Sub-attonewton force detection at millikelvin temperatures

A 290-nm-thick single-crystal silicon cantilever has been cooled in vacuum to a temperature of 110 mK in order reduce its thermal motion and thereby i mprove the achievable force resolution. Since the thermal conductivity of t he silicon cantilever is extremely low at millikelvin temperatures, an impr oved optical fiber interferometer was developed to measure the subangstrom thermal motion with optical powers as low as 2 nW. At the lowest temperatur e, the cantilever exhibited a quality factor of 150 000 and achieved a nois e temperature of 220 mK, with a corresponding force noise of 820 zN in a 1 Hz bandwidth. (C) 2001 American Institute of Physics.