With the AFM head raised, we now slide an experimental sample under the AFM.
Upon command, the AFM will then use the vertical ball-bearing tracks to lower the head until the probe tip contacts the sample.
But how does it sense this contact? Unlike the STM, it does not use electrical current flowing into the sample (indeed, an advantage of AFM is that it can measure insulating samples).
Instead, it lowers the head until the cantilever bends just enough that the reflected laser beam becomes centered in the photodetector. From this point onward, the feedback circuits of the AFM will continue to move the head so that the laser is held at this position.
