Nanometer Linear Focusing of Hard X Rays by a Multilayer Laue Lens

We report on a type of linear zone plate for nanometer-scale focusing of hard x rays, a multilayer Laue lens (MLL), produced by sectioning a multilayer and illuminating it in Laue diffraction geometry. Because of its large optical depth, a MLL spans the diffraction regimes applicable to a thin Fresnel zone plate and a crystal. Coupled wave theory calculations indicate that focusing to 5 nm or smaller with high efficiency should be possible. Partial MLL structures with outermost zone widths as small as 10 nm have been fabricated and tested with 19.5 keV synchrotron radiation. Focal sizes as small as 30 nm with efficiencies up to 44% are measured.

Figure 1

Schematics of (a) flat MLL (all layers aligned to axis), (b) ideal MLL (each layer at its Bragg angle), and (c) tilted MLL (each half tilted to an average Bragg angle).

Figure 2

Calculated local 1st order diffraction efficiency of ideal, flat, and tilted MLL vs position $r_n$ for outermost zone widths $\Delta r_{\min }$ of 2 nm (blue) and 10 nm (red). The optical depth is $13.5\mu \mathrm{m}$, chosen to maximize the integrated efficiency for the ideal case with $\Delta r_{\min }=10\mathrm{nm}$.

Figure 3

Calculated integrated 1st order diffraction efficiency and spatial resolution for ideal, flat, and tilted MLLs vs outermost zone width $\Delta r_{\min }$.

Figure 4

(a) Measured focal line profiles for samples A, B, and C. The solid lines are Gaussian fits. (b) Far-field diffraction profile of sample C. The $Q_z$ value corresponding to the outermost zone ($r_n=15\mu \mathrm{m}$) is indicated by an arrow. Inset: transmission vs sample position at $\theta ={\theta }_B$ and $\theta \gg {\theta }_B$.