  In this paper, we present refractive index measurement using a double-slot
silicon waveguide-based Mach Zehnder interferometer. We present a double-slot
waveguide that offers the best sensitivity and limit of detection compared to
wire and single-slot waveguides. We demonstrate ultra-low loss coupling between
a single-mode waveguide and a double-slot waveguide and experimental proof for
double-slot excitation. The double-slot waveguide is used to demonstrate a
highly sensitive concentration sensor. An unbalanced Mach-Zehnder is used as
the sensor device to demonstrate concentrations of Potassium Chloride in
deionized water. A sensitivity of 700 nm/RIU and a limit of detection (LOD) of
7.142e^-6 RIU are achieved experimentally. To the best of our knowledge, the
demonstrated sensitivity is the highest for an on-chip guided waveguide sensing
scheme.
