Twisted crystal slows light

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Researchers from France and China have slowed the group velocity of light back to less than one billionth of its speed in a vacuum. They did so by embedding dye molecules in a liquid crystal matrix.

The new approach to manipulating light, conducted by a group from France’s Université de Nice-Sophia Antipolis and China’s Xiamen University, uses little power, does not require an external electrical field, and operates at room temperature, making it more practical than many other slow light experiments. 

The results of their experiments have been reported in The Optical Society’s open-access journal, Optics Express.

The technique may allow scientists to compare the characteristics of different light pulses more easily, which in turn can help them build highly sensitive instruments to measure extremely slow speeds and small movements, said Umberto Bortolozzo, one of the authors on the Optics Express paper. In a companion paper published in Optics Letters, Bortolozzo and colleagues from the Université de Nice-Sophia Antipolis and the University of Rochester describe an instrument that uses slow light to measure speeds less than one trillionth of a metre per second.

It has long been know that the velocity of light is lower in a medium than in vacuum, but the magnitude of this slow-down in typical materials such as glass or water is less than a factor of two. The key to achieving a significant drop-off in speed is to take advantage of the fact that when light travels as a pulse it is really a collection of waves, each having a slightly different frequency, said Bortolozzo. However, all the waves in the pulse must travel together. Scientists can design materials to be like obstacles courses that 'trip up' some of the waves more than others.  In order to exit the material together, the pulse must wait until it can reconstitute itself. 

Other research groups have manipulated the properties of atomic vapours or crystal lattices to slow light. Bortolozzo’s team used a liquid crystal that could operate in a simple setup, did not require external voltages or magnetic fields, and worked at room temperature and with very low optical power. They added a chemical component that twisted the liquid crystal molecules into a helical shape and then added dye molecules that nestled in the helical structures.  The dye molecules change their shape when irradiated by light, altering the optical properties of the material and hence changing the relative velocities of the different wave components of the light pulse as it travelled through.