Digest Journal of Nanomaterials and Biostructures Vol. 9, No. 3, July – September 2014, p. 1255 - 1262 HOW SHAPE AFFECTS PLASMONIC PROPERTIES OF METALLIC NANOSPHERES T. SANDU * , G. BOLDEIU National Institute for Research and Development in Microtechnologies, 126A, Erou Iancu Nicolae street, 077190, Bucharest, ROMANIA Using a boundary integral equation method we analyze the extinction spectrum and the near-field enhancement induced by surface plasmon resonances with respect to the shape modifications of metallic nanospheres. We found that the shape effects are greater on near- field enhancement than on extinction. The far-field properties are affected by global geometric changes like the volume modifications. The near-field, however, is rather affected by targeted local shape variations like the curvature variation of nanosphere surface. (Received February 3, 2014; Accepted September 29,2014) Keywords: Localized surface plasmon resonance; Metallic nanoparticles; Boundary integral equation; Surface enhanced Raman spectroscopy 1. Introduction Since its first practical application, the boundary integral equation (BIE) method has gained a lot of attention because it allows convenient calculations of the localized surface plasmon resonances (LSPRs) in metallic nanoparticles (NPs) [1]. The near-field properties as well as the far-field response depend on the eigenvalues and the eigenfunctions of the BIE operator [2, 3]. As a result the BIE method provides a quite clear picture of the LSPR physics and offers an analytical tool for the design of plasmonic nanostructures for various applications like sensing [4], imaging [5] or medical diagnosis [6]. In sensing applications there are two properties that are mainly used: (a) the shift of LSPRs by the change of the surrounding refractive index; and (b) the near-field enhancement of the impinging electromagnetic field around the metallic nanoparticles. Some design aspects of the shape changes have been treated in recent papers that calculate perturbatively [7,8] the variations of the LSPRs spectral locations with respect to shape changes. However, these papers [7,8] treated only the spectral shift of LSPRs but geometrical changes induce also changes in the coupling strength of the LSPRs to the electromagnetic field [9]. Thus shape variations change not only the positions of the LSPRs but also their strength and new LSPRs may emerge in the spectrum [9]. The BIE method also permits direct calculation of the near-field enhancement created by the LSPRs [3]. The near-field enhancement is potentially used in chemical detection with some spectroscopy techniques like surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared spectroscopy (SEIRS) [10]. In other words, the LSPR effect enhances the applied electric field around the surface of metallic NPs, hence it enhances the Raman and the infrared molecular signature [11]. Advances made in chemical or top-down methods cannot avoid variations in shape or size [12]. Metallic nanospheres are ones of the most used plasmon NPs which are fabricated either by wet chemistry [10] or by plasma based techniques [13]. On the other hand, even though the nanospheres have good crystallinity their shape might deviate from a perfect sphere for instance due to the crystalline facets. Shape variations change not only the eigenvalues but also the eigenfunctions and their coupling weights to electromagnetic fields. In the present work we will analyze the changes of both the far-field spectrum and the near-field enhancement with respect to * Corresponding author: titus.sandu@imt.ro