Theoretical Study On The Nonreciprocity Of Magneto-Optical Hybrid Structures

Light is a kind of electromagnetic wave.Controlling light is a hot topic in this word,which is also a long-standing problem in integrated photonics.Unlike electrons,a photon does not carry a charge and therefore it cannot be manipulated directly by electric or magnetic fields.Fortunately,one can make it by the interaction between light and matter.Faraday magneto-optical effect and Kerr magneto-optical effect are the effective ways to control light through adjusting the interaction between light and magneto-optical materials by magnetic field.However,the weak level of magneto-optical effect goes against the application of natural magneto-optical materials in visible wavelengths,so designing new materials is necessary for enhancing magneto-optical effect.Based on Maxwell equations and finite-element method,the electromagnetic property of magneto-optical effect is analyzed in detail,and then some approaches about the enhancement are introduced.By breaking the time reversal symmetry and the Lorentz reciprocity theorem,light filters and one-way devices are designed with magneto-optical materials,and the coherent backscattering effect is studied numerically from the random magneto-optical media.The main work can be summarized as follows:1.Focus on the case that linearly polarized light propagates along the magnetization direction of Bismuth iron garnet,the magneto-optical effect can be enhanced by nonresonant and resonant mechanisms.According to the effective medium theory,the permeability tensor of Bismuth iron garnet can be tailored after inserting metallic films.When diagonal terms of the effective permeability tensor are tuned to the vacuum permeability,the corresponding Kerr rotation can be enhanced up to 90°nonresonantly with wide bandwidth.The resonance of light in thick Bismuth iron garnet film can also enhance both Faraday and Kerr magneto-optical effects,while with small rotation and narrow bandwidth.2.The eigenstates along the magnetization direction of magneto-optical materials are left/right circularly polarized light.The decay-style filters for left or right circularly polarized light can be obtained when the wave vector of one eigenstate is designed to be imaginary by the effective medium theory.Left or right circularly polarized light can be filtered effectively with high quality when the thickness of the filter is longer than the decay length.In addition,Left or right circularly polarized light can also be resonantely reflected by Bismuth iron garnet film.However,both efficient and bandwidth of resonate-style filters are theoretically lower than those of decay-style filters.3.With the nonreciprocity of magneto-optical materials,one-way transmission of light can be realized both in the one-dimensional magneto-optical heterostructures and on the eage of the magneto-optical heterostructure.For the case that TM light propagates perpendicular to the magnetization direction,it is found by quasi-two-dimensional transfer matrix method that the spatial inversion symmetry can be broken when the unit of periodic heterostructures consists of three different films,in which one or more films are magneto-optical materials.And then,one-way transmission appears with high on-off ratio in the asymmetric part of the band structure.The divergent light manifests chirality along the eage of magneto-optical materials.With the help of metal or other magneto-optical material with opposite magnetization,TM light can be bounded on the eagle and performs one-way transmission with wide bandwidth.4.By finite-difference time-domain method,the coherent backscattering effect of two-dimentional random magneto-optical media is studied numerically,and it is found that the enhancement factor of the coherent scattering cone tends to 1.5,but not 2 any more.After analyzed the time-evolution data of multiple scattering,it is shown that stronger magneto-optical effect can accelerate the tendency.Furthermore,the distribution of singular values of the scattering matrix follows quarter circle law,indicating the multiple scattering is random in two-dimentional random magneto-optical media.