SU 8 used as optical waveguide in integrated optical microsensor for biological applications

We present preliminary experiments for an integrated optical sensor based on a Mach-Zehnder interferometer for biological applications. The sensor is sensitive to refractive index change produced by the presence of a biological species in the cladding of the optical waveguide. A "window" can be patterned in the upper cladding, so that the evanescent wave can be in direct contact with the environmental (the sensitive layer). We investigated as optical waveguides a new material, SU-8, a negative photoresist well known from the development of 3D micromachmed structures. We structured, by photolithographic techniques, rib and channel optical waveguides. We studied the influence of the silicon substrate on propagation losses and the possibility to use these losses for the selective attenuation of the higher order modes on the vertical direction. As biological materials we experimented collagen, which is a bio-polymer which can bind different enzymes or antibodies.