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Light detector on a chip shines path leading to new apps

Posted: 28 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Vanderbilt University? sensor? CPL? integrated circularly polarised light? light detector?

Vanderbilt University engineers have created what they claim as the first integrated circularly polarised light (CPL) detector on a silicon chip that paves the way for developing small, portable sensors that could expand the use of polarised light for drug screening, surveillance, optical communications and quantum computing, among other potential applications.

The detector was developed by a team directed by assistant professor of mechanical engineering Jason Valentine, Vanderbilt University, working with researchers at Ohio University.

"Although it is largely invisible to human vision, the polarisation state of light can provide a lot of valuable information," said Valentine. "However, the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturise. We have managed to get around this limitation by the use of metamaterials, materials engineered to have properties that are not found in nature."

Polarised light comes in two basic forms: linear and circular. In a ray of unpolarised light, the electrical fields of individual photons are oriented in random directions. In linearly polarised light the fields of all the photons lie in the same plane. In CPL, the fields lie in a plane that continuously rotates through 360 degrees. As a result there are two types of CPL, right-handed and left-handed.

Light detector chip

The chip in the hand does the same job as the conventional circularly polarized light detector on the right. (Anne Rayner/Vanderbilt)

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

Cuttlefish also produce varying patterns of polarised light on their skin, which has led scientists to hypothesise that they use this as a secret communication channel that neither their predators or prey can detect. This has led to the suggestion that CPL could be used to increase the security of optical communications by including polarised channels that would be invisible to those who don't have the proper detectors.

Wei Li, left, and Jason Valentine in the lab

Wei Li, left, and Jason Valentine in the lab. (Anne Rayner/Vanderbilt)

Unlike unpolarised light, CPL can detect the difference between right-handed and left-handed versions of molecules. Just like hands and gloves, most biological molecules come in mirror-image pairs. This property is called chirality. For example, cells contain only left-handed amino acids but they metabolise only right-handed sugars (a fact utilised by some artificial sweeteners that use left-hand forms of sugar that taste just as sweet as the right-hand version but which the body cannot convert into fat).

Chirality can be dramatically 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 University doctoral student Wei Li, who played a key role in designing and testing the device. "Portable detectors could be used to determine drug chirality in hospitals and in the field."

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