Quick pulse of laser light can cause dramatic shift in material's properties

Nicole Benedek

Guru Khalsa

When materials scientists want to change the properties of a material, they typically will introduce some sort of structural distortion or chemical change.

Nicole Benedek, assistant professor in the Department of Materials Science and Engineering, and postdoctoral researcher Guru Khalsa have what you might call a “bright idea” for a different approach to the manipulation of materials.

With theoretical techniques, Benedek and Khalsa predict that using intense mid-infrared laser light on a titanium perovskite can dynamically induce a magnetic phase transition – taking the material from its ferromagnetic ground state to a hidden anti-ferromagnetic phase. This dramatic shift could have useful applications, particularly in optical information processing.

“It would be a kind of optical switch,” Benedek said. “You have a material where it’s magnetic and ‘non-magnetic.’ It’s going between those two states with light.”

Their paper, “Ultrafast Optically Induced Ferromagnetic/Anti-Ferromagnetic Phase Transition in GdTiO3 From First Principles,” published March 12 in npj Quantum Materials, a publication of Nature.

Benedek’s research group uses theory and computational simulations to explore an area called “functional materials,” which do something useful when you apply an external stimulus, such as temperature, pressure, an electric or magnetic field – or, in this case, light.

Previous research into light-induced phase transitions, Benedek said, usually focused on hitting the material with a beam so high in energy that it excites the electrons to a degree that fundamentally changes the material, say from an insulator to a conductor.

“That’s interesting, but it’s a pretty blunt hammer,” she said. “With our work, we are interested in light pulses that are much lower in energy, and you are exciting specific patterns of atomic displacements in the material. Hopefully, not touching the electrons at all.”

And, she said, depending on how the atoms are displaced, you change the properties of the material in different ways. “It’s much more precise,” she said.

Both states involved in this work are magnetic, Khalsa said. In the ferromagnetic state, the electrons’ spins are all aligned the same way and the magnetism is measurable away from the surface (think of a kitchen magnet). With an anti-ferromagnet, the spins are aligned in such a way (both up and down) that they cancel each other out.

These two states are perfectly suited for information processing, Benedek said.

The other advantage to this type of material manipulation is that it happens extremely fast.

“You’re using light to give you very subtle structural changes,” Khalsa said, “but these changes are leading to a drastic change in the magnetic state.” “They’re happening really, really quickly – on the order of hundreds of femtoseconds [less than a trillionth of a second]. And you can go back and forth [between the two magnetic states].”

Future work will explore light-induced phase transition in other perovskites, including lithium niobate, which is used in mobile phones, piezoelectric (pressure) sensors and optical modulators, which are used in lasers. “We’re wondering how far we can go with this,” Khalsa said.

This research was supported by grants from the National Science Foundation to Benedek, as well as the Cornell Center for Materials Research, a National Science Foundation Materials Research Science and Engineering Center.

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Jeff Tyson