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Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces
Published on: July 26, 2016
Total internal reflection and evanescent gain
Jon Olav Grepstad1, Johannes Skaar
1Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; and University Graduate Center, NO-2027 Kjeller, Norway.
Abstract:
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 transverse dimensions. This instability can be ignored or eliminated in certain cases, for which evanescent gain prevails.
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