Technique to manipulate polarisation

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A technique has been developed by researchers at Brown University which uses stacks of metal plates to make a polarising beamsplitter. According to the team, the beamsplitter could be useful in a variety of systems that make use of terahertz radiation, from imaging systems to future communications networks.

Polarising filters for terahertz are generally said to be an array of metal wires. The researchers’ technique replaces the wires with a stack of closely-spaced steel plates, known as an artificial dielectric. Each pair of plates forms a parallel-plate waveguide.

When terahertz light is shined on the stack at a 45° angle, it splits the beam by exciting two waveguide modes. One beam of vertically polarised light passes straight through the device, while another beam of horizontally polarised light is reflected in a 90° angle from the original beam axis.

“This stack-of-plates idea has advantages over traditional methods of manipulating polarisation in the terahertz region,” said Professor Dan Mittleman. “It’s cheaper and physically more robust than other methods.”

“The artificial-dielectric concept also makes the device more versatile,” assistant professor Rajind Mendis said. “The device can be easily tuned for use at different terahertz frequencies simply by changing the size of the spacers separating the plates or by changing the illuminating angle.”

Another advantage is that with the addition of a second similar artificial-dielectric structure, the researchers could build an isolator which they claim could be an important component for future high powered terahertz devices.

In the imaging world, the ability to deliver and detect radiation at different polarisations could be useful in terahertz microscopy and material characterisation. In communications, polarised beams could enable multiple data streams to be sent down the same medium without interference.

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