A team of researchers has developed a method to control and manipulate the polarisation of light using ultra thin layers of semiconductor material. According to physicists at the Vienna University of Technology and researchers at Würzburg University, the breakthrough could open the door to completely new computer technology.
Described as an optical version of an electronic transistor, the research focused on a phenomenon known as the Faraday effect – the polarisation of light that can change when it passes through a material in a strong magnetic field. Professor Andrei Pimenov, who carried out the experiments at the Institute for Solid State Physics of the TU Vienna, said that until now the effect had only been observed in materials in which it was very weak. By using light of the right wavelength and extremely clean semiconductors, the researchers achieved a Faraday effect which was in orders of magnitude stronger than ever measured before. According to Pimenov, now light waves can be rotated in arbitrary directions – the direction of the polarisation can be tuned with an external magnetic field. An ultra thin layer of less than a thousandth of a millimetre is enough to achieve this. "Such thin layers made of other materials could only change the direction of polarisation by a fraction of one degree", said Prof Pimenov. If the beam of light is then sent through a polarisation filter, which only allows light of a particular direction of polarisation to pass, the scientists can, rotating the direction appropriately, decide whether the beam should pass or not. The key to this effect lies in the behaviour of the electrons in the semiconductor. The beam of light oscillates the electrons, and the magnetic field deflects their vibrating motion. This complicated motion of the electrons in turn affects the beam of light and changes its direction of polarisation. In the experiment, a layer of the semiconductor mercury telluride was irradiated with light in the infrared spectral range. "The light has a frequency in the terahertz domain – those are the frequencies, future generations of computers may operate with," Prof Pimenov said. "For years, the clock rates of computers have not really increased, because a domain has been reached, in which material properties just don't play along anymore." Pimenov believes a possible solution is to complement electronic circuits with optical elements. In a transistor, the basic element of electronics, an electric current is controlled by an external signal. In the experiment at TU Vienna, a beam of light is controlled by an external magnetic field. The two systems are very much alike. Before optical circuits for computers can be considered, the newly discovered effect will prove useful as a tool for further research. In optics labs, it will play an important role in research on new materials and the physics of light.