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, Leitenstorfers graduate student Carl Kbler 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 doesnt produce this shift by heating the VO2 lattice until
|Contact: David F. Salisbury|