In vacuo measurement of the sensitivity limit of planar Bragg grating sensors for monolayer detection

An exposed Bragg grating incorporated into a planar waveguide was used to form an optical device that acts as a refractive index sensor. The exposed evanescent field causes the Bragg peak to be sensitive to the refractive index of its surroundings. The corresponding shift in peak wavelength can be used to detect changes in this environment. Incorporation of a high index overlayer onto the surface has been shown to dramatically enhance this sensitivity. The sensitivity limit was measured via the deposition of a thin film of silica, demonstrating that the system can measure down to a single surface monolayer.