Abstract
Metal nanoparticle arrays have proved useful for different applications due to their ability to enhance electromagnetic fields within a few tens of nanometers. This field enhancement results from the excitation of various plasmonic modes at certain resonance frequencies. In this article, we have studied an array of metallic nanocylinders placed on a thin metallic film. A simple analytical model is proposed to explain the existence of the different types of modes that can be excited in such a structure. Owing to the cylinder array, the structure can support localized surface plasmon (LSP) modes. The LSP mode couples to the propagating surface plasmon (PSP) mode of the thin film to give rise to the hybrid lattice plasmon (HLP) mode and anti-crossing phenomenon. Due to the periodicity of the array, the Bragg modes (BM) are also excited in the structure. We have calculated analytically the resonance frequencies of the BM, LSP and the corresponding HLP, and have verified the calculations by rigorous numerical methods. Experimental results obtained in the Kretschmann configuration also validate the proposed analytical model. The dependency of the resonance frequencies of these modes on the structural parameters such as cylinder diameter, height and the periodicity of the array is shown. Such a detailed study can offer insights on the physical phenomenon that governs the excitation of various plasmonic modes in the system. It is also useful to optimize the structure as per required for the different applications, where such types of structures are used.
Highlights
Metallic structures at nanometric scale, smaller than the wavelength of light, have been studied in plasmonics for some time due to their ability to enhance local electromagnetic field in their vicinity.[1] [2] Such enhancement of the local field has been attributed to the plasmon polaritons excited in such structures
We have studied in details an array of gold nanocylinders with an underlying thin gold film in TM configuration
We demonstrated that the resonance frequencies of the localized surface plasmon (LSP) mode, in this configuration, decrease rapidly with increasing height (h2), while increase slightly with increasing diameter (D), which is opposite to the expected trends in arrays on a glass substrate
Summary
Metallic structures at nanometric scale, smaller than the wavelength of light, have been studied in plasmonics for some time due to their ability to enhance local electromagnetic field in their vicinity.[1] [2] Such enhancement of the local field has been attributed to the plasmon polaritons excited in such structures. The incident light can interact with the surface electrons of the conduction band of the metal, and the collective oscillations of these electrons can give rise to intense local electromagnetic fields at the metal-dielectric interface This resulting localized surface plasmon (LSP) has been effectively used for numerous applications such as surface-enhanced Raman scattering (SERS), drug delivery, chemical sensing, cancer therapy, and new photonic devices.[3] [4] [5] [6] The typical penetration depths of the confined field close to the metallic surface are of the order of a few tens of nanometers, and these nanoparticles can work as excellent transducers of local refractive index changes, which can be used for various applications of biomolecular detection. The plasmonic modes which participate in the coupling are rather both “broad states” and Fano resonance occurs only for coupling of “broad” and “discrete” states
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