Second-harmonic generation in thin films of copper phthalocyanine

We have observed the optical second-harmonic generation (SHG) from a polycrystalline film of copper phthalocyanine (CuPc) with thicknesses between 40 and 2000 Å. The nonlinear susceptibility of this film is ${\mathrm{\chi }}_{\mathit{Z}\mathit{Y}\mathit{Y}}^{\mathrm{}\left(2\right)\mathrm{}}$=1.89×(3/4π)×${10}^{\mathrm{-}7}$ esu where Z and Y are the directions 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 ${\mathrm{LiNbO}}_3$. The finite value of the nonlinear susceptibility indicates that the structure of this film lacks inversion symmetry. The polarization properties of the second-harmonic (SH) light show that the film lacks inversion symmetry perpendicular to the film surface. This result contradicts the observation by x-ray diffraction that indicates that the structure of this film is centrosymmetric. Our data indicate that the true crystal structure of this film is slightly deformed from the one determined by x-ray diffraction. The slight deformation makes SHG possible, and the observed strong SHG arises from a resonance of the SH light to an electronic transition. 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 perpendicular to the substrate.