Recent Entry

July 8, 2003

Well, I seem to have gotten a late start on my journal. Since the start of the Simons Program, I've been trying to figure out what project I want to work on. By going through various papers and past projects I was interested at first in Holography (which many people have worked on if you go through the past projects), and then in the Coherent Backscattering Effect. The Coherent Backscattering Effect, is a phenomenon in which photons travel "time-reversed scattering paths" and interfere constructively causing the appearance of a sort of cone of intense light in the backscattered direction. Over the weekend, I read (and understood most of!) one of the main papers on Coherent Backscattering written by Corey, et al.. I also did a few google searches on the topic, and found a really cool Java Applet which simulates the random walks that light takes, and the backscattering effect (you need a Java Virtual Machine installed in order to view it). I think that if I can find an application for this effect such as using it to study dense atomic media, then I would have a good project I can work with. Right now CBS is a pretty good nucleus for a project, but I don't think that it can stand on its own.

July 9, 2003

Today, we had a lab meeting and we discussed what everyone is working on, or plans to work on. Dr. Noe brought up how I am interested in photon localization, and Dr. Metcalf mentioned how he first learned of Coherent Backscattering, and told me about some people who I could contact that have been working on this topic for a while. I plan on and emailing Dr. Azriel Genack of Queens College, who has done a lot of work with localization and scattering. Afterwards, I spoke to Dr. Noe some more about CBS, and he told me of calculations that can be done with the data obtained by CBS through Brownian colloidal fluids to determine the size of the colloidal particles. This may be an interesting idea for a project.. I can look at the scattering intensities of particles undergoing Brownian motion, and then try to determine the particle size using a Fourier transform of the results, and possibly generalize the results. If I have some time, it might be interesting to analyze different structures of plastic spheres using microwaves to see how the intensity of the backscattered cone varies with the alignment of the spheres.

July 10, 2003

Today, I found a whole slew of new sources about CBS. Particularly interesting was the article entitled "Studies of coherent backscattering from opal photonic crystals" by J.D. Huang et al., since it discussed opals and opalescence, the topic which got me interested in scattering phenomena in the first place. I also found "Calculations of the Soil Hot-Spot Effect Using the Coherent Backscattering Theory" by Liang and Mischenko that seems to have an interesting application of CBS. During a lull when I took a break from searching for articles, I learned about polarization and Jones Matrices by playing around with the long spring and wondering what circularly polarized light would look like. Tom and Jose helped to clarify how the matrices explained the polarizations, and by looking at them I realized how the different polarizing lenses and plates affect light.

Tonight, I plan to finish reading the article about Calculating Hot-Spots and then read teh article about opal photonic crystals. Hopefully tomorrow I'll be able to get a start setting up a CBS experiment to get some data and actually see how it works. I've found a large number of diagrams depicting the setup, and Ming Lu (a previous student at the lab) has some helpful pictures that describe his setup.

July 11, 2003

Today I got a laser and a place to work! I spent a lot of my time today cleaning the desk and trying to locate the instruments I need for a CBS experimental setup, like a beam splitter, 1/4-wave plate, mirror and polarizer. I may need to adjust the classical setup by adding something like a narrow tube to the detector in order to filter out background noise, etc. Before I actually gather any sort of data for my project, I need to learn exactly how each of the components of the setup functions. I know about the linear polarizer and lenses in the setup, but I'm not sure how a 1/4-wave plate circularly polarizes light, or even what circularly polarized light is, exactly. I also need to figure out how the detection mechanism works. It doesn't look too difficult, but I think I should know it well in order to accurately interpret any results. Below is the setup Ming used in his project, to give an idea of what the setup looks like (the image has been gratuitously stolen from his jounal):

