Magnon-photon coupling in antiferromagnets in the presence of an external static magnetic field

The coupling of photons with long-wavelength magnons in antiferromagnetic Mn${\mathrm{F}}_2$ in the presence of an externally applied magnetic field in the body $c$ direction is investigated. It is found that two pure, circularly polarized modes can propagate in the crystal along the direction of the magnetic field. A study of the reflection spectra for normally incident plane-polarized radiation reveals that an antiferromagnetic crystal in an applied magnetic field can be used as an optical device to obtain pure, circularly polarized radiation.