1. The invention relates to the use in non-linear optics of azo dyes of formula I and the use of polymers containing azo dyes which include as characteristic monomer units bivalent groups derived from an azo dye of formula (I) wherein: D represents the residue of a diazo component derived from an aniline or from a five-membered aromatic heterocyclic amine whose heterocyclic ring includes one to three heteroatoms chosen from the group nitrogen, oxygen and sulphur and which can be anellated by a benzene, thiophene, pyridine or pyrimidine ring; R?