On-chip Raman sensing enabled by large-scale photonic integration is a promising technology for biological and healthcare applications. In this contribution we give a review the current status of on-chip Raman sensing with a particular focus on the ultimate performances. We discuss the limitations in terms of detection limit and the different paths currently followed to get around them.
In recent years, there has been a growing demand for hand held and miniaturized spectroscopic Raman systems that can be employed in the field to distinguish and quantify different analytes. In this paper, a novel and integrable system to detect a Raman spectrum is presented. We present the system principle, sensor design, experimental set-up and primary measurement results. In a conventional Raman setup, the four important components are: a light source, sensor, spectrometer and detector. We utilize a tunable laser as light source in the new Raman detecting system to replace the spectrometer by scanning the pump wavelength. A Raman sensor based on silicon nitride platform which has small size and high signal-background ratio is demonstrated in this paper to enable the excitation and the collection of the Raman signal using a plasmonic slot waveguide structure. Besides the tunable laser and the Raman sensor, there are two basic devices in our system, a narrow band-pass filter and a power detector. In this work, the Raman signal of the measured molecule 4-nitrothiophenol (NTP) is obtained by scanning the pump wavelength from 735 nm to 786 nm. The light source and detector in our experiment are implemented by discrete components. Silicon photonics promises the integration of a complete on-chip Raman spectroscope where the tunable laser, detector, sensor and filter can be integrated in a millimeter sized chip. We analyze the primary results measured by the discrete devices and discuss the feasibility of the on-chip integration in the end.
We demonstrate a compact on-chip gas sensor based on a SiN photonic integrated circuit providing an estimated detection limit of 0.1ppm for a variety of gas vapors, including IPA, ethanol and acetone. The sensor consists of an integrated Mach-Zehnder interferometer having one arm coated with an absorptive mesoporous layer to enhance the interaction of the gas vapors with the waveguide evanescent field. Gas detection is accomplished by monitoring the spectral shift of the output signal induced by a change in the effective refractive index of the coated waveguide. Results pave the way to novel integrated solutions for environmental safety monitoring.
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.
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