Among the various X-ray optics for synchrotron radiation producing micrometre and sub-micrometre beams with high intensity, the X-ray waveguide (WG) provides the smallest hard X-ray beam (few tens of nanometres) with a gain of up to 100 [1].

Resonances take place at given angles in planar WGs constituted by a three layer structure where the middle guiding layer, has the highest refractive index. The X-ray beam which is produced at the WG end has a spatial distribution with Full Width at Half Maximum (FWHM) equal to one half the guiding layer thickness d, and an angular distribution with FWHM simply given by: ~ /d, where is the X-ray wavelength. Two important features characterise the WG with respect to other optical elements: i) the beam dimension is independent on the X-ray source, but only depends on the thickness of the guiding layer; ii) the beam produced by the WG is essentially coherent because it is the result of an interference effect. A drawback of this optics is that, due to the divergence at the exit, a nanometre-sized spot on sample can only be obtained at a very short distance from the WG exit. Another limitation is that in planar WGs the beam is compressed only in one direction. A WG compressing the beam in two directions has been fabricated and tested [2], but its efficiency is quite low. If, however, the guided beam could be efficiently re-focussed at some distance from the WG end, both drawbacks could be overcome. In this way, the large working distance between the device and the sub-micrometre focus would leave free space for the sample environment (vacuum chamber, furnace, cryostat, magnets, high-pressure device, etc). Also, an improved signal/noise ratio could be achieved. Moreover, the beam from a planar WG could be focussed at the exit of a second WG perpendicular to the first one, thus realising a cross-coupled geometry for efficient compression in two directions. We have demonstrated the feasibility of such an approach with an experiment carried out at BM05.

Figure 155 shows a schematic view of the experimental arrangement. A planar WG constituted by a three-layer stack: C (20 nm) / Cr (130 nm) / C (3 nm), was adjusted in its first resonance mode TE0 under illumination by a monochromatic beam ( ~ 0.1 nm). Suitable slits allowed passage of the guided beam but cut the direct and the reflected beams. A 170 mm long Pt-coated Si mirror, located at a distance LM = 450 mm from the WG exit, was adjusted in order to refocus the guided beam at a distance L_KE = L_M (see Figure 155). An elliptical shape was given to the mirror with a dynamical bender, developed by the ESRF optics group. The deviation from the stigmatic ellipse in term of slope errors, measured with a Long Trace Profiler (LTP), was of the order of 2 µrad RMS. Knife-edge scans were performed optimising the mirror-knife edge distance L_KE for different incidence angles qI and mirror positions Y_M transversal to the incident beam direction.

Fig. 155: Schematic view of the experimental setup.

Figure 156 shows five beam profiles obtained at different L_KE values (closed points) for the _l and Y_M couple which gave the minimum spot size. The best scan in Figure 156 has a FWHM of 0.85 µm. Ray-tracing simulation (solid line in Figure 156) of the experiment was carried out. A good agreement with experiment has been obtained assuming a perfect alignment but a small slope error of 2 µrad. (i.e. the value found by LTP). Note the double peak present in both the theoretical and experimental curves for smaller distances L_KE. No good agreement could be obtained considering only mirror misalignment without slope error. Better results are expected with improved experimental conditions.

Fig. 156: Beam intensity profiles. Closed points: experimental result. Solid line: ray-tracing simulation. LKE is the variation with respect to the focal distance LF.

The experiment shows the possibility of submicrometre beam refocussing at large distance from the end of the optical device (about 365 mm in this case). This could not have been realised on a standard length beamline with other optical elements based on source demagnification, such as mirrors. The arrangement proposed has the potential to allow experiments with nanometre spatial resolution even with bulky equipment and with improved signal/noise ratio. The combination of two WGs' in cross-coupled geometry for efficient beam compression in two directions is also foreseen.

References
[1] W. Jark, A. Cedola, S. Di Fonzo, M. Fiordelisi and S. Lagomarsino, N.V. Kovalenko and V.A. Chernov, Appl. Phys. Lett. 78, 1192-1194 (2001).
[2] F. Pfeiffer, C. David, M. Burghammer, C. Riekel, T. Salditt, Science 297, 230-234 (2002).

Principal Publication and Authors
S. Lagomarsino (a), I. Bukreeva (a), V. Mocella (b), A. Surpi (a), T. Bigault (c) and A. Cedola (a), Nuclear Instruments and Methods A, submitted.
(a) Istituto di Fotonica e Nanotecnologie (IFN) - CNR - Roma (Italy)
(b) Istituto per la Microelettronica e Microsistemi (IMM) ­ CNR - Sezione di Napoli (Italy)
(c) ESRF