Mar 2 2017
Profile of the focused X-ray beam, without (top) and with (bottom) the corrective lens. Credit: Frank Seiboth, DESY
Special X-ray glasses have been custom developed by an international team of scientists in order to concentrate the beam of an X-ray laser stronger than ever before.
The inevitable flaws of an X-ray optics stack was almost completely eliminated by the separately developed corrective lens, which also concentrates three quarters of the X-ray beam to a particular spot with 250 nm (millionths of a millimeter) diameter, closely arriving at the theoretical limit.
The concentrated X-ray beam opens up totally new research avenues and also enhances the quality of specific measurements, according to the report presented in Nature Communications by the team surrounding DESY chief scientist Christian Schroer.
It is difficult to deflect or focus X-rays even though they follow the same optical laws as visible light.
Only a few materials are available for making suitable X-ray lenses and mirrors. Also, since the wavelength of X-rays is very much smaller than that of visible light, manufacturing X-ray lenses of this type calls for a far higher degree of precision than is required in the realm of optical wavelengths – even the slightest defect in the shape of the lens can have a detrimental effect.
Andreas Schropp, DESY
Schropp highlights that an extremely high level of precision has already been reached by the production of ideal mirrors and lenses, however the conventional lenses, produced from the element beryllium, are slightly too strongly curved near the center in general.
“Beryllium lenses are compression-moulded using precision dies. Shape errors of the order of a few hundred nanometres are practically inevitable in the process.”
As a result of this an increased amount of light is scattered out of the focus than unavoidable because of the laws of physics. Additionally, this light is quite evenly distributed over a rather bigger area.
Defects like these are considered to be irrelevant in several applications.
However, if you want to heat up small samples using the X-ray laser, you want the radiation to be focussed on an area as small as possible. The same is true in certain imaging techniques, where you want to obtain an image of tiny samples with as much details as possible.
Andreas Schropp, DESY
The defects were first meticulously measured by the scientists in their portable beryllium X-ray lens stack in order to optimize the focusing. The data obtained was then used by the scientists to machine a tailored corrective lens out of quartz glass by using a precision laser at the University of Jena. This was followed by using the LCLS X-ray laser to test the effect of these glasses at SLAC National Accelerator Laboratory in the U.S.
Without the corrective glasses, our lens focused about 75 per cent of the X-ray light onto an area with a diameter of about 1600 nanometers. That is about ten times as large as theoretically achievable. When the glasses were used, 75 per cent of the X-rays could be focused into an area of about 250 nanometers in diameter, bringing it close to the theoretical optimum.
Frank Seiboth, Technical University of Dresden
Almost three times as much X-ray light was focused into the central speckle than without it by using the corrective lens. In contrast, not much change took place in the full width at half maximum (FWHM), which is the generic scientific measure of focus sharpness in optics, and the FWHM remained at almost 150 nm, without or with the glasses.
The team at DESY’s synchrotron X-ray source PETRA III and the British Diamond Light Source analyzed the same combination of tailor-made glasses and mobile standard optics. The corrective lens resulted in a comparable advancement to that observed at the X-ray laser in both cases.
“In principle, our method allows an individual corrective lens to be made for every X-ray optics,” explains lead scientist Schroer, who is also a professor of physics at the University of Hamburg.
“These so-called phase plates can not only benefit existing X-ray sources, but in particular they could become a key component of next-generation X-ray lasers and synchrotron light sources,” emphasizes Schroer. “Focusing X-rays to the theoretical limits is not only a prerequisite for a substantial improvement in a range of different experimental techniques; it can also pave the way for completely new methods of investigation.
Examples include the non-linear scattering of particles of light by particles of matter, or creating particles of matter from the interaction of two particles of light. For these methods, the X-rays need to be concentrated in a tiny space which means efficient focusing is essential.”
The Technical University of Dresden, the Universities of Jena and Hamburg, KTH Royal Institute of Technology in Stockholm, Diamond Light Source, SLAC National Accelerator Laboratory and DESY were all part of the research.