What do you get when you combine some biomimicry, metamaterials and nanowires? It turns out to be the first integrated circularly polarized light detector on a silicon chip. Its development could usher in a new generation of portable sensors that can use polarized light for applications ranging from drug screening to quantum computing.
Researchers at Vanderbilt University have used silver nanowires to fabricate a metamaterial that is capable of detecting polarized light in a way not unlike the way cuttlefish, bees, or mantis shrimp do it.
“Although it is largely invisible to human vision, the polarization state of light can provide a lot of valuable information,” said assistant professor Jason Valentine in a press release. “However, the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturize. We have managed to get around this limitation by the use of ‘metamaterials’—materials engineered to have properties that are not found in nature.”
Polarized light comes in basically two forms—linear or circular. In contrast to non-polarized light, in which the electric fields of the photons are oriented in random directions, polarized light, whether linear or circular, features electric fields oriented in a single plane. (With circularly polarized light, the plane is continually rotating through 360 degrees.)
One of the distinguishing capabilities of circularly polarized light (CPL) is that it can discern the difference between right-handed and left-handed versions of molecules—a property known as chirality. Chirality is critically important in drugs because whether they are left handed or right handed determines their biological activity. For instance, there is the famous case of thalidomide, which in one chirality alleviates morning sickness in pregnant women and in the other causes birth defects. Having a portable sensor capable of detecting a drug’s chirality could be a game changer.
“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, in a press release. “Portable detectors could be used to determine drug chirality in hospitals and in the field.”
In research published in the journal Nature Communications, the researchers fabricated the portable CPL sensors by laying down nanowires in a zig-zag pattern over a thin sheet of acrylic affixed to a thick silver plate. This material is affixed to the bottom of a silicon wafer with the nanowire side up.
The nanowires create a sea of electrons that produces “plasmon” density waves, the oscillations in the density of electrons that are generated when photons hit a metal surface. These plasmon density waves absorb energy from the photons that pass through the silicon wafer. The absorption of the energy produces “hot” or energetic electrons, which generate a detectable electrical current.
The researchers found that they could make the zig-zag pattern of nanowires with a right- or left-handed orientation. When they arranged the nanowires in right-handed pattern, the surface absorbed right circularly polarized light and reflected left circularly polarized light. When arranged in a left-handed pattern, the opposite effect occurred. And when they arranged the nanowires to have both left- and right-handed patterns, the sensor could discern between left and right circularly polarized light.
The researchers concede that their current prototype is not efficient enough to be commercially viable. However, they have a few tricks up their sleeves that they believe will improve that efficiency in the next generation of their devices.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.