Ultrasensitive measurement of gas refractive index using an optical nanofiber coupler.

We report an ultrasensitive gas refractive index (RI) sensor based on optical nanofiber couplers (ONCs). Theoretical analysis reveals that a dispersion turning point (DTP) exists when the diameter of the coupler is below 1000 nm. Leveraging this DTP, the gas RI sensitivity can be significantly improved to infinity. Then we experimentally demonstrate a DTP and achieve ultrahigh sensitivities of 46,470  nm/refractive index unit (RIU) and -45,550  nm/RIU around the DTP using an ONC with a diameter of 700 nm. More importantly, the unique twin dips/peaks interference characteristics around the DTP offers further enhancement on the sensitivity to 92,020  nm/RIU. The demonstrated sensor not only shows vast potential in ultrasensitive pressure sensing, acoustic sensing, gas sensing, and gas phase biomarker detection, but also provides a new tool for nonlinear optics, ultrafast optics, quantum optics, and ultracold atom optics.