| Photonic crystal (PC) has the ability to control photonic motion and change the the behaviour of light, this ability plays important roles in the production of more compact and higher performance of optical devices. PC is a kind of novel optical microstructure materials, of which dielectric constants are periodic distribution, when the light is propagating in the structure of PC, the photonic bandgap (PBG) can be formed by Bragg Scattering. When the frequencies of the incident light are in the frequency range of PBG, the incident light can not propagate through the structure of PC. The characteristics of controlling photons in PC which are similar to the characteristics of controlling electrons in semiconductor, will have a significant impact on the application of photoelectric and all-optical devices in future.In general, if the working frequency range of PC device is wider, the effect based on the frequency jitter will be smaller, or the stability of PC device is better. So it has great significance to study the characteristics of energy band structure in the design of PC devices. In the recent years, the optical devices based on characteristics of energy band structure have great development, such as PC waveguides, PC fibers, PC polarization beam splitters (PBSs) and PC wave division multiplexers (WDMs).In the study of PC applications, researchers are trying to obtain more ideal results through designing the PC structures. Although the final performance of optical devices are improved by adding some microstructures such as defect structures, reflection increasing structures or antireflection increasing structures, resulting in the greatly increased difficulty in production. They also have not the structural flexibility in the production. It is difficult to adjust the structure according to the actual needs, such as the optical device is required to produce different output directions in the use of different environments. This requirement is difficult to meet without adding other waveguide structures to change the output directions. On the other hand, the ratio of PBG has become one of the important factors that affect the performance of devices, the PCs which have high ratio of PBG also have great prospects in the application. In this paper, several novel PC devices are proposed, using FDTD method and plane wave method to simulate.The focuses of this paper are mainly on two aspects:1. Research on the efficient beam splitter which is easy to product and restructureIn order to improve the transmission efficiency, the beam splitters which have been reported add various types of microstructures, and this approach will bring some difficulties in the production. A novel Y type PC beam splitter based on the effects of self-collimation and gradient change is proposed in this paper, containing the square lattice which has the effect of self-collimation, air defects and the gradient PC. The square lattice which has the effect of self-collimation increases the coupling efficiency, greatly reduces the energy loss before the beam was split. This beam splitter can be efficient without adding any microstructure. The beam splitter can also be convenient to adjust structure according to the actual needs of output directions. These advantages can be conducive to the production and application of the actual optical beam splitters.2. Research and application on the large PBG ratio of PC structuresThe PC structures with large PBG ratio have great significance in many fields, especially in the field of optical communications. The optical communication devices using the large PBG ratio of PC structures have large capacity of optical communications, and the optical devices using the large PBG ratio of PC structures also have remarkable optical characteristics such as high extinction ratio of the polarization beam splitter.A novel two-dimensional duplex PC structure similar to the graphene structure is proposed in this paper, according to the research hot spot-graphene structure in recent years. The dielectric constant ratio and dielectric filling ratio are studieded in the PBG characteristics of the PC structure. After the optimization of the dielectric constant ratio and dielectric filling ratio, three PC structures with large PBG ratio which are all similar to the graphene structure can be obtained, according to TE bandgap, TM bandgap and the complete bandgap. Among the optimized results, the maximum gap ratio of TE bandgap is larger than the current largest known, reaching 55.89%, TM bandgap ratio can reach 50.35%, very close to 52% that the largest TM bandgap ratio has been reported. The complete bandgap ratio can also reach 23.97%, which is a large complete bandgap ratio. These three large bandgap ratio structures can provide theoretical basis and reference for the study of the PC devices with high performance.A novel multifunction beam splitter based on two-dimensional PC structure similar to the graphene structure is propose in the application design of the proposed structure. The multifunction beam splitter contains the functions of polarization beam splitter and beam splitter, through the skillful design in the struture.When the multifunction beam splitter is used as polarization beam splitter, it can obtain not only high transmission efficiency, but also ultrahigh extinction ratio. The transmission efficiency is -0.04 dB and the extinction ratio is 80.54 dB at the TE output port and the transmission efficiency is -0.13 dB and the extinction ratio is 31.76 dB at the TM output port. The polarization beam splitter which has ultrahigh extinction ratio of the output with ensuring the high transmission efficiency can effectively improve the problems of polarization error caused by the insufficient performance of extinction ratio in the process of transmission. When the TE and TM bandgap structures as the TE and TM output ports are removed, the multifunction beam splitter can achieve wide frequency band and efficient function of 1:1 beam splitting.To sum up, the efficient beam splitter which is easy to product and restructure according to the actual needs and the two-dimensional duplex PC structures similar to the graphene structure which have large bandgap ratio are proposed. The application designs of the proposed structure are also proposed in this paper and the multifunction beam splitter which has good characteristics is obtained. The results obtained in this paper have important reference value for the technical fields of optical integrated, optical communications, optical detection and photoelectric sensors, etc. |
