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Studies of an Optimized Two-Channel Surface Plasmon Resonance Interferometer for Sensing Gas Flows
P. Yaney and J. Grote

The objective of this work was to demonstrate how joining surface plasmon resonance (SPR) with interferometry provides a high sensitivity molecular sensor, especially for low molecular weight gases. This paper reports how various parameters influence the performance of the twochannel surface plasmon resonance interferometer (SPRI) laboratory system. Much of the effort was to achieve good reproducibility and to reduce the sources of noise and drift. This SPRI design uses a Mach-Zehnder interferometer (MZI) that allows for measurements with a single detector or with a digital camera. Results both in detector and camera modes using the optimize system are reported. In the absence of functionalization of the gold film, it is assummed that sensing was primarily due to the nearfield photonic gradient forces generated by the evanescent electric fields at the gold film that forms a dielectric layer over the gold. The calibration of recent experimental studies in detector mode of dry N2 flow showed that the SPRI technique has a sensitivity of about 1 part in 107 RIU (RIU = refractive index units). The sensitivity to milliliters/min of anhydrous NH3 mixed in 1.5 liters/min of dry N2 flow is estimated to be <1 part in 108 RIU.

Keywords: plasmonics, SPR, interferometry, Mach-Zehnder, gas sensing, nitrogen, ammonia, near-field photonics, dispersion force

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