Abstract

The guided modes of thin absorbing Langmuir-Blodgett films of organic dyes surrounded by identical media were studied. A symmetric waveguide has no cutoff thickness and thus can be used to study very thin films of only a few monolayers. The modes can be excited by the frustrated total reflection technique using a high-index prism. The dye is modeled as having an isolated absorption band with a dielectric function represented by the Drode-Lorentz expression. The film is assumed to be uniaxial with the chromophore lying either parallel to the film or perpendicular to it. When the product of the transition moment by the relaxation time is large enough, the real part of the dielectric constant becomes negative at some wavelengths. For a parallel orientation, the film then supports two modes identified as the long-range and short-range exciton polariton modes that merge into a single surface mode for infinite film thickness. For the perpendicular orientation, a new type of exciton polariton mode appears for very thin films that is not supported as a surface mode when the thickness is infinite. Calculations show that a large enhancement of the field in the film on excitation by a prism is possible in many cases, opening up possible applications for nonlinear optics.

© 1988 Optical Society of America