Exploring the Mechanism Of Free-Electron Lasers: Part II

On Nov. 18, 1998, Laboratory Network featured an article about tabletop free-electron lasers (FELs; Tunable Laser Source Opens Door To Far Infrared Spectroscopy) that were developed at Dartmouth College (Hanover, NH). Since this article was posted, readers have asked for more information about how FELs work and how they are differentiated from other lasers. The developers of the tabletop FEL responded with an in-depth explanation of the mechanism behind their device.

How do FELs work?
Why are FELs traditionally large?
Why do FELs focus more easily than other lasers?

How do FELs work? (Back to Top)

FELs generate light in the visible, infrared, and far-infrared segments of the spectrum by running a particle beam past a rippled stationary magnetic field (electrons are more commonly used as the particle beams for FELs than protons). The rippled magnetic field makes the path of the electrons wiggle. Because of this, the stationary magnets are called "wigglers." It is this wiggling of the electrons that produces light from an FEL.

Because the electrons in the beam are moving quickly (with high energy), they experience a Doppler shortening effect similar to the change heard in a train's sound as the train nears an observer and then travels away. This effect causes the electron beam to vibrate faster as it travels past the magnets (wigglers), shortening the beam's wavelength.

In FELs, the laser-beam wavelength is determined by the product of the energy used to move the particles and the distance between the ripples in the stationary magnetic field. The challenge in an FEL is to design a wiggler with the correct period so that electrons traveling at a given energy produce light at the desired wavelength.

Why are FELs traditionally large? (Back to Top)

Free-electron lasers (FELs) are traditionally large because they contain particle accelerators. In particle accelerators, charged particles (like protons or electrons) are flung around a ring at high speeds, creating the high-energy stream of charges, or electron beam, used in applications such as FELs. Typical FELs operate with electron beams that range from a few megavolts up to a few hundred megavolts. To create this magnitude of energy, the accelerator must be huge—often occupying a gymnasium or a tunnel. The largest particle accelerator in the world is located at the DOE's Fermilab (Batavia, IL; www.fnal.gov).

High-energy accelerators can produce light in more areas of the spectrum than low-energy accelerators because they have more available energy to influence electron vibrations through the wigglers. Traditionally, FELs combined large wigglers with large, high-energy accelerators to create gymnasium-sized devices. This combination can generate high-electron energies (10s-100s of megavolts) that allowed the instruments to access very short wavelengths in the ultraviolet and long X-ray regions of the spectrum. FELs could also access longer wavelengths in the infra red and visible sections of the spectrum.

Why do FELs focus more easily than other lasers? (Back to Top)

In all lasers, there is an upper energy level and a lower energy level. When a laser operates, electrons travel from the upper level to the lower one and give off radiation in the form of photons. (The levels are fixed in place by the atomic structures surrounding them.) The energy released is proportional to the difference between the levels. This is a function of quantum mechanics.

The frequency of a photon is proportional to energy. Because wavelength is inversely proportional to frequency, the wavelength gets longer when the level separation is decreased. For low frequencies, the separation between energy levels is small. The product of wavelength and frequency is equal to the speed of light—a constant.

In order to study a particular part of the spectrum, a laser must be created that has the desired wavelengths. The challenge, then, is not to focus the laser, but to create useful wavelengths.

There are two ways to change the wavelength produced by a FEL: change the energy and speed of the electrons or make the beam and wiggler very small. To make the Dartmouth SEM-based FEL discussed in the November 18, 1998 Laboratory Network article, the research team followed the latter approach and used a small beam and small metal grating as a wiggler. The resulting low-energy beam (10s of kilovolts) was ideally suited to long-wavelength, far-infrared (between 1-mm and 10-µm) operation.

Because the Dartmouth researchers chose to reduce the size of the electron beam (with a correspondingly smaller rippled structure), they were able to avoid using a large accelerator. The tabletop FEL developed at Dartmouth is useful because it does not interfere with experimental methods, complementing existing IR and far-IR lasers instead of creating competition.

High-energy electron beams required to power traditional FELs are only available to a small number of labs. The Dartmouth work is important because it uses smaller, less-expensive, lower-energy electron-beam systems that most researchers could build or buy.

There are some research lasers that operate today in the far infra red spectrum, but there are currently no commercial vendors for obtaining them. In the past, there have been vendors who sold long-wavelength lasers such as these.

For more information, e-mail John.Swartz@Dartmouth.EDU.