Photorefractive Polymer Provides High Optical Gain and Long Lifetime

Michael D. Wheeler

SAN DIEGO -- Scientists have unveiled a durable, optically efficient polymer that improves on several years of research toward an inexpensive photorefractive material. Moreover, they have demonstrated its use as an efficient phase conjugated mirror, one in a long list of applications for such a material.
The commercial potential has prompted many researchers to develop inexpensive, organic polymers with the same photorefractive properties as their more expensive crystal counterparts. Those properties include optical gain: When a scientist directs two laser beams into a photorefractive material, some energy is transferred from one beam to another; thus, one beam is amplified and the other is attenuated. Part of the energy also transfers to the material, changing its index of refraction.
In early photorefractive polymers, this process yielded an optical loss: The materials absorbed too much of the lasers' light, resulting in no amplification. They also had an unacceptably short life span.
Researchers led by W.E. Moerner at the University of California at San Diego have developed a material comprising three 120-µm thick layers of the composite polymer PVK:PDCST:BBP:C₆₀ sandwiched between glass. The team reported that when it irradiated the multilayer polymer with Kr-ion and diode laser beams, the material exhibited an unusually high optical gain, amplifying the optical signal by a factor of five for a three-layer stack in an electric field of 60 V/µm.
The researchers found that if they reflect one beam off a mirror that is moving very slowly, the beam's ever-so-slight frequency shift actually improves gain to a factor of 500. Furthermore, the polymer does not decompose as rapidly as some others that have been reported with high gains, said Anders Grunnet-Jepsen, a team member.

Gaining on a solution
Spurred by this success, the group used the material to assemble a self-pumped phase-conjugating mirror. When a light image is projected through a distorting medium toward a mirror and back through that medium, the original image is doubly distorted. However, if this mirror is replaced with a phase-conjugating mirror, the polymer acts to "undo" the distortions when the light is reflected. The concept could be extremely useful for imaging applications, such as astronomy.
Besides optical storage and interferometry, Grunnet-Jepsen said it could be used in tracking distant objects and in lenseless imaging in photolithography.