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Grating Couplers for Chip-Integrated Optofluidic Fluorescence Quantification

Fluorescence-based assays are widespread in life sciences and used for instance in diagnostics, cell sorting, or to resolve cellular and tissue structures. Optofluidic engineering can help improve such assays, especially when sensitivity or throughput are important. Towards optofluidic integration, we combine a silicon nitride photonic chip including grating couplers for exciting and detecting fluorescence signals out of plane with a soft lithography based microfluidic chip. This work investigates the structures of grating couplers for wavelengths of 405 nm and 532 nm by simulations and shows as a proof of principle different measurements with fluorescein-concentrations (10 μM up to 10 mM) in deionized (DI) water using an excitation-wavelength of 405 nm. By exciting through a grating coupler and recording the fluorescence emission through a glass fiber, a limit of detection (LOD) of 73 μM is measured. In addition, the fluorescence detection through grating couplers in microchannels is demonstrated. This is considered as a first step towards a fully integrated optofluidic chip interface for high density fluorescence assay multiplexing.