ABSTRACT
The optical beam deflection method is the most used method in both AFM and nanomechanical sensors for measuring the deflections of the lever, both dynamic and static. This method is based on the reflection of a light beam on the surface of the cantilever that is collected by a segmented photodiode or position sensitive detector
(PSD). The light sources commonly used for this type of action are light emitting diodes (LED) and diode lasers.Photodiodes divided into two or four quadrants are responsible for the conversion of light energy of the laser spot that reaches each segment into a measurable electric signal, which can be filtered, amplified, compared with the signal coming out from the other segments, and finally monitored. The deflection of the cantilever changes the position of the reflected laser spot on the photodiode surface (due to changes in the angle of reflection), and consequently, the amount of light reaching each segment, correspondingly varying the differential electric signal. By the appropriate selection of the sum scheme along the four quadrant segments of the photodiodes it is possible to discern, in principle, displacements in two orthogonal axis, which can be useful to independently measure two kinds of modes of vibration of the cantilever: flexural and torsional vibrations.In typical experimental conditions for static measurements, the reflected light spot on the surface of the cantilever is centered on the photodiode, so that all segments receive the same amount of light at the beginning of each measurement. Thus, when the cantilever is bent due to the differential surface stress caused by physical or chemical changes on the cantilever surface, the laser spot displaces over the surface of the segmented photodiode. The position of the laser spot and therefore the deflection of the cantilever can be determined using the outputs of each of the quadrants.
