A Broadband SPP Unidirectional Coupler Based on a Groove-Doublet Structure

Zhi Li School of Physics, Peking University, Beijing, China Huimin Liao School of Physics, Peking University, Beijing, China Jianjun Chen School of Physics, Peking University, Beijing, China Xiang Zhang School of Physics, Peking University, Beijing, China Song Yue School of Physics, Peking University, Beijing, China Qihuang Gong School of Physics, Peking University, Beijing, China

The manipulation of light propagation is a basic subject in optics and has many important applications. One of the most efficient ways to control light propagation is to exploit interference effects. Here, by manipulating the surface-plasmon-polariton (SPP) excitation properties of two nanogrooves on a metal surface and exploiting the interference between them, we successfully demonstrate unidirectional launching of SPPs using a groove-doublet structure. By decreasing the groove separation to three-quarter of the SPP wavelength, a broad bandwidth of roughly 200 nm with high extinction ratios of ≥10 dB is successfully achieved. Further reduction of the groove separation to one-quarter of the SPP wavelength results in an abnormal shrinking in the bandwidth. Such an anomalous phenomenon cannot be explained by taking into account the interference between propagating SPPs and the dispersion effects of individual grooves. It is found that quasi-cylindrical waves (CWs) give important contributions to the abnormal bandwidth shrinking. Although it is a negative factor for the device performance in the current case, CWs actually provide another free degree of freedom to manipulate light besides SPPs. Such effects provide new opportunities for the design of ultracompact optical devices.

z_li@pku.edu.cn