  Total internal reflection occurs for large angles of incidence, when light is
incident from a high-refractive-index medium onto a low-index medium. We
consider the situation where the low-index medium is active. By invoking
causality in its most fundamental form, we argue that evanescent gain may or
may not appear, depending on the analytic and global properties of the
permittivity function. For conventional, weak gain media, we show that there is
an absolute instability associated with infinite transversal dimensions. This
instability can be ignored or eliminated in certain cases, for which evanescent
gain prevails.
