Poled photochemical stability

Assessing thermal and photochemical stability is important. Thermal stability can be readily measured by measuring properties such as second harmonic generation as a function of heating at a constant rate (e.g., 4-10 °C/min) [121]. The temperature at which second-order optical nonlinearity is first observed to decrease is taken as defining the thermal stability of the material [2,3,5,63,63]. It is important to understand that the loss of second-order nonlinear optical activity measured in such experiments is not due to chemical decomposition of the electro-optic material but rather is due to relaxation of poling-induced acentric... 

Achieving sufficient lattice hardening is important for several reasons in addition to securing necessary thermal stability for the poling-induced electro-optic activity [27-31,100-102]. As noted in the previous section, photochemical stability and optical loss (associated with material processing) are frequently strongly dependent on lattice hardness. 

Although very large hyperpolarizabilities have been obtained with long and unprotected polyene bridges, these materials are chemically and photochemically unstable or it is impossible to prepare them in adequate yield and purity, and therefore, they are not suitable for practical applications. On the other hand, chromophores based on fused ring systems, such as naphthalene benzimidazoles [38] or phthalocyanines [39,40] possess good nonlinearity and thermal stabilities over 350°C, but they are poorly soluble in spin-casting solvents and they cannot be effectively poled. 

Amoco Ultradel 9000D aromatic polyimides (Fig. 2, below) are a family of 7-butyrolactone (GBL) soluble, fully imidized, fluorinated polyimides developed for integrated optical applications. Thermal or photochemical cross-linking imparts aTg approaching 400 °C and provides stability for poled polymer systems. Excellent optical transparencies have also been demonstrated in these materials. 

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