Smallest ever nanobeam for high-energy X-ray imaging

22-04-2017

Nanoscale beams are essential for the direct imaging of nanoscale objects. However, producing suitable nanoscale X-ray beams is a significant engineering challenge. With a new multilayer mirror system, beamline ID16A now provides routine imaging on the nanoscale with a sub 13 nm high-energy X-ray beam. This is a world first.

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Samples from diverse fields such as life sciences, materials and environment sciences contain structural details of interest at length scales down to the nanometre. Several existing X-ray techniques can be used to image such a nanostructure and even map elemental composition, provided that the beam can be focused to a nanometre focal spot. The team of ESRF scientists and engineers working at beamline ID16A have now put into operation a high-energy beam with a sub-13 nm beam spot size, which is a world first at energies above 20 keV. Such a tiny and intense beam (6 billion photons per second) permits imaging of thick and highly absorbing samples with unprecedented spatial resolution, in addition to extending the range of detectable elements for X-ray fluorescence imaging both at the 2D and 3D level.

Peter Cloetens, scientist in charge of beamline ID16A, is very enthusiastic about this achievement, “These nanofocusing results, close to 10 nm, at very high energy and with a very high flux, are really unique worldwide. This will create new opportunities in the fields of materials and life sciences”.

ID16A is a new 185-m-long nano-imaging beamline at the ESRF used to study nanotechnological materials, environmental and biological samples. Techniques available include phase-contrast imaging, X-ray fluorescence imaging, and ptychography, all being combined with tomography to obtain tridimensional imaging. The beamline already has precisely controlled temperature and vibrational stability. Improvement to the X-ray focussing optics with a new shaped Kirkpatrick-Baez mirror system with a multilayer surface coating was the key to reducing its focal spot size.

Kirkpatrick-Baez (K-B) mirror systems are used to focus the beam of many ESRF beamlines. A K-B system consists of two mirrors, one for horizontal and one for vertical focussing. ID16A’s K-B mirrors required the highest possible level of perfection for the surface of these two small mirrors, only 36 and 70 mm in length. The elliptical cylinder figures of the mirror surfaces were prepared (JTEC Corporation, Japan) using deterministic figuring/polishing techniques based on technologies developed at Osaka University. The mirrors were coated at the ESRF’s Multilayer Laboratory with 120 bilayers of W/B4C, with a spacing of 2 nm. Installed permanently in the optics hutch of the beamline, the mirrors are supported by an in-house designed optomechanical assembly.

Christian Morawe head of the ESRF Multilayer Laboratory described the multilayer coating: “Multilayers increase the available optical aperture in order to reduce the diffraction limit of the focusing mirror while maintaining a high photon flux. The main difficulty was to coat the 36 mm and 70 mm substrates with a steep thickness gradient while using a 7-m long deposition machine.”

The focal spot was characterised firstly by conventional methods, which overestimated the beam size and were inefficient for such penetrating radiation, and secondly by a method based on high-energy X-ray ptychography. Thus, the defocused beam was retrieved by ptychography and subsequently propagated to the focal position using mathematical methods for the proper beam size estimate. Julio da Silva, scientist at ID16A beamline and leading author of the referenced publication commented, “This corresponded to the ptychography experiment with the highest X-ray energy ever performed. In the future, the augmented coherence of the X-ray beams from the ESRF-EBS will further improve the performance of such a technique at high energies.”

X-ray beam at ID16A at the focus position.

X-ray beam at ID16A at the focus position. The FWHM values were calculated as 12.0 nm for the horizontal focus and 12.6 nm for the vertical focus, with a measured flux of 6 × 109 photons/second.

Publication
Efficient concentration of high-energy X-rays for diffraction-limited imaging resolution, J.C. Da Silva, A. Pacureanu, Y. Yang, S. Bohic, C. Morawe, R. Barrett, and P. Cloetens,  Optica 4(5), pp. 492-495 (2017);  DOI: 10.1364/optica.4.000492

 

Text by Gary Admans

Top image: The sample environment of beamline ID16A. Credit: ESRF/F. Villar.