Polarizers May Enhance Remote Chemical Detection
[Writer] This is research news from U-I-C – the University of Illinois at Chicago. Today, chemistry professor Robert Gordon describes an experiment performed in his laboratory on polarized light, and how the findings may be used to enhance a detection device used in hazardous situations.
Here’s Professor Gordon.
[Gordon] A number of years ago, I was reading a book by Brian Greene called “The Fabric of the Cosmos” in which he talks about the present understanding of cosmology, where the universe came from and the underlying physics. This has always been a hobby of mine, and in (the book) he talks about the fact that the universe is filled with microwaves, and these microwaves are a remnant from the original Big Bang which occurred 15 ½ billion years ago. One of the interesting things about these microwaves is that they’re very directional and they are polarized – which I’ll explain in a second. And from the direction of the microwaves and from the polarization, astrophysicists can figure out what the early universe looked like.
Now, why was this interesting to me? First of all, let me explain what polarized means. Light is a wave – an electromagnetic wave which travels in straight lines. The disturbance of the electric field is always perpendicular to the direction the light moves in, so you can think of it as a wiggle that’s wiggling up and down while the wave moves forward. If it’s always wiggling up and down in the same plane, we say it’s polarized. It could be random, in which case we say it’s unpolarized. And what astrophysicists found was that the light of the microwave background of the universe has a small amount of polarization, which tells you something about some order in the early part of the universe that led to the order or directionality of the light that we’re seeing today.
Now this interested me because one of the things that my group has been doing for the last 4 or 5 years is studying what happens when a laser beam hits a surface and ejects some of the material from the surface. And we find that when the material comes off, it gives off light. Because the material is ionized, it produces a plasma and this plasma gives off radiation. So in a sense, it’s like a little Big Bang. The laser hits the surface, the light is focused, and the temperatures on the surface reach millions of degrees. It’s nothing like the temperatures of the Big Bang, which is many orders of magnitude higher. But still, in its own right, it’s like a little Big Bang. And it occurred to me that maybe the light coming off the crystal would have some polarization, which would tell you something about the nature of the “Big Bang” when the laser hit the surface. So I suggested to my students after reading Brian Greene’s book that they look to see if the light coming off is indeed polarized. And I expected we’d see 1 or 2 percent, and to our astonishment, we saw immediately more than 50%, and with a little persistence we found as much as 100%. That’s to say the light coming off was totally polarized.
This is a common experience. You have polaroid sunglasses which can protect your eyes by filtering-out the sunlight because when sunlight reflects off a surface – when any light reflects off a surface – it becomes polarized by the nature of the reflection. And by using a polaroid filter, you can filter out the light by rejecting the polarized light. So that’s a common, everyday experience.
So why is this interesting to us? First of all, it told us something fascinating about the way the laser interacts with the material and that there’s some highly directional process that we still haven’t figured or sorted out, but something directional in the electrons and or ions coming off the surface that is producing a directionality as they oscillate and produce the light. But it’s also a very practical thing because when you look at the light coming off a material it will show a spectrum of lines which corresponds to the elements of the material. So for example, if you’re looking at a dime, you’ll see lines from copper and lines from nickel, which are the main components of the dime, and if you didn’t know what the dime was made of, by looking at the spectrum of the lines produced by the laser-produced plasma, you could figure out from the lines that the material was made of copper and nickel. So this is a practical technique. It’s called laser-induced breakdown spectroscopy, or LIBS. And it’s used for all sorts of things, such as looking at bio- hazardous areas where you don’t want to get too close to the object. You can shine a laser at it and see the light coming out and figure out what it’s made of. Or, you could use it if you wanted to detect a potential explosive, and you’re looking for compounds that are known to be an explosive – nitrogen-containing compounds. You could shine a laser beam at it. If the pulse of light is intense enough and short enough, it won’t detonate the explosion but it will cause a little piece of it to vaporize. And by looking at the spectrum of that piece that’s vaporized, you can figure out what it’s made of and decide if it’s a bomb or not.
Now there’s one problem with that in the practical, everyday use of it, and that is, not only do you get the spectroscopic lines but you also get a large background of light that’s smeared over all wavelengths. And it’s this background of light which is strongly polarized. That’s what we found in our laboratory.
What most people would do in a situation like this is they would use a shutter – just like a shutter on a camera. Because one of the properties of this background is that it comes out very, very quickly, but the line spectrum lasts much longer. So by putting a shutter on your camera, you could open the camera lens after the background has disappeared. The problem is you have to respond in a very, very short period of time – less than a microsecond, which is much shorter than most camera shutters perform. The problem with that is, it’s very expensive. So rather than shuttering-out the background, what we found is we can use a polarizing filter to filter-out the background – since it’s the background that’s 100% polarized, whereas the lines themselves are unpolarized, or very weakly polarized.
For example, we looked at a spectrum of carbon which came off a graphite sample. Without the polarizing filter, the carbon spectrum was so smeared out by the background, you could barely tell there was carbon in it. But as soon as we put the filter in there and aligned the filter to reject the polarized light, the carbon spectrum emerged like from a (film) negative – like a photograph, it just suddenly came out of the noise. So that’s an application I think would be useful.
But then going back to our original interest of understanding the science, there’s a really intriguing question – where does the directionality come from? We have all kinds of ideas, but it’s still exploratory. One idea we think is, the laser beam is causing electrons in the material to oscillate back and forth like an antenna, and when they’re ejected from the material this oscillation persists once it gets into the plasma. And since they’re oscillating in a unique direction that’s determined by the polarization of the laser, the polarization of the laser therefore gets transferred to the polarization of these oscillating electrons and that, in turn, gets transferred to the light emitted from these electrons, which is why the light seems to be polarized. But right now these are speculations which we hope to test in the future.
One thing I neglected to say is that laser we’re using is a femtosecond laser. It’s called an ultra-fast laser – which means the electrons are ejected much, much faster than it takes time for the material to heat up. But recently we discovered that much more conventional lasers – nanosecond lasers – which are more commonly found, also produce a similar effect. That will make the process more practical because these are less expensive and more commonly available. But also probably involves a different physical mechanism because on the nanosecond scale, you do have thermal behavior occurring, so that’s another research direction which we intend to pursue.
[Writer] Robert Gordon is professor and head of chemistry.
For more information about this research, go to www-dot-news-dot- uic-dot-edu (www.news.uic.edu) … click on “news releases.” … and look for the release dated March 10, 2009.
This has been research news from U-I-C – the University of Illinois at Chicago.