The Design Of Photonic Crystal Circulator Based On Magneto-Optical Material

Photonic crystal,also named as the photonic band gap material,is a kind of new artificial material which permittivity or permeability is periodically arranged in space.The biggest characteristic of the photonic crystal is the existence of photonic band gap,which the electromagnetic wave can not propagate in the photonic band gap.In addition,based on the photonic band gap,we can control the electromagnetic wave by inserting defect rods in the photonic crystal.With the development of modern technology and the continuous emergence of new materials,a series of photonic crystal functional devices are gradually designed.These new functional devices have much smaller size and better performance than traditional optical devices.The relative permittivity or permeability of magneto-optical material takes the form of a second-rank tensor under an external DC magnetic field.When the electromagnetic wave propagates in it,it can provide magneto optical effect,which is the unique nature for magneto-optical material.Therefore,inserting magneto optical materials in photonic crystals will bring a lot of interesting phenomena.Meanwhile,these interesting phenomena can provide a large number of important ideas for designing some complicated and targeted photonic crystal functional devices.This paper focuses on photonic crystal circulator based on magneto-optical materials.First,we insert magneto optical materials into photonic crystal waveguide,and then adjust the coupling between photonic crystal waveguide and magneto-optical cavity to design several photonic crystal circulator.These photonic crystal circulators have the characteristics of compact,easy integration and short range efficiency,so it can be used efficiently in photonic crystal integrated optical circuits.The main work of this paper is as follows:1.Compact,low-loss and broadband photonic crystal circulator.In most of these designs,one or more magneto-optical resonators and several magneto-optical rods are required,so that it is difficult to make them compact in size,which would reduce the integration of the structures.In addition,the bandwidth of these structures is generally not wide enough,which also sets some limitations to their applications.In this paper,by analyzing the resonant mode and Voigt effect of electromagnetic wave in magneto-optical microcavity,we obtained the electromagnetic wave rotation angles for different form of magneto-optical materials,and at last we design a compact and broadband photonic crystal circulator.Only one ferrite rod is required to be inserted in our structure.Firstly,the performances of circulator based on the star-type,circle,and square ferrite rod are compared,showing that the circulator with the star-type ferrite rod performs better than the other two ones.And then,based on the star-type ferrite rod circulator,four cases of improvement,in which the background rods around the center ferrite rod are replaced respectively by the backward-triangle,forward-triangle,backward-semicircle,and forward-semicircle rods,are investigated to modulate the coupling between the center magneto-optical micro-cavity and the corresponding waveguides.The results show that,with proper parameters,all the four cases can greatly improve the output properties of the circulator,and different cases have its own advantages.The mechanism behind these improvements is also discussed.Finite-element method is used to calculate the characteristics of the circulator and Nelder-Mead optimization method is employed to obtain the optimized parameters.The ideas presented here are useful for designing broadband,low insertion loss,and high-isolation circulators which have potential application in integrated photonic crystal devices.2.Polarization-independent circulator based on ferrite and plasma materials in two-dimensional photonic crystal.Most of photonic crystal circulators can only operate with a specific polarization type(i.e.,TE or TM polarization only),which may greatly restrict their potential applications.In practice,e.g.,in biosensing applications,a single-polarization incident wave may produce reflected or transmitted waves that have both TE and TM polarization components.It is therefore necessary to perform further research to produce polarization-independent circulators for higher signal processing efficiency.First,on the basis of analyzing the wave equations in ferrite and plasma materials,TE and TM circulators are realized with ferrite and plasma in PhCs,respectively.Then,by properly combining these two types of circulators together,a polarization-independent circulator is achieved and investigated.The results show that,for both polarizations,the insertion loss and isolation for the polarization-independent circulator are less than 0.15 dB and more than 20 dB,respectively.Finite-element method is used to calculate the characteristics of the circulators and Nelder-Mead optimization method is employed to obtain the optimized parameters.The greatest advantage of the proposed structure is that it is polarization-independent,which makes it highly efficient in practical applications and the structure is very compact to integrate in practice.