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1 July 2011

Magnets + Lasers: An enlightening research duo

How do lasers help shine a light on Mag Lab research? Read and see for yourself! 

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View inside cell 3The view inside Cell 3, one of the protected rooms where high-powered lasers are operated.

Lasers are used in many eminently practical everyday applications —  from printers and surgeries to  stonecutting and police speed traps… we could go on for a really, really long time. But lasers are also the stuff of alien weapons in sci-fi movies, narrowly avoided James Bond deaths, and a host of other nebulous, cinematic weirdnesses that can cloud their common, practical uses.

At the MagLab, we use lasers for some pretty out-there experiments, with the end goal of learning more about the world (and hopefully, some new ways to improve components of those practical things listed above, too). But how is light (because that's all lasers are, after all) a tool for scientific discovery? Read on and become enlightened... 

Why bother with lasers?

Data, that's why. Most of the lab's scientists study solid matter — its potential, its limitations, and how it behaves in extreme environments. Optics spectroscopy is a set of techniques that provide interesting information for physicists. The lab conducts magnetic field research because magnetic fields interact with matter inside atoms' charges, and charged particles inside that matter interact with the magnetic field in kind. Put another way, we use a magnetic field to push on an interesting bit of matter, the matter pushes back, we measure that interaction, and presto: science!

What do lasers have to do with studying matter?

Lasers emit photons (photons = particles of light). Light itself also interacts with charged particles, so when researchers add it to an experiment, they can figure out more about what's happening using information gleaned from the particles of light. Because light is such a powerful force, understanding these interactions means understanding what's going on inside a material at the most basic level.

How do you get a laser beam inside a magnet (and back out)?

L.A.S.E.R.

The term laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation.

Great question! A typical research magnet looks like a giant steel-clad cannister, and it's designed for maximum magnetic field, not maximum access. Experimental samples, as well as the experiment's measurement equipment, are all crammed down into the bore — a central opening in the top of the magnet body that's typically smaller than a ping-pong ball.

With the top of a working magnet stuffed full of measurement equipment and the experimental sample, how do you get a laser in there too? You go in through the bottom. After you've shot your laser beam in there, you've got to retrieve, collect and analyze the light you've used, because the way the light changes is the source of your data. Crazy, right?

To get your light back, there are two things you can typically do, says Steve McGill, a lab scientist whose research often contains an optics component.

"The first is to collect the light reflected from a sample. With reflection, you have to get the light in, and you have to get the light back out. If the sample isn't perfectly flat and level, the light will still come in and hit it fine, but when it comes back, it will do so at an angle and bounce off the side of the tube. If it's even a little bit slanted like that, you have to pull your sample out and reconfigure everything."

The other option is to insert fiber optic cable, which carries light into the magnet, and then another piece to get your data out. The fiber works as a conduit for light, so you can bend it and move it around corners and you don't have to worry about the careful positioning. Fiber optic cable has several significant limitations — most importantly, it's incapable of carrying some of the data scientists are looking for.

What does data from lasers look like?

THE FIRST LASER

Theodore Maiman built the world's first laser in 1960 using a small ruby rod to amplify the light particles and focus them into a beam.

As explained above, light is changed by its interaction with the sample in the magnetic field. A scientist examines the properties of the light when it comes back, and how it's changed tells you something about the sample. It's like reading a mystery story backwards — you know what's happened, but a researcher must figure out why and how.

There's been a long history of people studying how light interacts with samples, and scientists have come up with different kinds of experiments that make sense of those changes. There are almost as many types of experiments as there are types of scientists, but we'll focus on a few common things they measure.

"One of the things I like to do is enumerate all the different aspects of light usable in this kind of experiment — you build out your experiment based on this menu of different properties," McGill explains.

Light is part of the electromagnetic spectrum —  a vast array of energy that travels in different wavelengths. Radio and TV signals, microwaves, X-rays, gamma rays and visible light all have their own widely varying signature wavelengths. Radio waves, for example, are a kilometer long, while microwaves are only a couple of millimeters.

When he's planning an experiment, McGill selects a laser of a certain frequency — this means how often a wave of light repeats. Light's photons are packets of energy, and that energy is proportional to the frequency of the light. The higher the frequency of the light, the more energy it has.

Polarization is the term for the orientation of a laser's wavelength — whether our wave is oscillating horizontally, vertically or, circularly (which can in turn be clockwise or counterclockwise!). Polarization's interaction with a magnetic field makes for some interesting, highly varied data, and this is the meat of scientists' optical investigations.

These kinds of experiments can show McGill and others how much a sample has been magnetized, among other interesting changes. To give a couple of examples in many experimental options, if a sample's opaque, the light that hits the sample and bounces back can be measured for magneto-optical Kerr effect — the effect being the change to that light. If the sample is translucent or transparent, you can measure the Faraday effect instead. While it's the same measure of how the light has changed, you're just doing it with the light that passed through the sample.

How do you get rid of a laser beam when you're done with it?

SAFETY FIRST

When working with lasers, scientists wear special goggles designed to block out the harmful light.

You can't just put high-energy beams of light in the recycling bin at the end of the day, so a few kinds of ingeniously simple devices have been invented to dispose of already-used laser beams properly. McGill's laser beams are small, so they're sent into a small box called a beam stop or a "black hole." The interior of the box is set at many different angles that bounce the light around and don't let it escape, so the beam is lost.

Another common device used to "throw away" laser beams is a box of razor blades, stacked blade side out. The thin profile of the blades split the beam up into smaller parts that are scattered between the blades and do not emerge.

Are the lab's lasers visible to the naked eye?

In movies, lasers look like colored beams of light rocketing through the air, burning holes in metal walls and deserving villains alike. But that is precisely the kind of thing we try to avoid at the MagLab. Our lasers are powerful and potentially blinding, so they're operated in a tightly-controlled, safety-conscious environment.

"We have lasers that are continuous, unbroken beams, and we have pulsed lasers, which are chopped up bits of interrupted light. With pulsed light, you're dumping all the energy into those little chops, and even a small amount of that very high energy can hurt you irreparably. You don't want to make a mistake with those," says McGill.

Though the beams actually are sometimes red, green or blue like in the movies, most visitors and non-optics lab employees will never see one. Even scientists don't see the laser beams very often if everything's going as it should.

"Laser beams look cool," McGill explains, "but photons interact with dust, smoke and other particles in the air to produce visible light, and in an experimental environment, that kind of stuff would be ruining your data."

Depending on the type of experiment he's running, McGill does see the light when it hits the small window or lens it may pass through or bounce from.

What's next for optics research?

MAGLAB LASERS

The MagLab has 15 lasers at its Tallahassee facility.

The lab's Split Florida Helix magnet has changed the way scientists conduct optics research by offering four port s on the side of the magnet's body — at the very places where high magnetic fields are normally trying to rip the magnet apart. This feat of engineering allows a more pure experiment for McGill and his colleagues, allowing direct access to the high magnetic field without the fuss of bouncing a light sample into the magnet's bore or the limitations of fiber optic cable.

An x-ray diffractometer is also being built at the MagLab to help scientists understand even more about the three-dimensional structure of crystalline materials as they change in high magnetic fields in real time. 

With new funding from the National Science Foundation, the MagLab will partner with the Cornell High Energy Synchrotron Source (CHESS) to open the High Magnetic Field (HMF) Beamline. This new facility will offer researchers a space for precision X-ray studies in high magnetic fields up to 20 tesla, greater than what is available at any other synchtrotron. 


This story was originally published in Issue 7 of flux magazine, a discontinued publication of the National High Magnetic Field Laboratory.