Development of Solid State Non-Linear Absorbers - Dyes in Polymer Hosts
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incident light intensity. The paper, in particular, will concentrate on the incorporation of the RSA dye chloro aluminium phthalocyanine (CAP) 6 into the polymeric host material poly 2-hydroxyethyl methacrylate (HEMA) leading to the production of good optical quality solid state samples with no deterioration in non-linear optical performance compared to liquid samples prepared under similar conditions. This result is in marked contrast to the incorporation of the dye 1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide (HITCI) into a polymer matrix where, although excellent optical quality samples were produced, comparison with a liquid sample showed gross changes had occurred following the polymerisation process. The main aim of this study has been to identify some of the necessary steps, precautions needed and difficulties encountered in the stages leading to the production of solid state optical absorbers. The paper will describe the production of samples in terms of key parameters such as solubility, integrity of the dye molecules in the polymer host, hardness of the resulting polymer and the ability to achieve a good optical finish. In addition it will also give details of some thermal annealing trials which were carried out in an attempt to relieve some of the internal stresses which commonly occur as a result of the polymerisation process and details of anti-reflection coating experiments in order to reduce Fresnel losses. PRODUCTION OF POLYMER DISKS The choice of polymer matrix for encapsulating CAP was based on solubility trials carried out using a number of monomers. These included styrene, methyl methacrylate (MMA), hydroxy propyl acrylate (HPA) and 2-hydroxyethyl methacrylate (HEMA). The best solubility of CAP was 195 Mat. Res. Soc. Symp. Proc. Vol. 374 @1995 Materials Research Society
found in HEMA and HPA presumably as a result of their more polar nature compared to the other two monomers. Trials were conducted by dissolving CAP in HEMA and polymerising thermally in glass sample vials placed in an oven at 105'C. The samples were maintained in an inert atmosphere of nitrogen. The contents of the vials were seen to solidify within 24 hours with no discolouration observed. After a further 3 days at 105'C the vials were allowed to cool down to room temperature. Although the solubility of CAP in HPA was good, HPA was dismissed as a polymer host on the grounds that initial polymerisation trials produced very soft rubbery material that was difficult to polish. The polymer preforms once removed from the glass vials were machined on a lathe to a thickness some 0.25mm greater than the required final thickness to allow for material loss at the subsequent polishing stage. The samples were initially polished using a 3 micron diamond paste dispersed in a mineral oil (Dialap Fluid). Mineral oil was used because water based polishing media caused the material to soften as a result of the uptake of water by the polymer. Once conditions had been established, the material was found to remain hard and polish well. Where
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