Complex-angle analysis of electromagnetic waves on interfaces

Abstract

Electromagnetic wave is reflected and refracted at interfaces, satisfying Fresnel-Snell law which is required by conservations of energy and momentum. If the incident angle is lower than the critical angle, we can use this Fresnel-Snell law, but the Fresnel-Snell law is modified in the case of existence of dissipation ($\tilde{n} = n + i\kappa, \kappa > 0$) or in the condition of total internal reflection. In the cases, we have to extend the angle of refraction from real number to complex number ($\theta \rightarrow \tilde{\theta} = \theta + i\psi, \psi \neq 0$). In this paper, by using complex-angle approach, we analyse the behaviour of electromagnetic waves in various kind of interfaces: dielectric - dielectric system, dissipative dielectric - dielectric system, and metal - dielectric system. We show that iso-frequency curves in wavenumber space is opened in the case where $n, \kappa > 0$, and closed either when $n \rightarrow 0$ or when $\kappa \rightarrow 0$ ("Lifshitz transition" of electromagnetic waves). Excess momentum (wavenumber) and anomalous spin (circular polarization) emerging with the transition are also discussed.