Hybrid photon-plasmon Mach-Zehnder interferometers for highly sensitive hydrogen sensing.

By using PdAu nanowires as plasmonic waveguides, hybrid photon-plasmon Mach-Zehnder interferometers by integrating single-crystal PdAu alloy nanowires with silica optical microfibers are demonstrated. Based on an evanescent wave coupling technique using optical fiber tapers, surface plasmon polaritons are efficiently excited and propagated in suspended PdAu nanowires. The interference spectra show attractive properties such as broad and flexible in situ tunability with wavelength spacings ranging from ∼1 to tens of nanometers, and high extinction ratios of over 20 dB. The hybrid Mach-Zehnder interferometers show a higher sensitivity to hydrogen gas than a single-nanowire sensing approach, and the lengths of PdAu nanowires used are less than 20 μm, which are 2 or 3 orders of magnitude shorter than the lengths of Pd coatings in existing fiber-optic hydrogen sensors. Other advantages including good reversibility and low-power operation are also obtained.