Next-Generation Light Source Development Focused on Deep Ultraviolet Free-Electron Laser
LASER VERSUS SYNCHROTRON
What do bar-code scanners, CD and DVD players, dental drills, fiber optic telecommunications, metal cutting and welding, cornea surgery, and tattoo removal have in common? They all use lasers.
Unlike light coming from a more common source such as a light bulb, laser light is intense and has special properties. It is highly monochromatic, meaning that light produced by a laser essentially has one wavelength, or color; it is collimated, meaning that laser light is concentrated in a narrow beam; and it is coherent, meaning the emitted photons are in step, or in phase, with each other in time and space.
Since the 1958 discovery of the process called light amplification by the stimulated emission of radiation, which was first postulated by Albert Einstein, lasers have made possible the development and commerce in billions of dollars of consumer goods, medical devices, industrial equipment — and scientific tools. Because of an inherent limitation, however, lasers cannot produce ultra-short wavelength light below 100-200 nm.
Very intense ultraviolet light is produced by one of the two synchrotrons at the NSLS, as well by other such accelerator-based light sources. Within a synchrotron, x-ray, ultraviolet, and infrared light is emitted as electrons are raced in a circular orbit to near the speed of light. In addition to intensity and its broad spectrum, synchrotron light has many special features: it is collimated, polarized, and pulsed; and it has a broad spectrum, so synchrotron light can be tuned to a particular wavelength.
An FEL such as Brookhaven’s is combining the intensity and coherence of laser light with the broad spectrum of synchrotron light. But, as Yu explains, the intensity of DUV-FEL light surpasses that of a synchrotron because of coherence.
HOW COHERENCE WORKS
“Using HGHG as the basis for Brookhaven’s FEL research and development
is exciting not only because of its immediate usefulness in a fourth
generation source of intense, highly coherent deep ultraviolet light, but
also because of its potential to be extended to much shorter wavelengths,
including hard x-rays.”
- Li Hua Yu
In a synchrotron or an FEL, electrons accelerated to near the speed of light are sent through devices called wigglers, which force the electrons to oscillate. The more the electrons are sent back and forth by a series of magnetic fields with alternating directions, the more intense the light that is generated.
In a synchrotron, this very intense light is emitted by electrons randomly, or incoherently, as a jumble of waves. In the DUV-FEL, however, intense light is produced by all the electrons coherently, or in the same time phase. The reason that light from the DUV-FEL is up to 10 million times more intense is its coherence. “Coherence is as if, instead of having a group of people sing the same song at different times, they sing it in unison,” says Timur Shaftan, an accelerator physicist also working on the DUV-FEL.
The electrons within an FEL can “sing in unison” in one of two ways: either by HGHG or by what is called self-amplified spontaneous emission (SASE). Through SASE, electrons interact with light emitted by their fellow electrons, creating small groups of electrons. “Within each group, the electrons sing in unison, but the songs between any two groups are out of sync,” explains Adnan Doyuran, a post-doctoral student on the DUV-FEL project.
Through the HGHG process, the electrons interact with light produced by a laser. This interaction also produces groups of electrons, but, “This time, not only is each group a united ‘choir,’ but all groups also ‘sing’ together,” adds Henrik Loos, another DUV-FEL post-doc. It is this “super choir” of electrons that emits light many times more intense than that generated by the SASE process, which is why the DUV-FEL project members chose HGHG over SASE.