Theoretical Study Of Improving The Unidirectional Transmission Of Light In The Photonic Crystal Waveguide Heterostructure With Total Reflection Interface

In recent years,with the development of communication technology,people have increasingly requirements about the communication.In the optical communication system,the smooth transmission of optical signals has become the focus of the research.If the light is scattered or reflected when it transmits from one end to the other,it will affect the input signal and the entire transmission effect.So it is necessary to study the unidirectional transmission of light in the optical path.At present,there are many ways to realize the unidirectional transmission of light.The main research methods are using the Faraday magneto-optical effect,adding non-linear materials to the photonic crystal structure,introducing defects to break the spatial or time inversion symmetry of the structure.However,these methods have some disadvantages in the practical application process.For example,we need to add magnetic field outside the structure to achieve optical rotation when using the Faraday magneto-optical effect;the high requirement of incident optical power is difficult to achieve when adding non-linear materials to the structure;the forward transmittance is low and the unidirectional transmission effect is not good enough when breaking the spatial space or time inversion of the structure.In later studies,researchers achieved a unidirectional transmission of light through a bandgap mismatch between the heterostructure with different band characteristics,one of which is the forbidden band,one is the conduction band.Using this method to realize a possible unidirectional transmission not only requirements the special band structure of the photonic crystal,but also needs to adjust the lattice arrangement at the interface.And in general,it is hard to find the suitable lattice arrangement of photonic crystal at the heterogeneous interface,so the use of it is limited.In this paper,we design the heterostructure with a total reflection tilted interface which can achieve total reflection.As long as the materials of the structure are suitable and the tilted angle of the heterogeneous interface is appropriate,the unidirectional transmission of light in TE and TM mode can be realized.We add photonic crystal waveguides to the basis of heterostructure.And the light can transmit along the photonic crystal waveguides,which can reduce the reflection and scattering of light in the homogeneous medium,so the efficiency of unidirectional transmission can be improved.The main contents and results are as follows:1.Using silica?SiO₂?and germanium?Ge?designs a kind of basic heterostructure which can realize unidirectional transmission.The critical angle of incidence to achieve total reflection in this structure is 20.47 degrees.Under the total reflection conditions,the angle between tilted interface and horizontal direction is 60 degrees.The transmittance of the double polarization mode light in the basic structure is close to 1 in the wavelength range of 1000²000 nm,the forward transmittance of TE mode is 0.66,and the forward transmittance of TM mode reaches to 0.74.2.In TE mode,a photonic crystal waveguide is added to the background of SiO₂ on the left side of the structure,in which the photonic crystal has complete bandgaps and the dielectric column is Ge,the heterostructure one is obtained.This structure realizes a unidirectional transmission with the forward transmittance being substantially higher than 0.7 and the transmission contrast ratio closing to 1 in the wavelength range of 1000²000 nm.A photonic crystal waveguide is added to the background of on the right of the heterostructure one,in which the photonic crystal has complete bandgaps and the dielectric column is SiO₂,the heterostructure two in TE mode is obtained.The forward transmittance of this structure is further increased in the wavelength range of the complete bandgaps,but the backward transmittance is also increased correspondingly,which result in a decrease in the transmission contrast ratio.The width of the photonic crystal waveguide in the background on the left and right of the heterostructure two are called d1 E and d2 E.The unidirectional transmission characteristics of the heterostructures are analyzed.The backward transmittance increases gradually when the value of d1 E is increased,which affects the efficiency of unidirectional transmission.When the value of d2 E is increased,the forward transmittance increases gradually,and the backward transmittance decreases significantly,the transmission contrast ratio gradually increases and tends to be steady.3.In TM mode,the photonic crystal waveguide one is added to the background of SiO₂ on the left side of the structure,in which the photonic crystal has directional bandgaps and the dielectric column is Ge,the heterostructure zero in TM mode is obtained.When the photonic crystal is replaced with a photonic crystal with complete bandgaps,which is the photonic crystal waveguide two,we get the heterostructure one.Based on the heterostructure one,a photonic crystal waveguide is added to the background of Ge on the right side of the structure,we get the heterostructure two,in which the photonic crystal on the right has complete bandgaps and the dielectric column is SiO₂.The unidirectional transmission characteristics of light in these three structures are analyzed respectively,and the forward transmittance increases gradually.The forward transmittance is basically higher than 0.9 in the wavelength range of 1480¹680 nm in the heterostructure two.Change the width of photonic crystal waveguide on the left and right of the heterostructure two which called d1 M and d2 M respectively,The unidirectional transmission characteristics of the heterostructures are analyzed.When d1 M gradually increases,the forward transmittance decreased gradually,when d2 M gradually increases,the forward transmittance increases and becomes more stable.4.In TE mode and TM mode,we respectively select new materials to constitute structures which can realize total reflection of light.Add photonic crystal waveguides to the structures respectively and research the unidirectional transmission characteristics.In TE mode,we use materials of air and silica as backgrounds,and place dielectric columns with different parameters to the two backgrounds which can form photonic crystal waveguides.Adjust the position of receive waveguide on the left.This structure realizes unidirectional transmission with forward transmittance close to 0.98 and the transmission contrast ratio higher than 0.9 in the wavelength range of 1392¹723nm.In TM mode,we choose materials of gallium arsenide and germanium as backgrounds.We add triangular lattice photonic crystal waveguides to the two backgrounds of the structure,in which the dielectric column is air.Adjust the position of receive waveguide on the left.This structure realizes unidirectional transmission with forward transmittance basically higher than 0.95 and the transmission contrast ratio higher than 0.82 in the wavelength range of 1000²000 nm.