Transverse magnetic (TM) waves in a four-layer slab waveguide structure are studied for optical sensing applications. The structure consists of a semi-infinite substrate, a thin metal layer, a medium with negative permittivity and permeability as a guiding layer, and a semi-infinite layer as a cover. The proposed sensor is operated in reflection mode in which the angular position of the reflectance peak is used to detect small changes in the refractive index of the cover medium. The optimal structure parameters that correspond to the sharpest and highest peak are presented. The results reveal that for aluminum metal layer, a thickness of about 9 nm represents the optimum metal thickness. Moreover, the thickness, negative permittivity, and negative permeability of the guiding layer are found to have great impacts on the performance of the proposed optical waveguide sensor
