The evanescent field surrounding the core of an appropriately designed waveguide can be very sensitive to the local refractive index of the cladding surrounding the core. In this study, a planar waveguide has been fabricated on a chip that contains underlying (buried) silicon photo-detectors. The detectors are located near the bottom of the lower cladding of the waveguide. The detectors convert the optical signal received from the evanescent field to electrical signals. A change in signal received by a detector indicates a change in the refractive index of the upper optical cladding. For example, a change in refractive index may indicate the presence of adsorbed species on the upper surface of the waveguide. The detector operates in real time and can indicate the degree of coverage of a species that absorbs to the waveguide surface through a relative shift of the evanescent field. The buried detector format provides significant opportunities for localized detection of chemical or biological analytes in complex milieu through monitoring of the evanescent field. The effect of different polymers and organic molecules bound to the surface of the waveguide on the detected optical signal will be presented as proof of principle. In addition, the predicted response to the refractive index changes via numerical modeling will be compared with the measured detector response.