Micro-mirrors will help make ultrafast X-ray moving images

A new method for using X-rays to study the structure of materials could give engineers a low-cost technique to produce nanometre-scale resolution, even with moving images.

The technique, developed at the US Department of Energy’s Argonne National Laboratory, uses very small mirrors to steer, filter and pulse precise bursts of X-rays.

The Argonne team has designed a microelectromechanical system (MEMS), etched from a silicon wafer, which incorporates a diffracting mirror that is fabricated with a pair of hinges on either side that allow it to oscillate in a see-saw motion. A set of minute capacitors, designed with ‘finger’-like structures that can mesh together with one side attached to the mirror and the other to the baseplate, act as the motor to power the oscillation (this is known as a combdrive).

Electron micrograph of the device, showing the mirror (with rounded ends), hinges and combdrives

While such MEMS have recently found uses in several areas of optics, this is the first time they have been applied to X-rays, the Argonne team claimed. This is despite the fact that polished crystalline silicon is the material of choice for many applications of X-rays in optics; however, silicon crystal X-ray optics are typically two to five orders of magnitude larger and 6-15 orders of magnitude heavier than MEMS devices, which in this case are about 10µm thick and 500µm square.

In a paper in Nature Communications, the team claimed that the device can reflect X-rays from a synchrotron source (like the UK’s Diamond Light Source in Harwell) to generate nanosecond-duration pulses of X-rays at over 100kHz repetition rates. This, they claim, could allow ultrafast medical applications of X-rays to study biological processes, or in materials science where it could allow scientists to study processes associated with electron transfer, spin and structural changes in real time.

According to Argonne nanoscientist Daniel Lopez, one of the lead authors on the paper, the device works because of the relationship between the frequency of the mirror’s oscillation and the timing of the positioning of the perfect angle for the incoming X-ray. “If you sit on a Ferris wheel holding a mirror, you will see flashes of light every time the wheel is at the perfect spot for sunlight to hit it. The speed of the Ferris wheel determines the frequency of the flashes you see,” he said. “The advantage of this new device is that it provides a very cheap way to generate and manipulate X-rays, and it can be adapted to virtually any X-ray facility in the world that already exists.”

Argonne X-ray mirror
The X-ray MEMS device uses a crystalline silicon mirror about 500µm square

Another of the lead authors, Jin Wang, said that the devices could allow the production of 3D ultrafast moving images from the X-ray studies.

Commenting on the research, Prof Paul Evans of the University of Wisconsin-Madison called the Argonne team’s work “incredibly exciting”. “It creates a new class of devices for controlling X-rays,” he said. “They have found a way to significantly shrink the optics, which is great because smaller means faster, cheaper to make, and much more versatile.”