Nonlinear optical properties of high glass-transition temperature polyphosphazene films

Polyphosphazenes with high glass-transition temperatures have been prepared according to a newly developed synthetic route. The 4-[(4′-nitrophenyl)azo]phenoxy chromophore has been incorporated as a side-chain group in a concentration around 30% by weight. The nonlinear optical (NLO) properties of polyphosphazene films have been studied. The three components of the second-order NLO susceptibility tensor for corona poled films are χ15(2)=9.3×10−24 C/V2 (2.5×10−9 esu), χ31(2)=9.6×10−24 C/V2 (2.6×10−9 esu) and χ33(2)=28.2×10−24 C/V2 (7.6×10−9 esu) for a poling field of 6 kV and a temperature of 140°C. The Kleinman symmetry and the condition χ33(2)=3χ31(2) are approximately obeyed within the experimental error. At room temperature (RT), the thermal stability of the second-order NLO response is very high since the χ15(2) value decreases by less than 10% in 70 days. The third-order NLO susceptibility has also been determined and reaches 0.38×10−31 C m/V3 (0.31×10−12 esu) for the unsubstituted polymer and 3.2×10−31 C m/V3 (2.6×10−12 esu) for the substituted (side-chain) one.