Metamaterial Detects Circularly Polarized Light

A metamaterial detector for circularly polarized light (CPL) may open the door for development of small, portable sensors for applications from drug screening to communications and beyond.

"Although it is largely invisible to human vision, the polarization state of light can provide a lot of valuable information," said Vanderbilt University professor Jason Valentine. "However, the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturize."

Valentine and colleagues at Vanderbilt and Ohio University got around this limitation by using a metamaterial detector on a silicon chip.

The metamaterial consists of silver nanowires laid down in a submicron zigzag pattern. Depending on the direction of the zigzags, the metamaterial absorbs either right or left CPL. The absorption process generates a detectable electric current in the silicon chip. By including both right-handed and left-handed surface patterns, the sensor can differentiate between right and left CPL.

A circularly polarized light detector on a chip (left) performs the same function as a conventional, optically based detector (right). Courtesy of Anne Rayner/Vanderbilt University.

At 0.2 percent, the prototype's efficiency is a far cry from that of conventional photodetectors. But the detector already has advantages over previous solid-state circularly polarized light detectors, the researchers said.

One used chiral organic materials that are unstable in air, worked only in a narrow range of wavelengths and had a limited power range. Another was based on a more complicated multilayer design that only worked at low temperatures.

CPL can be used to determine the chirality (right- or left-handedness) of molecules. Chirality can be important in drugs because their biological activity is often related to their handedness.

For example, one form of dopamine is effective in the management of Parkinson's disease, while the other form reduces the number of white blood cells. One form of thalidomide alleviates morning sickness, while the other causes birth defects. The number of chiral drugs in use today is estimated to be 2,500, and most new drugs under development are chiral.

"Inexpensive CPL detectors could be integrated into the drug production process to provide real-time sensing of drugs," said Vanderbilt doctoral student Wei Li. "Portable detectors could be used to determine drug chirality in hospitals and in the field."

Humans cannot readily distinguish the polarization state of light, but there are a number of species that possess "p-vision," including cuttlefish, mantis shrimp, bees, ants and crickets.

Cuttlefish also produce varying patterns of polarized light on their skin, which has led scientists to hypothesize that they use this as a secret communication channel that neither their predators nor their prey can detect. This has led to the suggestion that CPL could be used to increase the security of optical communications by including polarized channels that would be invisible to those who don't have the proper detectors.

Funding for development of the detector came from the National Science Foundation, Office of Naval Research, U.S. Army Research Office and Volkswagen Foundation.

The research was published in Nature Communications (doi: 10.1038/ncomms9379).