Breaking the Barrier Toward Nanometer X-ray Resolution

A team of U.S. researchers overcame a major obstacle for using refractive lenses to focus X-rays. This new technique will allow the effective focusing of X-rays down to extremely small spots, and is a significant breakthrough in the development of a new, world-leading light source facility that has potential for new applications in nanoscience, energy, biology, and materials research.

At U.S. Department of Energy's (DOE) Brookhaven National Laboratory's National Synchrotron Light Source (NSLS; Upton, NY,USA), the scientists exceeded a limit on the ability to focus "hard,” or high-energy, X-rays known as the "critical angle.”

The critical angle is the maximum angle that light can be deflected, or bent, by a single surface. For perspective, one should envisage a beam of laser light traveling toward a glass lens. Depending on the characteristics of the lens material and the angle at which the beam is pointed, the light can be refracted, that is, transmitted through the lens but deflected. However, when this light approaches the lens at angles less than the critical angle, the beam does not pass through the lens but is instead reflected. This results in a maximum deflection angle for light that passes through the lens.

"One measure of the quality of an X-ray optic is how small a focused spot it can make,” said NSLS researcher Dr. Ken Evans-Lutterodt. "The problem is that nature does not allow a single lens to deflect the X-rays very much. This limits how small of a spot you can create, and this translates to some fuzziness in the image. To get a sharper image, you need a lens that's more able to deflect the X-rays.”

In 2003, three Brookhaven researchers--Drs. Evans-Lutterodt, Aaron Stein, and James Ablett--were the first to notice the critical angle limit while evaluating the properties of a so-called kinoform lens for focusing hard X-rays. The research team proposed a solution to the critical angle problem of a compound kinoform lens, and both the problem and proposed solution were also suggested later by other researchers in the field.

In the current study, the researchers implemented their theory by creating a compound lens from a series of four kinoform lenses placed one after the other. Using this setup at NSLS beamline X13B, they demonstrated that the critical angle can be exceeded with hard X-rays, while still focusing like a single lens.

This is an important step for the National Synchrotron Light Source II (NSLS-II), a state-of-the-art synchrotron facility that will produce X-rays up to 10,000 times brighter than those generated by the current NSLS and could lead to advances such as alternative-energy technologies and new drugs for fighting disease. One of the key goals of the facility is to probe materials and molecules with just 1-nm resolution--a capability needed to study the intricate mechanisms of chemical and biological systems.

Next, the researchers will measure the resolution their new lens system produces, and will continue to fabricate and test optics that push further beyond the critical angle, and closer to the 1-nm benchmark.

The study's findings were described online in the September 28, 2007, issue of the journal Physical Review Letters.


Related Links:
Brookhaven Laboratory's National Synchrotron Light Source