Optical properties and waveguiding in films of a nonlinear polymer: Difluorophenyl-polydiphenylenevinylene (DFP-PDPV).

Third-order optically nonlinear polymeric materials built of rigid-rod molecules are often insoluble and transmit visible light poorly. We report on optical and waveguiding properties of a new soluble derivative of poly(p-phenylenevinylene) (PPV): a pi-conjugated polymer poly[biphenyl-4,4'-diyl-1,2-di(4-fluorophenyl)vinylene], (DFP-PDPV). We found that solvents used for processing of the DFP-PDPV polymer may influence the film optical properties. Changes in absorption spectra, birefringence, optical attenuation and the nonlinear: refractive index were observed in the films made by spin coating and solution cast using different solvents. The films are birefringent and dispersive. Refractive indices vary from 1.76 to 1.63 for the TE polarisation of the incident light and from 1.70 to 1.60 for the TM polarisation for wavelengths from 476.5 nm to 1.55 mu, respectively. The birefringence is an order of magnitude lower than that in films of unsubstituted PPV. DFP-PDPV is suitable for fabrication of optical waveguides. Low loss waveguiding layers could be prepared. Propagation losses were measured at 632.8nm, 810nm and 1064nm. The losses decrease at longer wavelengths reaching the level of 1 dB/cm at 1.06 mu. The waveguiding properties of DFP-PDPV films depend on the solvents used in processing of the polymer. We investigated thin films of DFP-PDPV for their potential for nonlinear waveguide applications. The modulus of nonlinear refractive index /n(2)/ in the range (0.9-1.5) x 10(-14) cm(2)/W was measured in DFP-PDPV films at 800nm using a femtosecond degenerate four wave mixing (DFWM) technique. These studies supplement the results of nonlinear optical studies of this polymer in solution using the Z-scan technique described in Proceedings of SPIE 3473, 79-90 (1998)(1).