Highly doped silicon-based hyperbolic metamaterials for mid-infrared bio-chemo sensing applications

Mid-infrared (IR) absorption spectroscopy operating in the wavelength interval between 2.5–20 µm (4,000 – 500 cm−1) has been used for the label-free detection of various molecules, from proteins to gases, thanks to their absorption band in certain wavelengths in the mid-IR region [1, 2]. This sensing scheme has lead to a wide variety of applications from environmental monitoring to the biomedical analysis. Various plasmonic (metallic) nanostructures have been studied in order to enhance the sensitivity of mid-IR absorption spectroscopy [2]. We report the enhancement of IR light absorption by analytes within hyperbolic metamaterial (HMM) structures composed of highly doped silicon (Si) plasmonic nanotrenches, as illustrated in Fig. 1(a). Highly doped Si can be a plasmonic material with negative permittivity for mid-IR wavelengths [3]. Such plasmonic nanostructures can function as hyperbolic metamaterials and to enhance mid-IR light absorption by molecules [4]. As a proof of concept, we use a 8 nm thick titania (TiO2) layer as a model analyte, conformally coated around the plasmonic trenches, absorbing light at the wavelength of 14 µm, as shown in Fig. 1(b). The Si-based sensing platform is advantageous for its CMOS compatibility, mass-production, multiplexing of sensing unit.