Slow Light Observed in Biological Film

Gary Boas

The phenomenon of slow light promises to contribute to advances in all-optical switching for telecommunications. Scientists have demonstrated slow light using materials such as atomic vapors, ruby crystals and quantum dots. However, because of their fixed lifetime and transition rates, inorganic materials do not allow wide all-optical control over the group velocity of photons. This sort of flexibility is crucial for all-optical switching.

Now, researchers at the University of Massachusetts Boston have achieved controllable ultraslow light using biological bacteriorhodopsin thin film. The material offers broad control over the group velocity, from less than 0.1 mm/s to the speed of light, said D.V.G.L.N. Rao, the principal investigator of the study.

This is possible because the bacteriorhodopsin molecule has two significant chemically isomerized states, the B and M states. The B is the stable state, and the M state has a relatively long lifetime, varying over orders of magnitude. The B state has an absorption band at 570 nm; the M state at 410 nm. Using the 568-nm beam from an argon-krypton laser from Coherent Inc. of Santa Clara, Calif., and the 442-nm output of a HeCd laser, they demonstrated controllable switching between the two states in a 100-μm-thick polymer film doped with bacteriorhodopsin molecules. This allowed them to modulate the rate of delay and to control the photons’ velocity.

In this way, Rao said, organic materials could enable all-optical switching applications requiring optical buffers and controllable delays. With optical routers, for example, a tremendous amount of information arrives at a single point, and users will want to delay some of that, depending on the priority. Work remains to be done before this is possible, however. “Right now, we are working with relatively long pulses,” he explained. “For real-world application, we may need to work with much shorter pulses. That is what we are planning to do.”

Physical Review Letters, Dec. 16, 2005, 253601.