Monday, June 15, 2009

A Refined Sense of Direction

Today (Friday) I spent the morning looking over Mittleman’s "Sensing with Terahertz Radiation" mainly as an attempt to better understand the terahertz regime. The notes that I took from the first section of the text were mainly about the history of THz, the introduction of TDS to THz and what benefits this has over CW spectroscopy, and also some other introductory facts about THz.

The next two sections that I read were about spectroscopy and imaging. Reading of typical characteristics of THz frequency (1THz corresponds to energy of 0.004eV, a temperature of 50K, and a wavelength of 0.3mm) and types of THz interactions (three distinct categories dependent on values of Q resonance) gave me a better understanding of a typical THz laser and how different samples are analyzed. These two sections also mention the practicality of THz radiation and how THz technology is sometimes the most effective technology for a given problem.

I also met with Antoine today and talked a little more about where I am at and where I might want to go next. He basically confirmed that LOB is currently using a technique to get a spatial profile but that it is probably not the best method, hence my role in determining a better method. He did not explain the method they use here too much, and I have not had ample time to look into it yet, but I do know that they use some sort of aperture that opens and closes to allow a certain fraction of light through each time it is open. He made it sound like this aperture was scanned through the waist of the beam and then a profile was made from this.

Though I do not yet understand completely the method used in LOB, I do understand that their problem is a result of diffraction. I am told that they are able to produce an image which looks Gaussian, but the “tails” of the curve are possibly not accurate due to diffraction (i.e. if you cut the tails off of a Gaussian curve maybe around were the concavity changes then that is what may limit your accuracy). I need to investigate this diffraction problem to determine whether the effect is negligible or not. In doing so, Antoine mentioned a certain convolution which might describe this effect, but I have yet to look into this.

I went to the library today and added two more books to my pile: Diffraction, Fourier Optics, and Imaging, by Okan K. Ersoy and Introduction to Fourier Optics, by Joseph W. Goodman. The plan is to read through some of the sections on diffraction and Fourier analysis.

Also, I read through some of the initial article that I was given while still in Michigan to see if I could pick up on anything that I may have missed. Simply as a note to myself, I found that they used THz-TDS to measure ionic content in living tissues (specific ions being: K+, Na+, and Ca++). Again, this was near-field THz, so they performed transverse scans over the sample and their image was 150μm by 1000μm. The article is not directly related to my task of creating a spatial profile, but I thought it would be good to re-read it since it was the one which was initially given to me for reading.

Finally, I have received the comments by John on the past couple of posts and this is something I will be looking into on Monday and should have answers to by Tuesday or Wednesday.

No comments:

Post a Comment