The twisting of the cantilever is measured in the light lever AFM by monitoring the left to right motion of the deflected laser light with a photodetector. Typically, the photodetector has four quadrants. The vertical force is measured by monitoring the top and bottom two quadrants, and the lateral forces are measured by monitoring the left and right two quadrants. Figure 4-18 illustrates the sensing system used for measuring lateral motions of the cantilever. Both a topography and frictional image may be measured simultaneously.

FIGURE 4-18 In LFM the probe torsional movement is monitored with a four section photodetector having a left (L) and right (R) section.

Figure 4-19 is an example of lateral force images on a very flat piece of gold. The topography image of this sample shows no features, however the LFM image shows a pattern drawn on the surface.

FIGURE 4-19 LFM image of a flat sample patterned using AFM lithography. At the left is the LFM signal scanning in one direction, at the right is the LFM signal scanning in the other direction.

Physical mechanical properties such as the surface hardness or stiffness of materials can be directly measured with the vibrating phase technique.When scanning a surface in vibrating mode, the feedback either keeps the relative phase or vibration amplitude of the cantilever constant, (Section 4.1.2). Thus while scanning, if the probe traverses a surface area having different mechanical hardness (stiffness), and the vibration amplitude is held constant, there will be a change in the phase signal. Figure 4-20 illustrates the operating principle of vibrating phase mode.