Focusing Better with X-Rays

Hank Hogan

Designing for fine-focusing helps when building telescopes and microscopes. However, the refractive optics used in x-ray versions of such instruments typically have had small numerical apertures, reducing performance. Compound kinoform lenses — refractive lenses with a stair-step shape that gets around numerical aperture restricting absorption — have been fabricated and tested, but these still could be limited by the critical angle where total reflection occurs.

A scanning electron microscope image shows an x-ray-focusing kinoform lens. Etched in silicon, these lenses focus x-rays entering from the lower left. On top is a single kinoform lens, a lens with a stair step related to the Fresnel lighthouse lens. Below is a compound kinoform lens, with two of four stages visible. Such structures could be used in microscopes for x-ray refractive focusing optics once fabrication issues are resolved. Reprinted with permission of Physical Review Letters.

Now investigators from Brookhaven National Laboratory in Upton, N.Y., and from Alcatel-Lucent in Murray Hill, N.J., have devised a possible solution, demonstrating that a compound lens made up of individual kinoform lenses exceeds the critical angle.

“As far as I know for hard x-ray refractive optics, this has not been done before,” said Kenneth Evans-Lutterodt of Brookhaven National Laboratory. He noted that, in achieving this result, the group built upon earlier work done at Brookhaven and elsewhere using synchrotron light sources.

Exceeding the critical limit is important because a higher numerical aperture leads to a shorter focal length and, typically, to better resolution.

The researchers built their compound lenses with silicon, using reactive ion etching and other microfabrication techniques. They carved shapes in the silicon to a final depth of about 80 μm and arranged a set of four lenses in series, so that the second lens took the output of the first and so on. The focal lengths of the lenses decreased with every stage, with the first having a focal length of 0.1 m, and the last, 0.25 mm.

They assessed the optical properties of the construct by sending an x-ray beam through the lenses and onto a copper target, which fluoresced when struck by the x-rays. The researchers measured what happened as they scanned the beam across the lens. From these results they determined that they had exceeded the critical angle.

Results were not as good as what they had predicted, however, and they attributed the discrepancy to imperfections in the lens construction. They are studying the process with the hope of improving sidewall smoothness and lens performance. However, finding the right recipe may take some time and effort, Evans-Lutterodt noted. “Reactive ion etching has a lot of parameters to play with.”

The goal, he added, is to push the optics to yield as high spatial resolution and efficiency as possible.

Physical Review Letters, Vol. 99, 134801, 2007.