A cartridge for processing a sample includes (a) a planar waveguide with upper and lower planar surfaces defining an optical axis therebetween, wherein the upper planar surface has a plurality of capture molecules bound thereto, (b) a lens portion, coupled to the planar waveguide, for focusing and refracting a light beam propagating parallel to, but offset from, the optical axis such that the light beam couples into the planar waveguide and propagates therein along the optical axis at a non-zero, internal propagation angle β relative to the upper planar surface, and (c) a sample chamber for positioning the sample in contact with the plurality of capture molecules such that a target analyte of the sample is detectable through (i) an assay involving the target analyte and the capture molecules and (ii) evanescent illumination of the assay using the light beam within the planar waveguide.