Optical composite nanoparticles with broadband finite absorption in the limit of infinitesimally small losses

In this work we show that two conjoined subwavelength cap-shaped structures may be used as an efficient energy-harvesting nanostructure with unbounded absorption efficiency. Following an analytical method based on coordinate transformation, we show that anomalous plasmonic resonances arise when one of the two nanostructures is plasmonic, resulting in adiabatic focusing of surface plasmons toward the particle corners, which are responsible for large absorption efficiency over a wide and controllable frequency range. Over this continuous bandwidth our proposed composite nanostructure shows finite amount of absorption even in the limit of no intrinsic material loss.