Self - organizing photorefractive neural network

The three dimensional display of images of moving objects has been a dream for many decades. Ranging from a study of microscopic objects, such as cells, in their natural environment to three dimensional movies, this dream has been realized under certain circumstances. Short recordings of moving objects have been made holo-graphically in silver halide films. But apart from limited sensitivity, this type of recording requires the wetprocessing of film. Azobenzene-containing polymers have been under intensive investigation during the last decade as materials for digital as well as holo-graphic storage. However, since storage in these polymers involves the physical reorientation of long polymer chains, this process was thought to take place over several seconds, if not minutes. We have shown that holograms can be written in side-chain azobenzene polymers with a single pulse lasting 5ns from a pulsed laser. 1 The cyanoazobenzene side-chain polyester, P3aA, is prepared by transesterification of the cyanoazobenzene containing diol and diphenyl pththalate in the melt under vacuum. The diol is prepared in a sequence of reactions starting from bromopropyl alkylation of diethyl malonate, reduction to the corresponding 1,3-propanediol, ketal protection with ac-etophenone, coupling with cyanoazobenzene phenol and finally deprotection by acidic alco-holysis. Approximately 3 mg of the polyester material is dissolved in 150 microlitres chlo-roform and cast on to a clean substrate. The film is dried in an oven at a temperature of 90˚C for ten minutes. A polarization holographic setup is used to record holographic gratings (Figure 1). We use a commercially-available small-frame frequency doubled YAG laser, lasing at 532nm, as the source. This laser delivers Q-switched pulses of 5-7ns duration at 20 Hz, with a peak power output of 1.6MW/pulse. A polarization beamsplitter (PBS) is used in conjunction with appropriately-oriented quarterwave plates (QWP) to derive the orthogonally polarized beams. The two beams overlap on the polyester film. A HeNe laser is used to read-out the diffraction gratings. We find that after just one pulse from the laser, several orders of diffraction of the HeNe laser can be seen. The diffrac-tion efficiency in the first order exceeds 4% at a spatial frequency of 160lines/mm. Significantly, an atomic force microscopic scan of the irradiated polyester shows considerable surface relief. A peak-to-valley value of approximately 90nm was obtained at a spatial frequency of 900lines/mm. Several groups around the world have observed surface-relief in azobenzene polymers when irradiated with CW laser beams. However, the presence of a surface …