A new NLO-active lambda-shaped main-chain polyimide that comprises a two-dimensional carbazole chromophore was synthesized. This polyimide exhibits high thermal and temporal stability. It can endure temperatures of up to 240 degrees C for a transient time and maintain a large SH signal at 100 degrees C for a long time because embedding the two-dimensional chromophores into the polymer backbone effectively suppresses the randomization of the oriented dipole at high temperatures.