Abstract: A simple, label-less, sensitive, rapid, and inexpensive sensor incorporates a unique optical transduction design and novel materials optimized for chemical or biosensing. The sensor features a planar coupler with at least one input and two output optical waveguides. The system responds to changes in refractive index upon analyte binding to molecular receptors on the sensor surface. In the preferred embodiment the coupling region can support at least two electromagnetic modes, and the interaction of these fields determines the amount of light propagating down each of the output waveguides. The changes in refractive index at the surface of the coupled region affect the interaction of the two propagation modes and, therefore, the amount of light detected at each output.