Transmissive hybrid metal-dielectric metasurface bandpass filters for mid-infrared applications

Mid-infrared (MIR) spectroscopy plays a pivotal role in molecular identification and biosensing due to its ability to probe characteristic vibrational fingerprints of biomolecules. Plasmonic nanostructures have been explored for MIR applications but suffer from low efficiencies and broad spectral responses caused by intrinsic ohmic losses. All-dielectric metasurfaces, with low optical losses, offer an attractive alternative; however, their functionality is often restricted to reflection-mode operation. This work introduces a hybrid metal-dielectric metasurface designed to operate in transmission mode, specifically tailored for molecular identification in MIR biosensing applications. The metasurface comprises germanium (Ge) on aluminum (Al) cylinders atop a calcium fluoride (CaF₂) substrate, optimized to exhibits a high transmission efficiency of 80 % at a wavelength of λ = 4.2 µm, with a narrow full-width-half-maximum of 0.5 µm. By leveraging the hybridization of modes between the Ge and Al layers, the device enables precise spectral filtering. We demonstrate the potential of this metasurface for molecular identification and biosensing applications through numerical simulations and experimental validation. The straightforward fabrication process further highlights the practicality of this approach, paving the way for miniaturized MIR biosensing platforms.