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G. Antonacci, J. Goyvaerts, H. Zhao, B. Baumgartner, B. Lendl, R. Baets:
"Ultra-sensitive refractive index gas sensor with functionalized silicon nitride photonic circuits";
APL Photonics, 5 (2020), 8; S. 081301.

Portable and cost-effective gas sensors are gaining demand for a number of environmental, biomedical, and industrial applications, yet current
devices are confined into specialized labs and cannot be extended to general use. Here, we demonstrate a part-per-billion-sensitive
refractive index gas sensor on a photonic chip based on silicon nitride waveguides functionalized with a mesoporous silica top-cladding
layer. Low-concentration chemical vapors are detected by monitoring the output spectral pattern of an integrated unbalanced Mach-
Zehnder interferometer having one coated arm exposed to the gas vapors. We retrieved a limit of detection of 65 ppb, 247 ppb, and
1.6 ppb for acetone, isopropyl alcohol, and ethanol, respectively. Our on-chip refractive index sensor provides, to the best of our knowledge,
an unprecedented limit of detection for low gas concentrations based on photonic integrated circuits. As such, our results herald
the implementation of compact, portable, and inexpensive devices for on-site and real-time environmental monitoring and medical
diagnostics.

http://dx.doi.org/10.1063/5.0013577