We are currently trying to get an UHV AFM to work in our lab but we are having doubts regarding the spot size of the laser we are seeing on the camera when attempting to center the laser onto the cantilever. The spot size completely covers the entire cantilever itself and is really large in comparison to the spot size that is observed in our air AFM. We were under the impression that the spot size needed to stay within the area of the cantilever and not spill over in order to get a reliable signal on our oscilloscope. Since the laser diameter is so large, would this mean that the laser needs to be calibrated by a technician? Any help would be appreciated!
A longer beam path increases the motion of the reflected spot on the photodiodes, but also widens the spot by the same amount due to diffraction, so that the same amount of optical power is moved from one photodiode to the other. The 'optical leverage' (output signal of the detector divided by deflection of the cantilever) is inversely proportional to the numerical aperture of the beam focusing optics, as long as the focused laser spot is small enough to fall completely on the cantilever. It is also inversely proportional to the length of the cantilever.
This may help you.
Some UHV AFM gives the kit for aligning the laser on the tip outside (the chamber), usually not very fine. However, I won't trust the camera to judge the spot on the cantilever. As long as you get a fresh tip, see if you are getting the recommended signal (of laser) after alignment. Then you can go for tuning cantilever frequency. You should get a very high Q value in UHV system compared to air. If you are not getting all these, you must take help from technician as your laser is not focused (your fiber optical cable might need to be adjusted)
One important aspect is the size of the reflected laser spot relative to the size of the segmented photodiode / position sensitive detector (PSD) used to read out the deflection of the laser beam. Approximately, it should be about the same size as the PSD surface area, to ensure a linear response over a wide range of deflections and a constant sum signal. If it were bigger than the PSD surface, then part of the spot would be off the PSD edge and not contribute to the sum signal. Conversely, if its diameter were similar to the width of the gap between two adjacent segments, then the sum signal would go through a dip when moving the spot across the gap. From your question, it appears that the upper limit might be a problem. To check for the response linearity, I would mount a cantilever for contact mode, adjust the laser spot position, zero the PSD readings and run a force-distance curve on a stiff surface. Upon contact between tip and sample, the deflection signal from the PSD should increase in a linear manner. While running the FDC, it seems a good idea to keep an eye on the sum signal.
Hope this helps.
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