Si3N4 Grated Waveguide Optical Cavity based Sensors for Bulk-index Concentration, Label-free Protein, and Mechano-Optical Gas Sensing

摘要

A grated waveguide (GWG), which is a waveguide with a finite-length grated section, 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. Small changes in the environment of the GWG, which disturb the evanescent field of the GWG resonant modes, lead to a shift of its transmission spectrum. This effect can be exploited for sensing applications by detection of a bulk refractive index change or nanodisplacements of a cantilever suspended above the GWG. Here we present 3 applications: (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 GWG spectral shift is due to the antibody-antigen interaction and 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. Gratings were defined on Si3N4 waveguides using laser interference lithography. To demonstrate (1) concentration sensing, we filled a cuvette on the surface of the sensor with a phosphate buffered saline solution of 1 wt% (PBS1x). Evaporation of water from the open cuvette continuously changes the concentration, hence the bulk index, which is measured as a spectral shift of the sensor. Changes of the refractive index down to 2×10-5 RIU and concentration changes down to 0.01 wt% can be resolved, which is comparable with the resolution of ultrasonic sensors. For (2) protein sensing, it was found that the spectral shift of a peak in response to the antibody-antigen binding reaction changes with time t approximately according to an exponential function, with time constant 770 s. The reaction saturates after ~35 minutes. The total shift was approximately 342 pm, corresponding to the growth of an ad-layer of ~2 nm. The sensitivity of a micro-bridge device for (3) gas sensing was rather low due to the relatively large gap g of ~700 nm between the bridge and the GWG. During the H2 absorption process, the shift depends almost linearly on time, which is partly due to the initially rapid change of the gap size, g. The H2 desorption takes place at approximately half the rate of the absorption process.