Light from an optical fiber illuminates the metasurface, is scattered in four different directions, and the intensities are measured by the four detectors. From this measurement the state of polarization of light is detected. Credit: Capasso Lab/Harvard SEAS
What do astrophysics, telecommunications and pharmacology have in common? Each of these fields relies on polarimeters – instruments that detect the direction of the oscillation of electromagnetic waves, ie polarization of light.
When light is reflected or scattered off an object, its polarization changes and measuring that change reveals a lot of information. Astrophysicists, for example, use polarization measurements to analyze the surface of distant, or to map the giant magnetic fields spanning our galaxy. Drug manufacturers use the polarization of scattered light to determine the chirality and concentration of drug molecules. In telecommunications, polarization is used to carry information through the vast network of fiber optic cables. From medical diagnostics to high-tech manufacturing to the food industry, measuring polarization reveals critical data.
Scientists rely on polarimeters to make these measurements. While ubiquitous, many polarimeters currently in use are slow, bulky and expensive. Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences and Innovation Center Iceland have built a polarimeter on a microchip.
“We have taken an instrument that is can reach the size of a lab bench and shrunk it down to the size of a chip,” said Federico Capasso. “Having a microchip polarimeter will make polarization measurements available for the first time to a much broader range of applications, including in energy-efficient, portable devices.”
“Taking advantage of integrated circuit technology and nanophotonics, the new device promises high-performance polarization measurements at a fraction of the cost and size,” said J. P. Balthasar Mueller.
Capasso’s team was able to drastically reduce the complexity and size of polarimeters by building a 2D metasurface – nanoscale structure that interacts with light. The metasurface is covered with a thin array of metallic antennas, smaller than a wavelength of light, embedded in a polymer film. As light propagates down an optical fiber and illuminates the array, a small amount scatters in 4 directions. 4 detectors measure the intensity of the scattered light and combine to give the state of polarization in real time. This leaves the signal mostly intact, useful in optical telecommunications, where measurements must be made without disturbing the data stream.
In telecommunications, optical signals propagating through fibers will change their polarization in random ways. New integrated photonic chips in fiber optic cables are extremely sensitive to polarization, and if light reaches a chip with the wrong polarization, it can cause a loss of signal. Chip-based polarimeters could for the first time provide comprehensive and real-time polarization monitoring, which could boost network performance and security and help providers keep up with the exploding demand for bandwidth. http://www.seas.harvard.edu/news/2016/01/novel-metasurface-revolutionizes-ubiquitous-scientific-tool
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