Mixture containing dye in a liquid crystal host.

Researchers in France and China have found a novel method of slowing and stopping light pulses: by embedding azobenzene dye molecules in a liquid crystal matrix to act as a “speed bump.” The team slowed the group velocity of light pulses down to less than 1 mm/s and stored them for as many as 160 ms (Opt. Express 21, 19544).
The group, led by Umberto Bortolozzo of the Université de Nice-Sophia Antipolis in France, deposited the dyed liquid-crystal host, mixed with a chiral agent, between two glass plates separated by 25-μm-thick spacers. The dye molecules change shape under irradiation with light in the ultraviolet to green range. By selectively illuminating the liquid crystal region, scientists can make the effect occur in desired patterns, such as a diffraction grating.
In their experiments, the researchers sent 140-ms-long laser pulses through the liquid crystal setup and observed group delays of 31.3 ms, for a group velocity of 0.8 mm/s. The chiral structure in the liquid crystal is responsible for storing the pulses.
A different team led by Bortolozzo used a liquid-crystal “valve” to measure Doppler shifts as small as 1 μHz in a pair of laser beams (Opt. Lett. 38, 3107). Both lines of research may have future applications in interferometry, quantum computing and optical sensing.
Bortolozzo and his Nice colleagues were joined by scientists from Xiamen University in China on the first article and from the University of Rochester in the U.S.A. on the second paper.