Determination of cantilever's spring constant by using single-optic-fiber radiation-pressure technique

We generated sub-femtonewton radiation-pressure forces to drive a very soft silicon-nitride cantilever without highly reflective coating and determine its spring constant in the range of 10^-5 N/m. To be applicable to cantilevers of general dimensions and reflectivity, we built an experimental setup with a single optic fiber accessing a cantilever surface and an in-situ reflectivity measurement feature. A 1320-nm laser from a superluminescent diode was used to generate oscillating forces of 0.2–1 femtonewton, and a tunable laser to measure the cantilever’s displacement and its absolute distance from the fiber. The origin of the discrepancy between the obtained and bulk values of silicon-nitride reflectivity is to be discussed.