SIMES/PULSE Researcher Honored for Advancing Ultrafast X-ray Experimental Capabilities

September 5, 2012

Tim Miller, a graduate-student member of the SIMES and Stanford PULSE institutes, joint SLAC-Stanford programs, will receive the 2012 Melvin P. Klein Scientific Development Award for his leadership and ingenuity in establishing a new type of experimental capability that enables ultrafast X-ray experiments at the Stanford Synchrotron Radiation Lightsource.

Instead of taking a static snapshot of a sample using SSRL's X-rays, these “pump-probe” studies involve hitting the material first with a pump pulse – typically a laser – to cause a change in the material, such as a rearrangement of the atoms. Then the X-rays probe the material to see how it reacts over time, which gives the technique its other name: time resolution study.

With this technique, scientists can see the movements of atoms in materials and the forming and breaking of chemical bonds.

Miller worked with Jeff Corbett and James Safranek of the SPEAR3 accelerator physics team to carry out the first direct measurements of X-ray pulse duration while SSRL was running in "low-alpha" mode, in which fewer electrons are sped around the ring, resulting in fewer photons in shorter bunches without sacrificing the high measurement rate intrinsic to SSRL.

In the process, they discovered that SSRL’s low-alpha mode enables X-ray pulses down to a few picoseconds in length, short enough to gain access to a new world of dynamical processes.

Miller also took charge of synchronizing a laser with the X-rays on the beamline and carried out a proof-of-concept time resolution study of a material called silver selenide, a member of a class of materials called superionics that show promise as components in next-generation electrochemical devices.

This was part of a broader science project generally focusing on nanoscale phase transitions and involving experiments at both SSRL and LCLS.

"These were tour de force experiments that Tim carried out with great creativity and experimental elegance under very challenging experimental conditions," wrote Aaron Lindenberg, an assistant professor in Materials Science and Photon Science and Miller's advisor, in his letter nominating Miller.

Miller’s experiments were vital to successful implementation of time resolution studies for several reasons. Such experiments take place on incredibly fast timescales of picoseconds and sometimes even femtoseconds: trillionths to thousandths of a trillionth of a second. According to Miller, SSRL was already almost there. “Synchrotrons have been providing ultrashort pulses all along," he said.

SPEAR3, the synchrotron that provides SSRL with X-rays, routinely delivers pulses of about 20 picoseconds in length. That’s not as short as pulses from SLAC’s Linac Coherent Light Source, but it’s almost short enough to resolve processes that occur on a time scale of picoseconds to hundreds of femtoseconds. Almost, but not quite.

To make that final jump from 20 picoseconds to less than five, or less than one, SSRL must run in low alpha mode. Shorter bunches of electrons create shorter X-ray pulses, while fewer photons reduce the intensity of the X-rays hitting the sample; too many X-ray photons interrupt the process they're supposed to capture. The probe becomes a pump.

But to make this work, researchers need to know details of those X-ray pulses, such as their length and timing, and they also need a laser that is precisely synchronized to the X-ray pulses. Thus Miller’s prize-winning work.

One of the best properties of the laser, Miller said, is that it's portable. "That means you can do anything at any beamline – you can use any of SSRL's capabilities in low-alpha mode." The time resolution program has already expanded to Beam Lines 8-2, 10-2, 13-3 and 6-2, and their new capabilities are attracting new users.

Not bad for someone who says he never even saw a laser while earning his B.S. in Materials Science and Engineering and Biomedical Engineering at Carnegie Mellon University. "I didn't even know what a synchrotron was,” Miller said. “I worked on ultra-high-strength stainless steel.

"Graduate school has been a lot of fun," he said. "I've met more people and learned more about the inner workings of stuff than at any other time in my life."

The Klein Scientific Development Award was created in 2006 in memory of Melvin P. Klein (1921-2000), a pioneer in the use of spectroscopic diagnostics. It is given annually to an undergraduate or graduate student or postdoctoral fellow for outstanding research conducted at SSRL.

Miller will receive the award and give a presentation discussing his research on October 4 at the annual LCLS/SSRL Users' Meeting and Workshops.