Alastair Cunningham

Self-assembled plasmonic core shell clusters with an isotropic magnetic dipole response in the visible range

We theoretically analyze, fabricate, and characterize a three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain. Using two different bottom-up approaches that rely on self-organization and colloidal nanochemistry, we fabricate clusters consisting of dielectric core spheres, which are smaller than the wavelength of the incident radiation and are decorated by a large number of metallic nanospheres. Hence, despite having a complicated inner geometry, such a core-shell particle is sufficiently small to be perceived as an individual object in the far field. The optical properties of such complex plasmonic core-shell particles are discussed for two different core diameters.

Self-assembled plasmonic metamaterials

Abstract Nowadays for the sake of convenience most plasmonic nanostructures are fabricated by top-down nanofabrication technologies. This offers great degrees of freedom to tailor the geometry with unprecedented precision. However, it often causes disadvantages as well. The structures available are usually planar and periodically arranged. Therefore, bulk plasmonic structures are difficult to fabricate and the periodic arrangement causes undesired effects, e.g., strong spatial dispersion is observed in metamaterials. These limitations can be mitigated by relying on bottom-up nanofabrication technologies. There, self-assembly methods and techniques from the field of colloidal nanochemistry are used to build complex functional unit cells in solution from an ensemble of simple building blocks, i.e., in most cases plasmonic nanoparticles. Achievable structures are characterized by a high degree of nominal order only on a short-range scale. The precise spatial arrangement across larger dimensions is not possible in most cases; leading essentially to amorphous structures. Such self-assembled nanostructures require novel analytical means to describe their properties, innovative designs of functional elements that possess a desired near- and far-field response, and entail genuine nanofabrication and characterization techniques. Eventually, novel applications have to be perceived that are adapted to the specifics of the self-assembled nanostructures. This review shall document recent progress in this field of research. Emphasis is put on bottom-up amorphous metamaterials. We document the state-of-the-art but also critically assess the problems that have to be overcome.

All-optical control of localized plasmonic resonance realized by photoalignment of liquid crystals

Large shifts in the plasmonic resonances of a thin film of gold nanorods (GNRs) are induced through the modulation of the local refractive index of the neighboring dielectric medium. This change is enabled through light-induced surface reorientation of a nematic liquid crystal in contact with a nanometer-thin photoalignment layer coating the GNR film. The presence of isolated and well distributed GNRs, both before and after the photoalignment layer deposition, is shown through atomic force and scanning electron microscopy. Exposure of the photoalignment layer to polarized light is shown to reorient aligned nematic liquid crystal molecules to an orthogonal direction, thereby changing the local refractive index of the medium in close proximity to the GNR film. This large change in the local dielectric strength is shown to cause a broad red shift of the localized plasmonic longitudinal resonance and almost no shift in the local transverse resonance. The ability to remotely and quickly change the plasmonic properties of this GNR system marks a breakthrough towards the realization of all-optical plasmonic and photonic devices such as plasmonic colour filters.

Pigments based on silica-coated gold nanorods: Synthesis, colouring strength, functionalisation, extrusion, thermal stability and colour evolution

The intense plasmon absorption bands of gold nanorods (GNRs) with peak extinction coefficients up to 6.4×109 M−1 cm−1 as well as their expected high stability make GNRs promising candidates for the colouration of bulk materials. The comparison of the integrated absorption in the visible region of GNRs with those of commercial organic pigments shows that the colouring strength of GNRs is 4 to 8 times higher.In order to improve their stability, GNRs were encapsulated in a silica shell of around 15 nm thickness using an optimized Stöber method. The silica surface was modified with octadecylsilane to enable their dispersion in non-polar media. Different plastics were successfully coloured with a tiny quantity of bare and functionalised GNRs@SiO2. These rods were homogeneously dispersed using extrusion. The shape of the rods was effectively stabilised by the silica shell at high temperature during the extrusion process. Surprisingly, a slight modification of the rods colour was observed due to a decrease of the refractive index in the mesoporous silica shell. However, this effect is greatly limited after the functionalisation.

