Slab waveguides of poly(p-phenylenevinylene)s for all-optical switching: impact of side-chain substitution

Abstract

We have prepared thin film waveguides by spin-coating poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and three related alkoxy-substituted PPVs, which have a different degree of long alkyl side-chains. We use UV-Vis-NIR and FTIR spectroscopy, prism coupling, and third-harmonic generation spectroscopy to determine the intrinsic absorption coefficient, refractive index and anisotropy, birefringence, infrared dichroism, mode-propagation loss, and third-order nonlinear optical susceptibility, respectively. Our results reveal structure–property relations of the impact of side-chain substitution on the morphology of the films. At increasing amounts of long alkyl substituents, the aggregation tendency of the PPV chains is reduced, which causes strongly reduced mode propagation losses from 20 dB/cm to less than 1 dB/cm. We also see a dilution effect of the conjugated π-electrons of the polymer backbones within a shell of alkyl chains, and varied orientations of PPV main chain segments with respect to the layer plane. These insights help to design suitable polymers for all-optical switching, as well as for other applications in optoelectronics.