Sharpened focusing of hard x rays

Because of their high energy and weak interaction with matter, hard x rays (λ < 0.1 nm) are difficult to focus, whether by reflection, refraction, or diffraction. Recently, scientists at Argonne National Laboratory fabricated what they call a multilayer Laue lens (MLL)—alternating layers of metal and silicon deposited with nanometer precision in progressively thicker layers, then cut in cross section like a slice of layer cake. When illuminated nearly edge-on, with x rays skimming the interfaces, the layers form a sequence of Fresnel diffraction zones. The MLL succeeds both because of the precise zone tolerances and because the depth of the zones can be made microns long by merely cutting a larger section of the multilayer wafer. In tests, an incomplete MLL was tilted less than 1 mrad away from 0.064-nm (20-keV) x rays coming out of Argonne’s Advanced Photon Source, and focused the x rays to a line only 30 nm wide, better than previously possible. The researchers say that an ideal MLL should be able to focus hard x rays to a spot of 1 nm or less, which would make it useful for microscopes and microprobes, among other things. (H. C. Kang et al., Phys. Rev. Lett. 96 , 127401, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.127401 .)