Biomedical Engineering Reference
In-Depth Information
Proper alignment of the laser is very important in order to obtain best results from the
AFM. Poor alignment may reduce the sensitivity of the optical lever, could introduce
imaging artefacts or prevent imaging altogether. For example, laser light spilling over the
edge of the cantilever may reflect off the sample, and interfere with the laser light
reflecting off the cantilever, see Section 6.6.2. The highest sensitivity is generally obtained
with the laser spot centred over the position of the tip, that is, very close to the end of the
cantilever, and in the centre (as shown in Figure 4.3 by point 7). One trick to check the
laser is not on the edge of the cantilever is to observe the beam profile with white paper as
described above; the edge of the cantilever will change the shape of the laser beam spot on
the paper.
Having aligned the laser onto the cantilever correctly, it must then be correctly aligned
with the photodetector. To do this the photodetector is translated until the laser spot is
centrally located on the four segments, as shown in Figure 3.4. Sometimes there is a visual
display of the photodetector in the AFM software, and sometimes just a numeric display of
the signals from the photodetector segments. In both cases, the aim is the same, to get the
laser spot to the centre of the detector, i.e. to equalize the signals from all four segments.
This is a rather similar process to the laser alignment, and the only complication comes
when the spot is completely off the detector, in which case the user might not know which
way to turn the screws. If this is the case, the user simply turns the detector translation
screw all the way in one direction, and then all the way in the other until the alignment is
found, being careful not to apply too much pressure to the screws when the end of the
movement is reached. In addition, often a third control is inserted in the optical path, which
controls a mirror between the cantilever and the photodetector. This control directly
rotates the mirror, and serves as a coarse adjustment for the photodetector alignment.
This control is shown in Figure 4.4. In normal day-to-day operation of the AFM in air - for
instance, when exchanging one probe with a similar one - the adjustment of this control is
not required during the optical alignment procedure. There are two common reasons why
the control might need to be adjusted. The most common reason for needing to adjust it is
that when changing from air to liquid operation, the refraction of the laser at the liquid
Fig. 4.4. Full optical system for optical lever AFMs, showing the various adjustments required for
optical alignment (indicated by arrows).
