Oct. 3, 2003 A team of researchers at the University of Colorado at Boulder has taken another step in the quest to build a compact, tabletop x-ray microscope that could be used for biological imaging at super-high resolution.
By firing a femtosecond laser - a laser that generates light pulses with durations as short as 100 trillionth of a second - through a gas-filled tube called a waveguide, they were able to create more efficient "laser-like" beams in regions of the spectrum that were previously inaccessible.
The wavelength region over which they generate this "soft" x-ray light efficiently is called the "water-window" region, an important region for biological imaging, according to physics Professor Margaret Murnane. She also is a fellow of JILA, a joint institute of CU-Boulder and the National Institute of Standards and Technology.
The water window is an area in the spectrum where water is less absorbing than carbon, which means carbon absorbs more light and thus makes it easier to take images, according to Murnane. Current technology allows researchers to do work in this region, but requires a large-scale and expensive facility.
"With further work, this advance will make it possible to build a compact microscope for biological imaging that fits on a desktop," Murnane said. "Such microscopes could visualize processes happening within living cells, or perhaps even allow scientists to understand how pharmaceuticals function in detail."
A paper on the subject by graduate student Emily Gibson, physics Professor Henry Kapteyn, Murnane, Ariel Paul, Nick Wagner, Ra'anan Tobey, David Gaudiosi and Sterling Backus of the CU-Boulder department of physics and JILA appears in the Oct. 3 issue of the journal Science. Ivan Christov of Sofia University in Bulgaria, Andy Aquila and Eric Gullikson of the Lawrence Berkeley National Laboratory and David Attwood of the University of California at Berkeley and the Lawrence Berkeley National Laboratory also participated in the work.
"We were able to generate more efficient light in the water-window than in the past," said Emily Gibson, the lead author of the paper. "People have been able to generate small amounts of light in the water window with a laser, but our approach using fibers generates the light more efficiently, allowing you to have enough light to do useful things like take images of cells."
To create the "soft" x-ray beams, the research team led by Kapteyn and Murnane fired a laser through a gas-filled hollow tube called a waveguide. The intense laser light literally rips the atoms of the gas apart, creating both ions and electrons, according to Murnane. The laser beam then accelerates the electrons to very high energies and slams them back into the ions, creating "soft" x-ray light in the process, she said.
Unfortunately, some of the waves can be out of phase, canceling each other out and weakening the strength and coherence of the output beam, she said. However, by modulating the diameter of the guide, Murnane said they can arrange for the laser light and "soft" x-ray light to travel at the same speed along the same path, increasing the efficiency of the process.
As a result, a well-synchronized stream of photons fires out of the system, boosted up to a high-energy, "soft" x-ray wavelength. Many of the most important technologies of the 20th century, such as the Internet and MRI imaging, emerged from the use of electromagnetic radiation ranging from radio waves to the visible region of the spectrum, she said. In recent years fiber optics and photonics have revolutionized communications and created a new global society via the Internet.
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