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Scientists slow light to a crawl in liquid crystal mix

Researchers from France and China have discovered a way to put a speed bump in light’s path, reducing its velocity to less than one billionth of 300,000 kilometers per second, its top speed. Embedded dye molecules in a liquid crystal matrix were used to slow the light, which may find future applications in remote sensing and measurement science.

Reported today in the journal Optics Express, the new approach requires little power, operates at room temperature, and does not need an external electrical field, making it much more practical than other similar experiments. Slowing down light allows researchers to more easily compare the characteristics of different light pulses, which could lead to the development of instruments to measure extremely slow speeds and small movements.

Scientists have long known that light reduces in speed when travelling through matter, but the magnitude of this slow-down in materials like glass or water is less than a factor of two. The question became one of manipulating matter in order to achieve a more considerable light slow down.

The key is to take advantage of the way light travels. When travelling as a pulse, light is really a collection of waves, each with a slightly different frequency. However, all the waves in the pulse must travel together. Thus, researchers can design materials to act as obstacle courses that impede some of the waves more than others. The pulse must first pull itself together again before it can exit the material.

Other research teams have manipulated the properties of atomic vapors or crystal lattices to significantly slow light inside a medium. In this study, the team instead used a liquid crystal that could operate in a simple setup. A chemical component was added, which twisted the liquid crystal molecules into a helical shape. Added dye molecules, which nestled in the helical structures, changed their shape when irradiated by light, altering the material’s optical properties and changing the relative velocities of different wave components as the light traveled through. The helical structure of the matrix ensures a long lifetime of the shape-shifted dyes, making it possible to store a light pulse and release on demand.

This technique is well suited to sensing and interferometry applications, and could be used to develop a highly sensitive instrument that operates on a similar principle to a radar gun.

The next step for the team will be to test the approach in similar phase sensing applications. The researchers will attempt to extend the work to other molecular arrangements and different types of dye.