
373Introduction to Optical Coatings and Thin Film Production
Chapter 13
Gain, too, can be included in the admittance diagram. e easiest way to include
this is to change the sign of the extinction coecient when loss then switches to gain.
Figure13.27 shows the clockwise shrinking spiral locus that characterizes an absorb-
ing layer.
When gain is included, the spiral reverses and grows, although it is still described
clockwise at this stage. However, if the layer is thick enough, it reaches and passes
through the imaginary axis. At the crossing point, the reectance becomes 100% and
then greater than 100%. Beyond the imaginary axis in the second ...