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Silicon nitride waveguide platform for on-chip spectroscopy at visible and NIR wavelengths

On-chip spectroscopy in visible and near-infrared wavelength offers immense potential for various screening and diagnostic application. Silicon Nitride (SiN) is a promising material to realize on-chip spectroscopy as it is transparent over a broad wavelength spectrum; visible-to-mid-infrared. Besides, it is compatible with the CMOS process technology. Since SiN can be deposited using various deposition techniques, it is essential that an appropriate deposition process is chosen to achieve desired properties. We present a detailed study of SiN films deposited using PECVD and LPCVD process. The films were characterized for various, material and optical properties. We also performed waveguide transmission measurement on different SiN films and found a strong correlation with material properties such as stress and autofluorescence to achieve low-loss waveguide. Using the optimized material, we design, fabricate, and demonstrate single-mode SiN wire, spiral waveguides and grating fiber-chip coupler at visible (λ =520nm) and near infra-red (λ = 780nm) wavelengths used for spectroscopy. Furthermore, we use a waveguide to efficiently excite fluorescence or Raman active molecules drop casted as the top cladding of the waveguides. A fraction of the isotopically emitted fluorescence or Raman signal couples back to waveguide and rest is emitted normal to the surface. We discuss the theoretical efficiencies of both the methods of collection and compare it with the conventional microscope-based spectroscopic system. Waveguide based spectroscopic sensing offers higher efficiency due to increased light-matter interaction along the length of the waveguide. We presented detailed design, simulation, and experimental demonstration of grating coupler waveguide system suitable for son-chip spectroscopy application.