Integrated optical displacement sensors for scanning force microscopies

Abstract

In this thesis, we have studied the use of integrated optical waveguide devices acting as integrated displacement sensors on cantilevers for scanning probe microscopes. These displacement sensors include integrated optical waveguide devices such as Bragg gratings, ring resonators, race track resonators and waveguide Michelson interferometers fabricated on a cantilever to measure the displacement of the cantilever tip due to the forces between surface and the tip. The displacement of the cantilever tip is measured directly from the change of the transmission characteristics of the optical device. As the cantilever tip displaces, the stress on the cantilever surface changes the refractive index of the materials that make up the integrated optical device which cause variations in its optical transmission characteristics. We have also studied an optical waveguide grating coupler fabricated on the cantilever for the same purpose. In two different embodiments of this device, light is either coupled in or out of the waveguide via the waveguide grating coupler. The displacement of the cantilever alters the direction of the scattered light and measuring the power of the scattered light with a position sensitive detector allows for the detection of cantilever i tip displacement. The novel design proposed in this work provides very high displacement sensitivity of the order of 10−4˚A−1 .