To improve the orientational stability of the chromophore induced by an electric field in a second-order nonlinear optical polymer, we have synthesized a thermally stable and cross-linkable azo dye, 4-[(4‘‘-aminophenyl)sulfonyl]-4‘-[N,N-bis(2-hydroxyethyl)amino]azobenzene. The transparent films with silica as matrix and organic chromophore as cross-linking agent were then prepared by using a sol−gel process. The resulting films heated at 200 °C under a high-corona field exhibited the nonlinear optical coefficient d 33 = 32 pm/V in Maker-fringe measurement. Long-term stability of second-harmonic coefficients in these films was observed even at 160 °C.