I spent most of today (Thursday) in catching up with my notebook and MATLAB graphics from this past week, though I was able to adjust the lens and stage in order to take a better range of data in the y-direction. The idea is pretty self-explanatory, so I do not feel as though I need to go into it too much -- I made it possible to scan over more distance in the y-direction by moving the lens.
The new y-scan is shown in the plot below. The second plot which directly follows is simply this new scan and the scan from yesterday.
It is pretty obvious that the two scans from today and from yesterday are very similar, with the scan today being a bit more symmetrical. What is interesting to look at is what is happening on the outer edges of this scan from today; after the signal goes to zero. We see that the signal increases a bit and it almost looks as though it could be similar to a diffraction pattern. However, I do not think this is the case.
What happens is that we get the signal by the lock-in being able to read a chopped signal and reference it with when the signal is on and off. When the excitation beam is not incident on the Si then the lock-in is not reading a chopped signal -- it is just reading a stream of signal which ends up just being noise, which is most likely what we are seeing here. It is possible to simply think of it as there being a threshold, after which point the signal does not mean anything. This is what we are seeing.