Ultra-sensitive active whispering gallery mode sensor in an optofluidic microcapillary with Vernier effect of coupled modes

Ultrahigh sensitivity is achieved in an optofluidic microcapillary active sensor by utilizing the Vernier effect of coupled whispering gallery modes. A refractive index sensitivity of 2924 nm/RIU is obtained experimentally, which is 7 times higher than the sensitivity limit of a conventional whispering gallery mode sensor. By reducing the wall thickness of the microcapillary to decrease the difference of free spectral range of the coupled whispering gallery modes, further improvement in refractive index sensitivity can be as high as 48426 nm/RIU. This sensing strategy allows us to design a robust, easily fabricated ultra-sensitive optical sensor for microfluidic sensing applications.