Light Takes It Slow In Liquid Crystal Matrix

This is a picture of a sample: 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. Image Credit: Dong Wei

Researchers have undertaken a practical and deceptively simple approach to slowing down light, using similar materials to those found in LCD screens.

Often experiments designed to slow light are focused on the end goal of manipulating a light pulse for optical communication applications. However, in this case it is hoped that the technique may have positive consequences for interferometers, sensors, and other measuring devices such as police radar guns.

One of the primary benefits is that slowing light down means that comparisons between different light pulses can be made much more easily, thus leading to a higher movement and speed sensitivity in devices.

It has long been known that light travels through materials at a slower speed than through a vacuum. For reference, the speed of light through a vacuum is 299792 km/s, through air it is around 299705 km/s, and through water it is about 225056 km/s. However, these speed reductions are often not enough to be practically useful, so much research has been undertaken to discover how materials can be altered to slow light to a greater degree.

This recent research was undertaken by a team from Université de Nice-Sophia Antipolis and Xiamen University, and was conducted under surprisingly simple conditions – under room temperature and without the need for an external electric field.

The material chosen to slow the light pulses was a liquid crystal, which was then impregnated with a chemical component that moulded the crystal into a helix, which dye molecules were then added to.

The dye molecules alter shape when light hits them (in this case provided by a green laser beam), changing from rod-like shapes, to ‘V’ shapes. This essentially gets in the way of the light trying to pass through the dye, meaning the light can be slowed substantially or even stored.

This image shows the molecular structure of the liquid crystal helix and the embedded dyes. The dyes are represented by red rods on the right of the picture. When the dyes are illuminated with light, they change from a rod-like shape (called a trans state) to a V-shape (called a cis state). The shape change can delay the passage of a light pulse or store a memory of that pulse. Image Credit: Optics Express

Now it is the team’s intention to test the method out with different dye molecules and other molecular arrangements. Umberto Bortolozzo, an author on the Optics Express paper outlining the findings, explains some of the future possibilities below:

"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."

This is a picture of the experimental setup for slow light in the liquid crystals medium: A green (532nm) laser beam is directed to the sample where it induces the structural changes of the dyes which are responsible for the slow light effect. Image Credit: Umberto Bortolozzo

Original source: The Optical Society

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G.P. Thomas

Written by

G.P. Thomas

Gary graduated from the University of Manchester with a first-class honours degree in Geochemistry and a Masters in Earth Sciences. After working in the Australian mining industry, Gary decided to hang up his geology boots and turn his hand to writing. When he isn't developing topical and informative content, Gary can usually be found playing his beloved guitar, or watching Aston Villa FC snatch defeat from the jaws of victory.


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