July 14, 2003

Today was a sort of stream of tangents which ended on a constructive note. It began when Dr. Noe showed us that by inserting a polarizing lens at 45-degree angle between two linear polarizers at 90 degree angles to each other, one can see an image through the three lenses. By looking at the Jones Matrices for the lenses, I saw that this should work because a 45-degree polarizer between to linear polarizers prevents the matrices from multiplying out to the zero vector... but I wanted to know the relationship between the angle of the rotating polarizer (in the middle) and the amount of light let through. Pat and I designed an experiment using a phototdector and three polarizing lenses to measure the variation on the amount of current induced on the photodector by a laser (the laser I plan on using for my project), by the angle at which the rotating (middle) polarizer is situated. As we took down the data for the experiment, we noticed that the current kept fluctuating in the multimeter. The fluctuation was a little weird, so we decided to look at the output of the laser in the oscillator, and found that it was hashy... which didn't seem to make sense. During all this, I learnead about how the laser was built, the various modes that get gain inside the laser cavity, and how these modes could shift as the laser heats up. It turns out that it's the laser that was causing the fluctuations, but I don't think that should cause too many problems when I'm taking my data, I'll just have to keep it in mind when I have to look at and interpret the data. The graph below shows the results of our experiment, and the theoretical fit of the data (calculated by using the light's electric field as a vector and calculating the intensity based on the angle of incidence to each polarizer) I made using Gnuplot (which I learned today!):

July 16, 2003

Today Tom and I set up a scattering experiment in the back to measure the amount of laser-light scattered from the sample ceramic material he has. This setup is similar in many ways to the setup that I'm going to be using for my experiment. Today after the lunch meeting, I talked to Professor Metcalf about polarizers and Jones Matrices, and I think I have a pretty clear idea about how polarizers and materials that affect electric field orientation and phase operate on light. The next few things that I need to do are to figure out if I need to use a beam expander, and to figure out the distances in my setup. I also want to try to find information on fractal aggregates and any relatively accessible and safe aggregates that I could use for an experiment. Also, I want to email Professor Genack... which I haven't yet done to see if he has any input on a coherent backscattering project. Also, I'm going to write up a small website for the side experiments I did on polarizers and what I learned about Jones matrices.

July 17, 2003

Today I attended a fascinating lecture that Professor Metcalf gave on Quantum Mechanics. He clarified some of the questions I had about the wave equation which I read about in Jose's textbook, without even writing it out! I also worked on my experimental setup and began drawing it out on a piece of paper. I'll see if I can make a vector drawing of the projected setup on my computer to post up here, and I'll note the drawing in my lab journal. I also started working on a website about the experiment I did with polarizers, and a webpage about coherent backscattering that someone might find useful. I'll add a link to them once they're readable.

July 21, 2003

Today, Sage and I did a small scattering experiment using Tom's detector and sample to see what kind of data we would get, and try to interpret it. We definitely got some kind of curve, and tried to figure out what kind of theoretical curve to use to fit the data. A graph of our results, and a possible theoretical fit based on the sine of the angle between the detector and the sample can be found in Sage's journal. We made the graph using gnuplot, which is getting easier and easier to use everytime!

Also today I attended the second lecture Professor Metcalf gave on Quantum Mechanics. I understand a lot of it, but Quantum Mechanics is still very confusing since it dictates that you cannot ever get a definite answer from just the pure mathematics which describe nature. After the lecture, I continued working on my setup, and mapping out my space on the desk on a piece of paper so I know exactly what it should look like before I actually set it up

July 22, 2003

I haven't really had a chance to work on those two pages I want to make... I may be able to do that tonight and just email it to myself. I've also been trying to better understand the math behind the CBS effect; I found a slide presentation online which could help me to may help me. I've also been seeing a lot of the same math repeating in a lot of the papers I've read, which I'm slowly beginning to pick up on. I thin that once I understand the basics, I'll get the special derivations in each individual paper. I'll update my sources page when I get back to the dorms and put that up tonight. I still need to find out more about fractal aggregates to see if I want to use them in my project, and I need to continue working on my setup.

July 23, 2003

Today, Alex Ellis, a High School student who worked in the lab last year visited the Laser Center. He gave a presentation on his project on Astigmatic Laser Mode Conversion. We also talked about getting started with writing papers, and time management in general. He gave some pretty good advice about when to start writing my paper, and when to really start gunning with a project.

I also continued trying to understand the math behind CBS, and I understand why the enhancement factor was predicted to be about 2 in the theoretical model of CBS. It's because when two waves are in phase when being scattered back, their sum is squared, and when they are out of phase, their squares are summed, thus when the destructive waves are subtracted, a term with a factor of two is left over, predicting an enhancement of two. I still don't quite understand the major reason why the shape of the intensity distribution is conical, but I think I'll understand by the end of the week.

