A material's behavior is heavily influenced by its symmetries. One unique symmetry of interest to many physicists is chirality. Chiral materials have non-superimposable mirror images, like a right and left hand. For physicists like Fahad Mahmood, Rafael Fernandes and Jorge Noronha, the nonlinear interaction between chiral materials and light is of particular interest. How do these materials respond when light triggers effects beyond straightforward, linear response?
"If I have a shiny crystal and I put a red laser on it, I'll get red light back; that's a linear response, as the frequencies - or colors - of the incoming and outgoing light are the same" Mahmood said. "You can go a little further and try to excite some frequency so that it sends back a different color: you put red light on something, and it shines back as green, blue or yellow. That's nonlinear response."
Physicists have known about nonlinearity for more than a century, but the Illinois Grainger trio is the first to examine this response in chiral materials subjected to an external magnetic field. Using Mahmood's existing setup of terahertz spectroscopy at low temperatures, the team examined tellurium, a known chiral material. To their surprise, ultrafast laser pulses directed at the material revealed amplification at certain frequencies. In other words, the outgoing terahertz signal grew as a function of time.
"This result is interesting because the amplification doesn't happen at all frequencies," Fernandes said. "It's like when you sing at a loud enough frequency and glass starts to vibrate. But this glass is going to sing back to you, and louder. It's a very unusual phenomenon and it only happens when you apply the magnetic field."
Noting that the unique effect resulted from a dynamic interaction between the magnetic fields, light waves, and tellurium's chiral structure, the group then borrowed principles from fundamental physics to explain their observation.
"Electrons in a chiral material have a preferred way of moving - their spin is coupled to their movement," Noronha said. "The magnetic field is reinforcing that movement along a certain direction, and that's what we call this instability: it makes things spin faster. And we saw this not as a fundamental particle property, but as some emergent property in a material - that the material itself has this handedness that the electron inherits from the (tellurium) crystal."
Although similar behavior may have occurred in extreme environments in the early universe, or in the tiny droplets of quark-gluon matter formed during the ultrafast collision of heavy nuclei, this is the first known observation of such an effect in materials, suggesting that fundamental instabilities once thought to exist only in exotic systems can also occur in everyday systems consisting of electrons and atoms.
The Illinois scientists believe their findings may eventually be used to develop new methods of controlling light and energy in chiral materials. But most importantly, they view their work as an example of what's possible through collaboration. This research reflects a rare cross-disciplinary effort, combining the expertise of Mahmood in condensed-matter experiments with Fernandes's condensed-matter theory and Noronha's background in nuclear physics theory.
"The training, techniques, and language are very different across different branches of physics, and it's not that common for the different branches to talk to each other," Noronha said. "But Illinois has a long history of bringing these disciplines together, and we are very happy to be carrying forward that tradition."
Yijing Huang, a former IQUIST postdoctoral fellow, and Nick Abboud, an Illinois Physics graduate student, also contributed significantly to this work.
Research Report:Dynamic magneto-chiral instability in photoexcited tellurium
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