Refractive index modification of polysilane films by UV-light irradiation

The refractive index modification of polysilane films by the Ultra-Violet (UV) light irradiation was investigated. Poly(methylphenylsilane) (PMPS) and other polysilanes involving phenyl group attached directly to a silicone atom showed high refractive index (n(init) similar to 1.70) and very large refractive index reduction (Deltan = -0.14) at lambda = 633 nm compared with alkyl substituted polysilanes. The effective reduction in these polysilanes was found to be caused by the elimination of phenyl substituents in addition to the shortening of the (sigma-conjugation due to the photo-oxidation of silicon backbone for shorter wavelength UV light exposure. Thus, UV-exposed polysilanes turned to almost the same siloxane-like structure consisting of Si-O-Si backbone with alkyl substituents, and their refractive indices converged to a lower value of about 1.55. These results indicate that the molecular design of the side groups to enhance the initial refractive index can expand the dynamic range (Deltan) in the refractive index modification of the polysilane film.