2-D Photonic Crystal Heterojunction Based On Total Reflection For Light Unidirectional Transmission

In the optical communication system, unidirectional transmission device isused to barrier the reflected light and ensure the stability of optical system. Inorder to meet the development trend of optoelectronic integration, preparation ofintegrated optical unidirectional transmission is particularly important. Photoniccrystal (PC) is suitable for photoelectric integration for its lattice constant iscomparable to the transport wavelength. Currently, the main way to obtain theunidirectional transmission of light is joining the magnetic or nonlinear materialinto the photonic crystals. The Faraday rotation effect of magnetic material orthe nonlinear optical effect of nonlinear material can achieve unidirectionaltransmission of light. However, the Faraday rotation effect requires an externalmagnetic field, and the nonlinear optical effect requires a high power incidentlight. Neither is very convenient in practical applications.To overcome these shortcomings above, an available way to create unidirectional optical transmission is via spatial inversion symmetry breaking.One way is using the directional bandgap mismatch. However, the requirementof bandgap makes the structure design more difficult. In this paper, based ontotal reflection principle, a2-D photonic crystal heterojunction for unidirectionaloptical transmission is designed. It does not depend on directional bandgapmismatch any more. The finite-difference time-domain method and the planewave expansion method are used for studying transmission property, fieldintensity distribution and band maps of the two photonic crystals. The mainstudies and conclusions are as follows:1. Based on total reflection principle, a2-D square-lattice photonic crystalheterojunction is designed. It can achieve unidirectional transmission for TEmode centered in1550nm. Transmission peak is located at the wavelength of1560nm with transmittance of0.5and transmission contrast of0.93. The widthof unidirectional transmission is about500nm. Studying on the field intensitydistribution and the band maps, we find that, when the heterojunction interfacemeets the requirements of total reflection, it can achieve unidirectionaltransmission, even if the heterojunction is made of two PCs which both haveconduction band in the operating frequency range. In order to have a goodperformance of unidirectional optical transmission, the optimal angle betweenheterojunction interface and the horizontal direction is450, due to the materialsand the structure. Furthermore, the forward transmission peak moves to thedirection of long wave while the cylinders’ radius or the lattice constant increases.2. The unidirectional transmission property for TM mode of2-D photoniccrystal heterojunction is analyzed. It can also achieve unidirectionaltransmission without directional bandgap mismatch. Besides, we also find that,if the distance between heterojunction interface and its adjacent dielectriccylinders changes, the forward transmittance and the unidirectional transmissionrange will change. Further, if the position of right waveguide moves up within600nm from the center, the unidirectional transmission will not be influenced.At last, the dependence of unidirectional transmission on the refractive index ofdielectric cylinders is studied. When the refractive index is larger than that ofboth the background materials, and the gap is larger, the wave width ofunidirectional transmission is wider.3. Based on total reflection principle, a2-D square-lattice photonic crystalheterojunction for unidirectional transmission of dual-polarization light centeredin1550nm is designed. The directional bandgap mismatch is also needless forunidirectional transmission of dual-polarization light, due to the total reflectionon heterojunction interface. Finally, the structural perturbation is studied. Toensure the unidirectional transmission of dual-polarization light centered in1550nm, the horizontal position perturbation of heterojunction interface is within20nm and the radius perturbation is within4nm around140nm. Theseconclusions provide the basis for experiment preparation.