Silicon photonics is rapidly emerging as a mature technology platform for the fabrication of photonic integrated circuits. It builds on the technology base of the CMOS-world and allows to implement advanced photonic functions on a small footprint chip with high accuracy and yield. For operation at telecom wavelengths above 1 micrometer one typically uses silicon-on-insulator wafers with waveguides with a silicon core. For short-wavelength operation, below 1 micrometer, one can use a silicon nitride (SiN) core instead of a silicon core. This results in a platform for operation in the visible and near infrared, with moderately high refractive index contrast and low loss photonic components. Operation at short wavelengths can be beneficial for a variety of reasons, including the possibility to use low cost high performance sources and detectors and the compatibility with sensing in an aqeous environment.
The SiN CMOS-platform has been used to demonstrate a variety of spectroscopic sensing functions. In essence the SiN chips may contain sensing structures, whereby the evanescent tail of the guided light is interacting with the analyte, as well as spectrometric functions to read out the spectrum resulting from the interaction with the analyte. This approach has allowed to demonstrate refractive index biosensors, spontaneous Raman spectroscopy and surface-enhanced Raman spectroscopy. In the latter case the SiN waveguides are enriched with gold nano-antennas to enhance the local field strength seen by the analyte. The spectrometric functions can be based on arrayed waveguide gratings, echelle grating spectrometers or Fourier Transform spectrometers.