An electromagnetic plane wave incident obliquely from a transparent medium onto the surface of an absorbing medium can be reflected with 0 or π phase shift if (i) the wave is p (TM) polarized, and (ii) the complex relative dielectric function ε is such that 0 ≤|ε| 2/2Re(ε) ≤ 1. Furthermore, the locus of ε such that the reflection coefficient for the p polarization is real at the same angle of incidence, is a circle, and that of ε½ (the complex relative refractive index) is Bernoulli’s lemniscate.
Journal of the Optical Society of America (1917-1983)
R. M. A. Azzam, "Reflection of an electromagnetic plane wave with 0 or π phase shift at the surface of an absorbing medium," J. Opt. Soc. Am. 69, 487-488 (1979)