Optimized leaky mode spectroscopy with a single planar film

A transparent dielectric layer coated on a high refractive index prism forms a planar leaky waveguide. The leaky mode spectrum of this layer strongly depends on the imaginary part of the layer refractive index κ. As an example the leaky mode spectrum of a doped polymer layer is studied theoretically and experimentally. For κ=0 no modes are detectable. In the range of κ≈10−4, so-called “optimized leaky modes,” can be observed while, in the range κ⩾10−3, broad resonances with reduced reflectivity can be observed. The imaginary part of the polymer refractive index was adjusted by doping with the dye malachit green with an absorption maximum at λ=620 nm.