmonic crescents. The self-assembly of particles used as a mask and modified colloidal lithography is applied to produce arrays of planar and 3D gold crescents over large areas. The chirality of the individual 3D crescents leads to the formation of chiral lattice modes, that is, surface lattice resonances that exhibit collective optical activity. Collaboration with the groups of Matthias Karg and Peter Banzer. Read our paper in Advanced Materials.
Chiral plasmonics is a fascinating research field that is attractive to scientists from diverse backgrounds. Physicists study light–matter interactions, chemists seek ways to analyze enantiomeric molecules, biologists study living objects, and material engineers focus on scalable production processes. This tutorial-style review addresses these issues with the goal to provide a comprehensive introduction into chiral plasmonic nanostructures. It starts with a brief introduction of the relevant physics involved in chiral light−matter interactions. A brief guide about how to adequately characterize samples follows. Subsequently, an overview of fabrication techniques that produce chiral substrates over large areas is given, and the strengths and weaknesses of the different approaches are discussed. The focus is on simple and robust processes that do not require clean room facilities and can be implemented by a much larger scientific audience. Collaboration with the group of Lisa Poulikakos. Read our review in Advanced Optical Materials.
Here, a colloidal lithography approach is used to produce macroscopic arrays of sub-micrometer 3D chiral plasmonic crescent structures over areas >1 cm^2. The chirality originates from symmetry breaking by the introduction of a step within the crescent structure. This step is produced by an intermediate layer of silicon dioxide, onto which the metal crescent structure is deposited. It is experimentally demonstrated that the chiroptical properties in such structures can be tailored by changing the position of the step within the crescent. These experiments are complemented by finite element simulations and the application of a multipole expansion to elucidate the physical origin of the circular dichroism of the crescent structures. Collaboration with the group of Peter Banzer. Read our paper in Advanced Optical Materials.
Colloidal lithography utilizes self-assembled particle monolayers as lithographic masks to fabricate arrays of nanostructures by combination of directed evaporation and etching steps. This process provides complex nanostructures over macroscopic areas in a simple, convenient, and parallel fashion without requiring clean-room infrastructure and specialized equipment. The appeal of the method comes at the price of imperfections impairing the optical quality, especially for arrayed nanostructures relying on well-ordered lattices. Here, we use a correlative approach to connect nano- and microscopic defects occurring from the colloidal lithography process with the resulting local optical properties. Correlative optical and electron microscopies reveal the individual role of packing order, organic impurities, and solid polymer bridges. Our findings show that simple cleaning processes with solvents and oxygen plasma already improve the optical quality but also highlight how well-controlled self-assembly processes are required for predictable optical properties of such nanostructures. Read our paper in Langmuir.
All of my first-authored work resulted from strong, great and fruitful collaborations. Here, I simply grouped all the work that I co-authored. Please have a look at the stunning nano-fabrication and nano-physics projects from my colleagues and collaborators below, including papers on SERS sensing, plasmonic nanoparticles, chiral sensing, structural colour, silicon pillar arrays and transparent electrodes.
We reviewed the physics and fabrication of ‘Bioinspired Photonic Pigments from Colloidal Self-Assembly’ in Advanced Materials. The natural world is a colourful environment. Stunning displays of colouration have evolved throughout nature to optimize camouflage, warning, and communication. The physics governing the interaction of light with structural features and natural examples of structural colouration are briefly introduced. It is then outlined how the self-assembly of colloidal particles, acting as wavelength-scale building blocks, can be particularly useful to replicate colouration from nature. The importance of absorbing elements, as well as the role of surface chemistry and wettability to control structural colouration, is discussed. Finally, approaches to integrate dynamic control of colouration into such self-assembled photonic pigments are outlined.
My former student and collegue Lukas J. Roemling compared 3D photonic crystals with (2+1)D ones, resulting from stacking of inidividual monalyers. This approach gives a lot of freedom, and allows incooperation of active ingredients, and the less-ordered structure of the (2+1)D colloidal crystal scatters light within a larger angular range under diffusive illumination.
Self-assmebly of core-shell particles can be used to fabricate complex silicon pillars with plasmonic, optical and chemical properties for applications in catalysis and energy. Colloidal lithography masks were used in a collaboration with Fedja Wendisch and Gilles Remi Bourret:
Check out the work on complex nanoparticles in confinement and their optical properties by my colleague and friend Dr Junwei Wang. We looked at colloidal clusters with complex crystal structures and MOF particle assemblies.
Understanding the optical properties of complex gold nanoparticles (with Annette Andrieu-Brunsen)
