Mid-infrared filters based on subwavelength grating structures supporting high-Q resonance modes

Abstract

We propose a notch filter based on a subwavelength grating structure, featuring ultra-narrowband suppression characteristics within the mid-infrared spectrum (3–5 μm). The device leverages Fano resonance generated by the coupling between Fabry–Pérot cavity modes and surface plasmon polaritons. By introducing controlled nanoscale perturbations within the unit cell, a high-Q resonant state is excited, resulting in a narrow linewidth of 1.08 nm in the transmission spectrum. The resonance depth can be freely adjusted via the grating duty cycle, while the quality factor increases with the degree of asymmetry, reaching up to 3900. The structure also exhibits a strong response to variations in the ambient refractive index while maintaining a high quality factor (>800), and demonstrates a high sensing sensitivity of 798 nm/RIU. This work establishes a versatile and robust platform for high-performance mid-infrared filtering and sensing applications, including refractive index detection, molecular spectroscopy, thermal imaging, and gas sensing.