Thursday, June 18, 2009

Beam Alignment and More Reading

Today (Tuesday, June 16), I spent time doing both reading and some laser alignment in the lab. The reading consisted primarily of the article which I have been referring to about spatially profiling the THz beam with an n-type Si wafer. Reading through the majority of this article cleared up many things.

The first thing of interest was that this list gave a variety of other groups and sources for which I may refer and which discuss alternative methods of attaining a spatial beam profile. Perhaps the most notable being a technique which combines an electro-optic (EO) technique with a charge-coupled device (CCD) camera. As I may have mentioned in an earlier post or comment, the problem with most of these techniques is that the detector is positioned at the observation point, which means having to rearrange the apparatus to do spectroscopy, imaging, etc. There is also mention of an older method which uses a bolometer and knife-edge method to measure the focusing profile from a surface emitter (surface emitter I think meaning THz generator). This is presumably not the best method because there is no frequency dependence (i.e. it measures the profile composed of all the beams).

As far as this aspect of the paper goes, I am mainly interested in looking up some of these papers and trying to see what other types of methods there are.

What this group wanted to do was develop a new technique for attaining a beam profile at various frequencies (well, the whole range of frequencies (the whole signal) is taken and then a single frequency of interest is taken from that). Their method involved scanning an optical beam of nm wavelength over the Si and detecting the change in THz signal caused by optical excitation in the semiconductor. The altered signal is measured by a photoconductive antenna placed a focal distance away from the Si wafer.

What happens physically is that the transmittance of the THz beam through the Si decreases as the free charge carrier density increases due to excitation of electrons and the change into a semi-metal. This means that the amount of change of the THz amplitude is proportional to the amplitude of the THz wave at the point where the nm beam is incident on the Si. Thus we find the amplitude distribution of the THz wave by chopping the CW nm optical beam and lock-in detecting the change in amplitude by scanning over the surface of the Si.

What I have just written about is many of the actual notes which I took during the re-read. However, I would also like to better address the questions that John asked in a recent comment.

The first question deals with how the group distinguished between the three frequencies that it mentions creating a spatial profile of.

The other question (which went hand-in-hand with the previous one) was about how this group measured total power, since this would make it indecipherable to tell which part of the spectrum is which and in turn make it impossible to tell spatial profile by frequency.

Because I have yet to answer these completely, I will do so in another post later this week.

During the afternoon, I was able to go into the lab and begin working a little with one of the microscope setups. I had to move the alignment laser to another location, change the stand it was in, and align the beam. Just to note, the laser is a class 1 infrared laser so that the beam can go through the optical components. I got as far as securing the laser into the holder and getting it to the approximate correct height, but in trying to get the horizontal angle set, I was having some trouble. I want to set the horizontal angle correctly before setting the laser in place on the table. I was going about doing this by trying to pass the beam through two slits drilled in vertical rods. (For lack of a more concise description, this is the idea of setting two holes far apart and by passing the beam through those two holes, it will be aligned properly… the farther the two holes, the better).

The plan is to finish aligning the laser on Wednesday or Thursday.

2 comments:

  1. Sounds like the group measuring the beam profile at different frequencies is not measuring total power, but the full THz pulse waveform at each position of the laser on the Si wafer. I gather that they then Fourier transform the time-domain waveform (via FFT) and look at the amplitude of one specific frequency component at a time vs. laser position. Sounds like a good technique really; I like the sound of it the more I hear. I guess I wouldn't have thought that the decrease of field amplitude caused by the laser light on the Si would have been significant enough to get good data, but if they can do it, then it's a nice method.

    Is the EO/CCD paper from RPI? If it's the one I am familiar with, they were using a pretty high power laser to get a high enough intensity in their probe beam so that they could expand it and do THz beam profiling without scanning anything. Nice to do if you have a high power short-pulse laser.

    Also, a surface emitter is a type of THz source. It uses the built-in electric field that can occur at the surface of a semiconductor to accelerate the photoexcited charge carriers and create the THz pulse.

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  2. John,

    That sounds just like what this group did in fact do. I am still reading more into it, but this is the idea.

    Yes, the paper is in fact from RPI... and I found it online last week. I have not yet had a chance to read it, but will hopefully do so shortly.

    I thought a surface emitter was a type of source... this clears things up for me.

    Alex

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