Ultrafast infrared light pulses cause thin film to ‘breathe’

Cornell Engineering researchers have demonstrated that, by zapping a synthetic thin film with ultrafast pulses of low-frequency infrared light, they can cause its lattice to atomically expand and contract billions of times per second - strain-driven "breathing" that could potentially be harnessed to quickly switch a material's electronic, magnetic or optical properties on and off.

The research was publishedSept. 12 in Physical Review Letters. The paper's co-lead authors are former postdoctoral researcher Jakob Gollwitzer and doctoral student Jeffrey Kaaret.

Stretching and squishing a material to induce strain is a common method to manipulate its properties, but using light for that purpose has been less studied, according to Nicole Benedek, associate professor of materials science and engineering, who co-led the project with Andrej Singer, associate professor of materials science and engineering in Cornell Engineering.

"On the theory side, whenever you try to do anything with light, it immediately becomes very complicated," Benedek said. "When a material interacts with light, we don't really know what is happening at a detailed level, and so you have to try and glean as much information from experiments as you can to build a model."

Benedek used computational theory to predict the optimal frequency of light and other experimental parameters that, in combination with the right material, would achieve a "dynamic" strain that could be reversed.

"Normally, when we grow materials under strain, once the material is synthesized, that's it, the strain can't change. It's just in there," Benedek said. "But this dynamic strain is a very short change in the shape, and then it goes away."

The researchers determined they could get the desired deformation by firing picosecond bursts of terahertz light, which is essentially at the same low frequency as phonons, a type of lattice vibration that are the sonic equivalent of photons and travel through material as soundwaves.

"The atom can swing around its position, like a child on a swing," Singer said. "If you swing it at the right frequency, you can increase the amplitude of that atom, and that's exactly what we're doing. We tune the frequency and excite a specific atomic motion that results in the quick expansion of the lattice. The light generates an entirely new material state that wouldn't be possible to make otherwise."

The researchers needed the right material to pair with that process. They chose lanthanum aluminate, which, as thin films go, isn't very flashy. In fact, it's quite boring.

That's why they selected it.

"In its normal state, it doesn't really have any exciting properties," Benedek said. "Because the theory is very difficult, we wanted something that was as simple as possible, and there has been interest from the light community in this material. So we picked it because it was going to make our lives easier. But then it turned out to be very interesting."

The team turned to Darrell Schlom, Tisch University Professor in the Department of Materials Science and Engineering (Cornell Engineering), who synthesized the material via oxide molecular-beam epitaxy. The experiment was conducted with a free-electron laser by collaborators at the Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory.

Analysis confirmed that zapping the phonons with ultrafast bursts of terahertz light induced the predicted strain. But that's not all: The researchers also discovered that the process permanently enhanced lanthanum aluminate's structure.

"That was something that we didn't expect," said Singer, whose group performed the X-ray characterization. "This particular material has domains of the same structures, oriented in different ways, and separated by domain boundary. The excited phonons create a new structure that forms at this domain boundary and propagates laterally across the film surface. We induce a more crystalline, more ordered state."

All materials have their limits and can only be stretched and compressed so much. But using low-frequency light now opens up new opportunities, such as switching between two different states in the same material, turning on and off electronic and magnetic properties, and inducing structural rearrangements for superconductivity.

"The combination of theory, synthesis and characterization allows us to understand how light interacts with the family of complex-oxide materials and access properties beyond what's possible with standard methods," Singer said.

Co-authors include Schlom; Ankit Disa '10, assistant professor in applied and engineering physics; former postdoctoral researchers Eren Suyolcu, Guru Khalsa and Sören Buchenau; postdoctoral researchers Oleg Gorobtsov and Yorick Birkhölzer; John Harter, Ph.D. '13; doctoral students Rylan Fernandes, Jayanti Higgins and Ziming Shao; and researchers from the Linac Coherent Light Source at the SLAC National Accelerator Laboratory, the Leibniz Institute for Crystal Growth and the University of California, Santa Barbara.

The research was supported by the Department of Energy's Office of Basic Energy Sciences and the Cornell Center for Materials Research with funding from the National Science Foundation's MRSEC program.