The Genetic Optimization And Application Of One And Two Dimensional Photonic Crystals And Electromagnetic-absorption Materials

Photonic Crystals (PCs) are usually defined as a class of artificial material with periodic variation of refractive index. Due to the existence of photonic band gaps (PBGs) and effect of photon localization in PCs, it becomes possible for people to control the behavior of photons and fabricate the photonic integrated devices. During the past twenty years, great progress has been achieved in both theoretical and experiential studies on PCs, and the optimum design has played an important role in these successes. Recently, as a representative host of new generation of global optimum algorithms, the Genetic Algorithms (GAs) have attracted many researchers'attentions and been wildly used in designing kinds of optical electromagnetic devices, due to its robustness and powerful capacity of random search. The main subject of this paper is focused on the application of GAs in the design of one-dimensional (ID) PCs, two-dimensional (2D) PCs, and electromagnetic-absorption materials (EAMs). The concrete research contents and results can be summarized as follows:(1) Based on GAs and the Transfer Matrix Method, we have proposed a general approach to design 1D PC omnidirectional reflectors. In particular, via using the model of quasi-periodic photonic heterostructure made by nonquarter-wave films, we have obtained GA designs of ID PC omnidirectional reflector for the optical telecommunication band (800-2000nm), the visible-light band (390-780nm), and arbitrary frequency band (0-2ω0) respectively.. For examples, the omnidirectional reflector for the optical telecommunication band is made by Te and SiO2 and has a working bandwidth of 800-1620nm, which has covered three main telecommunication windows centered at 0.85μm,1.3μm, and 1.55μm. The omnidirectional reflector for the visible-light band is made by SnS2 and SiO2 and has a working bandwidth of 446-779nm, which has covered most area of the visible-light band. The omnidirectional reflector for arbitrary frequency band is made by Te and SiO2 and has a much wider working bandwidth than those reported in literatures. To verify these designs, we have specially done an experiment for the GA design of optical telecommunication band. The omnidirectional reflector was fabricated via the pulsed laser deposition method and we used a near-infrared spectroscope to measure its reflectance spectra, which has shown a good agreement with the theoretical result.(2) Based on asymmetrical unit cell, we have proposed a method to design 2D PCs with large complete PBGs. As the 2D PCs with square-lattice for example, two kinds of asymmetrical unit cell was proposed which are constructed by a number of hexangular or round dielectric rods, respectively. Based on these two unit cells, we have obtained optimum designs of 2D PCs with large complete PBGs in higher-order and under-light-line frequency respectively via using the GAs in conjunction with the Plane Wave Expansion method. For examples, the optimum unit cell comprising five hexangular rods for higher-order frequency owns a large complete PBG with center frequency at 1.0502×2πc/a and an absolute bandwidth of 0.1314x2πc/a. The optimum unit cell comprising four round rods for under-light-line frequency owns a large complete PBG with center frequency at 0.4084×2πc/a and an absolute bandwidth of 0.0618×2πc/a. Furthermore, we have adopted the multiple-scattering theory, the boundary condition theory, and the photon-states density theory to give explanation of physics mechanism for the above designs.(3) Currently, the main problem in designing 2D PC waveguide directional emitters is how to simultaneously optimize the emitting efficiency as well as control the angle of directional emission. In this paper, based on the Moreno's model on directional emitter, we have corrected its physics mechanism by using the antenna theory and then proposed a developed model with asymmetrical corrugated surface to solve the above-mentioned problem. By employing the GA in conjunction with the Finite-difference time-domain method, we have optimized such asymmetrical corrugated surface and obtained several highly-efficient designs of directional emitters with different beaming angles. In addition, based on the revised physics mechanism, we have proposed a new thought to designing directional emitters by adopting an asymmetrical grating-like surface behind the termination of 2D PC waveguide. The GA optimized results have demonstrated that highly-efficient designs of directional emitters with different beaming angles can also be obtained from the grating-like surface. At the same time, these directional emitters have higher emitting efficiency as well as simpler structure than those obtained from asymmetrical corrugated surface. They can be applied as a type of high-performance energy coupler in the photonic integrated systems.(4) Using the transmission line theory, we have proposed three objective functions to evaluate the performance of flat-type EAMs. According to the electromagnetic parameters of different nanometer materials, we have designed some flat-type EAMs with two-objective and three-objective characters by using the self-adaptive GA and the fast Pareto multi-objective GA NSGA-Ⅱ. By comparing these designs, it is found that the multi-objective GA can make better tradeoff between objective functions and provide more high-quality designs than single-objective GA. For example, by using the NSGA-Ⅱ, a four-layer absorber with thickness of 2.8071 mm is obtained to provide average reflection coefficient of-11.95 dB and average reflection bandwidth of 0.52 in 2-18 GHz, considering arbitrary incident angles and both TE and TM polarizations. It can be used as a new generation of high-performance EAM with characters of strong-absorption, broad-band, and thin thickness.As a type of powerful global optimization algorithm, the GAs can be used to solve kinds of design problems in complicated electromagnetic devices and play an important role in improving the design efficiency as well as verifying the physics mechanisms in devices. The GA designs of PCs and EAMs in this paper can provide both theoretical and engineering references for researchers in this area.