The Optofluidic Light Cage – a novel concept for single nano-object tracking and surface-enhanced Raman spectroscopy

• Applicants: Professor Dr. Stefan Maier; Professor Dr. Markus A. Schmidt
• Subject Area: Experimental Condensed Matter Physics ; Analytical Chemistry ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 434371252

Optofluidics targets the investigation of light/liquid interaction and has strong impact in bioanalytics, with upcoming applications increasingly demanding integration of optofluidic components into microfluidic environments. Two applications being particular challenging within integrated microfluidics are single nano-object tracking and surface-enhanced Raman spectroscopy (SERS). The first relies on analyzing the trajectories of diffusing nano-objects via elastic light scattering, yielding critical information about nano-particle properties and nano-scale interactions. SERS employs local field enhancements provided by plasmonic nanostructures to boost otherwise small Raman cross sections. Both applications demand smallest sample volumes and interfacing with microfluidics, which ideally uses planar waveguides combined with microfluidic circuitry. Due to a low refractive index, guiding light inside water cores demands sophisticated microstructured claddings. Within planar waveguide technology hollow core anti-resonant waveguides have been developed, while complex fabrication and a difficult access to the core have prevented their widespread use. Therefore, a planar integrated waveguide platform that can be interfaced with on-chip microfluidics with straightforward access to the core region is still missing up-to-date.Within this project we plan to develop and investigate a so-far unexplored optofluidic platform – the optofluidic light cage – that allows for waveguide-integrated nano-object detection and SERS. The light cage concept was recently introduced by the authors and relies on confining light inside a hollow core via the anti-resonant effect by lattices of freely suspended high-aspect ratio (>1000) strands with micrometer diameters over centimeter distances. The open space between the strands is a unique feature of the light cage concept, yielding unique opportunities for both applications. Within nano-object detection it enables fast diffusion into the core region, direct microscopic detection and high nano-object throughput. Functionalizing the strands by plasmonic nanoparticles or metallic layers will allow for the molecular identification of substances via SERS with strongly suppressed Raman background. Implementation will rely on 3D nano-printing using direct laser writing, which will enable for interfacing light cages with microfluidics. The project involves theoretical investigations of the underlying physics, design and simulations, implementation and characterization, and application within the two mentioned areas. In summary, the proposed project yields a novel type of optofluidic platform that can be interfaced with microfluidics and has the potential to open new applications and to reach new performance levels particular within bioanalytics and chemistry.

• DFG Programme: Research Grants