Brewster phenomena in a lossy waveguide used just under the cut-off thickness

The authors determine the propagation constants of the leaky modes supported by a leaky waveguide by studying the poles of the extension into the complex plane of the reflection coefficient. Under a cut-off thickness, the pole no longer exists, but a complex zero appears. For convenient values of the guide parameters, the zero crosses the real axis. Thus, there exists a real incidence corresponding, for a propagating plane wave, to a Brewster absorption in grazing incidence. The prediction is confirmed by direct calculations of the reflection coefficient, whose behaviour near the absorption peak is studied.