Diffraction efficiency, photorefraction

Figure 9 Diffraction efficiency of photorefractive grating in the composite systems. Note that high diffraction efficiency for the composites containing the intramolecular charge transfer dopants 3 and 4 occurs at lower applied voltages than those for the intermolecular charge transfer dopants.

Figure 10 Diffraction efficiency of photorefractive grating as a function of optical intensity for composites containing either 3 or 5/NI. The composite containing 3 saturates at higher diffraction efficiencies than the composite containing 5/NI.

Photorefractive polymers have large diffraction efficiencies (ti approaching unity), but because they respond on times Is, they have TDE values 1 s ... 

If bifimctional molecules do not form glasses, they can be incorporated into polymer binders. Examples of bifunctional molecules are shown in Fig. 24. DTNBl (molecule (b) in Fig. 24) was doped in PMMA and Cgg was used as a sensitizer. Diffraction efficiencies of 7%, sub-second grating growth times, and net two-beam coupling gain coefficients of 34 cm" were observed in such samples [81]. DPANST (molecule (c) in Fig. 24) was doped into PBMA [84]. Recently, ms response times were reported in photorefractive glasses based on the bifunctional chromophore DRDCTA (molecule (d) in Fig. 24) doped with the plasticizer DOP (molecule (b) in Fig. 23) and Cgg as a sensitizer [89, 90]. 

Fig.25. Normalized diffraction efficiency vs applied field measured in 105 pm-thick samples of the polymer-dispersed liquid crystal TL202 PMMA ECZ TNFDM (circles) and the photorefractive polymer DHADC-MPN PVK ECZ TNF (squares)...

E. J. Smiley, D. J. McGee, C. Salter, and C. R. Carlen. Diffraction efficiency and phase stability of poly(V-vinylcarbazole)-based photorefractive polymer composites as a function of azo-dye concentration. 7. Appl. Phys., 88(8) 4910-4912, October 2000. 

We have prepared multi-component photorefractive polymers for optical data storage applications (8). This photorefractivity is a combination of two functionalities electro-optic effect and photoconductivity. We have achieved holographic diffraction efficiencies up to >30% in our composites. 

The material has to simultaneously possess photoconductivity and electro-optical effect to have photorefractive properties. Typical candidate materials have low glass transition temperature, frequently reduced by the plasticizer. Diffraction efficiency is improved by addition of the plasticizer because chromophore groups have higher rotational mobility and increase their contribution of birefringence to the total refractive index modulation. ... 

The chromophores serve for the formation of efficient photorefractive gratings and they are photo-chemically active, probably by 2 -n 2 photochemical reactions, when triplet sensitized. The photorefractive external diffraction efficiency is highly dependent on the glass transition temperature of the composites. A low glass transition temperature favors the efficiency. 

Meerholz K, Volodin BLSandalphon BL, Kippelen B, Peyghamharian N. A photorefractive polymer with high optical gain and diffraction efficiency near 100%. Nature 1994 371 497-500. 

Smiley EJ, McGee DJ, Salter C, Carlen CR. Diffraction efficiency and phase stability of poly(V-vinylcarbazole)-based photorefractive... 

Marder et a/./ describe the synthesis of several new photorefractive polymers, which are composed of a new type of nonlinear optical chromophore attached to conjugated polymer, poly(p-phenylene-thiophene). Since the NLO chromophore is labile in many reaction conditions, the Stille coupling reaction was used to prepare these polymers. The resulting polymers exhibit high PR performances. An optical gain coefficient of 158 cm at a field of 50 V/ m and a diffraction efficiency of 68% at a field of 46 V/ m for polymer PI were obtained, which are among the best values for fully functionalized PR polymers to date. 

On the other hand, formation of photorefi active silicone composite with good performance was reported A carbazole-substituted polysiloxane that was sensitized by 2,4,7-trinitro-9-fluorenone was used as a photoconducting medium and l-[4-(2-nilrovinyl)phenylpiperidine was added as an optically nonlinear chromophore. The photorefractive property of polymer was determined by diffraction efficiency using a 100 pm-thick film. The maximum diffraction efficiency (rj max) of 71% was obtained at the electric field of 70 V/pm. 

Photorefractive materials alter their refractive index in response to light. These materials can be used to record thick phase holograms with very high diffraction efficiency. This recording process can also be reversed, either by uniform exposure to light or by heating. These materials, too, have rather low sensitivity, but research is continuing to produce improvements. 

---