A niobium pentoxide waveguide sensor for on-chip mid-infrared absorption spectroscopic methane measurement

The advantages of integrating infrared (IR) waveguide gas sensors on a chip include compactness, low power consumption, and high selectivity. Mid-IR waveguide sensors could be made of a variety of metal oxides with merits of low cost, high stability, transparency in the visible-IR spectrum, and non-toxicity. However, because most metal oxides have low refractive index, they are rarely used as waveguide core materials. To overcome this limitation, we chose niobium pentoxide (Nb2O5), a transparent material with high refractive index in the 0.35-10 mu m wavelength range, as the core layer. We fabricated a Nb2O5 rectangular waveguide gas sensor using magnetron sputtering and lift-off process. The Nb2O5 waveguide with an external confinement factor of 11.5% was used for on-chip methane measurement at 3.291 mu m based on wavelength modulation spectroscopy. A detection limit of 348 parts per million was achieved with an optimal averaging time of 61.2 s for a 2 cm long waveguide. The reported Nb2O5 waveguide gas sensor not only broadens the waveguide sensor core material family from silicon, chalcogenide (ChG) glass to metal oxides, but also provides detailed preparation and characterization methods for such kind of waveguide device.