Surface-confined molecules play a crucial role in several technologies such as bio and chemical sensing, affinity chromatography, and biocompatibility of materials. Guided-wave optics is a sensitive technique used to investigate surface-confined chemical and biological species. A high refractive-index film confines light along its axis; a portion of the guided beam extends beyond the waveguide interface, probing the adsorbed molecules near the surface. A more sensitive waveguide platform occurs when the guiding film is very thin and allows only one electromagnetic mode to propagate, but in this case the power and wavelength range that can be coupled is limited. We developed a high numerical aperture optical system with an integrated diffraction grating to couple a broad spectral range of light in and out of a single-mode, .5 micron-thick waveguide. Absorption spectroscopy using the waveguide has been demonstrated for the oxidized and reduced states of the protein cytochrome c. The waveguide spectroscopic technique developed here is a simple, efficient, and reproducible approach for studies in molecular monolayers.