On‐Chip Saturated Absorption of CO ₂ in a Silicon‐on‐Sapphire Waveguide

ABSTRACT We report on the first demonstration of saturated absorption in a molecular gas driven by the evanescent field of a planar photonic waveguide. The extreme optical confinement of the waveguide mode facilitates saturation at power levels significantly lower than those typically required in free‐space configurations. To overcome dominant atmospheric background absorption, we utilize a double‐resonance pump‐probe scheme targeting the CO 2 ν 3 → 2ν 3 hot band. A 2.7 µm pump enables a 4.3 µm continuous‐wave probe to achieve optical saturation at milliwatt powers ( P sat ≈ 7.9 mW at 3 Torr) within a silicon‐on‐sapphire (SOS) waveguide. We analyze the resulting spectral line shapes, showing how longitudinal propagation losses moderate power broadening, and characterize the system's pressure‐dependent collisional dynamics. This establishes a viable, low‐power architecture for integrated chip‐scale mid‐infrared frequency references, with myriad applications such as network time keeping, LiDARs, and more.