Wednesday, July 15, 2009

Another Lateral Scan

Today (Thursday) I spent doing some more catching up and also taking another lateral scan of the Si wafer. This scan was done at a fixed position in the x-direction at approxtimately the "bump" that we see in both x-direction lateral scans (see the scans from Monday to see the bump... this was also done using the same, more-focused lens). This was to try and get an idea of the shape of the beam at this one point. Below is a plot of this.

We can see that the shape is indeed rather symmetrical, just with the other scan in the y-direction. There is a fair amount of skewness, as well. The general shape is Gaussian, but it seems to be too "rigid" to be a proper distribution. The whole idea from this scan is that the profile is rather different than what we would expect for a perfect Gaussian distribution.

The problem with doing a series of scans like this over multiple strips of the Si wafer is that we are able to find some rather interesting behavior, but do not have a very good idea what is happening in regions other than those which are scanned. This can be solved in a variety of ways, the easiest being switching out the current translational stage with an automatic one and writing a simple program to profile the entire beam. The problem with this is time and lack of equipment. I have about two weeks remaining, there is not a translational stage laying around that I could use, and there are perhaps more interesting things for me to be looking at.

This being said, I would like to comment on the noise in the signal that I am reading off the lock-in. In case I have not mentioned this before, I take these data manually, and to try and deal with the noise I take and average the highest value and the lowest value over the duration of a few moments. The signal on the lock-in happens to oscillate back and forth, and these maxima and minima are approximately what I average to get the value that I use to plot.

Antoine and I tried to cut-down on this noise later in the day by changing the chopping frequeny, but the best frequency we found was about 283 Hz (frequencies of about 250 Hz and 300 Hz gave a lot of noise due to the surronding environment). In an attempt to see if this averaging technique changes much the final data, I plotted all three sets of data over position (which includes the maximium, average, and minimum). All three series of data seem to correspond well with each other (which simply means that the difference between maximum and miniumum remains relatively consistent, regardless of the position on the wafer -- thus even further suggesting that it is in fact noise). A plot of this is shown below (Note, this is from the same scan that I am describing above, but I see this effect in every scan that I plot maximum, minimum, and average on).

We thought of some ways to reduce this noise, but could not figure anything out right away. I think the easiest thing would be to connect an oscilloscope directly to the lock-in that is able to look at the signal over time and average the signal even more than what the lock-in already is. From this we could hopefully attain a more accurate average value. This idea would not work, however, since we do not have such an oscilloscope around the lab. Also, because of the consistency with the change in signal due to noise, we might as well ignore this for now.

From here I would like to further look into the differences in signal over position and also to continue trying to learn about the waist of the beam as a function of wavelength.


  1. I forget... did you say a few weeks ago what your time constant was on the lock-in? Using a larger time constant should provide you the same noise reduction as the manual averaging that you have been doing. (depending on the source of the noise)

    One question is: how stable is the reference-frequency signal coming from the chopper control box? If that varies much relative to the lock-in measurement bandwidth (inversly proportional to the time constant), then the lock-in reading could get noisy. It's probably not an issue, but I did have experience years ago with a chopper that had an electronic frequency output that was not well synchronized with the mechanical chopping frequency (and it was not stable, either). There are also numerous other noise sources that could cause your problems.

    Is the noise bad when you look at the THz signal without the extra excitation beam on the Si and you chop the THz beam? When you look at the THz beam this way, then the lock-in signal corresponds to the difference between the THz being on and the THz being off. When you look at the THz when chopping the excitation beam to the Si, then the lock-in signal corresponds to the difference between the THz signal transmitting through the unexcited Si and the THz signal transmitting through the excited Si. Therefore, I would expect the signal with the THz chopped to be appreciably larger than the signal with the excitation beam to the Si blocked. Is that true? I am also curious as to how different these signals look from each other. (In this case, that means only over the few picoseconds immediately surrounding the main THz transient.)

  2. If I remember correctly, the time constant that I was using was 1 second. I know that I used this for the first few scans that I took (the ones which took about 1.5 hours to take), but I also think that I used this for the constant when profiling the beam.

    As far as the reference-frequency signal, this is something that I will have to ask Antoine or Guilhem about later this coming week.

    I have not looked at the noise too much with the THz chopped without the excitation beam, but if I remember correctly, simply blocking the excitation beam still read an oscillating noisy signal. This would maybe suggest looking into what you mention in your second paragraph, as it would mean that the noise must come directly from the chopper.

    I will have to check on a few of these topics in lab later this week, and I ought to have answers very soon. Hopefully I can better understand this noise, though I do not think it is the most important thing right now given the amount of time I have.