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How to control polarization of light

October 03, 2016

The results of this case can be used in the manufacture of the so-called spatial light modulators serving as a base for all LCDs, for example. They are arranged so that each pixel can switch the light with some speed, making it brighter or weaker, and this switching is performed by the rotation of the polarization of light. According to one of the authors, Tatyana Dolgova, senior researcher of the Laboratory of Nanophotonics and Metamaterials, Faculty of Physics, MSU, the new fast spatial light modulators can be used when creating a holographic memory, three-dimensional displays, as well as the accurate refractive index sensors and magnetic field sensors.

The recording speed in the three-dimensional holographic memory is directly dependent on the spatial light modulator's switching speed. This speed is highly limited in liquid crystals, as they perform the polarization rotation by rotating the LCD molecule itself, which takes tens of milliseconds. Scientists have proposed to carry out the rotation not by a mechanical turn, but by the effect, discovered by Faraday. Its essence lies in the fact that the plane of polarization of the light is rotated as it passes through a magnetized material.

In 1998, one of the authors of the article, Japanese physicist Mitsuteru Inoue proposed the concept of spatial light modulators based on new nanostructures - magnetophotonic crystals. These micro crystals contain optical resonators - a system of two parallel mirrors. Today the main scope of its use is a significant "slowing down" the light. A photon caught in such a resonator, moves between the mirrors and comes out after a significant delay. So if a polarized light passing through the crystal is placed to the magnetic field, the Faraday effect increases with each pass from mirror to mirror, and ultimately becomes much more noticeable.

"We are working on magnetophotonic crystals together with Professor Inoue almost from the beginning, and during these fifteen years have learned a lot about these amazing nanostructures," says Tatyana Dolgova. "And finally we got to the ultra-fast light modulation. In our experiments with the real crystals we have ensured that the light is about ten times slower than if it is simply in the air. And it increases the Faraday rotation by an order of magnitude!"

According to Dolgova, there is no paradox between the "slowing down" the light and the resulting ultrafast modulation. "The 'slow' light is actually only relatively slow compared to the speed of light in a vacuum, but it is still incomparably fast compared to the speed of the liquid crystal molecules rotation."

In their experiments, the MSU physics have ensured that the plane of polarization of the "slow" light is turned so quickly that it is significantly different even between the beginning and the "tail" of a 200-femtosecond laser pulse. (Femtosecond is one quadrillionth of a second, or one millionth of a nanosecond) Dolgova said that the magnitude of the effect obtained in the Moscow State University is still insufficient for practical use, however, the limitations are not fundamental. Physicists have shown clearly: ultrafast modulation of light in magnetophotonic crystals is possible and has more than good prospects.

Despite the fact that the liquid crystal modulation rate is enough for conventional screens the super switching speed is necessary in the such devices in nanophotonics where photons are used instead of electrons to perform some logic or counting, photonic switching, optical recording, - namely, for the prospect of creation of photonic computers. And now a group of Professor Inoue demonstrates the samples of the three-dimensional holographic memory and displays for playback of 3D images and video, working with fast magnetophotonic spatial light modulators.
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Lomonosov Moscow State University

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