Optimizing nanofocusing in plasmonic composite lenses comprising nano-ring grooves and nano-disks

Improving focal intensity and reducing focal spot size of Surface plasmon polaritons (SPPs) nanofocusing are essential requirements in plasmonic lens research. These advancements hold great potential for applications in optical tweezers, on-chip biosensing, and nanoscale light-matter interactions. Here, we propose the plasmonic composite lens design integrating circular-ring grooves with a disk lens, where the focal spots of the ring and disk overlap spatially and constructive interference in time. The optimized composite lenses reduce the minimum focal spot size from 316.3 nm to 240.5 nm while enhancing the focal intensity by a factor of 8.58 to 19.4. Comparative analysis of the excitation and electric field evolution of SPP in lenses, we confirm that the focusing enhancement is attributed to energy coupling and interference effects of backward-transmitting SPPs on the disk. Additionally, the lens enables active focal spot manipulation through oblique incidence of circularly polarized light via spin-dependent interference effects, offering potential applications in chiral macromolecule detection, smart pixels, and multifunctional integrated photonic devices.