Novel Nonlinear Optical Material with Poly(3-hexylthiophene) Thin Film Prepared on Quartz Glass Surface Modified by Self-Assembled Monolayer

The third-order nonlinear optical properties of regioregular poly(3-hexylthiophene) (RR-P3HT) thin films prepared on fused glass substrate were evaluated. The surface modification by hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (ODTS) was performed on the glass substrate to form self-assembled monolayer (SAM) layers. The formation of SAM layers on the glass substrate increase the contact angle of the solution and the optical property of the RR-P3HT thin films is enhanced due to the excellent orientation and alignment of the thin film. The pi-conjugated macromolecule thin films can be prepared by spin-coating and drop-casting methods and the structure and orientation alignment of thin films can be controlled by the solution processing and deposition techniques. The nonlinear optical property, third-harmonic generation of RR-P3HT thin films has been evaluated by Maker-fringe method. The third-order nonlinear optical susceptibilities chi((3)) (-3 omega; omega, omega, omega) of drop-cast RR-P3HT thin films on quartz glass substrate were estimated from optical third-harmonics (TH) intensity measurement. An Nd:YAG laser with a wavelength of 1064 nm, pulse width of 5 ns and a repetition frequency of 10 Hz was used to evaluate the TH intensity. The effect of surface modification of quartz glass substrate by HMDS and ODTS on the RR-P3HT film structure was also investigated. The orientation alignment and crystallinity of the RR-P3HT thin films were evaluated using X-ray diffraction (XRD) and UV-vis absorption spectra. The UV-vis and XRD profile reveals the better orientation and crystallinity of the RR-P3HT thin film after surface modification by HMDS and ODTS. Moreover the incident angle dependences of third harmonic (TH) intensity was measured and the TH intensity of RR-P3HT thin film prepared on glass substrate with SAM layer was found to higher than that of non-treated substrate. The SAM layers significantly enhances the optical property of the material and the third-order nonlinear optical susceptibility chi((3)) of RR-P3HT thin films on quartz glass surface modified by ODTS is 1.90 x 10(-8) esu. The third-order nonlinear optical susceptibility chi((3)) of RR-P3HT thin film was found to higher than poly(p-phenylenevinylene) (PPV).