New optical device opens path for extreme focusing of X-rays
Adaptable refractive correctors for X-ray optics
An innovative new type of optical component for X-rays has been developed by a scientific team in the Optics and Metrology Group at Diamond Light Source. This new optical component is designed to correct for the effect of imperfections in the optical elements used for focusing of X-rays. It works by introducing a controlled change to the X-ray's phase. It is known as an "adaptable refractive corrector" - so called because the corrector uses refraction and can adapt the correction to the unique imperfection of any optical element. The researchers have designed and tested such a component at Diamond obtaining reductions in the effect of the imperfections in a range of mirror and lens focusing optical elements by a factor of up to 7. This development is expected to have application to new developing techniques such as hard X-ray microscopy at the nanometre scale.
X-rays, like visible light, are a form of electromagnetic waves but with much smaller wavelength (by a factor of about 10,000 for hard X-rays). And like visible light, X-rays can be focused by lenses or by curved mirrors and modern X-ray sources such as synchrotron rings and X-ray free electron lasers generate X-rays with properties that allow focusing down to a spot of size at the 10 nanometre level. This is possible because of the small wavelength of the X-rays, however specialised highly perfect X-ray focusing optical elements are required to preserve the X-ray phase and realise this. The ability to focus X-rays to this level opens-up the possibility of measuring the properties of materials at resolution approaching that of the atomic spacing.
The design and manufacture of optics for focusing X-rays requires a high degree of expertise. Reflection of X-rays from mirror surfaces only occurs at an extremely small angle to the surface and focusing by lenses is so weak that large numbers (100s) of X-ray lenses must be used to obtain useful focusing. The precision required of the optics is highly demanding. As David Laundy, Lead Investigator on the project explains:
Real X-ray optical elements, whether reflection or refraction based, inevitably introduce unwanted phase changes onto the X-rays that even at the level of a fraction of the X-ray wavelength degrade the focus. We can measure these phase changes and it tells us that in order to achieve the ultimate in focusing of the X-rays, a mechanism for X-ray phase correction is required.
Adaptable refractive correctors are the new optical element that was devised and developed by the scientific team. Each corrector allows changing of the phase of the X-rays in order to correct for imperfections in X-ray optical elements. Each consists of two separate phase plates that allows a spatially dependent phase correction to be applied to the X-rays. Then, uniquely, the phase correction can be varied in form and size by independently adjusting the positions of the two phase plates. This gives the possibility of using the same design of adaptable corrector to correct different optical components and different optical layouts. In addition, a number of the adaptable correctors can be cascaded giving greater control over the correction. The correctors were fabricated in the polymer SU-8 using microfabrication techniques at Indus-2 Synchrotron Source and were then brought back to Diamond for testing on its B16 Test Beamline.
To correct the focusing X-ray optics, the X-ray phase error caused by the focusing optical component was measured by the scientific team. This was done using a highly sensitive technique in which a precision fabricated gold knife edge was scanned through the focus of the X-rays while the X-ray intensity was measured on a two-dimensional X-ray detector downstream. This enabled the phase error at every point on the focusing optical component to be quantified. This information was then used to optimise the adaptable refractive corrector introduced into the X-ray beam just before the focusing optical element adjusting the alignment of the two plates individually to obtain the smallest residual phase error. Following optimisation, the corrector gave a significant correction in the phase error in both X-ray mirror and lens focusing optical elements which would allow the optics to achieve focusing close to the theoretical "diffraction" limit. Kawal Sawhney, head of the Diamond Light Source Optics Group commented:
On modern synchrotron sources, increasingly nano-sized X-ray beams much smaller than 100nm are required with a strong interest to get to sub-10nm beam sizes. However, the X-ray Optics required to achieve this is beyond the present-day fabrication technologies. The adaptable refractive correctors have great potential to generate a near perfect optical system not only for fully exploiting new sources but also developing new techniques.