Tunable optical effects in natural hyperbolic magnetic media

• Main Author: Macêdo, R.
• Published: University of Glasgow 2016
• Subjects: 621.381; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.712620

Anisotropic media have been the object of study in the fields of optics and photonics for decades. More recently, we have seen the emergence of a new class of anisotropic materials, the so-called hyperbolic media. These new structures rapidly gained attention for displaying several optical effects such as all-angle negative refraction with low loss. This thesis presents a theoretical investigation of anisotropic magnetic crystals working as hyperbolic media. These type of structures are not often associated with hyperbolic behaviour. The aim is to show that magnetic crystals are not only an alternative way to obtain hyperbolic dispersion but that they can also be controlled by externally applied fields. The general theory is applied to manganese fluoride (MnF2), a reasonably well studied antiferromagnet, in a configuration in which spins cant in response to applied magnetic fields. The work can be divided into three main strands. The first is to treat the surface reflection and illustrate how light beams can be laterally shifted from the position predicted by geometrical optics i.e., the so called Goos-Hänchen effect. In antiferromagnets these shifts can be controlled with an external magnetic field. Moreover, the possibility of nonreciprocal displacements is investigated, for both oblique and normal incidence, due to an inherent nonreciprocity of the polariton phase with respect to the propagation direction. Reciprocal displacements are also studied in the absence of an external field. The second strand of the work is a study of how all-angle negative refraction can occur in a uniaxial antiferromagnet close to the magnon resonance frequency. This behavior is based on the fact that, in such cases, the antiferromagnet acts as an indefinite permeability medium, i.e., not all its permeability tensor components are of the same sign. If an external magnetic field is applied, the angle of refraction becomes tunable, and can be made to change sign. The final part of this work proposes tunable slab lensing in natural magnetic media using an externally applied magnetic field. Natural hyperbolic magnetic materials not only display imaging obtained from slab lenses with plane parallel sides but also allow one to modify the focal length of a slab lens with an externally applied magnetic field. This is possible because antiferromagnets are gyrotropic and support magnetic polaritons whose frequencies are sensitive to magnetic fields. In addition, an investigation of the caustic formation in this type of lenses is presented for low temperatures, when damping should be small. Slab focusing is also studied at higher temperatures.