I went to two seminars which the LOB hosted this morning. The first was about a new form of microscopy developed by Jerome Mertz and his group at Boston University. This type of microscopy is called “HiLo Microscopy” and is intended to remove the out-of-focus background image artifacts that are common in microscopy when using devices like the confocal microscope. In essence, his technique involved optical sectioning and compiling together two complementary images – one of which was taken with a high-pass filter and the other of which was taken with a low-pass filter.
The second talk was about collagens and a little about multiphoton microscopy, but I did not understand this talk as well as the first.
This afternoon I read into some sources a bit more, one of which was the source I mentioned in my last post by the Moscow group. My main interest in reading this was to look at different techniques that physicists are using to attain such a beam profile. The article mentions the more traditional approach of using autocorrelation functions or spatial Fourier transforms, the former is one which I am not particularly familiar with and would like to read into a bit more. This group is able to attain a pulse shape and beam profile by using measurements from two different techniques: partially overlapping time intervals which are used to understand the temporal dependence (pulse shape) AND multidirectional bands which are used to understand the spatial dependence (beam profile). The article is more about the algorithms developed to solve this problem than the actual experimental design and was therefore more helpful in understanding the mathematics rather than the design.
Another article that I skimmed through was “Spatiotemporal transformations of ultrashort terahertz pulses” by a group from the Czech Republic. The article was from 1999 and was published in the Journal of the Optical Society of America. I found this article intriguing in that it discussed how THz pulses reshape in specific optical components. For instance, a THz beam is reshaped through any focusing optics and this reshaping can be described with an ABCD transformation (something I will read about in the photonics text).
Finally, I began reading through a little of Mittleman’s text on Imaging with Terahertz at the end of the day and intend to read more of this on Friday.
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Alex,
ReplyDeleteNote that the phase shift, or reshaping, due to focusing is a result of a phenomena known as the Gouy shift. We demonstrated this first here in my lab, with a student named Boh Ruffin. I will e-mail you a copy of this paper.
I am not so sure I would get too caught up in the temporal measurement of the pulses, since the group at LOB already has a means to do this with the gated PC (photoconductive) detector, plus they really want to know the spatial profile.
Can their near-field aperture with a coupled PC receiver just be used to give a high-res beam profile? If they really want wavelength dependence too, then the aperture may actually modify the time-domain shape (i.e., screw up the pulses) too much. They can probably answer this question in about 5 seconds.
Have a good WE. I still gotta track down Steve to ask about Skype.
- John
John,
ReplyDeleteI have read across some of the Gouy shift material (mainly in Saleh's Photonics text), but I have yet to fully understand it. I received the paper that you sent me and plan on looking through it soon.
Alright, I did not really plan on looking at the temporal pulses too much anyway... the reference just happened to be about both the spatial and the temporal (which seems to be a recurring theme of many papers).
In terms of the near-field aperture, I think maybe this works... and it sounds a lot like the current method they use. If this is in fact the method used here at LOB, then I think the problem of diffraction arises. Also, like you said, this probably also changes the time-domain shape. I can ask this to Antoine soon to get a more definite answer, but he may end up just posting a comment to this to answer instead.
Alex
Also, note that I try to explain the method used at LOB in the post directly after this one (as in the next most recent). It may not be particularly clear, but it is a start to understanding the current setup.
ReplyDeleteAnother thing that I forgot to mention is that I think the most beneficial aspect of the technique of spatial profiling that I am investigating (with the Si wafer and 800nm laser to create charge carriers) is that the device used to attain a spatial profile is not in direct line of the THz beam (well, the Si wafer is, but when it is in its unexcited state most of the THz beam transmits through). I have read that most other techniques of spatially profiling involve putting an obstructive object in the way of the beam, whereas this new technique seems to avoid that.