  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
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 sensitivity 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.
