High-Transmission Mid-Infrared Bandpass Filters using Hybrid Metal-Dielectric Metasurfaces for CO₂ Sensing

Mid-infrared (MIR) spectroscopy is widely applied in many applications such as gas sensing, industrial inspection, astronomy, and imaging. While thin-film narrowband interference filters are cost-effective for MIR sensing, their complex fabrication limits their suitability for miniaturized systems. Plasmonic nanostructures, though explored for MIR applications, suffer from broad spectral responses and low efficiencies due to the ohmic losses inherent in metals. All-dielectric metasurfaces, with low intrinsic losses, have been proposed as alternatives for MIR spectroscopy. However, their operation is typically limited to reflection mode. In this work, a hybrid metal-dielectric metasurface operating in transmission mode for MIR spectroscopy is introduced. Composed of germanium (Ge) atop aluminium (Al) cylinders on a calcium fluoride (CaF₂) substrate, the metasurface achieves high transmission efficiency (80%) at λ = 2.6 µm and a narrow full-width-half-maximum of 0.4 µm. The transmission response arises due to the hybridization of modes between the Ge and Al structures. Numerical simulations are demonstrated, a straightforward fabrication method, and successful deployment as an in-line optical filter for CO₂ gas detection, achieving a detection limit of ≈0.04% (≈400 ppm). This work highlights the potential of hybrid metasurfaces as in-line gas sensing filters in MIR spectroscopy.