Ultra-Wideband Mid-Infrared Chalcogenide Suspended Nanorib Waveguide Gas Sensors with Exceptionally High External Confinement Factor beyond Free-Space
Mingquan Pi, Chuantao Zheng, Huan Zhao, Zihang Peng, Gangyun Guan, Jialin Ji, Yijun Huang, Yuting Min, Lei Liang, Fang Song, Xue Bai, Yù Zhang, Yiding Wang, Frank K. Tittel
Abstract
On-chip waveguide sensors are potential candidates for deep-space exploration because of their high integration and low power consumption. Since the fundamental absorption of most gas molecules exists in the mid-infrared (e.g., 3–12 μm), it is of great significance to fabricate wideband mid-infrared sensors with high external confinement factor (ECF). To overcome the limited transparency window and strong waveguide dispersion, a chalcogenide suspended nanorib waveguide sensor was proposed for ultra-wideband mid-infrared gas sensing, and three waveguide sensors (WG 1 –WG 3 ) with optimized dimensions exhibit a wide waveband of 3.2–5.6 μm, 5.4–8.2 μm, and 8.1–11.5 μm with exceptionally high ECFs of 107–116%, 107–116%, and 116–128%, respectively. The waveguide sensors were fabricated by a two-step lift-off method without dry etching to reduce the process complexity. Experimental ECFs of 112%, 110%, and 110% were obtained at 3.291 μm, 4.319 μm, and 7.625 μm, respectively, through methane (CH 4 ) and carbon dioxide (CO 2 ) measurements. A limit of detection of 5.9 ppm was achieved for an averaging time of 64.2 s through the Allan deviation analysis of CH 4 at 3.291 μm, leading to a comparable noise equivalent absorption sensitivity of 2.3 × 10 –5 cm –1 Hz –1/2 as compared to the hollow-core fiber and on-chip gas sensors.