2ND-HARMONIC GENERATION IN THIN-FILMS OF COPPER PHTHALOCYANINE

We have observed the optical second-harmonic generation (SHG) from a p olycrystalline film of copper phthalocyanine (CuPc) with thicknesses b etween 40 and 2000 angstrom. The nonlinear susceptibility of this film is chi(ZYY)(2) = 1.89 x (3/4pi) x 10(-7) esu where Z and Y are the di rections perpendicular and parallel to the film surface, respectively. This value is unexpectedly large, approaching a quarter of the value for the well-known nonlinear optical crystal LiNbO3. The finite value of the nonlinear susceptibility indicates that the structure of this f ilm lacks inversion symmetry. The polarization properties of the secon d-harmonic (SH) light show that the film lacks inversion symmetry perp endicular to the film surface. This result contradicts the observation by x-ray diffraction that indicates that the structure of this film i s centrosymmetric. Our data indicate that the true crystal structure o f this film is slightly deformed from the one deter-mined by x-ray dif fraction. The slight deformation makes SHG possible, and the observed strong SHG arises from a resonance of the SH light to an electronic tr ansition. We propose a mechanism of this deformation-induced SHG that is based on the electronic orbitals of this molecule. In order for the SHG to be possible, the molecular plane of CuPc must be nearly perpen dicular to the substrate.