Grated Waveguide-based Optical Cavities as Compact Sensors for Sub-nanometre Cantilever Deflections, and Small Refractive-index Changes

The paper reports on theoretical and experimental results of integrated optical (IO) cavities defined by grated waveguides in Si3N4 and Si, for the accurate detection of cantilever deflection and bulk index changes. Microcantilever-based sensors can be used to detect molecular adsorption, which causes changes in the surface stress [1], leading to deflection of the cantilever. We propose a novel and highly sensitive integrated read-out scheme to detect small deflections of a cantilever in close proximity to a grated waveguide structure. It is well-known that a partly grated waveguide defines a cavity in the grated region for wavelengths outside the stopband. In particular, for wavelengths next to the stopband sharp spectral features can be observed, which are related to the strong dispersion in that wavelength region. A cantilever, if placed in the evanescent field region of the grated waveguide, will lead to the occurrence of propagating modes for wavelengths inside the stopband, and so to resonances inside the stopband, as shown in fig. 1. As can be seen from the figure, on decreasing the gap between grating and cantilever the first near band-edge resonance peak is pulled inside the stopband; simultaneously its spectral width goes down. This effect can be used for the detection of cantilever displacements. Assuming that a peak shift can be detected with a resolution δλ of 0.001 times its spectral width, it follows from fig. 1 that a displacement of ( / ) (50 / 2) 0.2 pm 5 pm g g δ λ δλ = ∂ ∂ = = can be detected with the considered compact device having a length of only 98 μm. As an example to the above, theoretical results for a hydrogen trace-gas sensor, utilizing palladium as an H2 absorbing material, will be presented during the conference.