Shaping quasi-transparent nanotubes into Maximally strong EM scatterers

Abstract

The problem of enhancing the electromagnetic (EM) scattering for almost transparent nanotubes via shape modification of their cross section, is studied in this work. An isogeometric analysis approach, in a boundary element method setting, is employed to evaluate the local electric field, which is expressed in terms of the exact same basis functions utilized in the geometric representation of the cylinder boundary. In this way, the overall scattering power becomes computable via proper integration of the far field magnitude around the nanotube and shape optimization can be directly performed with the aim of maximizing the scattering enhancement compared to an equiareal circular nanotube. The optimization framework uses: (i) a hybrid approach combining global optimizers with gradient-based local algorithms for accurately determining the shape at the final stages, (ii) a series of parametric models generating valid non-self-intersecting nanotube shapes, and (iii) an isogeometric-enabled boundary element method solver approximating the value of the electric field with high accuracy. The optimized nanotube shapes give much higher total scattering response than their circular counterparts. In a wide range of operating conditions (such as nanotube’s electric conductivity or cross section area), the optimized shapes assumed a distinct spiky shape which was further studied with respect to the direction of excitation. Apart from boosting scattering for quasi-transparent nanotubes, the developed methodology can be adapted to solve the inverse problem, namely, determining the nanotube shape from its scattering signal, as well as extended to address similar problems with finite arrays of nanotubes comprising EM metasurfaces.