Surface-Enhanced Plasmon Splitting in a Liquid-Crystal-Coated Gold Nanoparticle

Abstract

We show that, when a gold nanoparticle is coated by a thin layer of nematic liquid crystal, the deformation produced by the nanoparticle surface can enhance the splitting of the nanoparticle surface plasmon. We consider three plausible liquid crystal director configurations in zero electric field: boojum pair (north-south pole configuration), baseball (tetrahedral), and homogeneous. From a calculation using the Discrete Dipole Approximation, we find that the surface plasmon splitting is largest for the boojum pair, intermediate for the homogeneous, and smallest for the baseball configuration. The boojum pair results are in good agreement with experiment. We conclude that the nanoparticle surface has a strong effect on the director orientation, but, surprisingly, that this deformation can actually enhance the surface plasmon splitting.Colloidal suspensions and emulsions in anisotropic host media, or coated with anisotropic media such as liquid crystals have attracted considerable recent interest, since these systems exhibit a novel class of colloidal interactions [1] or the possibility of forming tetravalent binding sites of colloids [2]. The alignment of the principal axis of anisotropic media (known as the director) around a colloidal surface is a important feature; this new colloidal interaction comes from homeotropic alignment, perpendicular to the surface, while tetravalent binding sites from tangential alignment to the surface.These systems have also been studied for optical applications [3,4,5,6]. For example, related to the emerging nanotechnology, a recent experiment has demonstrated nanoscale control of optical properties on a length scale much less than a light wavelength [5]. The authors observed the surface plasmon splitting of a gold nanoparticle coated with a thin layer of nematic liquid crystal: the surface plasmon resonance frequency in the scattering cross section was shown to depend on the angle between the director and the polarization of incident light. They also showed that, by rotating the director using a static electric field, they could control the change in surface plasmon frequency.A recent calculation [6] showed that this method could indeed produce a measurable change in the surface plasmon frequency, assuming that the director was oriented in the same direction everywhere within the coating or the host media. But in reality, the director field should be influenced near the metal surface [7], and thus the alignment of the liquid crystal director field near the nanoparticle surface may reduce the surface plasmon splitting to hinder some possible applications of this system which involve electrical control of its optical properties. Thus, it is important to investigate quantitatively how an inhomogeneous director field may influence the optical properties of these coated metal particles.In this Letter, we report on novel optical phenomena arising from the surface deformation (or alignment) of the liquid crystal director field. Our main finding is that this deformation can actually enha...

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