The exploration of light for applications in optical integrated devices has received world wide attention during the past decade.Organic compounds are of interest because of their large hyperpolarisabilities β and fast response times as well as offering many diferent types of processing geometry. It has been generally understood in the past that the requirements which must be satisfied by the microscopic system (i.e.the molecule) in order for the macroscopic system (i.e. the crystal ) to display second harmonic generation are the following: (a) the molecule must have a dipole moment μ (b) the molecule must have large components of hyperpolarisability β (c) the molecules must adopt a mutual orientation in the unit cell which avoids centro-symmetry .
In practice, TATB was found to have a very large optical susceptibility,although it did not satisfy conditions (a) and (c). With regard to condition (c), it was suggested that structural defects might explain the appearance of second harmonic generation in a centro-symmetric crystal. For this reason we decided to re-examine the problem using electron microscopy and diffraction.