Maximal Electromagnetic Coupling Between Arbitrary-Shaped Nanotubes

The interaction of electromagnetic waves with pairs of nanotubes having arbitrary-shaped cross sections is investigated. The study starts by thoroughly examining nanotube couples with circular boundaries to identify the structural, textural and excitation parameters that enhance the electric field concentration. Such an initial step generates a design space across which the regions of interest are determined to formulate a shape optimization problem targeting at the maximization of the signal internally to the nanotubes by modifying their boundary surface while maintaining the values for the remaining operational parameters. An IsoGeometric-Analysis-based Boundary Element Method (IGABEM) is employed for estimating the electric field around nanotube boundaries, which is subsequently used towards the overall solution of the boundary value problem. A combination of global and local optimizers along with geometric parametric models that generate valid nanotube cross sections complement the IGABEM method in the quest of optimal designs. The achieved optimal shapes attain substantial boost in the concentration of the electric field which can easily exceed, by 30 times or more, the corresponding value obtained by circular nanotube pairs. This superior performance is maintained for a large range of wave angles and nanotube areas and, accordingly, the robustness of the obtained results and their potential applicability in a wide range of applications, is demonstrated. Finally, the computational method developed in this work can be easily extended for analyzing a finite array of nanotubes, while insights gained by the reported optimal shapes pave the way for their optimization and fine tuning in collective setups like electromagnetic gratings and photonic metasurfaces.