Fluorescence enhancement in large-scale self-assembled gold nanoparticle double arrays

Localized surface plasmon resonances excited in metallic nanoparticles confine and enhance electromagnetic fields at the nanoscale. This is particularly pronounced in dimers made from two closely spaced nanoparticles. When quantum emitters, such as dyes, are placed in the gap of those dimers, their absorption and emission characteristics can be modified. Both processes have to be considered when aiming to enhance the fluorescence from the quantum emitters. This is particularly challenging for dimers, since the electromagnetic properties and the enhanced fluorescence sensitively depend on the distance between the nanoparticles. Here, we use a layer-by-layer method to precisely control the distances in such systems. We consider a dye layer deposited on top of an array of goldnanoparticles or integrated into a central position of a double array of goldnanoparticles. We study the effect of the spatial arrangement and the average distance on the plasmon-enhanced fluorescence. We found a maximum of a 99-fold increase in the fluorescence intensity of the dye layer sandwiched between two goldnanoparticle arrays. The interaction of the dye layer with the plasmonic system also causes a spectral shift in the emission wavelengths and a shortening of the fluorescence life times. Our work paves the way for large-scale, high throughput, and low-cost self-assembled functionalized plasmonic systems that can be used as efficient light sources.

Bottom-up Organisation of Metallic Nanoparticles

This chapter deals with bottom-up strategies that allow one to prepare amorphous assemblies of metal nanoparticles. Within these assemblies the nanoparticles couple to each other, affecting the effective electromagnetic properties of the materials. As a consequence, besides the properties of the individual particles, parameters such as number of individual particles within the assembly, geometry of the assembly and average distance between particles within the assembly can be used to design the optical properties of a material. It is therefore highly desirable to control these parameters with high precision, which is the art of self-assembly. Compared to top-down lithographic methods the bottom-up self-assembly approach is cheap and enables the fabrication of large area two-dimensional or three-dimensional samples, making it attractive for applications. In the following, after an introduction, different strategies that were used in the past to assemble nanoparticles into defined structures are briefly discussed. Such strategies rely on templates such as liquid crystals, DNA or surfactants. A versatile approach, which relies on charge-driven self-assembly mediated by charged surfaces and polyelectrolytes, is then discussed in more detail. This approach easily allows one to build large scale amorphous layered structures of nanoparticles with high control of parameters such as distance between particles within one layer and distance between the layers. The method is not restricted to flat surfaces and can be used to coat for example silica beads, resulting in core–shell structures. An attempt has also made to rationalise the observed optical properties in terms of coupling between particles within the different assemblies, thus paving the way to the design of materials with novel electromagnetic properties.

Cloaking dielectric spheres by a shell of plasmonic and polaritonic nanoparticles

This paper is dedicated to the study of plasmonic cloaking, based on the use of appropriate core-shell systems that may act as a cloaking devices for a finite range of frequencies. This cluster consists of an amorphous arrangement of metallic (gold or silver) and/or polaritonic nano-particles, which could be approximated in the quasistatic limit by an effective medium, having interesting properties such as a negative or very low permittivity and/or permeability in the optical domain with moderate losses. We first derive the effective properties of a shell made of such small spheres using the Maxwell-Garnett and Clausius-Mosotti formulas. We then numerically show that a dielectric core sphere is almost made invisible at optical frequencies with a scattering reduction of more than 70 percent. We finally derive some analytical expressions that we have compared to rigorous numerical simulations.

Plasmonic nanoparticles assemblies: preparation, structural, and optical properties

Self-assembly techniques are used to build complex amorphous structures from plasmonic particles. The assembly makes use of surface chemistry and intermolecular interactions between surfaces, surfactants, polymers and particles. The resulting two- or three-dimensional structures have optical properties that derive from the coupling between particles. A high control of the structural parameters on the nanometer scale can easily be achieved. In contrast to top-down techniques relatively large areas can be prepared in a versatile manner thus paving the way to applications as functional devices. Several structures are discussed such as layered arrays of gold nanoparticles, core-shell structures and hierarchical structures. The optical properties of these structures are also presented and compared with simulations. Some of the structures are of interest for plasmonic cloaking whereas other might find applications as substrates for sensing by surface-enhanced Raman spectroscopy.

Bottom-up metamaterials with an isotropic magnetic response in the visible

A theoretical framework to analyze the optical properties of amorphous metamaterials made from meta-atoms which are amenable for a fabrication with bottom-up technologies is introduced. The achievement of an isotropic magnetic resonance in the visible is investigated by suggesting suitable designs for the meta-atoms. Furthermore, two meta-atoms are discussed in detail that were fabricated by self-assembling plasmonic nanoparticles using techniques from the field of colloidal nanochemistry. The metamaterials are experimentally characterized by spectroscopic means and the excitation of the magnetic dipole moment is clearly revealed. Advantages and disadvantages of metamaterials made from such meta-atoms are discussed.

Gold nanoparticles embedded in flexible materials: new frontiers in Plasmonics

Fabrication of samples showing plasmonic properties is fundamental for the realization of devices that can exhibit peculiar electromagnetic properties. Here we illustrate results of experiments performed on systems with Au nanoparticles embedded in flexible substrates.