Study On Photonic Crystal Heterostructure To Realize Unidirectional Transmission Of Light In A Wide Range Wavelength

In the optical light path,the light can propagate in backward direction because of the reflection on the interface of devices when it passes optical devices,which affects the performances of active devices,so building the all-optical diode is indispensable in the optical devices in order to reduce the impact of backward light.Because the rapid motion of photons in photonic crystals could be controlled effectively,and transmitting large capacity information by using photons instead of electronics has become practical requirement for the development of science and technology,the photonic crystals can be used to design an all-optical diode in micro-nano scale to lead light to one-way transmission efficiently.The all-optical diode based on magnetic photonic crystals satisfies the requirements of high contrast ratio,but it needs to bring in external magnetic field to control the Faraday rotation effect of magnetic materials,which is not benefic for all-optical integration.And the unidirectional transmission devices with nonlinear materials in photonic crystals have high forward transmittance and contrast ratio,in which high light intensity is needed to produce enough large optical nonlinear effects.Using directional band mismatch of photonic crystals can also achieve unidirectional transmission,whose forward transmittance and contrast ratio are is too low due to the scattering effect of photonic crystals,so it can’t meet the requirements of optical communication and optical information processing.All these three methods are not conductive to the application of integrated optics circuit.We found that the heterostructure interface with total reflection interface enables the backward incident light to reflect totally in the interface and reduces the backward transmittance.So in this article,the photonic crystal heterostructures are constructed to widen the scope of unidirectional transmission of light waves and increase the transmission contrast ratio.The band structure and the figures of the electric field are analyzed,so the research contents and results are as follows:1)Basing on total reflection in heterogeneous interface,the transmission spectrums of light transmission and electric diagrams are observed.It finds that the forward incident light can reflect in the interface and not output along the incident direction,when light propagates in the reverse direction,its backward transmittance is zero due to the great total reflection in the interface,but the forward transmittance is very low.So in order to improve the forward transmittance,the structure by building a 2D dielectric photonic crystal PC1 in the left of interface is designed to reduce the reflection and improve the transmittance in the interface;Then the other structure with a 2D dielectric photonic crystal PC2 in the right of interface is also designed and it can collimate light waves to the original incident direction and improve the forward transmittance due to the collimating effect,so the photonic crystal heterostructure with great unidirectional transmission performances is achieved.2)In the 2D photonic crystal structure,there are differences of light transmission with the different polarization mode.Because the light of TM mode is controlled easily by dielectric rods with high refractive index immersed in medium with low refractive index,while the light of TE mode is controlled easily by dielectric rods with low refractive index immersed in medium with high refractive index.In order to get efficient polarization-independent light unidirectional transmission,the 2D photonic crystal heterostructure composed of PC1 and PC2 is designed.Based on the total reflection interface and the self-collimation of photonic crystals,the unidirectional transmission results are realized by optimizing for the period of photonic crystals and the width of output waveguide.For TM mode,the highest forward transmittance can be up to 0.87,in the optical communication of 1550 nm it has forward transmittance of 0.834 and the transmission contrast of 1,and the width of unidirectional transmission between 1423.81 nm to 1920 nm is about 500 nm;For TE mode,the forward transmittance is up to 0.754,in the optical communication of 1550 nm the forward transmittance increases to 0.678,the transmission contrast is 1,unidirectional transmission region is located in 1447.94 nm to 2000 nm with the width of 550 nm.So it achieves the polarization-independent unidirectional transmission around 1550 nm in the optical communication band,which has the transmission contrast of 1,but the forward transmittance of 1550 nm for TE mode is not high enough,so its transmission performance is needed to improve.3)In the photonic crystal structure,which has air holes immersed in medium,it is benefit for TE polarized light to propagate.In order to further improve the TE mode of unidirectional transmission performance,the structure of the 2D photonic crystal heterostructure with air holes is constructed based on the total reflection interface and self-collimation effect.Through optimizing the distance between the heterointerface and the adjacent air hole and the width of output waveguide,the highest forward transmittance can reach to 0.75,the width of the transmission peak around 1550 nm is about 260 nm and transmission contrast reaches to 1,so this photonic crystals heterostructure with air holes in in the optical communication band has greatly unidirectional transmission performances for TE mode with the width of 600 nm.