Solution box: SOLBOX-08

Nanowires are popular components of nano-optical systems because they can be useful in many related applications [1], such as optical transmission [2-4], sub-wavelength imaging [5, 6], and energy harvesting [7]. These structures are usually made of silver (Ag) or gold (Au), which are active at optical frequencies with strong plasmonic responses and which provide the favorable characteristics of nanowires. For example, by using a transmission line involving an arrangement of nanowires, electromagnetic energy can be carried to distances long with respect to wavelength. As the technology in this area develops, nanowires with improved geometric properties [8] – such as regularity, cross-sectional preciseness, and surface smoothness – become available, further expanding their usage. Naturally, electromagnetic simulations of nanowires [9-11], especially using their three-dimensional full-wave models, are essential to studying and understanding these important structures, as well as to designing them. In this issue of Solution Box, an optimization problem involving a nanowire transmission line to be improved by a coupler is presented (SOLBOX-08). Specifically, a pair of nanowires with a sharp 90° bend, which leads to significant deteriorations in the power transmission capability due to reflections and diffractions, is considered. The purpose was to design an efficient coupler in a limited space around the corner, in order to improve the transmission as much as possible. In the sample solution that is also considered in this issue, cubic nanoparticles were used to reduce the reflections and to improve the power transmission. Starting from a full grid, the existence and absence of each nanocube was decided based on an optimization via genetic algorithms (GAs). The trials during the optimization were efficiently performed by using the Multilevel Fast Multipole Algorithm (MLFMA) [12, 13]. This has been designed for accurate analysis of plasmonic objects [14-16] without resorting to approximate and asymptotic techniques. Different numerical solutions, analysis methods, and optimization tools to design more efficient couplers, probably leading to better transmission capabilities, are welcome. We are also looking for alternative solutions to previous problems (SOLBOX-01 to SOLBOX-07), which can be found in earlier editions of this column. Please consider submitting your contributions to Ozgur Ergul (ozergul@metu.edu.tr).