Preparation of Materials for Optical Application

Materials with non-linear optical (NLO) properties are of particular interest to the telecommunications industry and specific to the technology of photonics [150]. The impregnation of polymeric materials with organic, azobenzene dyes is a possible route to the preparation of these materials. Poling techniques are required to avoid the polar molecules aligning antiparallel to each other, which avoids centro-symmetric arrangements [151]. A thorough review of the preparation and assessment of polymeric materials with azo-dyes for non-hnear optics has recently been prepared by Yesodha et al. [152]. 

The use of CO2 to prepare such materials involves the plasticization of the polymer matrix followed by the impregnation of the chromophore. The apphcation of an electric field results in the ahgnment of the dye molecules, and this is followed by depressurization of the system, which results in the rapid escape of CO2 and the consequential freezing of the dye alignment in the matrix. A specific benefit of this method is that the rapid release of CO2 enables the matrix to be frozen , and hence the dye molecules do not have time to reorientate [76]. A general problem with the polymeric guest-host system is the instability of the system due to polymer chain relaxation, which can result in the loss of the necessary alignment. Supercritical fluid treatment of such materials allows one to process them at lower temperatures because of plasticization and thus possibly to achieve better orientation of the dyes in the polymer matrix. 

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