Incorporating news from O plus E magazine, Tokyo
TOKYOResearchers led by Tomiki Ikeda, a professor at the Tokyo Institute of Technology, have developed a holographic recording material that contains polymer liquid crystals and makes use of the optical isomerization of azobenzene. Optical interference patterns are recorded as changes in the index of refraction of the material, resulting in a phase hologram.
When light illuminates azobenzene that exists in the trans state, the material isomerizes to the cis state (see figure). The cis state is unstable with respect to heat, so it eventually returns to the trans state.
A polymer azobenzene liquid-crystal layer approximately 500 nm thick was grown on a glass foundation. When a 488-nm beam is incident upon this layer, the optical isomerization reaction causes a transition from the trans to the cis state. This causes the molecules to orient themselves isotropically. Because the index of refraction between the cis and trans states differs by 0.08, holograms can be recorded by creating interference bands on the foundation using an appropriate optical system. Write time is only 200 ns. Interference bands with a line pair of 1 µm have already been recorded. Holograms of three-dimensional objects have also been successfully recorded and played back.
Light causes azobenzene to isomerize from the trans to the cis state (top). Light at 488 nm changes liquid-crystalline azobenzene to an isotropic state with a change in index of refraction of 0.08 (bottom).
The data can be erased by raising the temperature above the glass transition temperature of 70°C. Holograms can thus be rerecorded using the same material. Because the cis state is unstable, the molecules gradually return to the trans state. However, the positions of the molecules remain the same. The material is extremely stable at room temperature and the data do not disappear even after a year.
The research group believes that this material can be used not only in holographic displays, but in optical-memory devices and holographic optical elements.
Courtesy O plus E magazine, Tokyo