(223h) On-Chip Infrared Spectroscopy with Near-Field Detection

The miniaturization of infrared spectroscopy can provide a cheap and portable platform for molecular identification across a large application space. Infrared (IR) spectroscopy enables the identification of molecules by correlating absorption peaks to vibrational modes of atomic bonds such as C=O. Two major technical challenges associated with on-chip IR spectroscopy include i) the lack of successful integration of broadband infrared emission sources, and ii) the selectivity of the photodetector to spectrally resolve different wavelengths. To overcome both these challenges, we introduce an on-chip infrared spectrometer equipped with a set of “all-in-one” infrared sensors that both source and detect outgoing infrared radiation to an analyte. Each sensor has a unique narrowband emission/detection window, which cumulatively makes for a broadband IR detection range. A single sensor is made from a suspended, resistive calorimeter embedded in a substrate and separated from a membrane-support by ~500 nanometers (see Figure 1). Nanoscale separation distances ensure the calorimeter only sources and detects if the analyte has a vibrational mode (absorption peak) that overlaps with the narrowband, near-field emission/detection window of the calorimeter. We demonstrate a silicon nitride calorimeter that can distinguish between water and ethanol by spectrally matching the near-field detection window to distinct vibrational modes. Overall, our near-field detection technique paves the way for broadband on-chip spectroscopy with the potential to decrease the cost of an IR spectrometer to below $50.