Scientists have discovered a centrosymmetric crystal that behaves as though it is chiral – absorbing left- and right-handed circularly-polarized light differently. This counterintuitive finding, from researchers at Northwestern University and the University of Wisconsin-Madison in the US, could help in the development of new technologies that control light. Applications include brighter optical displays and improved sensors.
Centrosymmetric crystals are those that look identical when reflected through a central point. Until now, only non-centrosymmetric crystals were thought to exhibit differential absorption of circularly-polarized light, owing to their chirality – a property that describes how an object differs from its mirror image (such as our left and right hands, for example).
In the new work, a team led by chemist Roel Tempelaar studied how a centrosymmetric crystal made from lithium, cobalt and selenium oxide interacts with circularly polarized light, that is, light with an electromagnetic field direction that rotates in a helical or “corkscrew-like” fashion as it propagates through space. Such light is routinely employed to study the conformation of chiral biomolecules, such as proteins, DNA and amino acids, as they absorb left- and right-handed circularly polarized light differently, a phenomenon known as circular dichroism.
The crystal, which has the chemical formula Li2Co3(SeO3)4, was first synthesized in 1999, but has not (to the best of the researchers’ knowledge) been discussed in the literature since.
A photophysical process involving strong chiroptical signals
Tempelaar and colleagues found that the material absorbed circularly polarized light more when the light was polarized in one direction than in the other. This property, they say, stems from a photophysical process involving strong chiroptical signals that invert when the sample is flipped. Such a mechanism is different to conventional chiroptical response to circularly polarized light and has not been seen before in single centrosymmetric crystals.
Not only does the discovery challenge long-held assumptions about crystals and chiroptical responses, it opens up opportunities for engineering new optical materials that control light, says Tempelaar. Potential applications could include brighter optical displays, polarization-dependent optical diodes, chiral lasing, more sensitive sensors and new types of faster, more secure light-based communication.
“Our work has shown that centrosymmetric crystals should not be dismissed when designing materials for circularly polarized light absorption,” Tempelaar tells Physics World. “Indeed, we found such absorption to be remarkably strong for Li2Co3(SeO3)4.”
The researchers say they took on this study after their theoretical calculations revealed that Li2Co3(SeO3)4 should show circular dichroism. They then successfully grew the crystals by mixing cobalt hydroxide, lithium hydroxide monohydrate and selenium dioxide and heating the mixture for five days in an autoclave at about 220 °C.
The “tip of the iceberg”
“This crystal is the first candidate material that we resorted to in order to test our prediction,” says Tempelaar. “The fact that it behaved the way it does could just be a great stroke of luck, but it is more likely that Li2Co3(SeO3)4 is just the tip of the iceberg spanning many centrosymmetric materials for circularly polarized light absorption.”
Some of those compounds may compete with current champion materials for circularly polarized light absorption, through which we can push the boundaries of optical materials engineering, he adds. “Much remains to be discovered, however, and we are eager to progress this research direction further.”
“We are also interested in incorporating such materials into photonic structures such as optical microcavities to amplify their desirable optical properties and yield devices with new functionality,” Tempelaar reveals.
Full details of the study are reported in Science.
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