A Near Infrared Broadband Surface Plasmonic Lens Constructed with Multiple Couplers with Unidirectional Propagation Property

Based on the unidirectional propagation property of a single coupler composed of two parallel rectangular slots with different lengths, a near infrared broadband circular surface plasmonic lens is proposed. Multiple identical couplers are arranged on the two concentric circles each of which has half of the total couplers. The numerical investigation using the finite difference in time domain method is performed. The results show that when a radially polarized beam is directed onto the surface plasmonic lens from the side of the substrate, a highly enhanced and confined, dominantly longitudinal polarized nanoscale electromagnetic field spot can be obtained in the center of this circular lens. Various simulations are conducted to optimize the nanostructure. It is demonstrated that, with the optimized lens nanostructure, the extinction ratio of the intensity of the hot spot to the maximum intensity of the off-lens areas is more than 24 within the bandwidth of more than 224 nm. This characterization attributes to the unidirectional propagation property of a single coupler. The high extinction ratio can minimize the outward propagating surface plasmonic energy which may cause the noise and unnecessary coupling with the adjacent nanostructure in the circuit.