Two-Dimensional Vertical Photonic Crystal Waveguide Array for Multi-Analyte Mid-Infrared Sensing

Two-dimensional photonic crystal waveguides (PCWs) enhance light–matter interactions through slow-light effects and resonant field confinement. We report the design, simulation, and experimental realization of silicon VPCW arrays exhibiting dual mid-infrared (mid-IR) resonances: a symmetric Lorentzian-type and an asymmetric Fano-type resonance. Each VPCW is formed by introducing a hollow air-core defect into a hexagonal lattice of deeply etched air holes in silicon. By tuning the defect radius and lattice constant, these resonances were aligned with the absorption lines of acetaldehyde (C₂H₄O) (λ ≈ 5.88 μm) and water vapor (H₂O) (λ ≈ 6.05 μm), enabling multiplexed gas detection. Simulations predicted large group indices (n_g ≈ 1064 for C₂H₄O and n_g ≈ 204 for H₂O), consistent with slow-light features. Experimentally, Lorentzian and Fano resonances were observed, with linewidth broadening attributed to fabrication imperfections, at photonic band-edge wavelengths where slow-light modes are expected. This work demonstrates the first realization of dual Lorentzian–Fano resonances in VPCWs under normal-incidence light excitation, establishing a CMOS-compatible platform for compact mid-IR absorption spectroscopy and multiplexed gas sensing.