Magneto-Optical Nonlinearity Of Antiferromagnet/Dielectric Systems

The resonant frequencies of typical antiferromagnetic(AF) materials are dis-tributed over the far-infrared and millimeter ranges. It is theoretical and practical significant to study the nonlinearity of the AF, because the resonant frequencies are situated in terahertz(THz) frequency range. Scientists are attempting to de-velop the THz technology for the future communication and detection. The AF nonlinearity is weak, enhance the nonlinear effects of AF is useful primarily for application. The second harmonic generation(SHG) technique is a common tool to produce new light sources at frequencies where no laser is available, and this method is widely used as one method of frequency conversion. Optical bistabil-ity(OB) provides a basis for various optical devices, such as optical switches, op-tical memories and optical logic gates. Notably, the nonlinearity mentioned re-sults from the magnetic dipole-dipole interaction between electromagnetic waves and AF dynamic magnetizations. For AF films, in order to observe the AF nonlinearity one needs a very strong electromagnetic wave (or strong light) to illuminate them. It means that there are some serious difficulties on the present technical level. Therefore, it is a very interesting subject to look for a method of enhancing the nonlinear effect at a proper incident power. In this thesis, the AF nonlinear theory is established and it is used to disscuss the SHG and bistable states. The innovations of the work and main results are listed as follows:1. The magnetization and susceptibility are obtained from the dynamical equations of magnetization in a uniaxial antiferromagnet with two sublattices, such as FeF2,MnF2and CoF2, whose damping term is of Gibbs style. We obtain the nonlinear susceptibilities in the style of explicit function in the presence of external magnetic field. Those results structure a solid foundation for disscussing nonlinear effects of AF.2. We present some methods to increase the second harmonic genera-tion(SHG) of an AF film (AFF)for a fixed input power in Voigt geometry. Firstly, the film is put between two different dielectrics, and an obliquely incident elec-tromagnetic wave is used to generate the SH waves. We find that the SH outputs depend sensitively on the incident angle and dielectric constants of the dielectrics. We find that structural design can enhance the outputs of SHG to240times of the single film.3. We calculate the SHG from an AFF embedded in a one-dimension photonic crystal in the Voigt geometry. The electromagnetic wave localization on the AFF leads to a giant enhancement of the SHG. The numerical results based on three typical structures show that the highest power of SH outputs reaches350times of a single AFF for the same incident power. Three narrow and obvious SHG bands are found.The SHG nonreciprocity is very obvious.4. We present some methods to increase the SHG of the AF film for a fixed input power in Faraday geometry. Firstly, we calculate the SHG from the AF film(AFF). For TE wave incidence, four obvious SHG bands are found. For TM wave incidence, one obvious SHG band is found. However, no SH waves can be seen for vertical incidence of pumping wave. Secondly, we calculate the SHG from the AFF embedded in a one-dimension photonic crystal. The numerical re-sults based on three typical structures show that the highest power of SH outputs reaches4400times of a single AFF for the same incident power. We find that the best structure and the best layer-thickness. The SHG nonreciprocity is very obvi-ous.5. To explore the third-order nonlinear effects of the AFF, we study the properties of optical bistability (OB) of the AFF. We find the criterion for the presence of the OB. The OB can be found only in a certain range of frequencies and incident angle. We also examine the AFF thickness dependence of the OB regions. The switching threshold of OB is enlarged as the incident angle in-creases.