Supertwisty light proposed

Researchers suggest the existence of a never-before-imagined property of electromagnetic fields

Light, it seems, can be like Shirley Temple’s curls: more twisty than anyone could possibly have imagined.

TWISTED A proposed experiment to probe superchiral light would send circularly polarized light (green helix) through molecules of different chirality or ‘handedness’ (labeled here as L and R, for left-handed and right-handed). Adam Cohen and Yiqiao Tang/Harvard University; figure by Lu Men

In a paper to be published in Physical Review Letters, researchers suggest that electromagnetic waves, including light, can possess an excess twistiness beyond what physicists would ordinarily expect. The effect is probably limited to microscopic realms, but scientists had never before even speculated that it could occur.

“There’s this thing in the electromagnetic field that nobody has noticed all this time,” says Adam Cohen, a physical chemist at Harvard University who led the work. “That’s what makes it interesting.”

If its existence can be confirmed in the lab, the supertwisty light might one day have applications in drug synthesis, biosensors or other fields, he says.

Cohen’s team started out by studying molecules that have the property of “chirality” or “handedness,” meaning they come in two forms that mirror each other like right and left hands. A molecule’s chirality is detectable only when the substance interacts with other chiral things such as circularly polarized light, whose electric field traces out a helical pattern, like a Slinky, as it moves through space. When circularly polarized light hits a chiral molecule, that molecule produces a signal that shows whether it is the left-handed or the right-handed version.

Cohen wondered whether there were regions of space in which electromagnetic fields, including light, could be more chiral than in others. He and Harvard coauthor Yiqiao Tang ran some calculations and concluded that this could, in fact, be true. In tiny regions of space, they say, this “superchiral” light would twist around at rates hundreds of times higher than ordinary circularly polarized light.

The twistiness, they say, is an inherent characteristic of the light. “I think of it as a physical property of the electromagnetic field, just like intensity,” Cohen says. It would, however, occur only in regions of space — probably rare — where conditions of the electromagnetic fields are just right.

The work is “interesting from a fundamental point of view” because it introduces a measure of chirality that no one had thought of before, says Akbar Salam, a theoretical computational chemist at Wake Forest University in Winston-Salem, N.C., who was not associated with the work. “The question is whether this can be realized experimentally.”

Cohen is already building the apparatus in his lab to try to do that. If the researchers can confirm the existence of superchiral light, it might be used one day to build sensors to detect chiral molecules, which might, for instance, indicate the possibility of life. Amino acids are the building blocks of proteins but can form through biological or nonbiological processes. On Earth, the biological versions are almost exclusively left-handed, while the nonbiological forms are made of equal amounts of right and left forms. In theory a Mars probe, Cohen says, could carry a lightweight superchiral sensor to test any amino acids found on Mars for biological origin.

In addition, many drugs come in both left- and right-handed versions, and the two can have radically different effects. Pharmaceutical companies constantly look for new ways to influence the outcome of their drug-making chemical reactions so they end up with only one handedness or the other. Superchiral light might potentially be useful for controlling such reactions, Cohen speculates.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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