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The ESRF has the X factor

20-06-2011

Demonstrated at the ESRF 15 years ago, refractive lenses are now operating at seven synchrotrons in five countries.

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Rainbows are the products of refraction, when the many wavelengths of sunlight are deflected at slightly different angles as a ray enters and exits a water droplet. A pair of refractive lenses positioned in front of the eyes corrects for impaired vision, and the refraction of radio waves by the upper atmosphere allows a short-wave radio transmission in Brazil to be picked up in China. This desirable property of electromagnetic radiation is virtually non-existent for X-rays, though. X-rays barely undergo reflection either, unless at very small angles. It’s no wonder that Röentgen used an “X” to name the rays that he had stumbled across.

A century later, in 1996, X-ray refraction was demonstrated at the ESRF by Antatoly and Irina Snigirev, Victor Kohn of the Kurchatov Institute in Russia and Bruno Lengeler of the Institute of Physics at RWTH Aachen University. Refractive lenses for X-rays work like glass lenses for visible light, but have much larger curvatures and are made from low-Z materials, such as beryllium, carbon, aluminium and silicon to keep absorption to a minimum. Because the deflection of hard X-rays is very small, tens or hundreds of lenses have to be placed in series to obtain useful focal lengths, which can be adjusted by simply adding or removing individual lenses. Such a compound-tunable X-ray lens was installed at the ESRF’s ID11 beamline in 2009.

 

Focal point

The original ESRF refractive lens in 1996 comprised 30 closely spaced, 0.6 mm diameter holes drilled into an aluminium block and was able to focus a 14 keV X-ray beam to a spot size of 8 microns. The following year, high heat load beryllium and aluminium lenses were installed at five beamlines to focus and collimate beams. Today, refractive optics are in place on half of the ESRF’s beamlines and are in wide use at other synchrotrons, providing focal points ranging from a few millimetres to tens of metres and spot sizes ranging from tens of nanometres to tens of microns.

Without refractive lenses, the diffraction, fluorescence and imaging capabilities of the ESRF would not be as impressive as they are. More traditional diffractive optics, such as aperiodic gratings called Fresnel zone plates, are well developed for soft X-rays. However, adapting them for more energetic beams is difficult. “Refractive and diffractive optics are competing in microbeam focusing and imaging,” says Anatoly Snigirev. “But Fresnel zone plates are limited to energies of 15 keV, whereas refractive lenses have an energy range from a few keV to hundreds of keV.”

Since 2000 it has been possible to make composite refractive lenses from silicon. Microelectronics-fabrication technology allows deep vertical structures to be etched perfectly in a smooth surface, and is now being transferred to materials such as diamond that have low X-ray absorption and low thermal expansion. “Diamond lenses are a dream and the future, but still require some technology development,” says Snigirev.

In 2009, Snigirev and co-workers from France and Russia proposed a new type of hard X-ray interferometer based on two parallel arrays of compound refractive lenses etched in silicon, allowing new ways to study advanced nanoscale materials, such as photonic crystals (Snigirev et al. 2009). Faults in self-assembled colloidal crystals that can seriously affect the optical properties of photonic crystals have already been revealed by the microfocusing capabilities of compound refractive lenses (Hilhorst et al. 2009), and more recently the Snigirevs, in collaboration with researchers at Moscow State University, demonstrated a high-resolution transmission X-ray microsope based on parabolic refractive lenses that is ideal for studying periodic mesoscopic structures (Bosak et al. 2010).

This year, parabolic refractive aluminium lenses were installed close to the source inside the ESRF storage ring to measure the vertical emittance, which is now routinely of the order of 3–4 pm and therefore approaching the diffraction limit of conventional optics. “The imaging resolution of refractive lenses is in theory better than that of pinhole cameras, and having two different emittance diagnostics systems allows a cross-check of the results,” says Friederike Ewald of the Accelerator and Source Division.


Bruno Lengeler says that most textbooks in optics claim even now that there are no refractive lenses for X-rays. “The development in the past 15 years has clearly shown that this statement is far too pessimistic.”


Matthew Chalmers

 

References
A. Bosak et al., Adv. Mater. 22, 3256 (2010).
J. Hilhorst et al., Langmuir 25, 10408 (2009).
A. Snigirev et al., Phys. Rev. Lett. 103, 064801 (2009).

 

This article appeared in ESRFnews, June 2011. 

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Top image: Rare refraction: unlike visible light, X-rays have a refractive index close to and slightly below unity, meaning that a large number of concave lenses is needed to focus an X-ray beam.