Single-fiber radiation-pressure technique for calibrating ultra-soft cantilevers with non-perfect reflectance

Ultra-soft cantilevers have played important roles for pushing the sensitivity limit in several areas such as magnetic resonance force microscopy for singe electron spin detection, dynamic force magnetometry for half fluxoid quanta observation, and so on. In this work, to determine their mechanical impedance, we developed the techniques of single-fiber radiation-pressure calibration as well as local reflectance and transmission measurement, applicable to various cantilevers even with micro-scale width or without perfect-mirror surface. From the optic fiber interference signal and analysis for 200 nm-thick silicon nitride cantilevers, we obtained the reflectance value of 0.35, much larger than its bulk value, which agreed with the estimation considering thin film effect and was also consistent with separately obtained transmission values. It reveals the radiation pressure's dependece on the thickness, suggesting optimal tuning of cantilever thickness. Radiation pressure modulation of femtonewton level was adopted in actuating the cantilevers for mechanical impedance determination, and the preliminary results are to be shown and discussed.