Gold nanorods (GNRs) have great potential widespread applications in biomedical imaging, drug delivery and photothermal therapy due to unique surface plasmon resonance (SPR) ranging from visible to near infrared (NIR) region, facile synthesis and easy functionalization. The frequency of SPR is not only determined by the nature of the metal, but also by a number of other parameters, such as particle size and shape, the presence of a capping shell on the particle surface, or the dielectric properties of the surrounding medium. In this study, use is made of finite element method (FEM) for the calculation of the complex effective permittivity of the gold-core nanorod coated with polymer shells. Such a permittivity allows the determination of the absorption spectrum of these clothed nanoparticles. A simple scheme based on FEM is developed, which enables us to compute the optical properties, such as the effective dielectric function and absorption cross-section of coated-GNRs which are embedded in a dielectric matrix. The influences of the aspect ratio, shell thickness and dielectric constant of the shell on the longitudinal and transversal resonances modes are investigated.

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