In 2005, a research collaboration teaming Haglund and René Lopez (now at the University of North Carolina, Chapel Hill) with Andrea Cavalleri and Matteo Rini from the Lawrence Berkeley National Laboratory tested the vanadium dioxide transition with an ultra-fast laser that produced 120-femtosecond pulses. (A femtosecond is a quadrillionth of a second. At this time scale, an eye blink lasts almost forever. In the three-tenths of a second it takes to blink an eye, light can travel 56,000 miles. By contrast, it takes 100 femtoseconds to cross the width of a human hair.)

Using this laser, the researchers determined that VO2 film can flip from transparent to reflective in a remarkably short time: less than 100 femtoseconds. This was the fastest phase transition ever measured. However, the mechanism that allowed it to make such rapid transitions remained a matter of scientific debate.

Now, in a two-year collaboration with the Leitenstorfer group, the Vanderbilt researchers have used a laser with even shorter, 12-femtosecond pulses to “strobe” the vanadium dioxide transition with the fastest pulses ever used for this purpose. The result? “This transition takes place even faster than we thought possible,” says Haglund. “It can shift from transparent to reflective and back to transparent again in less than 100 femtoseconds, making the transition more than twice as fast as we had thought.”

In order to identify the driving mechanism for the rapid change of state in vanadium dioxide, Leitenstorfer’s graduate student Carl Kübler developed a method that converts the near-infrared photons produced by their 12-femtosecond pulse laser into a broad spectrum of infrared wavelengths that bracket a well-known vibration in the vanadium dioxide crystal lattice. At the same time, the Vanderbilt researchers figured out how to grow VO2 film on a diamond substrate that is transparent to infrared light.

This allowed the researchers to show that the energy in the laser beam goes directly into the crystal lattice of the VO2, driving it to shift from its transparent, crystalline form to its more compact and symmetric metallic configuration.

The laser light doesn’t produce this shift by heating the VO2 lattice until it melts, as the conventional wisdom about phase transitions suggested. Instead, the researchers found that the stream of photons directly drive the oxygen atoms from one position to another by a process that is rather like pumping a swing in time with its natural frequency.

“People have believed for a long time that what happened in this phase transition was that the electrons get excited and then, somehow or another, the crystal structure changes,” says Haglund. “But it turns out that the change in crystal structure is associated with this coherent molecular vibration.”

Such a rapid transition is only possible because the difference between the metallic and semiconductor geometries is extremely small. “You can think of the movement that results as a breathing motion of the oxygen ‘cage’ that surrounds the vanadium ions,” says Haglund. “That makes it possible for the structure to change from the semiconducting to the metallic states. It’s a little like taking a deep breath to get into last summer’s clothes.”

Henri Ehrke and Rupert Huber from the University of Konstanz, and Andrej Halabica from Vanderbilt University also collaborated in the study, which was funded by the National Science Foundation and the Alexander von Humboldt Foundation.