Sensitivity enhancement of integrated optical near field sensors

Abstract

Sensors based on materials with high refractive indices are desirable for sensing applications where a low penetration depth of the evanescent field into the covering analyte medium is required. To enhance the proportion of power carried into the covering medium while keeping the penetration depth low, a waveguiding device can be coated by a high-index film of metal oxide. We present numerical calculations as well as experimental comparisons between the evanescent fields of titanium dioxide-coated and uncoated waveguides in lithium niobate. The experiments were performed by using a Scanning Near-Field Optical Microscope (SNOM) in collection mode, which is an appropriate tool to measure and characterise evanescent fields. The coating of the waveguide leads to an enhancement of the power carried out into the covering medium by a factor 15 while the penetration depth remains the same in the range of a few 10 nm.