Study On Polarization-independent Unidirectional Transmission Of Optical Waves In Photonic Crystal Heterostructures

The integrated optical path has many advantages that the integrated circuit cannot match:large information capacity,fast processing speed,high switching response speed,and strong anti-electromagnetic interference capability.Analogous diodes,integrated optical systems also have a basic device:an optical diode based on unidirectional transmission of light waves,an optical device limits the transmission of light in a particular direction.At present,optical integrated devices are developing toward the micro-nano scale,so designing micro-nano-scale optical diodes has become an urgent need for the development of the times.Photonic crystal?PhC?can integrate more promising photonic integrated chips,which opens up the possibility of replacing electronic integrated chips in the future.In theory,the symmetric light transmission devices based on two-dimensional?2D?PhCs will show the advantages of high contrast ratio,low insertion loss,broad operation bandwidth,independent on polarization.They have micro size and can be easily manufactured with standard semiconductor fabrication technique.However,it does require further research and development to fully explore the unique advantages of the photonic diodes based on 2D PhC structures.In this thesis,the polarization-independent unidirectional transmission of the light wave based on 2D PhC heterostructures are studied.The heterostructures are composed of 2D PhCs with different structures.The unidirectional transmission are achieved by breaking the spatial reversal symmetry without requiring the nonlinearity or active materials.The design principle is the generalized total reflection?TR?with an inclined interface between the two PhC structures.This principle allows polarization independent symmetric transmission in a broad wavelength region without required the band gaps of the PhCs.The photonic band structures and equal frequency characteristics of 2D PhCs are analyzed to improve the forward transmittance and the contrast ratio,and enlarge the bandwidth.High-performance polarization-independent photonic diode are achieved.The research contents and simulation calculation results are presented as three corelated projects:1.Unidirectional transmission of linearly polarized light in the heterostructureHere the silicon and air are chosen as the materials to compose the photonic diode structure.The silicon is chosen because it is a key material in modern integrated circuits and has a high refractive index,which has found broad application in optics and photonics.In this study,the goal is to achieve unidirectional transmission for both TE and TM polarized light based on the TR principle.As a result,the forward transmittance is higher than 0.5 in the wavelength range of 157 nm?center at the wavelength of1550 nm?,the contrast ratio is higher than 0.9.In addition,for both TE and TM polarized light,the forward transmittance at 1550 nm?the telecommunication wavelength?is higher than 0.5.In this way,we have demonstrated the unidirectional transmission for both linearly polarized light.2.Unidirectional transmission of circularly polarized light in the heterostructureHere we design a heterostructure for the unidirectional transmission of circularly polarized light.Based on unidirectional transmission of different linearly polarized light,it is possible to design that simultaneously meet the three requirements for unidirectional transmission of circularly polarized light,namely,the two polarization states have the same operating spectral range;the transmittance is the same;the phase difference is maintained,and the intensity distribution overlaps in space.In this way,the photonic diode is able to achieve unidirectional transmission of circularly polarized light,which is confirmed by calculating the polarization states at the output port.As a result,the forward transmittance of the circularly polarized light keeps over 0.6 in the wavelength range of 1497 nm to 1666nm with the contrast ratio close to 1,the bandwidth is 169 nm,and the forward transmittance is 0.65 at 1550 nm.Here,we demonstrated the photonic diode for the circularly polarized light.3.Unidirectional transmission of arbitrary linearly polarized light in the heterostructureHere a heterostructure that is suitable for unidirectional transmission of arbitrary linearly polarized light?including TM,TE,and 45°linearly polarized light?is proposed.In this study,we rotate the lattice direction of PhC₁,to increase the diffraction effect and use with the self-collimation effect of the PhC_2.In this way,the forward transmitting light can propagate through the photonic diode with high efficiency.In the meantime,the backward optical transmission is suppressed by the generalized TR effect.As a result,the unidirectional transmission of the light of the arbitrary linear polarization states is achieved.The results show that the forward transmittance is higher than 0.66 at the wavelength of 1550 nm,and the contrast ratio is higher than 0.9.A forward transmittance of more than 0.6is achieved in the wavelength range of 1533 nm to 1634 nm with a bandwidth of 101 nm,and the transmission contrast ratio is higher than 0.9.In this study,we demonstrate a photonic diode for arbitrarily linearly polarized light.