ABSTRACT
The essence of the method is that pushing on the tip of the fibre with a force F produces a torque r = FL, where L is the length of the fibre. The cantilever simultaneously deflects, Ax — F/kx, and twists, AO = r/ko. The deflection, and hence the force and torque, is obtained from the differential vertical photodiode voltage Ax = cYoAVvert, where the bare sensitivity factor, an, is measured in the constant compliance regime without the attached fibre. The actual sensitivity factor with the attached fibre, CY l, is greater than this because only part of the piezo-drive movement goes into deflecting the cantilever, Ax < Az (the rest is soaked up by the twist). The bare vertical sensitivity factor has to be measured in a separate experiment and depends on the positions of the laser, the photodiode, and the cantilever mount. With practice, it is possible to obtain better than 10%
