# Localized Surface Plasmon Fields Manipulation on Nanostructures Using Wavelength Shifting
## 1 Introduction  
A surface plasmon (SP) is a longitudinal electron oscillation formed between a metal and a dielectric interface. SP coupling occurs at a specific energy and momentum, collectively referred to as the SP resonance (SPR) condition. The SPR condition is highly affected by the refractive index in the proximity of the interface. Thus, the SPR phenomenon is widely used in optical sensor design for a high-sensitive chemical or biological target detection [1]. When it applied within or collaborates with noble metallic nanostructures, highly localized SP field can be achieved, which we call LSP. LSP has been widely investigated for various studies, including ultrasensitive chemical or biological sensing which capacitates better sensitivity compared to the conventional SPR sensors [2, 3, 4, 5, 6, 7, 8]. The role of created LSP for sensitivity improvement of a sensor is amplification of the optical response resulted from the minor refractive index changes of sample area. In addition, LSP has received considerable attention for improving imaging resolution down to subwavelength level by being applied for a microscopic system [9, 10, 11, 12, 13]. For the image resolution improvement, nanostructures are designed to extremely localize SP field at the nanoscale small gap or region [14, 15, 16]. The extremely localized field, wherein the full-width half-maximum (FWHM) is below the diffraction limit, excite fluorescent agents at the subwavelength scale. Thus, super-resolution images can be achieved, overcoming diffraction limit, with an imaging reconstruction process [12, 17, 18]. In practice, the improvements in sensitivity of a sensor and image resolution are primarily determined by the spatial distributions of the LSP fields. Moreover, it has been reported that the LSP fields can be effectively utilized to trap a particle owing to their strong gradient force [19, 20, 21, 22]. It is well-known that the LSP field distributions on metallic nanostructures can be spatially engineered by modulating the conformations or materials of nanostructures. There are some researches that exploiting spatially engineered LSP field distributions. It can be applied for the optical lithographic methods by utilizing the strongly generated varied spatial distributions of LSP fields as a lithographic source at nanoscale. To fabricate accurately the complicated and creative nanostructures as we desired, the spatial distributions of LSP should be thoroughly engineered through selecting appropriate materials or managing conformations and composition of nanostructures [23]. The diversely engineered spatial distributions of LSP fields were also utilized for the biosensing and imaging applications [24; 25; 26]. Beyond the aforementioned applications exploiting static LSPs field distribution, demands for developing a technique which enables dynamic control the static states of LSP field distributions are increasing. Recently, control of the LSP fields on the metallic waveguides has been introduced for functional plasmonic circuit and drawn great attention [27; 28]. In this study, the LSP field distributions on diverse metallic nanostructures were calculated for exploring the feasibility of manipulating LSP fields by modulating the wavelength of the incident light. For that, plasmonic fields generated on a nanoscale rod array (I-shaped), an array connected with two nanoscale rods at right angles (T-shaped), and an array with three nanoscale rods at 120\({}^{\circ}\) to each other (Y-shaped) were calculated with four wavelengths of the light source which is commonly used. Three different nanostructures were examined to test feasibility for manipulating position of LSP fields by tuning incident wavelength shifting. The near-field distributions on them at different incident wavelengths were calculated via a rigorous coupling wave analysis (RCWA) method.