| In this thesis, we consider the polaritonic band structures and the potential appli-cations of the superlattices composed of single phase multiferroic materials. The3d electron shell of the single phase multiferroic materials is partially filled and charge distribution has no space inversion symmetry which allow the coexistence of ferromag-netic and ferroelectric orders. The general multiferroic materials are also piezoelectric and piezomagnetic materials, they have both the piezoelectric and piezomagnetic prop-erties. The domains have been polarized and magnetized simultaneously and adjacent domains have opposite polarization and magnetization, which have been arranged peri-odically along a certain direction and constituting the multiferroic superlattices. There-fore, the superlattices have been modulated periodically by the piezoelectric, piezo-magnetic and magnetoelectric constants, which leads to the phonon branch in bulk media to be folded and provides the possibility for the intersection of the photon and folded phonon branches. Thus the coupled polariton band structures are created. The time-space reversal symmetries of the superlattices are broken due to the coexistence of polarization and magnetization in a single domain, thus the polaritionic band structures are nonreciprocal with respect to the forward and backward propagation directions. The current difficulty is that the material with strong piezoelectric and piezomagnetic coefficients still has not been found to date. For the typical compound GaFeO3with relatively complete material parameters, the electromechanical coefficient is0.17, but the magneto-mechanical transducer coefficient (METC) is6to7orders of magnitude smaller. Therefore, the influence of the piezomagnetic effect in the actual situation is negligible. However, for METC superlattices with comparable electromechanical and magneto-mechanic coefficients, an extremely asymmetric polaritonic band structures are demonstrated, such band structure can be used to construct the slow light and one-way waveguide.As the piezomagnetic effect is so weak, we consider a superlattice composed of the ferroelectric (PMN-PT) and ferromagnetic CoFe2O4materials, the reasons for selecting these materials are that the large transverse electromechanical coefficient for the PMN-PT is0.41, and the METC for CoFe2O4is0.47. When an electric field is applied to a material, lattice stress is produced. The stress is then passed across the interface and affects the magnetization via piezomagnetic effect. The net result is the strong magne-toelectric effect. Such superlattices can be divided into several types depending on the domain arrangements and number of domains in the unit cell. The superlattices with one electric and magnetic domains is particularly interesting. The polaritonic bands for the forward direction have normal band gaps and for backward direction demon-strates a linear dispersion relation. Such feature can be utilized to construct microwave slow-light delay-line. For a unit cell of3.5microns and the length of superlattices of1mm, the frequency is in the microwave range. The delay times for the slow-light near the first and second band gaps are480ns and1052ns, respectively, which is much less than the decay time in PMN-PT and CoFe2O4domains. The forward and backward branches can be switched easily by flipping the external field. The resulting change of the refractive index can be made as larger as124.The whole thesis consists of five chapters.The first chapter gives a short review on the origin of magnetoelectric effects and applications of the single-phase multiferroic materials and the composite magnetoelec-tric materials. The applications include the slow light devices and one-way waveguide.In the second chapter, the dynamic equations and transformation matrix method of polaritonic excitons are derived.The third chapter describes the non-reciprocity of polaritonic band structures in the single-phase multiferroic superlattices, it is due to the broken space inversion and time reversal symmetry. We also discuss the possible applications.The fourth chapter concerns the field tunable multi-channel microwave delay-line constructed with the piezoelectric and piezomagnetic superlattices.The fifth chapter is a summary of this thesis. |
