Several types of waveguiding structures are known that can support the propagation of a finite number of bound electromagnetic modes. Two such structures are the dielectric slab and the optical fiber. In both structures the electromagnetic field associated with the bound modes extends beyond the central region; that part of the field that penetrates into the surrounding medium is termed evanescent. In this paper we use first-order perturbation theory to treat the effects caused by a surrounding medium with gain on the bound modes of the dielectric slab. A noteworthy effect is the amplification of these bound modes in accordance with formulas we present and which arises by evanescent-wave interaction with the surrounding medium.
K. O. Hill, A. Watanabe, and J. G. Chambers, "Evanescent-Wave Interactions in an Optical Wave-Guiding Structure," Appl. Opt. 11, 1952-1959 (1972)