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March 16th, 2012

Pump-and-probe laser method shines new light on rapid phase transitions

Published on March 16th, 2012 | By: pwray@ceramics.org

Lost vibrations: In the insulating state, the VO2 crystal structure vibrates at four well-defined frequencies when hit by a low power laser pulse. These vibrations modulate the reflectivity over time as shown in the left image. In the right image, the power of the laser has been turned up and the VO2 has stopped vibrating, indicating that the phase transition has occurred. Credit: Fritz Haber Institute.

We received word that an international team of researchers has developed a way to use an ultrafast optical system to track phase transitions in materials that occur in time spans that may be as short as trillionths of a second. The laser-based system provides a “table-top” alternative to more complicated X-ray-based systems for investigating phase transistions.

 

Working under grants from NSF and the Alexander von Humboldt Foundation, investigators at Vanderbilt University and the Fritz Haber Institute of the Max Planck Society (Germany) decided to focus on the phase transitions of vanadium dioxide, which undergoes some phase transitions so rapidly that they, so far, have been tough to follow. For example, in a paper published in Nature Communications, the authors say that the shift between the transparent and reflective phases in VO2 is the fastest phase transition known.

 

“This means that there is a lot that we still don’t know about the dynamics of these critical processes,” says professor of physics Richard Haglund in a Vanderbilt news release. Haglund directed the team of researchers from Vanderbilt.

 

The group wanted to explore how it is that VO2 can shift from a transparent, semiconducting phase to a reflective, metallic phase extremely quickly. This phase change can be induced by heating the material above 150°F, and it is this property that has made VO2 an attractive material for thermochromic windows. VO2 can also be forced to undergo the phase transition by hitting it with a pulse of light from a laser, and thus it is also a candidate material for optical switches and faster computer memory.

 

Haglund has wondered for many years about how and why VO2 achieves this rapid transition. Back in 2005, he directed a study, the results of which were published in Optics Letters, in which he noted, “Phase transitions in solids generally occur at the speed of sound in the material, but vanadium dioxide makes the switch 10 times faster. So far no one has succeeded in coming up with a definitive explanation for that rapid a change.”

 

In this most recent work, Hagland’s group at Vandy created and characterized the VO2 thin films. The VO2 film was then given to the FHI group, who used an adaptation of the “pump-and-probe” method using a femtosecond infrared laser. Their method can simultaneously launch a phase change and optically track the progress of the phase change by measuring the reflectivity of the surface of the material.

 

“With this new technique, we were able to see a lot of details that we’ve never seen before,” said Haglund. He says they learned that the electrons in the material rearrange themselves first, followed by a movement of the atoms as the material shifts from its semiconductor to metallic-phase orientation. According to the group’s paper, these insights into VO2 can be used to design high-speed optical switches.

 

“The real power of this technique is that it is sensitive to atomic changes inside the material which are usually observed using expensive large-scale X-ray sources. Now we can do the experiment optically and in the lab on a tabletop,” says Simon Wall, an Alexander von Humbolt fellow at the Fritz Haber Institute.

 

The Nature Communications paper is titled, “Ultrafast changes in lattice symmetry probed by coherent phonons” (doi:10.1038/ncomms1719).


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