Symmetrical Crystals Can Absorb Light Asymmetrically

Just when scientists thought they knew everything about crystals, a Northwestern University and University of Wisconsin-Madison collaboration has uncovered a hidden secret.

Centrosymmetric crystals are a special type of material that is fully symmetrical in every direction from a central point. Previously, scientists thought only non-centrosymmetric materials could exhibit chiral behavior — a property in which an object acts differently from its mirror reflection. But, for the first time, researchers have found a centrosymmetric crystal can act “chiral” despite its symmetry.

In the new study published in the journal Science, the research team investigated how a specific centrosymmetric crystal interacts with circularly polarized light, which twists like a corkscrew in either a clockwise or counterclockwise direction. Previously, scientists assumed a centrosymmetric material would absorb both directions of light equally. Instead, the crystal absorbed light in one direction more than the other — exhibiting apparent chiral behavior.

Not only does this finding challenge long-held assumptions about crystals and chirality, it also opens new opportunities for exploring common centrosymmetric materials to develop new technologies that control light. Potential applications could include brighter optical displays, more sensitive sensors and new types of faster, more secure light-based communication.

“This discovery is surprising to many in the scientific community whom, for a long time, thought this principle was impossible,” said Northwestern’s Roel Tempelaar, who supervised the theoretical exploration leading to this work. “Crystals are really categorial because of their symmetry. You can infer a lot about their behaviors — without doing complicated calculations or measurements — just by looking at their symmetry. Now, we realize that sometimes there is more than meets the eye.”

Tempelaar is an assistant professor of chemistry at Northwestern’s Weinberg College of Arts and Sciences. He co-led the study with Kenneth Poeppelmeier, the Charles E. and Emma H. Morrison Professor of Chemistry at Weinberg, and Randall Goldsmith, a professor of chemistry at the University of Wisconsin-Madison.

Two ‘flavors’ of molecules

A fundamental aspect of the molecular world, chirality occurs when an object and its mirror image cannot be perfectly superimposed on top of one another. A pair of shoes, for example, are a mirror image of one another. Consequently, a person cannot wear their left shoe on their right foot. It simply wouldn’t fit because feet and shoes are both chiral. On the other hand, a sphere and its mirror image do perfectly match, which is an example of achiral behavior.

Although it might seem like an obtuse concept, chirality profoundly influences how molecules interact with each other and with biological systems — affecting everything from the flavors of foods, effectiveness of medicine and the very nature of life itself. The pharmaceutical drug thalidomide is a tragic example of how chirality can critically affect human health. While thalidomide can be effective for treating morning sickness during pregnancy, the mirror image of the same compound can cause severe birth defects.

“Many molecules come in two ‘flavors,’” Tempelaar said. “They have a ‘right-handed’ form and a ‘left-handed’ form. By studying their interactions with chiral light, you can diagnose what type of molecule you have. Sometimes the right-handed version is a powerful drug, but the left-handed version is poison.”

An unexpected twist

After previously developing computational chemistry methods for studying the chirality of molecules, Tempelaar and his team were curious if they could apply their calculations to crystals. Their modeling suggested that a mechanism could lead to apparent chirality in crystals under specific conditions, ultimately showing that it could work even under centrosymmetry.

“Owing to its high degree of symmetry, a centrosymmetrical crystal traditionally is not expected to exhibit chirality,” Tempelaar said. “But we were optimistic about our level of predictivity because we had already rationalized this particular phenomenon in a variety of molecular samples. We didn’t expect the physics in a crystal to behave any differently.”

Tempelaar and his group predicted that a centrosymmetrical crystal made from lithium, cobalt and selenium oxide should be able to absorb light in a chiral manner. To test this theory, Poeppelmeier’s team synthesized the crystal. According to long-accepted principles, the crystal should not absorb clockwise and counterclockwise spinning light differently.

But the scientists found it did.

“To our knowledge, no centrosymmetrical crystal has been reported to do this,” Tempelaar said. “But we had success with the first material we tried. We are very excited about this. It’s opened our eyes to start looking at a variety of centrosymmetric materials in a new way.”

The study was primarily supported as part of the Center for Molecular Quantum Transduction with funding from the U.S. Department of Energy.