Mid-infrared nanophotonics for biochemical sensing

The mid-infrared (IR) wavelength region is widely utilized for absorption spectroscopy to detect molecules in gas, liquid, and solid phases by taking the advantage of characteristic infrared absorption peaks unique to molecular bonding. MidIR absorption spectroscopy offers a unique label-free detection method with a wide
variety of applications from environmental monitoring to medical diagnosis. However, the challenge lies in the huge mismatch of molecular sizes, typically on the order of nanometer as opposed to microns of mid-IR wavelengths, making detection of low concentration of them extremely challenging. Various nanophotonic phenomena enable us to localize mid-IR light at nanoscale to enhance light absorption by molecules and thus sensitivity. We present an overview of nanophotonic phenomena, such as waveguide modes, both propagating and localized plasmon modes, together with micro- and nanostructured platforms that host these phenomena, including dielectric planar waveguides, photonic crystals, plasmonic, and phononic nanostructures.