Optical characterization of alignment and effective refractive index in carbon nanotube films.

We have characterized the thickness and effective refractive index of carbon nanotube forests by fitting reflectance measurements in the visible and near infrared ranges. The measurements were performed with polarized light. An effective medium layer consisting of a mixture of graphite and air was used to simulate the nanotube film. The proposed model accurately described the behaviour of the reflected s-polarized component (Rs), which allowed for the precise determination of the thickness and porosity of the films, in very good agreement with SEM measurements of film thickness. The p-polarized component (Rp), on the other hand, could not be described in terms of the developed model. In badly aligned samples, where there is a mixture of Rs and Rp behaviour, the model fails to fit the Rs component as well. This effect can therefore be taken as an indirect indication of lack of alignment in the samples.