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Researchers demonstrate the optical equivalent of an electrical transistor

09 July 2013

Researchers at the Vienna University of Technology have built a transistor that is able to function using light instead of electricity.

The oscillation direction of a light wave is changed as it passes through a thin layer of a special material
The oscillation direction of a light wave is changed as it passes through a thin layer of a special material

TU Vienna researchers have managed to turn the polarisation of beams of light – simply by applying an electrical current to a special material. This way, a transistor can be built that functions with light instead of electrical current.

Changing the polarisation direction of light without incurring losses is difficult. The TU Vienna team has now managed to overcome this using light in the terahertz part of the spectrum. An electrical field applied to an ultra-thin layer of material can turn the polarisation of the beam as required, producing an efficient light transistor that they believe can be miniaturised for use in optical computers.

Certain materials can rotate the polarisation direction of light if a magnetic field is applied to them. Known as the Faraday effect, it is, however, minutely small.

Two years ago, Professor Andrei Pimenov and his team at the Institute of Solid State Physics at TU Vienna, together with a research group from the University of Würzburg, managed to achieve a massive Faraday effect as they passed light through special mercury telluride platelets and applied a magnetic field.

At that time, the effect could only be controlled by an external magnetic coil, which had disadvantages. "If electro-magnets are used to control the effect, very large currents are required", says Andrei Pimenov. Now, the polarisation of terahertz radiation simply by the application of an electrical potential of less than one volt has been achieved. This makes the system much simpler and faster.

While a magnetic field is needed to change the the polarisation, it is no longer the strength of that magnetic field that determines the strength of the effect, but the amount of electrons involved in the process. This can be regulated simply by electrical potential. Hence only a permanent magnet and a voltage source are needed - an arrangement that is comparatively easy to manage.

Optical transistors
If light is passed through a polarisation filter, dependent on the polarisation direction, it is either allowed to pass through or is blocked. The rotation of the beam of light (and thus the electrical potential applied) therefore determines whether a light signal is sent or blocked.

"This is the very principle of a transistor", says Pimenov. "The application of an external voltage determines whether current flows or not, and in our case, the voltage determines whether the light arrives or not."

According to the researchers, this is the optical equivalent of an electrical transistor. 

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