Optical second-harmonic generation from oligothiophene films

We have studied surface second-harmonic generation from thin layers of several oligothiophenes deposited by vacuum sublimation on fused silica substrates. Although these oligothiophenes have a symmetric pi-electron system, they show a second-harmonic response which is strongly dependent on the conjugation length. Their average orientation was also determined by analyzing the polarization dependence of the second-harmonic signal.