Filterless Nondispersive Infrared Sensing using Narrowband Infrared Emitting Metamaterials

For many industrial and manufacturing applications, detecting and identifying low concentrations of harmful gases and byproducts are performed using nondispersive infrared (NDIR) sensors. These simple devices utilize a broadband IR emitter, thermopile detector, and a spectrally narrow bandpass filter tuned to a vibrational resonance of the analyte of interest. However, such filters are expensive to fabricate and limit the NDIR to operation at only a single frequency, unless filter wheels are employed, which expand the size and complexity of the device considerably. Here, we create a nanophotonic infrared emitting metamaterial (NIREM) fabricated from thin films of doped CdO grown on patterned sapphire substrates (PSS) that exhibit narrowband thermal emission. By coupling a sufficiently narrow line width emitter with a simple broadband detector such as a thermopile, the functionality of the NDIR sensor can be replicated without the need for the narrow bandpass filter. Unlike many metamaterial-based emitters, our device emits both p- and s-polarized light with near-unity emissivity at angles ranging from 0° to 40° off the surface normal without complicated and expensive lithography steps. As a proof of concept, we implement this NIREM for CO2 gas detection within an FTIR spectrometer, demonstrating performance comparable with a conventional blackbody/filter combination. This demonstrates that the NIREM concept can provide a suitable plug-and-play replacement for NDIR devices as they can be implemented in a form-factor commensurate or significantly reduced in comparison to the current state of the art. In principle, by incorporating multiple NIREM dies tuned to emit at different frequencies, multiple vibrational modes could be sequentially detected, making the approach amenable to identification and quantification of complicated molecules within a single NDIR configuration.