Planar-Waveguide Interferometers for Chemical Sensing

Interferometry is an optical technique that compares the differences experienced by two light beams traveling along similar paths. Planar waveguides have evanescent fields sensitive to changes in the index of refraction in the volume immediately above the waveguide surface. Placing a chemically sensitive film within this region provides the basis for chemical sensing. Film–analyte interactions change the index of refraction, causing the propagating light speed or phase to change in a direction of opposite sign to that of the index change. To measure this change, a reference propagating beam, which is adjacent to the sensing beam, is combined optically with the sensing beam, thus creating an interference pattern of alternating dark and light fringes. When chemical or physical changes occur in the sensing arm, the interference pattern shifts, producing a sinusoidal output. Waveguides and interferometers come in a variety of designs, but all rely on the evanescent field interacting with a chemically selective film to produce a measured response. The sensing mechanism can be passive (a physical change) or active (reactive sites in the film). Through a judicious choice of sensing films, interferometers can be designed to detect a wide variety of chemical and biological materials. Multi-interferometer devices with several different sensing films can be used to detect and identify a variety of different chemical or biological analytes either through specific sensing chemistry or through analysis of patterned response from an array of different films.