We propose here an application to sensing of annular aperture arrays (AAA). We theoretically investigate the optical properties of the reflective AAA device when illuminated in-plane. The cavity presents almost perfect absorption due to the waveguide mode resonance with strong field localization in the aperture. Additionally, the reflective cavity is modeled to be available for on-chip sensing with a theoretically expected sensitivity of 764 nm∕RIU (refractive index unit).