The theory of optical modes guided by a homogeneous thin film is developed in optical terminology. The properties of the modes are determined from the zeros of characteristic equations which describe the phenomena. When there are no losses, these equations are solved exactly. For the lossy case, simple exact solutions can not be found. A perturbation procedure is introduced which yields explicit first-order expressions for the optical constants N and K of any mode which can be supported by a lossy system. Under stated circumstances, the attenuation of the mode will be much less than that suggested by the loss tangents of the dielectrics comprising the system.
J. KANE and H. OSTERBERG, "Optical Characteristics of Planar Guided Modes," J. Opt. Soc. Am. 54, 347-352 (1964)