Bragg Fresnel lenses (BFLs) have been used extensively at third-generation synchrotron sources for hard X-ray microscopy and microdiffraction applications [1,2]. We have developed a new 2D focusing X-ray lens by combining a linear thin-film Fresnel lens and a multilayer mirror. In the multilayer Bragg-Fresnel lens (MBFL), the Fresnel zones focus the X-ray beam vertically by diffraction, whereas the multilayer mirror is bent to focus X-rays horizontally. This unique combination eliminates the need for a second mirror in conventional Kirkpatrick-Baez mirror-based microprobes.

The structure of a multilayer BFL is illustrated in Figure 122. The linear Fresnel zones are made of a thin Au layer deposited on a multilayer substrate after patterning by electron beam lithography. (An optical micrograph of the Au Fresnel zones is shown in the inset of Figure 122). For the prototype lenses that we fabricated, the multilayer consists of 200 layers of Mo/Si on a float glass substrate. The multilayer was fabricated by Xenocs, Grenoble. The layer spacing increases over the length of the substrate away from the source with a 4% gradient to maintain a large acceptance angle. The thickness of the Au layer is chosen such that the radiation going through the metallised zones are phase-shifted by to ensure high focusing efficiency. The thickness, which depends on the layer spacing of the multilayer, is about 12 nm for the test lenses. The length of the Fresnel zones is 20 mm, with an outermost zone width of 200 nm. An array of six lenses with different focal lengths was made on a single 15 mm x 60 mm substrate.

Fig. 122: Illustration of a multilayer Bragg-Fresnel lens made of Au Fresnel zones deposited on a multilayer substrate, which is bent to produce horizontal focusing; (inset) an optical micrograph of the Au Fresnel zones on the prototype lens.

X-ray characterisation of the prototype multilayer BFLs was conducted at the optics beamline BM5. The focusing properties of the lenses were measured using 10 keV and 12.4 keV radiation. Figure 123 shows the measured horizontal and vertical intensity profiles of the beam at the focal plane of the MBFL. The focus size is approximately 2 µm (vertical) x 12 µm (horizontal) from an incident beam size of 250 µm (V) x 200 µm (H). The vertical focus is close to the demagnified source size, but the horizontal focus was broader than expected, which we attribute to geometric figure errors caused by the simple mirror bender used in the experiment. Submicrometre spot sizes should be achievable with improvement in the bender and by choosing a larger demagnification ratio.

Fig. 123: Horizontal (left) and vertical (right) intensity profiles across the focused beam. The horizontal focus was measured with a 5 µm aperture whereas the vertical focus profile was derived from a knife-edge scan.

In conclusion we have demonstrated an X-ray focusing optic that combines diffraction and geometric focusing in one single device. The multilayer Bragg Fresnel lens requires only one binder to produce two-dimensional focusing and will be useful in a wide range of applications.

References
[1] Y. Hartman, AK. Freund, I. Snigireva, A. Souvorov, A. Snigirev, Nucl. Instr. and Met. Phys. Res. A, 385, 371-375, (1997).
[2] Y. Li, G.C.L. Wong, C.R. Safinya, E. Caine, E.L. Hu, D. Haeffner, P. Fernandez, and W. Yun, Rev. Sci. Instrum. 69, 2844-2848 (1998).

Authors
Y. Li (a), M. Yasa (a), C.R. Safinya (a), J. Als-Nielsen (b), C. Mammen (b), A. Freund (c), J. Hoszowska (c), C. Mocuta (c).
(a) University of California, Santa Barbara (USA)
(b) Niels Bohr Institute, Copenhagen (Denmark)
(c) SRF