July 28, 2003

Today, after yet another mind-blowing Quantum Mechanics lecture which I thoroughly enjoyed, I spent some more time thinking about the theory behind my project. I really need to work on my setup, but I really want to understand the math behind it. I spent the day reading about photon diffusion approximation in various media. I found some more interesting articles with easier explanations and definitions which I'll post on my sources page when I get around to updating it. I've been working on my CBS page with the new information I've learned, but it's not yet complete. Today I looked at the pinhole which Doug Bennett used on his setup, and played around with it trying to figure out the size. I also spent some time looking at websites on photomultipliers to try and figure out how they work. I need to set up the pinhole, reflection screen and PMT up so I can figure out how to align everything to gather data. Reading about the problems that Ming and Doug had have helped me a lot in thinking about the setup, and the pictures they put up are also excellent. Tomorrow, I'll spend the morning trying to get a PMT for my setup, and working on aligning everything to get rid of weird reflections which seem to occur when the laser hits the beam-splitter and the quarter-wave plate. Also, I have to make sure the quarter wave plate is aligned correctly to circularly polarize light, and the linear polarizer is adjusted to prevent reflections from hitting the detection mechanism.

July 29, 2003

Quite a few good things happened today. Earlier, Jose took me to clean off the beam-splitter I'm using in my setup, and Oleg showed us the necessary steps needed in cleaning coated optical components. I wrote these down in my journal as they can prove to be useful later on. The best part about it, though, was that when I put the newly cleaned beam-splitter back into my setup, ALL of the reflections that were annoying me were gone, save the one from the quarter-wave plate.

Later, Jose gave us a talk on Lasers, Cavities/Stability, and Mode Structure/Gain. It was really interesting, and gave me a lot more insight and understanding into exactly how lasers function. After the talk, Mr. Schorn, my science research teacher came in to talk to me about my paper, and to visit the lab. We talked for a long while about my project, and I tried explaining to him what I was working on. After we felt that he had a pretty good understanding, we talked about deadlines and how I should organize my introduction. I think it's important that I get started as soon as possible in writing my paper, so that I don't have to worry about cramming the entire paper-writing session into a period of three nights.

July 30, 2003

Today, all the Simon's Fellows visited the Brookhaven National Laboratory. It was a great experience, as I got to see the Relativistic Heavy Ion Collider, National Synchotron Light Source, and the Van de Graaf Accelarator that they have over there. Upon our return, Jose finished the talks he started yesterday about different types of lasers and gaussian optics/beams. There's been so much information to absorb over the last two days, but I think that I've managed to grasp and understand most if not all of it. It may be worthwhile to try and explore/derive the math behind the different topics that Jose touched upon to try and gain an even deeper and more useful understanding. I also continued working on my setup.. and tried to figure out ways around the reflections and the severe loss of laser signal which I am going to have to deal with due to the reflections by the wave plate and linear polarizer.

August 4, 2003

Today I did a lot of work in organizing all of my sources, and with the help of Dr. Noe, I started to make a chart in Latex of all the different setups I've read about. I also compiled a list of questions for the authors of the papers, and plan on emailing them tomorrow. I came back to the lab at night to work on setting up two lenses for expanding my beam. I managed to expand my beam, but not by much... I think I'll need a little more space to make the beam bigger which shouldn't be too bad. Afterwards, when I came back to working on the chart and list of questions, I found this rather interesting description of CBS and Localization on Ad Lagendijk's groups page:

The following example is useful in understanding this [CBS and Localization]. A typical textbook model of a random walker [photon] is a drunken sailor. Suppose that his wife is struggling to get him home, but he does not want to go. Whatever direction in which he tries to make a step, she wants him to go in the opposite direction and will pull on him (enhanced backscattering). In this way she hampers his motion and reduces his diffusion constant. If she is very strong she will pull very hard and he will not be able to make any step at all: he (and she) are localised. Thus the observation of enhanced backscattering can be seen to be at the heart of localisation.

Tomorrow, I'm going to work on my setup and come up with some sort of presentations for the Simons Fellows visiting on wednesday.

August 6, 2003

Here I am again, another late night at the lab. Today we gave our presentations for the Simon's Fellows who visited the lab, and I whipped together a quick description of what I've been doing here. I think the talks went pretty well... the best part, however was the laser-light show we put on afterwards, and when we got to explain our projects to everyone in the lab itself. I think that through explaining my project and answering questions (or trying to answer questions) about it, I learned a lot more about it.

