Detector quadrant photodiode

Figure 2 shows the brief principle of a laser-detected FFM. A sample is put on a piezoelectrical tube (PZT), which scans X, Y plane and controls the feedback of Z axis. The laser beam from a diode is focused on the mirror of the free end of a cantilever with lens, and the reflected beam falls on the center of a position-sensitive detector (PSD), a four-quadrant photodiode. When the sample contacts with the tip and relatively moves under the control of a computer, the reflected beam deflects and changes the position on the PSD due to the twist and deflection of the cantilever caused by the changes of surface roughness, friction force, and adhesive force between the sample and the tip. The extension and re-... 

Figure 7.2 A schematic diagram of nanometer position sensing. Light from the evanescent field scattered by the microparticle is measu red with a quadrant photodiode detector, whose differential outputs correspond to the x and y displacements and the total intensity depends exponentially on the distance z between the particle and the glass plate.

The SFM tip is placed in the near-field of the source and in contact with the sample surface acting as a detector of the SAW. Due to laser deflection detection with a 4-quadrant photodiode vertical and lateral oscillation of the cantilever can be detected with lock-in amplifiers leading to amplitude as well as phase signals... 

The original AFM used an STM to sense the movement of the cantilever in response to interactions with the sample. In most commercial AFM instruments, optical detectors have supplanted this type of electrical detector. The most widely used detection system uses laser beam reflection off the end of the cantilever into either a position sensitive detector or a quadrant photodiode. A change in the angle of the cantilever moves the spot on the detector, producing a change in the voltage out of the detector. 

Soon after the introduction of the AFM, it was recognized that small modifications allowed the detection of friction forces [275]. For this purpose, a scan direction perpendicular to the long axis of the cantilever is used. A friction force between the tip and the substrate will then lead to a torsion of the cantilever and this in turn induces a lateral movement of the laser beam on the detector. By using a quadrant photodiode as shown in Figure 3.8, this lateral shift can be recorded and allows simultaneous recording of surface topography and friction [276]. This mode of operation is called friction force microscopy (FFM) or lateral force microscopy (LFM). 

For calibration of vertical stiffness, the same procedure is carried out for the vertical detector signal but using a different correction formula for P derived by Brenner [327]. The second issue is coimected to the bandwidth of the quadrant photodiode, which should be at least one order of magnitude higher than Vq. This condition may not always be fulfilled espedally if silicon photodiodes are used in combination with infrared lasers. A detailed discussion on this topic can be found in Ref. [328]. 

In 1987 Mate et al. [468] used, for the first time, an atomic force microscope (AFM) to measure friction forces on the nanometer scale (review Ref. [469]). This technique became known as friction force microscopy (FFM) or lateral force microscopy (LFM). To measure friction forces with the AFM, the fast scan direction of the sample is chosen perpendicular to the direction of the cantilever. Friction between the tip and the sample causes the flexible cantilever to twist (Fig. 11.7). This torsion of the cantilever is measured by using a reflected beam of light and a position-sensitive detector in the form of a quadrant arrangement of photodiodes. This new method made it possible for the first time to study friction and lubrication on the nanometer scale. 

FIGURE 10. Schematic drawing of a ZEKE experimental setup QD, quadrant diode PD, photodiode CP, channel plate detector INT, integrator SMS, stepping motor drive FL, dye laser ADC, analog to digital converter PI/O, output/ input port SHG, frequency doubler X, point of interaction. Reproduced by permission of the American Institute of Physics from Reference 27... 

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