We present a novel scheme for ultrabroadband and omnidirectional perfect absorbers with compact multilayer film structure. Our proposed device shows an average absorption of ∼98% over a wide range of wavelengths ranging from 400 to 2000 nm. The ultrabroadband characteristics are achieved with strongly overlapped optical resonances by designing a tandem structure composed of three absorptive materials, while the overall structure features a graded refractive index profile to obtain a wideband antireflection property. In addition to the high efficiency and ultrabroadband absorption, our perfect absorbers exhibit a great angular tolerance up to 60°, which is attributed to not only relatively broad resonances but also negligible propagation phase shifts in ultrathin highly absorbing layers. Lastly, we explore the effect of the number of semiconductor–metal stacks on the performance of the absorber. The presented approach can have tremendous potential for various applications, such as solar–thermal energy harvesting, thermoelectrics, detection, and imaging.
A compact polarization splitter based on the directional coupler in horizontal multiple-slotted waveguide (HMSW) structures is proposed and designed by using a modified three-dimensional full-vectorial beam propagation method with perfectly matched layer absorbing boundary conditions. The results show that the birefringence of the HMSW is stronger than that of the single-slotted waveguide, leading to a reduction in length for the present device. Moreover, the HMSW can also enhance the optical confinement inside the slot regions. The coupling length of the directional coupler composed of two-parallel HMSWs as functions of the waveguide parameters are analyzed, and a compact device of 238.0 mm in length is achieved with the crosstalks of À27:9 and À35:6 dB for quasi-transverse-electric and quasi-transverse-magnetic modes, respectively. In addition, the evolution of the injected field along the propagation distance through the polarization splitter is also demonstrated.
Abstract-An Improved Differential Evolution (IDE) algorithm is proposed for optimization problems. With the novel mutant operation adopting sub-optimal individual, the convergence of Differential Evolution (DE) algorithm is accelerated without increasing the risk of premature. Five typical test functions are minimized using DE and IDE algorithms, and the results show the superior performance of IDE algorithm. Furthermore, the algorithm is applied to pattern synthesis of two antenna arrays. Broad nulls are formed in radiation pattern of a linear array to suppress broad-band interferences. In a microstrip patch array, the sidelobe level of array is decreased about 12.9 dB and the mainlobe can scan to the desired angle.
The one-dimensional photovoltaic absorber material Sb2S3 requires crystal orientation engineering to enable efficient carrier transport. In this work, we adopted the vapor transport deposition (VTD) method to fabricate vertically aligned Sb2S3 on a CdS buffer layer. Our work shows that such a preferential vertical orientation arises from the sulfur deficit of the CdS surface, which creates a beneficial bonding environment between exposed Cd2+ dangling bonds and S atoms in the Sb2S3 molecules. The CdS/VTD-Sb2S3 interface recombination is suppressed by such properly aligned ribbons at the interface. Compared to typical [120]-oriented Sb2S3 films deposited on CdS by the rapid thermal evaporation (RTE) method, the VTD-Sb2S3 thin film is highly [211]- and [121]-oriented and the performance of the solar cell is increased considerably. Without using any hole transportation layer, a conversion efficiency of 4.73% is achieved with device structure of indium tin oxide (ITO)/CdS/Sb2S3/Au. This work provides a potential way to obtain vertically aligned thin films on different buffer layers.
Abstract-In this paper, a new evolutionary learning algorithm based on a hybrid of improved real-code genetic algorithm (IGA) and particle swarm optimization (PSO) called HIGAPSO is proposed. In order to overcome the drawbacks of standard genetic algorithm and particle swarm optimization, some improved mechanisms based on non-linear ranking selection, competition and selection among several crossover offspring and adaptive change of mutation scaling are adopted in the genetic algorithm, and dynamical parameters are adopted in PSO. The new population is produced through three approaches to improve the global optimization performance, which are elitist strategy, PSO strategy and improved genetic algorithm (IGA) strategy. The effectiveness of the proposed algorithm has been compared with GAs and PSO, synthesizing a circular array, a linear array and a base station array. Results show that the proposed algorithm is able to adapt itself to different electromagnetic optimization problems more effectively.
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