We used silicon nitride (Si3N4) directional coupler (DC) interferometers to fabricate evanescent field biosensors for detection of aptamer hybridization. The DC detects temporal changes in refractive index (RI) as changes in the relative intensities of near-fields. Numerical simulation provided information for sensitivity of the DC as well as preferable dimensions of single mode waveguides. The result suggests that lengthening the DC improves its sensitivity. Another approach to realize high sensitivity was to make the coupling region of the DC longer for a given complete coupling length Lc. A folded DC is also presented based on calculations using Si waveguides. Here a meander-line with an array of segmented DCs and asymmetric 180°-curved waveguides was accommodated in a (100 μm)2 square area. Phases of propagating lights within two curved waveguides could be coincided. This cascade-connection of the segments of the DCs facilitated highly sensitive detection of biomolecules and a flexible sensor design. Signal dependences on device length and bulk RI change were evaluated by end-fire coupling measurements. The optical intensities agreed well with the corresponding fitting curve and the curve calculated based on the coupled mode theory.
K. Okubo, K. Uchiyamada, M. Yokokawa, K. Asakawa, and H. Suzuki, "Fabrication and characterization of silicon nitride directional coupler interferometer for sensing aptamer hybridization," Proc. SPIE 9725, Frontiers in Biological Detection: From Nanosensors to Systems VIII, 972506 (Presented at SPIE BiOS: February 14, 2016; Published: 22 April 2016); https://doi.org/10.1117/12.2211899.
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