Measuring the refractive indices of conjugated polymer films with Bragg grating outcouplers

Photoluminescence trapped inside a polymer film can be scattered out with a Bragg grating. The refractive index in the plane of the film and in the normal direction can be determined as a function of wavelength by measuring the scattering angles of the TE- and TM-waveguide modes. For the polymer poly[2-methoxy-5-(3′,7′-dimethyloctloxy)-paraphenylenevinylene], which is commonly used in light-emitting diodes, the refractive index in the plane of the film drops from 2.0 to 1.8 as the wavelength increases from 600 to 750 nm, while the refractive index in the normal direction is relatively constant at 1.57.