  Most integrated evanescent-field photonic sensors measure changes in either
the real part or the imaginary part of the complex refractive index of the
sample, i.e., refraction or absorption. Here we propose and experimentally
demonstrate a near-infrared sensor based on a silicon nitride Mach-Zehnder
interferometer which provides a direct measurement of the complex refractive
index. Our architecture exhibits a high sensitivity, achieving limits of
detection below $2\cdot 10^{-6}\,\mathrm{RIU}$ for both the real and imaginary
parts of the refractive index. We furthermore show that our sensor can be
employed as an integrated dispersion spectrometer.
