Fabrication of surface plasmon interferometric sensors exploiting multimode nanoslits

As phase-based sensors, surface plasmon interferometers offer higher sensitivity than resonant or attenuation-based plasmonic sensors. In this paper we realize surface plasmon interferometric sensors based on a multimode nanoslit used as a combiner. The phase difference in the surface plasmon waves, incident on the nanoslit, determines the resonant mode excited therein, and the radiation pattern that emerges therefrom. The device construction integrates on-chip grating couplers, gold sensing and reference surfaces, transparent claddings, sealed microfluidic channels, and a nanoslit in the gold film. The structure can be arrayed with individual microfluidic channels thereby enabling multiplexing. Nanofabrication of the devices using wafer-based processes is discussed in detail. Fabrication involves integration into a full process flow of techniques such as photolithography, electron beam lithography, focused ion beam milling, plasma etching, wafer bonding, and dicing, with several overlay and precision alignment steps. We also describe the design and realization of a test jig useful for mounting a chip under test, providing in-plane sealed microfluidic interfacing to several channels simultaneously, and enabling optical interrogation in the perpendicular direction using microscope objectives. Operation of the devices is demonstrated by refractometric (bulk) sensing experiments. The device concept is of strong interest for multiplexed biosensing applications, and the fabrication flow presented can be scaled to mass-manufacturing.