Determination of the optical permittivity and thickness of absorbing films using long range modes

Analytic theory and numerical modelling are presented for the prism coupling of light to long range surface modes supported by a thin absorbing film. If both transverse magnetic and transverse electric modes can be excited, then by comparing theory to angle dependent reflectivity data the optical permittivity and thickness of the film may be unambiguously established. An experimental confirmation is presented for thin films of indium tin oxide (ITO) on glass substrates. The environment of the ITO is made optically symmetric, to support the long range mode, by use of a fluid which is index matched to the glass substrate. Attenuated total reflection, with prism coupling through the matching fluid, gives, with suitable fluid thickness, sharp resonant modes in the angle dependent reflectivity. Comparison of such data with modelling theory yields the ITO parameters.