Ghost surface polaritons at oblique antiferromagnetic surface

Abstract

We investigated hybrid-polarization surface magnon polaritons at the oblique surface of antiferromagnet (AF) in an external magnetic field pointed along the AF easy axis, where the easy axis does not lie in the AF surface. According to the direction of surface-polariton propagation, there are two specific geometries. In geometry-I, where the direction of propagation is vertical to the easy axis, we found two hybrid-polarization surface polaritons (HSPs), which are highly nonreciprocal. They are one ordinary hybrid-polarization surface polariton (called a HSP still) and one ghost surface polariton (GSP-I). GSP-I can exist not only inside but also outside the AF reststrahlen frequency bands and originates from the external magnetic field. In Geometry-II with the easy axis in the propagation plane, we predicted another ghost surface polariton (GSP-II). GSP-II is a TE surface wave and is reciprocal. Another difference between the two GSPs is that the Poynting vector of GSP-I is of oscillation-attenuation but that of GSP-II monotonously decays with the distance from the surface and GSP-II exists only in the reststrahlen band without external magnetic field. This study further enlarges the family of surface polariton and widens the window of polariton application.