Monday, July 27, 2009

Aluminum Letters and Realignment of Microscope

Keeping the same exact setup as yesterday, Antoine wanted to look at what would happen if we placed an aluminum cutout of the letter "F" in the waist of the THz beam today (Friday). The basic idea is that we have a shape with easily-distinguishable features that we can put in the way of the beam to see if/how it effects the beam. Since the cutout was aluminum (with a paper back for support), we expect the THz to reflect off the letter and then we would not see this reflected light at the receiver.

This would be really interesting if it worked, as we would be able to build a profile of the resultant beam due to such an obstruction in the way. To look at this theory a bit, I made scans in the y-direction at two different x-positions -- x = max (16.5 mm) and x = 14.0 mm. For the scan at x = max, I used the scan from yesterday as a reference (which is maybe not the best reference), and for the x = 14 mm scan, I took a scan with the "F" in place and a scan with it removed to use as a reference. The only other problem with using such a scan for the reference is that the "F" is held in place with plastic wrap in a frame, and so there is going to be some reflection due to the plastic wrap. The plot for the x = max is shown below.

There seems to possibly be some sort of feature showing up around where the center of the beam is located, though it is not very easy to tell. One big problem ends up being that I do not know where I am scanning along the letter, so I do not know what features to be looking for. I think maybe the best thing to do with this is to be able to take some sort of a ratio of the two signals to see what the relative difference is of the two at each point.

The next plot shows the data I took at x = 14 mm (where the reference was taken right after the scan with the "F" in place).

It is very easy to see that these two scans are extremely similar in almost every way to each other. The only real difference comes in the relative amplitude of the signals, but that is probably due to the plastic wrap used to hold the letter in place and not anything else.

It is an overall very interesting thing to be looking at, though I think the best option is to be able to take a full roster scan of the cross-sectional area of the beam -- both a reference and one with the letter in place. I would like to look into this more, though I will have to see what time permits.

After taking these scans, Antoine wanted to adjust the parabolic mirrors to see if he could correct some of the skewness that I saw in the x-direction scan from earlier in the week. After he adjusted the mirrors, here is the scan that I acquired.

The shape is, in general, much more like what we would expect for a Gaussian beam, but is rather clearly skewed to the left. Also, for the "edges" of this scan, I have already mentioned that we can ignore them due to the lock-in and how it is working. Overall, the shape looks better, and if anything else, then this should suggest that this technique of profiling is indeed rather useful and with some more work might become a very accurate way of profiling such a beam.

In order to see how this scan differs from the scan taken earlier in the week, see the plot below.

The most important thing to notice from this is that changing the parabolic mirrors around and realigning the microscope will result in a change in beam shape to some extent. Adjusting the setup and then doing a scan by hand to test the profile is maybe not the easiest way to do this, but it is definitely a rather straight-forward way to see how the beam looks.

I hope to maybe look into some of these things a bit more in the final week, though there are a lot of things which I may focus on -- the waist size as a function of frequency, the N2 data, the "F" in the beam, etc.


  1. I think you may have hit on the most significant reason for the non-Gaussian nature of the THz beams, that is, a non-perfect alignment of the THz optics. With just the right alignment of the emitter, receiver, and parabolas, you should get the Gaussian shape in both planes. I had been thinking that the LOB group had done measurements on the beam profile in their set-up so that they already were confident that they had a Gaussian shape, but that was my fault for not asking about that. It now makes much more sense that the THz beam is a bit off alignment-wise, as compared to the excitation beam causing the profile distortion. I think it was still a valuable exercise to have you check this. Indeed, it ruled out the one potential cause, allowing you to see that the excitation beam incidence angle did not influence the beam profile and leading to the conclusion that the THz alignment was to blame.

    In regards to what to do with your limited time, I would suggest that if it seems most important to look for the frequency dependence of the beam waist, then you should do that by looking just at the y-direction THz spatial information. You don't have to sweep the x-direction info under the carpet, but you can acknowledge that there may be a slight alignment issue that gives you an x skew, and thus you have looked at the beam size versus frequency only in the y direction.

  2. I have talked with Antoine a bit today about this, and I feel as though I will try to still look at the frequency dependence of the beam waist (so long as I have time). I will probably spend most of my time in preparation for the presentation later this week, but time-permitting I will try to truncate the data I took on the spectroscopy setup and probably only look at the y-direction scans. Even if the data is not conclusive, it will be nice to at least have a better look at the possible effect of frequency on waist size.