Improved diffraction efficiencies

A systematic study on the effect of plasticization on the PR properties is reported by Bolink et al. [58], It is shown that ECZ can be used as an efficient plasticizer, leading to a large increase in the gain coefficient and the diffraction efficiency, which arises solely due to an improvement in the orientational mobility of the dispersed NLO molecules. 

Another approach is to use better engineered chromophores with added side groups that improve compatibility. Cox et al. have incorporated a racemic ethylhexyl group into the azo chromophore (l-(2/-ethylhexyloxy)-2,5-dimethyl-4-(4 -nitrophenylazo)-benzene, EHDNPB) (Scheme 3g) to increase the solubility of the dye in PVK, inhibiting crystallization and, at the same time, acting as a plasticizer [65], A 60% device diffraction efficiency and gain coefficient of 120 cm-1 are achieved with 55 wt% chromophore. 

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. ... 

A major improvement was the usage of the photo-conductive polymer PVK. This allowed the concentration of the charge transport agent to be increased, while completely excluding crystallization of the carbazole groups [154], As chromophore, an azo dye 2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA) was used, and TNF as a sensitizer. The compositions showed almost 100% diffraction efficiency at a laser intensity of 1 W cm and 90 V p,m biased voltage. However, the response time was slow, >100 ms. 

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. 

The current focus is on improving the stability of materials while maintaining faster responses and high diffraction efficiencies at lower electric fields. 

---