Photoinduced second harmonic generation in molecular crystals caused by defects

We have found optical photoinduced second harmonic generation in pyrene molecular crystals photoinduced by intrinsic defects. A correlation between the number of macro-vacancies (defects) measured by positron annihilation and second-order nonlinear optical susceptibilities indicates the appearance of additional non-centrosymmetry in the electron charge density distribution caused by the defects. Simultaneously, changes in the kinetics of phenomena in the picosecond time regime are demonstrated. The latter indicate the essential role of intra- and intermolecular contributions in the observed phenomena.