Bulk Sensing Using a Long-Range Surface-Plasmon Dual-Output Mach–Zehnder Interferometer

Optical bulk (refractometric) sensing of sample solutions is demonstrated using dual-output Mach–Zehnder interferometers built from long-range surface-plasmon polariton waveguides operating at a free-space wavelength of 1375 nm. The device of interest was constructed by embedding Au stripes in Cytop claddings and etching a fluidic channel through the top cladding of one arm of the interferometer to expose the Au stripe. Bulk sensing was carried out by flowing sequentially a series of solutions of different refractive index through the microfluidic channel. The optical powers of the two outputs responded sinusoidally and were complimentary, as expected in theory. Three detection schemes aiming at improving the detection limit are analyzed showing that the device benefits from a 2× larger dynamic range, and has the ability to suppress common perturbations, relative to a single output. A detection limit of ∼4 × 10⁻⁶ RIU is demonstrated, which can be further improved by lengthening the sensing channel. The device is promising for application as a biosensor.