Anyway, on to the exciting part of this entry: Tonight I took the CCD Camera/Computer from Allison's setup, as we won't really need it there anymore, and wheeled it to my corner of the lab. I positioned the CCD Camera using a stray reflection from the center of the beamsplitter that conveniently showed me where the exact backscattering direction was. I then attached it to the computer, and turned it on. When I opened the software that communicates with the camera, I saw some good news and some bad news. The good news was that there was very, very little background noise with all the lights turned out, but the bad news was that there was an ugly diffraction pattern from either a fingerprint or some defect in the linear polarizer I was using. Also, the 1/4-Wave plate was not in this specific setup, so that may cause some additional reflections and scattering that did not show up in this image. I tried to get rid of this blotch by turning the polarizer, but this only weakened it a little... I'm thinking that it's a fingerprint on one of the lenses, or the beam splitter. This problem can be solved by cleaning each of the components - but they're so nicely aligned! :-( At least I know where the backscattered point is, and I think that I'm at the focal point of my focusing lens, which is also a plus.

After all of these discoveries, I tried putting a piece of styrofoam where the sample will go, and I saw a neat speckling image behind the huge blotch! I'm not sure what it means exactly, but it was definitely exciting to see something show up on the camera that had to do with scattering. Tomorrow, I'll figure out how to capture the images from the camera, and put up my preliminary images here, also, I'll add the 1/4-wave plate to the setup and see how that changes what I'm viewing... I expect the signal to get weaker, but i don't think it will get so weak that the camera won't pick it up.

August 13, 2003

Well, I haven't updated my journal in a while, but I've been doing a lot of work over the last week! I was actually able to image some cones today, for the first time... after a tremendous amount of work on my setup. Tonight, I spent a lot of time conducting some mini-experiments on my setup to try and determine exactly where my images are coming from, and how well I can get them to resolve. I've been also trying to learn about the CCD Camera, and how to use it along with processing the images after I have them. Dr. Noe suggested that I make a separate page for all these small things that I've been working on, and I think that I will do that tomorrow. I just compiled a basic list of the topics on which I've been working for the last week, and tomorrow I'll add a description for each experiment along with pictures(!), results and some discussion. It's been a long trip just to get the images, but now things should become a bit easier. I just need to figure out how I can make sure that the images I'm looking at are definitely CBS, and then I need to try and come up with theoretical fits for the data. I'm really ecstatic about the images I've seen, and I think this could be a really cool project once I better understand the theoretical fits... I understand a lot of the math behind it, but when it comes down to the specific details of the photons moving inside the scattering medium, and then determining the intensity function by using the phase shift function, I get a little lost. Tomorrow, I'll take pictures of my setup, work on that page about my mini-experiments, analyze the pictures I've taken, and then try and reread papers that go in-depth on CBS theory. I made a list of things to accomplish today, and I'm up to number 7: Sleep.

August 20, 2003

I've spent the last week learning how to process the images and coming up with some questions about my project. I lost about a day and a half of data collection and interpretation because of the blackout, but I'm commuting back for the next two weeks to try and do as much as I can before school starts. I talked to Doug Bennett yesterday, and found the line-shape curve he used to fit his data. I spent this morning trying to plot the curve on my calculator before the lab was open, and I'll try and fit the profile that I have with a curve for the intensity. It comes from the Saulnier paper, but I don't really understand how it was derived. I need to read the background paper and fit the data. I've also been working on a site to describe my experimental setup (Click Here). Tomorrow, I plan on doing a small experiment with different concentrations of milk in order to see if I can detect a change in the width of the cone with the concentrations. Then, I need to determine what the actual mean free path is in the solutions I use. I'll need some sort of calibrating solution of a known density and composition.

August 28, 2003

This past week, I did some more experiments using different types of milk. Some of my better results can be found here. I've also been reading a lot about absorption coefficients and diffusion coefficients to get a better understanding of how they relate to the mean-free path and how they could be estimated with CBS. I've also written some of a draft of my paper (I think it's a good idea to get started on it early!). Tomorrow I hope to get some more definitive results and then I can spend the weekend analyzing them and writing up more of a complete paper before school starts. I'm still not sure about exactly how the theoretical fit is derived from a 'diffusion approximation', but I have a much better understanding of the math behind it and the phenomenon itself.