I. Introduction
Surface plasmons (SPs) localized at the mental-dielectric interface possess extraordinary optical properties, such as subwavelength wave confinement and enhanced local field, which have attracted a large amount of attention during the last two decades [1], [2]. Due to the momentum mismatch between the SP wave and a free-space photon of the same frequency, it is conventional to excite propagating SPs by a prism coupling configuration, such as the Kretschmann or Otto configuration. The SP wave can be excited at some specific incident angles and SPR phenomenon would occur when the incident light wave resonates with the matching SP wave, leading to a strong electromagnetic field on the metal surface. SPR is extremely sensitive to the refractive index (RI) of surrounding sensing medium owing to the strong field enhancement. Therefore, SPRs have been widely applied to chemical and biological sensing [3]. Noble metals, particularly gold and silver, is commonly used for SPR sensing for their relatively low ohmic loss. A kind of effective method to improve the sensitivity is to cover the conducting layer with high RI thin dielectric layer or two-dimensional (2D) materials. [4]–[7]. In addition, some also resort to the dielectric micro- or nano-structures placed on top of metallic layer to realize sensitive SPR sensing [8]–[10]. Although above mentioned methods have enhanced the sensitivity to a certain extent, further improvement of sensing performance hits a bottleneck imposed by the intrinsic loss of noble metals. Such condition worsens in the near infrared wavelength because of large imaginary part of refractive index leading to broadening of the resonant linewidth [11], [12]. Therefore, complex plasmonic nanostructures with characteristics of coupling of plasmonic modes are often introduced to shrink the linewidth [13], [14].