One of the important innovations in the Quesant AFM is the isotopic focal system. This is a patented optical arrangement which allows the cantilever and probe to be scanned in two directions, but automatically directs the laser beam so that it remains focussed exactly on the same spot on the cantilever tip. Figure 1 shows a schematic of the scanner tube. It consists of a PZT cylinder, which bends under the application of high voltages, and a ceramic extension tube, which has the probe holder on its end. Within the tube are three lenses, which form an image of the sample and cantilever at a point exactly in the middle of the PZT. Figure 1 also shows a line which represents the center line of the assembly with greatly exaggerated deflection. The PZT is uniform, and is excited with a field that is uniform. The mathematical shape that it will assume under these conditions is a circular arc. The centerline is therefore also a circular arc. If lines are constructed that are tangent to the PZT where it is attached to the head at the top, and where it is cemented to the unbending ceramic tube, they will intersect at a point at the midpoint of the PZT. This is known as the isotopic focal point, which means same place.

       The image of the cantilever formed at the isotopic focal point does not move as the scanner goes back and forth, since the lenses move with the ceramic extension tube. The laser beam, which comes from a solid state laser connected to the upper part of the head, once directed at the image of the cantilever, will continue to be centered on the cantilever tip very precisely, regardless of the scanner movement. The beam reflected from the cantilever is also corrected for any movement.

       A further advantage of the optical system is that it affords an unobstructed view down through the tube of the cantilever and sample. A microscope tube looks down on the isotopic focal point. It would be difficult to design an objective lens for the microscope to have a working distance as great as the entire scanner length. Fortunately, the isotopic focal system makes this simple by moving the cantilever image almost to the top of the tube.