We propose a novel design for a guided-mode resonance (GMR) grating sensor that is optimized for detecting small average index changes in an extended region of space, retaining sensitivity up to several tens of microns away from the grating surface at optical detection frequencies. This kind of sensors has high sensitivity in the half-space above the grating, close to the theoretical limit, together with a controllable - potentially very high -quality factor. It relies on a resonance with a "confined" mode of a sub-wavelength thick grating slab, a mode that is largely expelled from the grating itself. The small thickness assumption allows us to derive analytical expressions for many properties of these sensors, expressions that are then tested numerically using a rigorous coupled-wave analysis (RCWA) method, and in preliminary experiments.
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