I. Introduction
Localized surface plasmon resonance (LSPR) refers to light-induced collective oscillation of conduction band electrons in metallic nanostructures [1], [2]. The LSPR phenomenon is significant because it enables plasmonic nanostructures to remarkably absorb and scatter light across a range of the optical spectrum from ultra-violet to infrared wavelengths. Such widely tunable optical properties have been explored in many research fields such as bio-sensing [2], photocatalysis [3], [4], chemical sensing [5], energy conversion and harvesting [6], imaging [7], surface-enhanced spectroscopies [8]–[10], and optoelectronics [11]. Substrate-bound metallic nanostructures fabricated by planar processes have recently emerged as an alternative to solution-synthesized nanoparticles because of their precisely controlled dimension, large-scale uniformity, and feasibility toward device integration [12]. LSPR properties are known to depend on the composition, morphology, shape, and size of the nanostructures, as well as the immediate surrounding environment [13]. Consequently, the nanoplasmonic properties in substrate-bound nanostructures are altered by the substrate materials. It is well known that the LSPR of any metallic nanostructure is modified when it interacts with other metallic nanostructures in the vicinity. This coupling effect can either be caused by short-range near-field interaction or long-range far-field interaction [14]–[16]. While the near-field interactions have important consequences in achieving highly concentrated local electric field for surface-enhanced phenomena, the far-field interactions have led to exciting phenomena such as surface lattice resonance and plasmonic hybridization [17], [18]. Substrate-bound nanoplasmonic nanostructures are readily fabricated by electron-beam lithography or focused ion-beam, which provide a high degree of freedom in designing structural geometry [19]. However, they are not economical solutions due to extremely low throughput. Alternatively, nanosphere lithography (NSL) and colloidal lithography (CL) are two easy-to-implement, cost-effective masking techniques that produce wafer-scale ordered arrays on a planar substrate with controllable periodicity and geometry [12], [20]. It is worth noting that most nanoplasmonic entities produced by NSL or CL feature “small” diameter (~100 nm or smaller) of “thin” (~20 nm) nanodisks or holes with significant sparsity, from which radiative coupling is typically weak and negligible.