Light can be slowed to less than a billionth of its top speed by using embedded dye molecules in a liquid crystal matrix, which could lead to new technologies in remote sensing and measurement science. The molecular structure of the liquid crystal helix and the embedded dyes. The dyes are represented by red rods on the right. When the dyes are illuminated with light, they change from a rod-like shape to a V-shape. The shape change can delay the passage of a light pulse or store a memory of that pulse. Courtesy of Optics Express. Taking advantage of light’s travel as a collection of waves, each with different frequencies, researchers from the University of Nice Sophia Antipolis in France and Xiamen University in China designed obstacles meant to “trip up” some of the lightwaves. They found this technique to be more practical than many other slow-light experiments, as it operates at room temperature, uses little power and does not need an external electrical field. The experimental setup for slow light in the liquid crystals medium: a green (532 nm) laser beam is directed to the sample, where it induces the structural changes of the dyes that are responsible for the slow-light effect. Courtesy of Umberto Bortolozzo. To begin, the team used a liquid crystal similar to those used in LCD displays and added a chemical component to twist the liquid crystal molecules into a helical shape. The dye molecules were then added as obstacles, and they nestled themselves into the helical structures. The dye molecules change shape when irradiated by light, altering the optical properties of the material and changing the relative velocities of the different wave components as the light pulse travels through. In order to exit the material together, the pulse must wait for the lagging waves until it can reconstitute itself, slowing down the light’s travel. The helical structure of the liquid crystal matrix ensures a long lifetime of the shape-shifted dyes, making it possible to store a light pulse and later release it on demand. Slowing down light may help scientists compare the characteristics of different light pulses more easily, which can help in manufacturing highly sensitive instruments to measure extremely slow speeds and small movement, said researcher Umberto Bortolozzo. The mixture containing the dyes in the chiral liquid crystal host is in between two glass plates. The red color is given by the dyes, while the green reflection is due to the helical structure of the chiral host, which selects preferentially this wavelength for the reflection. Courtesy of Dong Wei. “Realizing slow and stopped light in these media is very exciting both for the fundamental research that discovers such new effects in soft matter systems, and for the new possibilities that these investigations could open in the fields of remote sensing and optical storage,” he said. The work appeared in Optics Express (doi: 10.1364/OE.21.019544). For more information, visit: www.osa.org