A rod-like fluorinated polyimide as an in-plane birefringent optical material 2: Control of optical retardation using spontaneous molecular orientation

Abstract

Following previous work, a fluorinated polyimide with a rod-like structure has been investigated as an in-plane birefringent optical material whose birefringence and thickness can be precisely controlled. Poly(amic acid) films fixed in a metal frame by two sides and thermally cured without any drawing resulted in a polyimide film with an in-plane birefringence (Δn) larger than 0.1 at 1543 nm. The optical retardation, which is defined as the product of Δn and the film thickness, was controlled by varying the curing and post-annealing temperatures and by using reactive ion etching. In situ measurements of the tensile stress and the generated retardation showed that the initial orientation at below 200°C was due to the large tensile stress caused by the film shrinkage during imidization and that the increased Δn at higher temperatures was caused by the spontaneous orientation of the polyimide molecules. The curing temperature dependence of refractive indices, optical transmittance in the visible and near-infrared region, and the wavelength dispersion of retardation of the in-plane birefringent polyimide films are also reported. Copyright © 1999 John Wiley & Sons, Ltd.