Gold “Nanograils” with Tunable Dipolar Multiple Plasmon Resonances (original) (raw)

In the emerging field of plasmonics, there has been much research interest in various kinds of metal nanostructures, in which confined electrons are forced to oscillate by an incident electromagnetic wave; the resulting electron oscillation can exhibit strong local-field enhancement at a particular resonant frequency. Such electron-photon coupled resonances, known as localized surface plasmon resonances (LSPRs), have resulted in unique, promising applications ranging from photonics to biotechnology. The strongly enhanced electromagnetic field around metal nanostructures is essential to increase nearly all light-matter interactions including surface-enhanced Raman scattering (SERS). According to the plasmon hybridization model, the plasmon resonance frequency is largely determined by the geometry of the metal structure itself, as a result of electrostatic interactions between confined electrons distributed over the surfaces of the metal conductor. It is also known that the plasmon resonance is scale-invariant, implying that metal nanostructures with the same shape but of different sizes will have essentially the same LSPR frequencies. Tunable plasmon resonances, together with enhanced local electromagnetic fields, are of practical importance for high fidelity biochemical sensors. Thus, metal nanoparticles with various shapes, including nanoshells, nanorings, nanorods, and nanorices have been proposed for exploiting the full potential of the geometry-dependent LSPRs. In shell-type geometries such as nanoshells and nanorings, interactions among electrons bound to the inner and outer surfaces of the shell give rise to so-called plasmon hybridization, resulting in a wide range of tunability. Nanocrescent moon structures that combine the geometrical features of both nanorings and nanotips can exhibit very strong local fields around the sharp edge of a shell opening. Arrays of individual plasmon nanostructures are also of special interest in the context of local field enhancement. For example, it is possible to utilize a local hot spot at the junction of a dimmerlike coupled structure or long-range dipolar interactions to obtain further enhancement of the local fields. Here, we report novel metallic nanostructures that share the features of shell-type plasmon geometries mentioned above. As shown in and b, the proposed structure is a vertically arranged ring with a varying diameter. The overall structural shape is featured as three rings with sharp edges (a typical ring diameter is several hundred nanometers) and is similar in appearance to a grail; hence the name 'nanograil.' For this geometry, three types of strong plasmon resonances coupled with each other at their respective edges are expected. As described below, these plasmon resonances can be controlled by varying the fabrication conditions in order to modify the geometrical features of the nanograil, which results in tunable LSPRs spanning from 600 to 2 000 nm. Furthermore, considerably large local-field enhancement arising from the sharp edges makes these nanostructures ideal platforms for SERS applications.