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23 September 2015 Some aspects of absorption and gain
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Absorption and gain are difficult to define precisely but in the sense we shall use the terms, they represent a failure of the sum of specularly reflected and transmitted light to equal that incident. A greater sum represents gain and a lesser, absorption. Although there is a variety of physical processes that can contribute to these phenomena, their quantitative expression can be achieved through a nonzero extinction coefficient k. In the normal way we encourage gain and discourage absorption, but, recently, an increasing interest in enhancing absorption has emerged. Calculations involving nonzero k are straightforward but understanding the consequences can be a little more difficult. The admittance diagram can help in this and especially in respect of lasers, amplifiers and perfect and coherent perfect absorbers. Normal thin-film calculations start from the light that enters the emergent medium through the rear surface and works backwards to find the conditions at the surface of incidence, but the normal model is capable of much more. It can readily handle a much wider range of conditions. In terms of the admittance diagram, it is simply necessary to open up the second and third quadrants of the complex plane. In those quadrants the net flow of energy is in the opposite direction and the rules for the loci are a little different from those in the more usual first and fourth quadrants.
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Angus Macleod "Some aspects of absorption and gain", Proc. SPIE 9627, Optical Systems Design 2015: Advances in Optical Thin Films V, 96270T (23 September 2015);


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