Compact integrated optical sensors based on a Si3N4 grated waveguide optical cavity

A grated waveguide (GWG) acts as an optical resonator, showing sharp fringes in the transmission spectrum near the stop-band edges of the grating. These oscillations are due to Fabry-Perot resonances of Bloch modes propagating in the cavity defined by the grated section. Any small structural changes in the environment of the GWG, which disturb the evanescent field of the GWG resonant modes, will lead to a shift of its transmission spectrum. Such an effect can be exploited for sensing applications, such as the detection of a bulk refractive index change or nanodisplacements of a cantilever suspended above the GWG. This paper presents 3 applications of the GWG: (1) a concentration sensor, based on the bulk index change of the GWG top cladding; (2) label-free protein sensing (PepN enzyme the major Suc-LLVY-AMC-hydrolyzing enzyme in Escherichia coli), where the spectral shift of the GWG response is due to the antibody-antigen interaction, leading to growth of an ad-layer on it; and (3) gas sensing, where the GWG detects stress-induced deflections of a doubly-clamped microcantilever (microbridge) with a Pd top layer due to H₂ gas absorption by the Pd receptor layer.