Sharper Focusing of Hard X-Rays

Sharper focusing of hard X-rays has been achieved with a device developed at Argonne National Lab.

Because of their high energy, X-rays are hard to focus: they can be reflected from a surface but only at a glancing angle (less than a tenth of a degree); they can be refracted but the index of refraction is very close to 1, so that making efficient lenses becomes a problem; and they can be diffracted, but the thick, variable pitch grating required for focusing is tricky to achieve.

The Argonne device is of the diffraction type, and it consists of a stack of alternating layers of metal and silicon, made by depositing progressively thicker layers (see figure at Physics News Graphics). When the X-rays fall on such a structure, nearly edge-on, what they see is a grating pattern (called a linear zone plate) consisting of a sort of bar-code pattern.

The Argonne device succeeds so well in focusing X-rays because the position of the zones can be controlled to within nanometer tolerances through the deposition process, and the depth of the zones that the X-rays experience can be made arbitrarily long -- microns long -- by merely cutting a thicker section of the multilayer wafer. In tests so far, one of these zone plates, very slightly tilted to the X-rays coming out of a synchrotron source, has focused 20-kiloelectronvolt X-rays to a line only 30 nanometer wide, better than previously possible.

According to Argonne researcher Brian Stephenson (stephenson@anl.gov, 630-252-3214), an ideal version of this kind of X-ray lens, which they call a Multilayer Laue Lens (MLL), should be able to focus X-rays to a spot of 1 nanometer or less. The likely uses for a better X-ray lens are in full-field microscopy (making a magnified X-ray image of a sample) or in scanning probe microscopy (by scanning the beam across a sample).

Kang et al., Physical Review Letters, 31 March 2006
Contact Brian Stephenson, stephenson@anl.gov, 630-252-3214
Image at Physics News Graphics