Nanoscale X-ray gratings will fly in next-gen telescopes

X-rays from space provide astronomers with important information about the most exotic events and objects in our universe, such as dark energy, black holes, and neutron stars. But X-rays are notoriously difficult to detect because many interesting cosmic sources are faint. And X-ray detection is optimal above the Earth's atmosphere, which makes collecting these high-energy rays very challenging and costly.

Now a group of researchers from Massachusetts Institute of Technology (MIT; Cambridge, MA) has fabricated new, highly efficient Critical-Angle Transmission (CAT) gratings for use in future improved space-based X-ray telescopes. The gratings combine the advantages of transmission gratings with the broadband efficiency of blazed reflection gratings via dense periodic arrays of atomically smooth nanometer-thin silicon mirrors. Light is reflected at very shallow angles to enable the structures to serve as efficient mirrors for the reflection and diffraction of X-rays.

New instrument designs based on these gratings could also lead to advances in fields beyond astrophysics, from plasma physics to the life and environmental sciences, as well as in extreme ultraviolet lithography, a technology of interest to the semiconductor industry. The concept behind CAT gratings might also open new avenues for devices in neutron optics and for the diffraction of electrons, atoms, and molecules.

Based on an invention by Ralf Heilmann and Mark Schattenburg of the Space Nanotechnology Laboratory (SNL) at the MIT Kavli Institute of Astrophysics and Space Research, the daunting fabrication challenges were overcome by graduate student Minseung Ahn of the Department of Mechanical Engineering at MIT in a yearlong effort, with the help of financial support from NASA and a Samsung Fellowship.

Motivated by technology goals for NASA's next-generation X-ray telescope, called Constellation-X, the new devices promise a five-fold improvement in efficiency of the transmission gratings onboard NASA's Chandra X-Ray Observatory (launched in 1999), which were also built at the Space Nanotechnology Lab. The improvement lies in the very efficient reflection of X-rays at very shallow angles from the sub-nanometer-smooth sidewalls of the silicon slats, through the spaces between the slats. In the earlier design, X-rays passed through a substrate of polyimide, which absorbed many of the rays and reduced the grating's efficiency.

The parallel silicon slats are as thin as 35 nm, separated by as little as about 150 nm. The slats extend many microns in the remaining two dimensions. "Imagine a thin, 40-foot-long, 8-foot-tall mirror, with surface roughness below a tenth of a millimeter," says Heilmann. "Then put tens of thousands of these mirrors next to each other, each spaced precisely an inch from the next. Now shrink the whole assembly--including the roughness--down by a factor of a million, and you have a good CAT grating."

Recent X-ray test results from a prototype device, obtained with the help of Eric Gullikson of Lawrence Berkeley National Laboratory (Berkeley, CA), confirmed that it met theoretical expectations. The results of this work were published in Optics Express (Vol. 16, No. 12) on June 9. They were also presented at the 52nd Intl. Conference on Electron, Ion and Photon Beam Technology and Nanofabrication in Portland, OR, on May 28, and will be presented again at the SPIE Conference on Astronomical Telescopes and Instrumentation in Marseille, France, on June 23.

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